15th International Conference on Muon Spin Rotation, Relaxation and Resonance

28 Aug 2022, 09:00 → 2 Sep 2022, 23:00 Europe/Rome

Science and Technology Campus, University of Parma

Adrian Hillier (STFC / UKRI) , Roberto De Renzi (University of Parma)

                         

The long delayed MuSR2020 conference will run Monday 29th August to Friday 2nd September, 2022, with a student day on Sunday 28th August. An in-person meeting will be held at the Science and Technology Campus, University of Parma, Italy.

The Call for Abstracts is now open for POSTER PRESENTATIONS ONLY, closing Monday 15th August, the closing date for conference registration. Limited Hybrid options continue to be available.

The conference is being jointly organised by the muon group at the University of Parma in Italy and the ISIS muon group at the Rutherford Appleton Laboratory in the UK. It will cover all aspects of the use of muon spectroscopy in condensed matter, materials and molecular sciences, while also considering applications of muons in other areas, such as nuclear physics, cultural heritage and the study of single event failures arising from the irradiation of microelectronics.

 

Conference Excursion Financial Times 19 settembre 2020.pdf

Conference Information

• How to insert a Figure when writing an abstract in Indico.pdf
• Living with COVID.pdf
• Pay registration at MuSR2020 Parma.pdf

Publicity Material MuSR2020_Conf_Poster_v9.12.pdf

Sunday, 28 August

09:30 → 11:00 Student Day: Tutorial

Conveners: Adam Berlie (Science and Technology Facilities Council) , Mark Telling (STFC)

09:30 Introduction

Speaker: Prof. Roberto De Renzi (University of Parma)

09:35 Introduction to muon spin spectroscopy

Speaker: Francis Pratt (STFC)

10:00 Computational techniques to support muon science

Speaker: Leandro Liborio (Scientific Computing Department, Rutherford Appleton)

10:25 Instrumentation for muon spectroscopy

Speaker: Dr Thomas Prokscha (PSI)

10:50 Discussion

11:00 → 11:30 Break

11:30 → 12:30 Student Day: Student Talks I

Conveners: Adam Berlie (Science and Technology Facilities Council) , Mark Telling (STFC)

11:30 Nuclear magnetic resonance of $^8$Li ions implanted in ZnO

ZnO is a wide direct bandgap (3.4 eV) semiconductor with promising electronic properties potentially useful in room temperature optoelectronic and spintronic devices. It can be used as a dilute magnetic semiconductor by tuning intrinsic or extrinsic magnetic defects while ZnO also demonstrates many unique surface effects such as a photogenerated metallic state. Imperative to utilizing these unique properties is understanding and controlling point defects in its hexagonal wurtzite structure that may lead to stable hole doping. We implanted a low energy (20-25 keV) beam of hyperpolarized spin-2 8Li ions and used β-detected nuclear magnetic resonance (β-NMR) to understand the stability, structure, and magnetic state of Li defects in ZnO [Adelman et al., arXiv:2109.08637v1]. Closely related to μSR used to characterize isolated hydrogen impurities in ZnO, β-NMR allows complementary investigations of light isotope dopants in the ultradilute limit.

Using 8Li simultaneously as the defect and probe, distinct Li sites are detected by measuring the coupling of the nuclear electric quadrupole moment to the asymmetric electronic charge distribution surrounding the 8Li nucleus. From 7.6 to 400 K, we find ionized shallow donor interstitial Li is exceptionally stable, verifying its role in self-compensation of the acceptor (Zn) substitutional. Like the interstitial, the substitutional defect shows no resolved hyperfine field above 210 K, indicating it is a shallow acceptor. By pulsing the 8Li beam, the spin-lattice relaxation is measured and indicates above 300 K the onset of correlated local motion of interacting defects. This is supported by resonance spectra collected with a CW frequency comb that enhances the amplitude of well-resolved quadrupolar multiplets and confirms a site change transition from disordered interstitial Li to the substitutional. The quadrupole hyperfine interaction exhibiting a T3/2 temperature dependence typical of non-cubic metals is also discussed.

Speaker: Jonah Adelman (University of British Columbia)

11:45 A wolf in sheep's clothing? Muon-induced magnetism in quantum spin ice

Compounds of the form $\rm{A}_2\rm{X}_2\rm{O}_7$ with the pyrochlore structures can exhibit classical or quantum spin ice behaviour if the crystal field environment of the $\rm{A}\rm{O}_8$ arrangement leads to the [111] easy-axis anisotropy. When Pr occupies the A-site, there is a low-lying electronic doublet and $\rm{Pr}_2\rm{X}_2\rm{O}_7$ compounds are found to be quantum spin ices$^1$. Pr$^{3+}$ is a non-Kramers ion and the presence of the muon can distort nearby $\rm{PrO}_8$ units and split the doublet ground states$^2$, resulting in an enhancement of the Pr nuclear moment due to hyperfine coupling with the electronic moments$^3$. We explore this effect using a theoretical model that takes account of the important interactions and compare our simulations with $\mu$SR data on samples of $\rm{Pr}_2\rm{X}_2\rm{O}_7$ (X = Sn, Hf, Zr) and new experimental data on $\rm{Pr}_2\rm{ScTaO}_7$, a candidate system that simultaneously realises spin ice and charge ice structures.

References:
1. A. Princep, Phys. Rev. B 88, 104421 (2013)
2. F. Foronda et al., Phys. Rev. Lett. 114, 017602 (2015)
3. B. Bleaney, Physica 69, 317 (1973)

Speaker: Hank Wu (Oxford University)

12:00 Negative muon spin relaxation in water and ice

Muons are the main component of cosmic ray particles on the earth, and most of the cosmic ray muons are injected into water or ice, which occupy more than 70% of the earth's surface. When negative muons ($\mu$$^-$) stop in H$_2$O, they are mainly trapped by oxygen nuclei and form muonic oxygen atoms O$\mu$$^-$, and about 15% of O$\mu$$^-$ atoms finally change to stable nitrogen isotopes $^{14}$N or $^{15}$N via the neutron emission after the muon capture process. The nitrogen isotopes produced by such a process may be chemically active due to their high recoil energy and may form various nitrogen compounds through reactions with water molecules. In this situation, $\mu^-$SR spectroscopy is suitable for studying the behavior of such active nitrogen in H$_2$O, since O$\mu^-$ atoms also act chemically as nitrogen. In the present study, we measured $\mu^-$SR spectra in water and ice to approach what kind of nitrogen compounds are formed by cosmic-ray negative muons, and how they affect the surrounding chemical environment.
Experiments were carried out at the D1 beamline in the Materials and Life Science Experimental Facility (MLF) of J-PARC. H$_2$O and D$_2$O samples were irradiated with a negative muon beam (47 MeV/c, double pulse), and ZF and LF-$\mu$$^-$SR spectra were measured. The result shows that the relaxation due to the nuclear dipolar field is observed in solid H$_2$O and D$_2$O at 200 K. The field distribution widths were deduced to be $\Delta_H$=0.27 $\mu$$s^{-1}$ and $\Delta_D$=0.066 $\mu$$s^{-1}$, for H$_2$O and D$_2$O respectively. The relationship between these two values is well explained by the difference in the spins and magnetic moments of proton and deuteron.
In this conference, we will discuss possible chemical states based on the present results.

Speaker: Yoko Kimura (Dept. Phys. Osaka Univ.)

12:15 Calculating muon sites and couplings from a high-throughput modelling perspective

In muon spin spectroscopy, the knowledge of muon implantation sites and hyperfine couplings is of importance to the analysis of the experimental data. Over the past decade there has been significant progress in calculating muon sites using first-principles methods such as density functional theory (DFT) [1,2].  However, the protocols required for muon calculations are both resource and task intensive. They are performed sequentially in steps with strenuous human intervention required to track, coordinate and analyse these calculations. The recent advent of the DFT-based high-throughput (HT) approach and the development of dedicated frameworks has opened the possibility of performing this type of sequential large-scale calculations in an efficient way. Here, we present our efforts towards the design and implementation of workflows within the AiiDA integrated platform for high-throughput DFT-based muon calculations aimed at i) the design of a user-friendly approach available to every muon user; ii) benchmarking the scope of sustainable DFT calculations. We started from identifying material selection criteria to exclude the well-known harder cases. We have benchmarked the workflow at its current stage over 16 magnetic compounds. Our preliminary benchmark results demonstrated the feasibility of this plan and have further allowed us to understand the workflow capabilities, its limitations and the likely improvements to be considered for more accurate results of the calculated muon properties. These improvements include; taking into account the muon charge states and spotting the right compromise between sustainable and accurate treatment of electronic correlation effects.

References
[1] J. S. Möller et al., Phys. Scr., 88, 068510 (2013)
[2] P. Bonfà et al., J. Phys. Soc. Jpn, 85, 091014 (2016)
[3] M. M. Isah et al., Ph.D. thesis, University of Parma (2022)

Speaker: Dr Muhammad Maikudi ISAH (Department of Mathematical, Physical and Computer Sciences, University of Parma, 43124 Parma, Italy)

12:30 → 13:45 Lunch

(self catering)

13:45 → 14:45 Student Day: Student Talks II

Conveners: Adam Berlie (Science and Technology Facilities Council) , Mark Telling (STFC)

13:45 Development and test of a TDC and amplifier circuit for a multi-channel positron detector.

In a continuous beam muon facility positrons are detected by relatively large plastic scintillators without position sensitivity. An idea has been proposed to make these positron detectors multi-channel and able to track the positron trajectories. This will ultimately enable 2-dimensional magnetic imaging of the sample with the µSR technique. To attain this “muon microscope” idea, large numbers of independent photosensors with high-timing resolution will be necessary.

Our group at KEK has developed an amplifier-shaper-discriminator (ASD) circuit named FGATI with 16 channels per chip and a high-resolution time to digital converter, called HR-TDC with a timing resolution on the order of picoseconds. Silicon photomultipliers (SiPMs) from Hamamatsu (MPPC) are employed to give electric pulses for the optical input [1-2]. We have been testing this new set-up at TRIUMF with a pulsed laser to understand the efficiency, transient response, timing resolution, and the data acquisition to a computer. We are now successfully detecting the rising and falling edge timing as well as the time-over-threshold (TOT) of the laser pulses.

The tested circuit will be a basis for the light detection and time recording from scintillation fiber arrays to be used for the multi-channel positron detectors. Multiple layers of such detectors will establish tracking the positron trajectory and aid with the development of the “muon microscope”.

This work is partially supported by a Grant-in-Aid for Scientific Research (No.JP21H04666) from Japan Society for the Promotion of Science (JSPS).

Reference
1 K.M. Kojima et al, JPS Conf. Proc., 21, 011062 1-6, (2018).
2 K.M. Kojima et al, J. Phys: Conf. Ser., 551, 012063, (2014).

Speaker: Marta-Villa de Toro Sanchez (Centre for Molecular and Materials Science, TRIUMF – School of Physics and Astronomy, The University of Edinburgh)

14:00 Inducing Quantum Criticality in CrCl$_3$ Under Pressure

Accelerated by the discovery of graphene, research on two-dimensional (2D) materials have attracted tremendous attention both from fundamental and applied sciences. Among the large number of 2D materials, chromium trihalides CrX3 (X = Cl, Br, I) van der Waals (vdW) magnets have also raised a large interest due to the existence of many magnetic subtleties that cannot be explained by their magnetic and/or structural transitions.
Numerous studies were performed on CrI3, but only a few have been reported so far on its analogue CrCl3. The 2D vdW CrCl3 compound is stabilized under a rhombohedral symmetry, consisting of 2D Cr layers arranged in a honeycomb web fashion and surrounded by octahedrally coordinated Cl, with weak vdW inter-layers coupling. The layer structure and inter-layer coupling make CrCl3 an ideal system to study under external stimuli such as pressure or magnetic field, where new intriguing states of matter can be unveiled. With such expectations, studies of CrCl3 under room temperature, high pressure have been reported[1]. However, its spin dynamics at low-temperature and high-pressure regime remain unexplored.
In this study, we present the results of our recent muon spin rotation (MuSR) investigations performed on hydrostatically pressured CrCl3. Our previous MuSR results at ambient pressure revealed successive transitions from paramagnetic to short-ranged-order-ferromagnetic then to antiferromagnetic states with strong spin dynamics as the temperature decreases[2]. When applying pressure, we observed that the magnetic ground state is gradually suppressed. A linear extrapolation points toward the suppression of magnetism at about $p_c$= 30 kbar indicating the possible existence of a quantum critical point at $p_c$.[3]

[1] Ahmad, Azkar Saeed, et al. "Pressure-driven switching of magnetism in layered CrCl3." Nanoscale 12.45 (2020): 22935-22944.
[2] Forslund, Ola Kenji, et al. "Spin dynamics in the Van der Waals magnet CrCl3." arXiv preprint arXiv:2111.06246 (2021).
[3] Ge, Yuqing, et al., in preparation.

Speaker: Yuqing Ge (Department of Physics, Chalmers University of Technology, SE-41296 Göteborg, Sweden)

14:15 Possible p-wave parity in Cr-based superconductor Pr$_3$Cr$_{10-x}$N$_{11}$

Superconductivity with a critical temperature $T_C$ $\sim$ 5.25 K was recently reported in the Cr-based superconductor Pr$_3$Cr$_{10-x}$N$_{11}$. The large upper critical field $H_{C2}$ $\sim$ 20 T, and the strong correlation between 3$d$ electrons derived from specific heat, suggest the unconventional superconductivity nature of this compound. We performed muon-spin rotation/relaxation ($\mu$SR) measurements on a high-quality polycrystalline of Pr$_3$Cr$_{10-x}$N$_{11}$ down to 0.027 K, and specific heat measurements under different magnetic fields up to 9 Tesla. Our $\mu$SR data indicate that time-reversal symmetry is broken in the superconducting state of Pr$_3$Cr$_{10-x}$N$_{11}$, and the superconducting energy gap is consistent with a $p$-wave model, which is also supported by the specific heat data.

Speaker: Changsheng Chen (Fudan University)

14:30 Data analysis for µSR experiments with negative muons.

Negative muons are often overlooked compared to their positive counterpart, partly due to the loss of around $\frac{5}{6}$ of the $\mu^{-}$ spin polarisation when a $\mu^{-}$ cascades down to the 1s muonic ground state after being captured by a nucleus. One needs to count for around 36 times as long to get statistics comparable to that of a $\mu^{+}$SR experiment. However, there has been a recent revival of $\mu^{-}$SR experiments, particularly in the study of hydrogen storage and battery materials [1,2]. When stopped in a material of atomic number $Z$, $\mu^{-}$ forms a muonic atom and cascades down to its ground state. The muon Bohr radius is 200 times smaller than the electron Bohr radius, and so this probe behaves like an ultra-dilute atom of apparent nuclear charge $Z-1$. The $\mu^{-}$ will be strongly hyperfine coupled to the nuclear spin of the capture atom, but if that nuclear spin is zero, such as an oxygen in MnO, the only coupling will be to the nuclear dipolar fields in a region very close to that capture nucleus. Because of these difficulties new analysis techniques have been developed in WiMDA [3] for the fitting of $\mu^{-}$SR data, and we have adapted the DFT+$\mu^{+}$ technique for the case of a negative muon. Both of these new techniques have been applied to MnO where the dipole field simulations show a large field at the oxygen site, and DFT+$\mu^{-}$ calculations show a Jahn-Teller-like distortion around the negative muon.

References
[1] J.Sugiyama et al Phys. Rev. Lett. 121, 087202 (2018).
[2] J. Sugiyama et al, Phys. Rev. Res. 2, 033161 (2020).
[3] F.L. Pratt, Physica B 289-290, 710 (2000)

Speaker: Mr George Gill (University of Oxford / ISIS)

14:45 → 15:15 Break

15:15 → 16:15 Student Day: Student Talks III

Conveners: Adam Berlie (Science and Technology Facilities Council) , Mark Telling (STFC)

15:15 Profiling defect and charge carrier density in the SiO$_2$/4H-SiC interface with Low-Energy Muons

Silicon carbide (4H-SiC) is a wide-bandgap semiconductor with promising applications in high-power and high-frequency devices. An advantage of SiC is that it is the only compound semiconductor that has the ability to form native silicon dioxide (SiO$_2$). The performance of SiC-based devices relies heavily on interface effects. However, characterization of oxidation-induced defects - both in the oxide and the semiconductor - is still challenging.
Low-energy muon spin spectroscopy (LE-$\mu$SR) can probe regions very close to the surface and interface up to a depth of 160 nm in SiO$_2$/SiC structures and is sensitive to charge carrier and defect concentrations.

We have studied SiO$_2$/SiC interfacial systems with thermally grown and deposited oxides using LE-$\mu$SR. The thermal SiO$_2$ has higher structural order, as indicated by the undisturbed muonium (Mu$^0$) formation. However, the oxidation process leads to strain in the oxide and to band-bending at the SiC-side of the interface, which affects the SiC faces differently: i) at the (0001) Si-face the results can be explained by the depletion of electrons at the interface and ii) at the (000$\overline{1}$) C-face a carbon-rich n-type region contributes to the increase of the diamagnetic fraction due to Mu$^-$ formation.
Further investigations have been conducted to understand the passivation effects of state-of-the-art post-oxidation annealing (POA) processes on the SiO$_2$/SiC interface. Particularly, POA in an NO environment leads to an increase in charge carrier concentration near the interface, likely due to N acting as a dopant, which can be quantified based on the measured diamagnetic fraction.

Speaker: Maria Mendes Martins (Laboratory for Muon Spin Spectroscopy, Paul Scherrer Institute)

15:30 Development of magnetic resonance imaging (MRI) system using beta-NMR technique

Today, the technology of magnetic resonance imaging (MRI) has been established and it is essential in the medical field. MRI is the method of making an in-situ image by utilizing nuclear magnetic resonance (NMR). However, the MRI technique has rarely been put to practical use for elements other than hydrogen because of the sensitivity issue. On the other hand, the technique of beta-ray-detecting NMR (beta-NMR) makes it possible to observe NMR for various elements with extremely high sensitivity by measuring the asymmetry of the beta-ray emission from polarized radioisotopes (RIs). By utilizing beta-NMR, we aim to create a 3-dimensional (3D) MRI system. We have developed a detector set with plastic scintillation fibers, which enables us to track back the trajectory of beta-rays. Moreover, by seeking the beta-ray asymmetry at each position in the sample, we can create a magnetic resonance image. We conducted experiments using a spin-polarized $^{12}\rm{B}$($I=1,T_{1/2}=20\;\rm{ms}$) beam at HIMAC heavy-ion synchrotron facility of the National Institutes for Quantum Science and Technology. We obtained the data from various samples of mixtures as well as simple substances. We have successfully obtained a 1D image of the beta-ray asymmetry for $^{12}\rm{B}$ in Si. The data analysis for 3D imaging are now in progress.
It is expected that this new technique will be applied to non-destructive and non-contact testing related to various fields such as medical and materials science.
In this conference, we will present our new results of the analyses. We will also show some idea that a combination of beta-NMR and mu-SR will expand this technique.

Speaker: Mr Takato Sugisaki (Dept. Phys. Osaka Univ.)

15:45 Time-reversal symmetry breaking in nonsymmorphic type-I superconductor YbSb$_2$

The interplay of superconductivity with nontrivial topological phases exhibit the fascinating topological superconductivity, which has attracted widespan attention from observing quasiparticle like Majorana fermions to its application in fault-tolerant quantum computation$^{1,2}$. It is proposed that the topological superconductivity can be realized in compounds having topological surface states and superconductivity$^3$. Only a few superconducting materials with nontrivial topological states have been discovered, and their superconducting ground state/pairing mechanism can not be adequately understood. Therefore, searching and studying the superconducting ground state of materials having nontrivial topological states is vital.
Here, we present the evidence of time-reversal symmetry breaking (TRSB) in the nonsymmorphic type-I superconductor YbSb$_2$, having a distorted Sb square net crystal structure similar to the other topological system ZrSiS$^{4,5}$. The microscopic muon spin relaxation and rotation investigation confirm the fully gapped type-I superconductivity with broken time-reversal symmetry in its superconducting ground state. This indicates that the nonsymmorphic RSb$_2$ superconductors are an interesting class of materials that exhibit unconventional superconductivity with fascinating properties and warrant great potential for future studies.

References:
1. X. L. Qi et al., Rev. Mod. Phys. 83, 1057 (2011).
2. M. Sato et al., Rep. Prog. Phys. 80, 076501 (2017).
3. L. Fu et al., Phys. Rev. Lett. 100, 096407 (2008).
4. R. Wang et al., Inorg. Chem. 5, 1468 (1966).
5. S. Klemenz et al., Ann. Rev. Mat. Res. 49, 185 (2019).

Speaker: Ms Anshu Kataria (PhD Scholar)

16:00 Anomalous electrical transport in frustrated intermetallic RCuAs$_2$ : the role of spin

The Kondo effect was a longstanding theoretical puzzle, describing the scattering of conduction electrons in a metal due to dilute, localised d- or f -electron magnetic impurities and resulting in a characteristic minimum in electrical resistivity with temperature. Extended to a lattice of magnetic impurities, the Kondo effect likely explains the formation of so called heavy Fermion systems and Kondo insulators in intermetallic compounds, especially those involving rare earth elements like Ce, Pr and Yb. The hybrisation of the 4f electron states with the conduction band and resultant screening of local moments, required for Fermi liquid behavior in the Kondo lattice, competes with interactions between localised moments. The diversity in the low temperature properties of heavy Fermion metals, as well as their highly tunable nature (with magnetic field, pressure, chemical substitution), make these systems invaluable in the investigation of the emergent properties of highly correlated quantum materials.

Counterintuitively, in a class of ternary intermetallic compounds of the type RCuAs$_2$ (R = rare earth) [1], the rare earths like Sm, Gd, Tb, and Dy with strictly localised 4f character, where the Kondo effect is not anticipated, also exhibit a pronounced minimum in resistivity well above their respective magnetic ordering temperatures. Even more surprisingly, no such minimum is observed for Pr, Nd, and even Yb based members of this series. Recent theoretical predictions suggest geometric magnetic frustration plays a role [2]. More generally, frustration is thought to be an important additional tuning parameter in the Kondo lattice model. A muon spin relaxation investigation of these materials is discussed, shedding light on the role of magnetic fluctuations in determining the electronic transport in heavy Fermion materials.

[1] E.V. Sampathkumaran et al, Physical Review Letters 91, 036603 (2003);
[2] Zhentao Wang et al, Physical Review Letters 117, 206601 (2016)

Speaker: Mae Abedi (Department of Physics, Simon Fraser University)

16:00 → 20:05 Conference Event: Registration

Convener: anna claudia gray (università di Parma)

18:05 → 20:05 Social Programme: Welcome Reception

Convener: Prof. Roberto De Renzi (University of Parma)

Monday, 29 August

09:00 → 09:20 Conference Event: Opening Remarks

09:00 → 14:00 Conference Event: Registration

09:20 → 10:20 Invited Talks: Yamazaki Prize Giving and Lecture

09:20 Yamazaki Prize Giving

09:40 Yamazaki Prize Lecture - The quantum muon

The key physical process at the heart of the muon-spin rotation ($\mu$SR) technique is that the spin of the positive muon precesses in a local magnetic field, a process that can be modelled either classically (torque on a magnetic dipole) or quantum mechanically (interference between components in a superposition). However, some aspects of the muon's interaction with its environment bring out features which are purely quantum mechanical and have no classical analogue. Understanding this requires an accurate modelling of the muon site, only possible with modern electronic structure (DFT+$\mu$) methods. I will review a variety of examples of muon experiments on organic, molecular and inorganic systems which will highlight some important qualities of this viewpoint and demonstrate the utitlity of "the quantum muon".

(Yamazaki lecture)

Speaker: Stephen Blundell (University of Oxford)

10:20 → 10:40 Break

10:40 → 11:40 Oral contributions: Spin liquids and related phenomena I

10:40 Role of Many-Body Quantum Effects in μSR Measurements

For unambiguous interpretation of experimental µSR data, a thorough understanding of quantum zero-point motion (ZPM) of muons in materials is essential. Namely, while ZPM of light nuclei like hydrogen and lithium is known to play a pivotal role in the structure and dynamics of many important classes of materials$^{1,2}$, quantum effects of muons in solids can be even stronger due to the lower mass of muons (~1/9 the mass of a proton) and can qualitatively change the measured µSR signal$^{3,4}$.

There has been much interest in using ab initio computation of muon stopping sites in materials to aid in the interpretation of µSR measurements. However, most computational techniques employed have either neglected quantum muon ZPM, or applied poorly controlled approximations to it with little clarity around the limits of their applicability. To address this, we have developed a unified description of light-particle ZPM in materials$^{4}$, clarifying the roles many-body quantum entanglement and anharmonicity play in determining the true ZPM regime. As proof of concept we applied these insights to our precision µSR quadrupolar level-crossing measurements on solid nitrogen, α–N$_2$, where they allowed us to significantly improve the accuracy of the extracted $^{14}$N nuclear quadrupolar coupling constant. This represents the first improvement in its accuracy in over 45 years, despite the ubiquity of solid nitrogen in nature, and a validation of our unified description of light-particle ZPM.

$^{1}$T. E. Markland and M. Ceriotti, Nat. Rev. Chem. 2, 0109 (2018). $^{2}$E. Snider et al., Nature 586, 373 (2020). $^{3}$S. J. Blundell, R. De Renzi, T. Lancaster and F. L. Pratt, Muon Spectroscopy: An Introduction (Oxford University Press, Oxford, 2021). $^{4}$M. Gomilšek et al., arXiv:2202.05859.

Speaker: Matjaž Gomilšek (Jožef Stefan Institute, Jamova c. 39, SI-1000 Ljubljana, Slovenia)

11:00 Studying spin diffusion and quantum entanglement with LF-$\mu$SR

LF-$\mu$SR studies of spin diffusion started with mobile solitons [1] and polarons [2] in conducting polymers. Spin 1/2 antiferromagnetic chains can also support diffusive spin excitations in a certain parameter range of the XXZ model [3], showing either diffusive [4] or ballistic transport [5]. Recent LF-$\mu$SR studies of layered triangular lattice quantum spin liquid materials such as 1T-TaS$_2$ [6] and YbZnGaO$_4$ [7] have shown spin dynamics that is extremely well described by a 2D spin diffusion model, fitting much better than previously proposed models for spin correlations. In YbZnGaO$_4$ the diffusion rate shows a clear crossover between classical and quantum regimes as $T$ falls below the exchange coupling $J$. That the spin diffusion approach works well in the high $T$ classical region might be expected, but it is found that it also works equally well in the low $T$ quantum region. This allows a $T$ dependent length scale to be derived from the data that can be assigned to a quantum entanglement length $\xi$. Another entanglement measure, the Quantum Fisher Information $F_Q$ [8] can also be obtained from the LF-$\mu$SR data and compared with $\xi$.

[1] K. Nagamine et al, Phys. Rev. Lett. 53, 1763 (1984); [2] F.L. Pratt et al, Phys. Rev. Lett. 79, 2855 (1997); F.L. Pratt et al, Physica B 326, 34 (2003); [3] B. Bertini et al, Rev. Mod. Phys. 93, 025003 (2021); [4] F.L. Pratt et al, Phys. Rev. Lett. 96, 247203 (2006); F. Xiao et al, Phys. Rev. B 91, 144417 (2015); [5] T. Lancaster et al, Phys. Rev. B 85, 184404 (2012); B.M. Huddart et al, Phys. Rev. B 103, L060405 (2021); [6] S. Manas-Valero et al, npj Quantum Mater. 6, 69 (2021); [7] F.L. Pratt et al, Phys. Rev. B 106, L060401 (2022); [8] P. Hauke et al, Nat. Phys. 12, 778 (2016).

Speaker: Dr Francis Pratt (STFC)

11:20 Exploting magnetic interactions in Kitaev anti-ferromagnet Na$_{2}$PrO$_{3}$

Our goal is to analyze the magnetic properties of the Kitaev material Na$_{2}$PrO$_{3}$ by comparing Neutron Scattering (NS) and Muon Spin Spectroscopy (μSR) experiments, with the addition of ab initio calculations.

Alkali-metal lanthanide oxides are an exciting field of study due to their frustrated geometry and possibly anisotropic magnetic interactions, as shown in Fig.1.

In this class of materials, also known as Kitaev materials, the SOC energy is comparable to that induced by crystal-field excitations (CEF), and the small spatial extent of f-electron orbitals promotes anisotropic Kitaev terms.

Na$_{2}$PrO$_{3}$ crystallizes with a monoclinic unit cell, where edge-sharing PrO$_{6}$ octahedra forms a honeycomb lattice. The effective paramagnetic moment is 0.99 μB, less than the free Pr$^{4+}$ ion moment (2.54 μB), and the origin of its small value is still under debate. In addition, it displays a magnetic ordering transition at $T_{N}$ = 4.6 K. Previous powder diffraction measurements could not detect any signs of magnetic ordering, despite evidence in specific heat and magnetometry measurements. Moreover, preliminary magnetic neutron diffraction results do not reveal any clear magnetic Bragg peaks, probably due to the low value of Na$_{2}$PrO$_{3}$ effective paramagnetic moment.

The principal question that motivated our work was to try to explain the small effective paramagnetic moment, considering the presence of the magnetic ordering. Thanks to the muon’s extreme sensitivity to small-moment magnetism, here μSR is highly relevant. From this, Na$_{2}$PrO$_{3}$ shows coherent oscillations of the muon asymmetry below $T_{N}$, reflecting the presence of an anti-ferromagnetic (AFM) ordering.

In comparison with experimental data, combined ab initio calculations and dipolar simulations were performed in order to elucidate the nature of AFM ordering inside this material and to try to explain the small value of the effective paramagnetic moment.

Speaker: Jonathan Frassineti (University of Bologna)

11:40 → 12:20 Invited Talks: Invited Talk I

11:40 What high resolution RIXS can tell us of cuprates (and of other quantum materials)

Resonant Inelastic X-rays Scattering (RIXS) is an energy loss spectroscopy made with x rays whose energy is tuned to a suitable absorption edge. When the instrumental resolution is good enough, RIXS spectra provide information on the energy, dispersion and symmetry of local and collective excitations, such as ligand field excitations, magnons and paramagnons, phonons, particle-hole pairs, charge density fluctuations and order. RIXS is a powerful complement of more traditional techniques like inelastic neutron scattering, Raman scattering, electron energy loss spectroscopy.
The rich physics of cuprates is very effectively captured by high resolution RIXS experiments made at Cu L3 and O K edges. This fortunate conjuncture has boosted the development of better and better instrumentation at synchrotrons and has served as one of the scientific cases for RIXS at XFELs. The field is expanding and experiments are leading to more insightful results, where the different degrees of freedom are organically studied.
After introducing the technique, I will provide a survey of results on cuprate parent compounds [1] and superconductors [2,3] and on infinite layer nickelates [4], which share several properties with high Tc superconductors.

References

1. Martinelli, L, Betto, D., et al. Fractional spin excitations in the infinite-layer cuprate CaCuO2, Phys. Rev. X 12, 021041 (2022)
2. Arpaia, R., Caprara, S., et al, Dynamical charge density fluctuations pervading the phase diagram of a Cu-based high-Tc superconductor, Science 365, 906 (2019)
3. Peng, Y.Y, Martinelli, L, et al. Doping-dependence of the electron-phonon coupling in two families of bilayer superconducting cuprates. Phys. Rev. B 105, 115105 (2022).
4. Krieger, G., Martinelli, L. et al. Charge and spin order dichotomy in NdNiO2 driven by SrTiO3 capping layer arXiv:2112.03341 (2021)

Speaker: Prof. Giacomo Ghiringhelli (Politecnico di Milano)

12:20 → 13:00 Oral contributions: Strongly correlated electron systems I

12:20 Insights into skymion-hosting materials from implanted muons

Low-dimensional magnetism continues to be of great theoretical and experimental interest, as reduced dimensionality supports strong fluctuations that can result in novel states and excitations. One theme in this field is the understanding of magnetism in reduced dimensions using notions from topology. Examples include topological objects such as walls, vortices and skyrmions, which can potentially exist in the spin textures of a range of systems. In recent years, the experimental discovery of skyrmions in magnetic materials and of their self-organization into a skyrmion lattice, together with their potential for use as high density, low-energy sensors and magnetic storage, has made the investigation of such magnetic topological objects particularly important$^{1}$.

Here we report insights gained from our muon-spin spectroscopy ($\mu^{+}$SR) investigations of materials with topological excitations, including: (i) order and dynamics in GaV$_{4}$S$_{8-y}$Se$_{y}$, a system hosting Néel skyrmions in which $\mu^{+}$SR shows how their stability is enhanced through chemical substitution and the application of pressure$^{2}$; (ii) the skyrmion-hosting multilayer system Ta/[CoFeB/MgO/Ta]$_{16}$, where low-energy $\mu^{+}$SR uniquely reveals changes in the magnetic structure with depth into the multilayer stack; (iii) Cr$_{1/3}$NbS$_{2}$, which hosts topological soliton excitations, and where we show that the magnetism is determined directly by features in the electronic bandstructure$^{3}$. These investigations demonstrate how the combination of $\mu^{+}$SR, magnetometry and electronic structure calculations, both to determine muon sites and more generally, can be used to achieve additional insights into the underlying magnetic behaviour.

$^{1}$T. Lancaster, Contemp. Phys. 60, 246 (2019). $^{2}$T.J. Hicken et al., Phys. Rev. Research 2, 032001(R) (2020); Phys. Rev B 105, 134414 (2022). $^{3}$T.J. Hicken et al. Phys. Rev. B 105, L060407 (2022).

Speaker: Tom Lancaster (Durham University)

12:40 Studying the evolution of the metallic state in LaNiO$_3$ from a single crystal to superlattices with $\beta$-detected NMR

The rare-earth nickelates (RNiO$_3$) are a prototypical example of a metal-insulator transition. Among the RNiO$_3$, LaNiO$_3$ is unique in remaining metallic, although highly correlated. Interestingly, superlattices with insulating interlayers of LaAlO$_3$, can be driven insulating and antiferromagnetic if they are thin enough$^{1}$. We have used $^8$Li $\beta$-detected NMR ($\beta$-NMR), to study LaNiO$_3$ as a single crystal, thin film, and in superlattices with LaAlO$_3$. We observe biexponential spin-lattice relaxation which we attribute to electronic phase separation$^{2,3}$. In the single crystal and bulk-like thin film, both phases appear metallic$^{2}$. However, in the ultrathin layers of the superlattices, the behaviour of one of the phases appears magnetic at low temperature$^{3}$.
1. A. V. Boris et al., Science 332, 937 (2011)
2. V. L. Karner et al., Phys. Rev. B 100, 165109 (2019)
3. V. L. Karner et al., Phys. Rev. B. 104, 205114 (2021)

Speaker: Victoria Karner (TRIUMF)

13:00 → 14:00 Lunch

14:00 → 14:40 Invited Talks: Invited Talk II

14:00 Unsupervised machine learning of muons experiments – why?

Zero-field muon spin relaxation experiments probe directly the intrinsic magnetic fields that arise spontaneously in a given material. The full understanding of such experiments requires a microscopic description of the material under investigation, including its electronic state and the complex interactions between the muon and the material’s electronic and structural degrees of freedom. However, paradoxically, such experiments can also yield crucial information about poorly-understood systems, well before we know enough about them for such detailed modelling. In this talk I will ask two questions: “How is this possible?” and “Can we do it better?” To address the first question I will review the particular cases of LaNiC$_2$ and LaNiGa$_2$, two closely related superconductors where the case for an exotic, time-reversal symmetry breaking pairing state is now well established, with muons experiments having played the key role. I will describe how we got to this point, emphasising the prudent use of phenomenological fitting functions and group-theoretical analyses. I will argue that while such approach cannot substitute detailed microscopic modelling (which has to have the final word) it can be crucial to get us to the point where the latter becomes feasible. I will then address the second question, specifically asking whether there is room for improvement in the way we tackle muons data phenomenologically. I will introduce the concept of unsupervised machine learning, using Principal Component Analysis and Auto-encoders as paradigmatic examples. I will propose that unsupervised machine learning can be used to find compact descriptions of muons data, helping with detection of phase transitions and material classification, without requiring either a microscopic theory or phenomenological fitting functions. I will illustrate this with muons data on real magnetic and superconducting materials and introduce simple software tools that can be used to carry out similar analyses.

Speaker: Jorge Quintanilla (University of Kent)

14:40 → 15:40 Oral contributions: Superconductivity I

14:40 Time-reversal symmetry-breaking charge order in a kagome superconductor

The kagome lattice, the most prominent structural motif in quantum physics, benefits from inherent nontrivial geometry to host diverse quantum phases, ranging from spin-liquid phases, topological matter to intertwined orders, and most rarely unconventional superconductivity. Recently, charge sensitive probes have suggested that the kagome superconductors AV3Sb5 (A = K, Rb, Cs) [1] exhibit unconventional chiral charge order. However, direct evidence for the time-reversal symmetry-breaking of the charge order remained elusive. We utilized muon spin relaxation to probe the kagome charge order and superconductivity in (K,Rb)V3Sb5 [2,3]. We observe a striking enhancement of the internal field width sensed by the muon ensemble, which takes place just below the charge ordering temperature and persists into the superconducting state. Remarkably, the muon spin relaxation rate below the charge ordering temperature is substantially enhanced by applying an external magnetic field. We further show [3] that the superconducting state displays a reduced superfluid density, which can be attributed to the competition with the novel charge order. Upon applying pressure, the charge-order transitions are suppressed, the superfluid density increases, and the superconducting state progressively evolves from nodal to nodeless. Our results point to the rich interplay and accessible tunability between unconventional superconductivity and time-reversal symmetry-breaking charge orders in the correlated kagome lattice, offering new insights into the microscopic mechanisms involved in both orders.

[1] Y.-X. Jiang et. al., Nature Materials 20, 1353 (2021).
[2] C. Mielke et. al., and Z. Guguchia, Nature 602, 245-250 (2022).
[3] Z. Guguchia et. al., arXiv:2202.07713v1 (2022).

Speaker: Dr Zurab Guguchia (Laboratory for Muon Spin Spectroscopy, Paul Scherrer Institute, Switzerland)

15:00 Two-component superconductivity in Sr$_2$RuO$_4$ studied by uniaxial and hydrostatic pressure $\mu$SR

After two decades of research, the symmetry of the superconducting state in Sr$_2$RuO$_4$ is still under strong debate. The long time favoured spin-triplet px + i py state is ruled out by recent NMR experiments (1). However, in general time-reversal-symmetry breaking (TRSB) superconductivity indicates complex two-component order parameters. Probing Sr$_2$RuO$_4$ under uniaxial pressure offers the possibility to lift the degeneracy between such components (2). One key prediction for Sr$_2$RuO$_4$, a splitting of the superconducting and TRSB transitions under uniaxial pressure has not been observed so far.
Here, we report results of muon spin relaxation (μSR) measurements on Sr$_2$RuO$_4$ placed under uniaxial stress (3). We observed a large pressure-induced splitting between the onset temperatures of superconductivity (T$_c$) and TRSB (T$_{\mathrm{TRSB}}$). Moreover, at high stress beyond the van Hove singularity, a new spin density wave ordered phase is observed.
To distinguish between a symmetry protected chiral state (d+id) and non-chiral accidentally degenerated order parameters (d+ig, f+ig) we also report $\mu$SR studies under symmetry conserving hydrostatic pressure. In these experiment no splitting between T$_c$ and T$_{\mathrm{TRSB}}$ is observed (4).
In this talk we discuss the implications on the superconducting order parameter in Sr$_2$RuO$_4$.

*This work was supported by DFG (GR 4667, GRK 1621, and SFB 1143).
(1) A. Pustogow, et al., Nature 574, 72 (2019)
(2) C. Hicks, et al., Science 344, 283 (2014), M. E. Barber, et al., Phys. Rev. Lett. 120, 076602 (2018).
(3) V. Grinenko, S. Ghosh, et al.,  Nat. Phys. (2021)
(4) V. Grinenko, et al., Nat. Comm. (2021)

Speaker: Hans-Henning Klauss (TU Dresden)

15:20 Ubiquitous Spin Freezing in Spin-Triplet Superconductor UTe$_2$

The novel superconductor UTe$_2$ is a rare material wherein electrons form Cooper pairs in a unique spin-triplet state with potential topological properties. Theoretically, spin-triplet superconductivity in UTe$_2$ may be explained in terms of pairing mediated by either ferromagnetic or antiferromagnetic fluctuations, but experimentally the magnetic properties of UTe$_2$ remain enigmatic. Here we report on a $\mu$SR study of independently grown UTe$_2$ single crystals that exhibit either a single or double phase transition in the specific heat near the onset of superconductivity. In the absence of an applied magnetic field, we observe an inhomogeneous distribution of magnetic fields in a sizeable volume fraction of all samples studied. The growth in the volume of the magnetic regions is halted by the onset of superconductivity at the critical temperature $T_c$. Upon further cooling, slow fluctuations of the local fields persist until a disordered spin frozen state appears below about one tenth of $T_c$. The $\mu$SR results are consistent with the formation of magnetic clusters in UTe$_2$ due to the influence of disorder on long-range electronic correlations or geometrical magnetic frustration associated with the ladder-like U sublattice structure. Our findings suggest that inhomogeneous magnetic clusters are responsible for the ubiquitous residual linear term and low-temperature upturn in the temperature dependence of the specific heat in UTe$_2$ below $T_c$. The omnipresent magnetic inhomogeneity may also have implications for the interpretation of other low-temperature experimental observations in the superconducting state of UTe$_2$.

Speaker: Prof. Jeff Sonier (Department of Physics, Simon Fraser University)

15:40 → 16:00 Break

16:00 → 17:20 Oral contributions: Muonic X-rays

16:00 Understanding Rome’s ancient past: An analysis of gold and silver coinage from the Roman Empire

The fineness and quality of a state’s coinage is often used as a proxy for its fiscal health, meaning the purity and chemical composition of coins are of real historical interest. Sampling of such objects is often at the surface or near-surface, but, in ancient coinages, these areas can be unrepresentative of the bulk alloy – sometimes radically so. In this paper, investigations on Roman gold and silver coinage using muonic X-ray emission spectroscopy (μXES) are reported.
Most Roman silver coins were produced from an alloy of copper and silver. Mints were able to disguise debasements from the general public by heating the silver-copper alloy blanks, oxidising the copper at the surface, and then soaking them in an organic acid. This stripped the copper from the surfaces of the blanks, causing a honeycomb structure of nearly pure silver to be consolidated as a rich layer when they were struck into coins. This technique could even be made to work on alloys that contained more than 80% copper. The result is that coins left the mint looking as if they were pure - at least on the surface: depth controlled μXES measurements reveal the true purities of such coins.
XRF analyses suggested some gold coins produced during the AD 68/9 Civil Wars held by the Ashmolean Museum were heavily debased, contra to existing analyses suggesting only minor reductions in purity at this time. With the techniques used on Roman silver in mind, μXES was used to eliminate the problem of ‘surface enrichment’ or compositional differences between ‘surface’ and ‘core’. The results determined that very impure gold coinages really were produced, with some being debased with copper to alter the colour of the alloy. The use of copper in this way by the Romans is some 185 years earlier than first thought.

Speaker: Adrian Hillier (STFC)

16:20 Non-destructive operando measurements of muonic x-rays on Li-ion battery

We have developed an elemental analysis technique with muonic x-ray on a Li-ion battery, taking advantages of muon and muonic x-rays, that is, accessibility of negative muons and high energy of muonic x-rays[1,2]. Especially, intense negative muon with low momentum at J-PARC enables us to investigate electrodes in Li-ion battery. There is no non-destructive method to observe Li directly deep inside the Li-ion battery. Elemental analysis with muonic x-rays has great advantages for that.

We have recently performed operando measurements of muonic x-rays on aLi-ion battery at J-PARC for the first time. By this technique, we have demonstrated the intercalation of Li in a cathode during charging and discharging. Also, we found that we can detect metallic Li deposition on a negative electrode using a difference in capture rates between metallic Li and C$_6$Li[3]. Using this technique, observing an increase in the metallic Li deposition during high-rate charge/discharge cycles is expected to be realized.

We will show the progress in operando measurements of muonic x-rays to study Li-ion batteries at J-PARC.

[1] M. Tampo et al., Proceedings of MuSR2014, JPS Conf. Proc.8, 036016,(2015).
[2] I. Umegaki et al.,"Detection of Li in Li-ion battery electrodes by muonic x-ray elemental analysis", MuSR2017.
[3] I. Umegaki et al., Analytical Chemistry, 92, 12,8194-8200 (2020).

Speaker: Dr Izumi Umegaki (KEK, Toyota Central R&D Labs., Inc.)

16:40 Developments on muonic X-ray measurement system for historical-cultural heritage samples in Japan Proton Accelerator Research Complex (J-PARC)

Negative muon elemental analysis, which can measure elemental compositional distribution in the depth direction from 100 nm to several centimeters in a cm-order area with a depth resolution on the order of μm, is a revolutionary technology that enables nondestructive analysis of samples that previously could only be cut and analyzed in cross-section.　In recent years, this technique has begun to be applied to historical cultural heritage, and has already been carried out on Japanese archaeological heritage, beginning to provide new insights into Japanese archaeological research. In this talk, we will report on the development of a negative muon X-ray measurement system for elemental analysis of historical cultural heritage at the KEK Muon Science Laboratory (MSL) in the Japan Proton Accelerator Research Complex (J-PARC). At MSL, machine time is very limited and fast measurement of archaeological samples is required. Therefore, we are developing a system to measure negative muon X-rays from archaeological samples at high speed. For this purpose, it is essential to improve the detection efficiency of the detector. Since the analysis of negative muon X-rays requires obtaining energy spectra over a wide energy range with high resolution, high-purity germanium semiconductor detectors (HP Ge) are used; for the pulsed muon source at J-PARC, the Ge detector can detect only one photon or less per pulse. Hence, the use of multiple Ge detectors is essential to obtain high detection efficiency. In addition, to obtain a high signal-to-noise ratio (S/N), noise sources must be identified and suppressed. By increasing the number of detectors and suppressing noise sources, we have succeeded in increasing detection efficiency by about 10 times compared to conventional systems.

Speaker: Motonobu Tampo (KEK)

17:00 Muon-Induced X-ray Emission (MIXE) at PSI

The Muon-Induced X-ray Emission (MIXE) technique, first developed in the 1980's mostly for studying fundamental science, has recently seen a wide usage in the field of applied sciences, which includes archaeology, battery research, meteorites, ancient paintings etc.
Probing deep inside the material (up to a few mm) and being non-destructive, this technique is sensitive to all the elements of the periodic table, except hydrogen.
The continuous muon source at Paul Scherrer Institute (PSI) along with the newly in-house made instrument is one of the most powerful setups for an efficient usage of this technique.
We present here recent developments of this dedicated detector setup for MIXE at PSI, used at the $\pi$E1 beamline, which can deliver negative muon rates between $\sim$1.5 kHz and $\sim$100 kHz for a momentum range between 20 MeV/c and 45 MeV/c, respectively.
This setup presently consists of 11 HPGe detectors, with an overall absolute efficiency of $\sim$5\% and a resolution of $\sim$1 keV (FWHM) for muonic X-ray energies at $\sim$100~keV.
In addition to the HPGe detectors, there are two scintillator detectors, utilized to detect the muon entrance time and as veto counter.
By making use of the continuous-wave character of the PSI beam, a clear distinction between X-rays, produced during the muon cascade, and $\gamma$-rays produced after the capture of the muon by the nucleus, is possible hence providing a second route for the elemental and isotopic determination.
This setup enables the determination of the quantitative elemental composition within $\sim$1 h of DAQ time.
A proof-of-principle experiment, using a simple three-layered sandwich sample has been recently published [1].
Several other experiments on precious objects from archaeology and meteorites along with operando battery samples have been performed and the analysis is in progress.

[1] S. Biswas, L. Gerchow et al., App. Sci. 2022, 12(5), 2541.

Speaker: Dr Sayani Biswas (PSI)

17:20 → 19:00 Posters: Poster Session I

17:20 → 19:00 Sponsor Event: Exhibition

Tuesday, 30 August

09:00 → 09:40 Invited Talks: Invited Talk III

09:00 Opportunities and challenges of molecular spins in quantum nanoscience

Implementation of advanced Quantum Technologies might benefit from the remarkable quantum properties shown by molecular spin systems based on the coordination bond. The versatility of the molecular approach combined with rational design has recently boosted the operativity temperature of molecules acting as bits of memory, otherwise known as Single-Molecule Magnets, or the coherence time of molecular spin qubits. The richness and tunability of the spectrum of spin levels make them particularly suitable for quantum error correction, while spin-spin interaction can be tuned to realize quantum gates and quantum simulators. Molecules can also be processed to be deposited on surfaces, allowing the realization of hybrid nanostructures. However, achieving the control of single molecules is also challenging, requiring to couple the electric field, which can be confined at the molecular scale, with the spin degrees of freedom of the molecule. Investigation of the spin dynamics at the level of the monolayer requires developing innovative tools and muon spin resonance might be an important resource.

Speaker: Roberta Sessoli

09:40 → 10:20 Oral contributions: Molecular chemistry and chemical physics I

09:40 Spin dynamics of V-based molecular magnets with integer spin values

In the present work, we investigate the spin dynamics of one-dimensional spin-integer molecular nanomagnets ((CH$_3$)$_2$NH$_2$)V$_7$MF$_8$(O$_2$CtBu)$_ {162}$C$_7$H$_8$, with M=Ni/Mn, in short V$_7$M [1,2,3], by means of magnetization, susceptibility and MuSR measurements. These heterometallic nanomagnets contain seven vanadium ions (s=1) and one Ni$^{2+}$ (s=1) or Mn$^{2+}$ (s=5/2) ion, arranged in the form of regular rings. The theoretical studies of rings with a finite number of integer spins indicate a gapped ground state and a significant deviation from the Landé rule, valid for semi-integer spins [4,5]. On the other hand, the infinite spin-integer chain exhibits a topological Haldane gap between the ground state and the first excited state [6]. As confirmed by experimental data, the ground state of V$_7$Ni and V$_7$Mn is expected to be antiferromagnetic, similarly to the molecular nanomagnet V$_7$Zn [1,2,7], and the exchange coupling constants among the nearest neighbour magnetic ions are estimated to be of the order of a few Kelvin degrees. Susceptibility and magnetization measurements at low temperatures display anisotropy effects when an external magnetic field is applied. The muon longitudinal relaxation rate $\lambda$ vs temperature, at magnetic fields $\mu_0 H \geq$ 500 G, in the range $1.5\leq T\leq 100 K$, follows a heuristic Bloembergen-Purcell-Pound model [8]. No effect related to a topological gap is evinced.

References
[1] F. Adelnia, PhD thesis in Physics, Università degli studi di Pavia (2016).
[2] F. Adelnia et al., Applied Magnetic Resonance 51, 1277 (2020).
[3] I. Villa, BD thesis in Physics, Università degli studi di Milano (2018).
[4] J. Schnack et al., Phys. Rev. B 63, 014418 (2020).
[5] D. Gatteschi et al., Oxford University Press (2011).
[6] F. Haldane, Phys. Letters A 93, 464 (1983).
[7] F. A. Rusnati, MD thesis in Physics, Università degli studi di Milano (2017).
[8] N. Bloembergen et al., Phys. Rev. 73, 679 (1948).

Speaker: Prof. Alessandro Lascialfari (Department of Physics, Università degli studi di Pavia, INFN and INSTM, Pavia, Italy)

10:00 Ultrafast molecular rotors in metal-organic frameworks: a combined $^{1}$H-NMR and $\mu$SR study

Typically, the solid state is not well suited to sustaining fast molecular motion - however, in recent years a variety of molecular machines, switches and rotors have been successfully engineered within porous crystals and on surfaces. Here, we report on a combined $^{1}$H-NMR [1] and $\mu$SR [2] study of fast-rotating molecular rotors within the bicyclopentane-dicarboxylate struts of a zinc-based metal-organic framework. Here, the carboxylate groups anchored to the metal clusters act as an axle while the bicyclic units are free to rotate. The three-fold bipyramidal symmetry of the rotator conflicts with the four-fold symmetry of the struts, frustrating the formation of stable conformations and favouring the continuous, unidirectional, ultrafast rotation of the bicyclic units down to cryogenic temperatures. As a remarkable consequence, the fast-motions regime for the $^{1}$H-NMR spin-lattice relaxation rate is maintained down to at least 2 K, as confirmed by its dependence on temperature and magnetic field. These results are confirmed by zero-field and longitudinal-field $\mu$SR experiments and, in particular, by the dependence of the longitudinal relaxation rate on temperature. At the same time, the experimental evidences suggest several implantation sites for the muons, among which one directly onto the rotating moiety. Muons thermalized in this latter site generate clear oscillations in the depolarization (shown in the picture) resulting from the dipolar interaction with the $^{1}$H nuclear moments on the rotors. We evidence a highly unusual dependence of these oscillations on temperature, suggesting a complex influence of the rotations on the muon implantation and diffusion.

[1] J. Perego et al., Nature Chemistry 12 845 (2020).

[2] G. Prando et al., Nano Letters 20 7613 (2020).

Speaker: Dr Giacomo Prando (Dipartimento di Fisica, Università degli Studi di Pavia, Italia)

10:20 → 10:40 Break

10:40 → 11:20 Invited Talks: Invited Talk IV

10:40 Challenges in next generation batteries for accelerating decarbonization

Batteries are a key-technology for accelerating decarbonization. The benefits of the development of advanced batteries are enormous: broader energy access, specifically for off-grid communities, the transport electrification that reduce the dependency from fossil fuels and the harmful local emission of nanoparticulates, better utilization of intermittent energy sources 1. Europe has decided to invest significantly in numerous projects and initiatives: the European Commission (EC) launched the European Battery Alliance in October 2017 to build a competitive manufacturing value chain in Europe for the creation of sustainable and fully recyclable cells and batteries [2, 3]. The EC funded the long-term research initiative Battery2030+ [4], thus guaranteeing accelerated support for research and innovation of advanced lithium-ion batteries and disruptive technologies such as Li metal solid state batteries, and the technologic platform Batteries Europe, which will coordinate the efforts and the resources of private and public partners to implement the research activities.
While Li-ion batteries will continue to play a major role in the energy storage, new and disruptive ideas are needed for the creation of sustainable batteries which pave the way to European competitiveness during the transition to a climate-neutral society.

References:
1 https://www.weforum.org/reports/a-vision-for-a-sustainable-battery-value-chain-in-2030
[2] https://ec.europa.eu/growth/industry/strategy/industrial-alliances/european-battery-alliance_en
[3] https://www.eba250.com/
[4] https://battery2030.eu/battery2030/about-us/challenges/

Speaker: Catia Arbizzani (Alma Mater Studiorum - Università di Bologna)

11:20 → 11:30 Sponsor Event: Quantum Design

11:20 Sponsor Event: Quantum Design

11:30 → 13:10 Oral contributions: Energy materials

11:30 H2 storage mechanism in fullerides studied with μSR

When positive muons (µ+) are implanted in insulating materials, they capture electrons to form muonium (Mu), a light isotope of H. This process makes muon spin resonance technique (µ SR) suitable for studying H interaction with matter, for example in hydrogen storage (HS) materials.
Among carbon-based materials, recently metal intercalated fullerides demonstrated to be promising for HS, representing de-facto a novel class of HS compounds: in particular, it has been shown that both lithium and sodium cluster intercalated fullerides can reversibly absorb relevant amount of hydrogen (up to 5 wt % in case of Li6C60), at thermodynamic conditions much milder than what observed in pure C60. However, the hydrogenation mechanism in these systems is not trivial and involves several processes, difficult to disentangle with conventional techniques.
In this work we show how uSR helped us to shed light on the hydrogenation process of these systems. In detail, we performed a µSR investigation of Li6C60 and Na10C60, either as-prepared or after hydrogenation, on the EMU and ARGUS beamlines, at ISIS-RAL. Interestingly, we found that in these compounds the formation of muonium is not inhibited, thanks to the presence of the intercalated partly ionized alkali clusters. Muonium was found to react with C60 to form adduct radicals, appearing as a missing fraction in the muon spin signal. This phenomenon is dependent on temperature and is invariably enhanced on cooling for all the investigated samples.
Such findings indicated that in these systems C60 hydrogenation is already feasible at cryogenic temperatures, with an efficiency even larger than at high T, while the high T needed for hydrogen storage in fullerides is only required to overcome the alkali metals mediated H2 dissociation barrier. Following this hint, we managed to further enhance the hydrogen absorption by co-intercalating transition metals nanoparticles (Pt, Pd) in the fullerides interstices.

Speaker: Daniele Pontiroli (Nanocarbon Laboratory, Dipartimento di Scienze Matematiche, Fisiche e Informatiche & INSTM, Università degli Studi di Parma, 43124 Parma, Italy)

11:50 Low energy muon study of the p-n interface in chalcopyrite solar cells

The slow muons technique provides a quantitative approach to characterize the effect of various cover layers on the passivation of bulk defects near the p-n junction of solar cells 1.
Several cover layers on top of the chalcopyrite Cu(In,Ga)Se2 (CIGS) semiconductor absorber were investigated in this work, namely CdS, ZnSnO, Al2O3 and SiO2.

The figure shows the depth profile of a measurement on a CdS/CIGS sample. The diamagnetic fraction is used as an indication of the perturbation of the lattice at the site of the muon. The lower part of the figure shows the model depth profile obtained after deconvolution of the experimental data with the range distribution function. The dip in the diamagnetic fraction near the interface indicates that the lattice is more perturbed in this near-interface region than further inward in the sample. We find that CdS provides the best defect passivation; the oxide materials are less effective.

1 Alberto, H.V. et al. “Characterization of the interfacial defect layer in chalcopyrite solar cells by depth resolved muon spin spectroscopy”, accepted for publication in Advanced Materials Interface, 2022.

Speaker: Helena Vieira Alberto (University of Coimbra, Portugal)

12:10 Negative muon spin rotation and relaxation for energy materials

A positive muon spin rotation and relaxation ($\mu^{+}$SR) has been widely used for assorted materials to study a microscopic internal magnetic field. However, the counterpart technique, $\mu^{-}$SR, is less common mainly due to a small asymmetry of the $\mu^{-}$SR signal, typically 1/6 to that of $\mu^+$SR, caused by the loss of the spin polarization during a capture process of $\mu^-$ by nuclei. This means that 36 times higher statistics are needed for $\mu^{-}$SR measurements to achieve a reliability comparable with the one of $\mu^{+}$SR. Fortunately, recent developments of the intense pulsed muon beam together with a multi-detectors counting system enable the measurement of the $\mu^{-}$SR spectrum within a reasonable amount of beamtime. As a result, we have developed a new tool to detect internal magnetic fields from a fixed view point, since the muonic atom (the bound state between $\mu^-$ and an element of the target material) should be stable up to the decomposition temperature of target materials. This is particularly important for research on energy materials, in which various atoms and/or ions are diffusing and such species could affect the local stability of the implanted $\mu^+$ at the interstitial site. Here, we summarize our $\mu^{-}$SR results on hydrogen storage material MgH$_2$ [1], cathode materials of ion batteries LiMnPO$_4$ [2] and Li[Ni$_{1/2}$Mn$_{3/2}$]O$_4$ [3], and an anode material Li$_4$Ti$_5$O$_{12}$ [4].

[1] J. Sugiyama et al., Phys. Rev. Lett. ${\bf 121}$, 087202 (2018).
[2] J. Sugiyama et al., Phys. Rev. Research ${\bf 2}$, 033161 (2020).
[3] J. Sugiyama et al., Z. Phys. Chem. ${\bf 236}$, 799 (2022).
[4] I. Umegaki et al., J. Phys. Chem. C ${\bf 126}$, 10506 (2022).

Speaker: Dr Jun Sugiyama (CROSS Neutron Science and Technology Center)

12:30 New insights into the photochromism of yttrium oxyhydride thin films from in-situ muon spin rotation (MuSR) and positron annihilation spectroscopy (PAS) studies

Thin films of rare-earth metal oxyhydrides, such as yttrium oxyhydrides (YH$_{3-2x}$O$_x$), show a pronounced photochromic effect where the transparency of the films decreases reversibly over a large range of sub-bandgap wavelengths upon exposure to UV light. This makes these materials suitable candidates for applications in smart windows. However, the exact mechanism behind the photochromic effect is unknown. We investigated the behavior of YH$_{3-2x}$O$_x$ thin films, with different O:H ratios, under dark and illuminated conditions using in-situ muon spin relaxation, employing low energy muons at the LEM spectrometer. Transverse Field (TF) measurements, complemented by ZF and LF experiments, revealed that the muonium (Mu$^0$) formation, inferred from the missing fraction in the TF depolarization curves, increases with increased O:H ratio corresponding to a larger semiconductor band gap. The temperature dependence of the muonium fraction was well described by a transition-state model, where Mu$^0$ formation and gradual Mu$^{+}$ recovery takes place, accompanied by the formation of a Mu$^+$-O$^{2-}$ complex and a polaron at the Y cation. The activation energy (E$_{A,dia}$) associated with Mu$^+$ recovery is dependent on lattice relaxation and is lower for thin films of higher H content (E$_{A,dia}$ $=29$–$45$ meV). In-situ illumination further reduces this energy barrier for all measured oxyhydrides, suggesting that the photochromic effect involves a reversible structural rearrangement during photodarkening. In the light of our muon spin rotation studies, we discuss several proposals for the identity of the light-absorbing species generated by the electron-hole pairs created upon UV illumination, such as the formation of metallic domains by H$^-$ diffusion, hydroxide formation, color centers, and dihydrogen formation. We complement our discussion with recent findings from in-situ positron annihilation studies on similar films, that suggest that hydrogen vacancies are formed, as well as metallic domains that may play an important role in the mechanism of the photochromic effect.

Speaker: Dr Stephan Eijt (Delft University of Technology)

12:50 Ion Diffusion in Na-ion Battery Cathode Material Na$_{0.5}$Mg$_x$Ni$_{0.17-x}$Mn$_{0.83}$O$_{2}$

While Li-ion batteries are considered the main candidate for mobile applications, compounds based on lithium’s heavier cousin, sodium (Na) have also started to receive a lot of attention lately as candidates for future batteries. One reason is that the Li-reserves are limited and if large scale energy storage become a reality in our future sustainable society, we might have to consider alternatives to the Li-ion technology [1]. During last decade, an increasing number of new Na-battery materials with improved performance have been discovered and the general interest for sodium-based energy storage have increased tremendously. Among the cathode materials the 2D layered P2 – Na$_{2/3}$Ni$_{1/3}$Mn$_{2/3}$O$_{2}$ compound [2,3] has shown promising storage capacity and operating voltages above 3.5 V. Unfortunately, this material displayed very poor cyclability i.e. short battery life times, directly related to structural transition during the charge cycles. A potential remedy was found by partly substituting Ni for Mg. The resulting Na$_{0.67}$Ni$_{0.3−x}$Mg$_{x}$Mn$_{0.7}$O$_{2}$ compound [4] also displayed improved Na-ion diffusion rates. In this study we have investigated the Na-ion self-diffusion by means of muon spin rotation ($\mu^+$SR) [5,6] for the compound series $0\le x \le 0.07$. We surprisingly find that even a very small amount of Mg substitution (x = 0.02) results in the best cycling stability and highest Na-ion mobility [7].

[1] G. Alexander, J.B. Goodenough, M. Månsson, et al., Physica Scripta 95, 062501 (2020)
[2] Z. Lu, et al., J. Electrochem. Soc. 148, A710 (2001)
[3] Z. Lu, et al., J. Electrochem. Soc. 148, A1225 (2001)
[4] G. Singh, et al., Chem. Mater. 28, 5087 (2016)
[5] Sugiyama, Månsson, Phys. Rev. Lett. 103, 147601 (2009)
[6] M. Månsson & J. Sugiyama, Phys. Scr. 88, 068509 (2013)
[7] Le Anh Ma, et al., Physical Chemistry Chemical Physics 23, 24478 (2021)

Speaker: Martin Mansson (KTH Royal Institute of Technology)

13:10 → 14:20 Lunch

13:40 → 14:20 ISMS Event: ISMS Executive Meeting

14:20 → 15:05 Facility Updates: Facility Updates I (Hybrid)

14:20 Present status of J-PARC MUSE

J-PARC MUSE is responsible for the inter-university user program and the operation, maintenance, and construction of the muon beamlines, namely D-line, S-line, U-line, and H-line, along with the muon source at MLF.
At D-line, which provides the world’s most intense pulsed negative and positive muon beams, various scientific studies, including those on industrial applications, archeology, and fundamental physics, have been performed. In FY2021, non-destructive analysis was carried out on samples brought back by Hayabusa2 from the asteroid Ryugu, which are thought to preserve the elemental composition of the solar system in its primordial state.
Stable operations have been achieved in S1 area of S-line for μSR. In addition, a group at Okayama University constructed a new experimental area, S2, in FY2020 for muonium 1s-2s measurement.
U-line, uses electrostatic lenses to focus low-energy muons obtained by laser ionization of thermal muonium to produce energy-variable and high time-resolution ultra-slow pulsed muon beams for various experiments. A muon spin spectrometer for materials science research using the μSR method has been installed in the U1A area, and is being upgraded and upgraded for the start of the inter-university user program. The spectrometer is located on a high-voltage stage and the depth of penetration into the sample can be controlled in the range from sub-keV to 30 keV.
The H line is a high-intensity muon beamline where experiments such as high-statistics fundamental physics experiments and transmission muon microscopy are planned. The first beam observed in the H1 experimental area, the first branch, in January 2022.
At present, the beam commissioning is being carried out in collaboration with several research groups that plan to conduct experiments at the H-line.

Speaker: Koichiro Shimomura (KEK/J-PARC)

14:35 TRIUMF Centre for Molecular and Materials Science Facility Overview

This document breifly describes the mission, governnace, operations, infrastructure and future directions of TRIUMF’s CMMS. The current muon and beta-detected NMR experimental facilites are revisted and the status of a number of pending beamline projects and spectrometer instalations are introduced.

Speaker: Dr Sydney Kreitzman (TRIUMF)

14:50 Progress on Muon Source Project at CSNS

A Muon station for sciEnce, technoLOgy and inDustrY (MELODY) has been listed in the CSNS II upgrade plan, and the infrastructure construction is scheduled to start by the end of 2022. Up to 5Hz of proton pulses will be extracted from the RCS ring to a stand-alone target station. One surface muon and one decay muon beamline are designed to provide multi-terminals for applications. In this report, we describe the design of MELODY and prospect for future applications.

Speaker: Yu Bao (Institute of High Energy Physics)

15:05 → 15:10 Sponsor Event: Nuclear Instruments (Hybrid)

15:05 Sponsor Event: Nuclear Instruments

15:10 → 15:25 Facility Updates: Facility Updates II (Hybrid)

15:10 Developing the MuSIC muon facility at RCNP, Osaka University

The MuSIC continuous muon beam facility was constructed in the Research Center for Nuclear Physics (RCNP), Osaka University. It has now been operating for about 7 years since the present intense muon beam was obtained in 2015 including the long-shutdown term for the accelerator upgrade (2020-2022). The RCNP is the international Cyclotron facility mainly for the nuclear physics, and has recently made a significant contribution to a number of scientific and engineering fields including the muon science. The first success in the MuSIC muon beamline was non-destructive elemental analyses of the muonic X-ray with a negative muon beam at the first phase and then, the first DC-$\mu$SR measurements was performed in 2019. During the long shutdown of the main accelerator, we are now developing a number of apparatuses for the DC-$\mu$SR at RCNP-MuSIC and after the shutdown, we will open them for user experiments.
In this talk, the current status of the facility and experiments will be reviewed and current $\mu$SR development will be introduced.

Speaker: Dai Tomono (Osaka University)

15:25 → 15:45 Break

15:45 → 16:40 Facility Updates: Facility Updates III (Hybrid)

15:45 Status of $\mu$SR facility in RAON

Rare Isotope Science Project (RISP) launched in 2011 to build the Rare isotope Accelerator complex for ON-line experiment for rare isotope science (RAON), ends the 1st stage of which completes one of two main accelerators (low-energy superconducting linac, SCL). Since 2019, $\mu$SR facility has been designed and constructed, composed of muon production target chamber, transport beamline, beam dump, except spectrometer. In this talk, we report the current status of $\mu$SR facility in RAON as an applicative facility of RAON and a tool for investigating materials.

Speaker: Dr Wonjun Lee (RISP, IBS)

16:00 Status of the Swiss Muon Source at PSI

The Laboratory for Muon Spin Spectroscopy (LMU) at PSI develops and operates the six muon instruments of the Swiss Muon Source (SμS). We give an overview of the current status, with an update on the commissioning of the new FLAME instrument and the upgrade plan of the μE4 beamline to increase the rate of low-energy muons by 50% in 2025. Furthermore, a new experimental facility is under development: the Muon-Induced-Xray-Emission (MIXE) instrument using negative muons for non-destructive, depth-selective elemental analysis of archeological artefacts, extraterrestrial samples and for operando studies of devices.

On a longer term, PSI is planning the major upgrade project IMPACT of the High-Intensity Proton Accelerator (HIPA). IMPACT ("Isotope and Muon Production using Advanced Cyclotron and Target technologies") aims for the production of radioactive isotopes for cancer diagnosis and therapy, and the installation of HIMB, the two "High Intensity Muon Beams". HIMB involves the replacement of the existing target M and the two beamlines πM1 and πM3 by a new target H with two very high-intensity surface muon beamlines μH2 and μH3 with muon rates up to 10¹⁰/s. This will offer unique new possibilities for muon applications [1]. Installation of this major facility upgrade is foreseen in a 1.5 years shutdown in 2027/2028. The project proposal is currently being under evaluation.

[1] M. Aiba et al., Science Case for the new High-Intensity Muon Beams HIMB at PSI, arXiv:2111.05788.

Speaker: Thomas Prokscha (Paul Scherrer Institute)

16:15 ISIS Facility Report

dummy abstract for ISIS facility report

Speaker: Dr Adrian Hillier (STFC / UKRI)

16:30 Final Discussion

16:40 → 17:20 ISMS Event: ISMS General Assembly (Hybrid)

17:20 → 19:00 Posters: Poster Session II

17:20 → 19:00 Sponsor Event: Exhibition

Wednesday, 31 August

09:00 → 09:40 Oral contributions: Strongly correlated electron systems II (Hybrid)

09:00 Discovery of Hidden Charge-Neutral Muon Centers in Magnetic Materials: Implications and Applications

Spin polarized muons are widely known as an extremely sensitive local probe of magnetism. Additionally, positively charged muons implanted into semiconductors and insulators often bind an electron to form a charge-neutral muon-electron bound state frequently referred to as a muonium center. While studied extensively in non-magnetic semiconductors and insulators as light analogues of corresponding hydrogen centers, charge-neutral muon states are rarely considered relevant in magnetic materials. Apart from the singular exception of antiferromagnetic MnF$_2$[1], no long-lived charge-neutral centers had been identified in magnetically ordered materials up-to-date.

Here, we present strong evidence that charge-neutral muon centers do exist in magnetic compounds. Detailed new $\mu$SR investigations of the antiferromagnets Cr$_2$O$_3$[2], Fe$_2$O$_3$[3] and MnF$_2$, in conjunction with density-functional-theory calculations, reveal that charge-neutral muon states are present in magnetic materials and can form with different electronic structures, analogous to the variety of muonium centers found in non-magnetic materials.

Crucially, we find that in magnetic materials, charge-neutral muon states do not display any signatures conventionally associated with muonium centers, making it difficult to distinguish them from the often assumed positive charge state. We demonstrate that the presence of the additional charge alters the local electronic and magnetic structure, affecting the $\mu$SR signal and its relationship with the intrinsic magnetic properties. Since the muon is used extensively as a sensitive magnetic probe, it is imperative to understand under what conditions charge-neutral states are formed in magnetic materials, and what impact they have on the observed $\mu$SR frequencies and damping rates.

[1] Uemura et al., Hyperfine Interact. 31 313(1986)

[2] M.H. Dehn et al., Phys. Rev. X 10, 011036 (2020)

[3] M.H. Dehn, J.K. Shenton et al., Phys. Rev. Lett. 126, 037202 (2021)

Speaker: Martin Dehn (UBC)

09:20 From $\mu$SR spectra to the magnetic interaction energy parameters: the MnSi helimagnet as a test case

For several decades the intermetallic compound MnSi has fascinated the community for different aspects of its physical and magnetic properties. Among these properties is the exotic temperature-magnetic field phase diagram. While this diagram was first established in the 1970s, the exact nature of one of the phases was only identified in 2009 as a lattice of magnetic skyrmions, i.e. a topological magnetic texture.
In this contribution we present recent developments in the interpretation of the muon response of MnSi in the helimagnetic and conical phases, respectively observed in zero and finite fields. These developments are based on a computation of the asymmetry spectrum in terms of the incommensurate magnetic structure parameters and the muon site and coupling.

In a first step we show the magnitude $m$ of the magnetic moment in the helical phase, the temperature dependence of which has attracted little attention in the literature, to decay as $T^2$ from its low temperature value. We interpret this decay as the result of spin waves excitations. The slope of $m$ vs $T^2$ determines the two dominant energy contributions in the traditional expression used for magnetic energy of the system.

In a second step, instead of the previously mentioned continuous field model, we consider a microscopic model for the energy, accounting for the presence of four magnetic Mn sites in the crystal unit cell and the symmetry elements of the P2$_1$3 space group in which MnSi crystallizes. The minimization of the energy is obtained for structures that somewhat deviate from the regular helical and conical phases. This result is consistently confirmed by fits to the asymmetry spectra which provide a quantitative determination of the microscopic model parameters. Directions for future developments are presented.

Speaker: Dr Pierre Dalmas de Reotier (Univ. Grenoble Alpes)

09:40 → 10:20 Oral contributions: Site calculations I

09:40 Quantum Information: How does it μve through fluorides?

The unitary evolution of a quantum system preserves its coherence, but interactions between the system and its environment result in decoherence, a process in which the quantum information stored in the system becomes degraded. A spin-polarized positively charged muon implanted in a fluoride crystal realizes such a coherent quantum system, and the entanglement of muon and nearest-neighbor fluorine nuclear spins gives rise to an oscillatory time dependence of the muon polarization that can be detected and measured. In this talk, we will show that the decohering effect of more distant nuclear spins can be modelled quantitatively, allowing a very detailed description of the decoherence processes coupling the muon-fluorine “system” with its “environment,” and allowing us to track the system entropy as the quantum information degrades [1]. Examples of this approach to various fluorides will be presented, using these methods to gain knowledge of the nature of the muon stopping site, distinguish between different crystalline phases of a compound, and identify Frenkel defects [2].

References
[1] J. M. Wilkinson and S. J. Blundell, Phys. Rev. Lett., 125 087201 (2020).
[2] J. M. Wilkinson et al., Phys. Rev. B, 89 L220409 (2021).

Speaker: John Wilkinson (STFC)

10:00 Insights into the magnetic ground state of Fe$_2$P from $\mu$SR, NMR and DFT perspectives

Fe$_2$P alloys have been proposed as promising for applications in magnetocaloric refrigeration due to their first-order magnetic transitions coupled to a magnetoelastic transition, which gives rise to a giant magnetocaloric effect in the vicinity of their Curie temperature [1]. The magnetic structure of Fe$_2$P has been investigated and known to order ferromagnetically, with magnetic moments along the c-axis. However, these earlier sparse and often very old literature on Fe$_2$P are characterized by inconsistencies in the quantitative description of the Fe$_1$ magnetic moment size and the presence of helical states below T$_c$.

Here, using a combined effort of two spectroscopic techniques, $\mu$SR and NMR, in addition to DFT calculations, we have accurately characterized the magnetic ground state of Fe$_2$P. We perform zero applied field measurements using both experimental techniques below the ferromagnetic transition T$_C$ = 220 K [2]. Our DFT calculations reproduce the experimental results and further allow us to improve their interpretation. We show a detailed characterization of the microscopic coupling between the electrons and P-nuclei or the muon in Fe$_2$P, which where then utilized to discuss the microscopic origin of the NMR and $\mu$SR resonances. Particularly, the computational predictions allow to identify correctly a previously mis-attributed signal from $^{31}$P nuclei, an information relevant for future experiments. This work completely characterizes the signal of two technique of election for the characterization of magnetic properties, thus providing an important base for further analysis of different alloy compositions.

References
[1] R. Hussain, F. Cugini, S. Baldini, G. Porcari, N. Sarzi Amadè, X. F. Miao, N. H. van Dijk, E. Brück, M. Solzi, R. De Renzi, and G. Allodi, Phys. Rev. B 100, 104439 (2019).
[2]Pietro Bonfà, Muhammad Maikudi Isah, Benjamin A. Frandsen, Ethan J. Gibson, Ekkes Brück, Ifeanyi John Onuorah, Roberto De Renzi, and Giuseppe Allodi. Phys. Rev. Mat. 5, 044411 (2021)

Speaker: Dr Ifeanyi John Onuorah (Department of Mathematical, Physical and Computer Sciences, University of Parma, 43124 Parma, Italy)

10:20 → 10:40 Break

10:40 → 11:40 Oral contributions: Site calculations II (Hybrid)

10:40 MuSpinSim: spin dynamics calculations for muon science.

MuSpinSim is a Python software to simulate muon ($\mu$SR) experiments. In particular, it simulates the spin dynamics of a system of a muon plus other spins such as electrons and atomic nuclei. MuSpinSim can simulate various common experimental setups used in $\mu$SR, such as zero, transverse and longitudinal field experiments; and it can simulate $\mu$SR experiments that are resolved in time, field, or temperature. Furthermore, MuSpinSim can account for the effects of hyperfine, dipolar, quadrupolar and Zeeman couplings, as well as simulate quantum systems exchanging energy with the environment with the Lindblad master equation. Finally, MuSpinSim can be used to fit experimental $\mu$SR data with simulations that use all of the capabilities described above. The fittings can be run in parallel on multiple cores, which significantly reduces the computational cost of the most expensive tasks. In this work, we present the Python package MuSpinSim with all the utilities it provides to facilitate simulations of $\mu$SR experiments, demonstrate the effectiveness of the method with some chosen example systems and show a prototype application of MuonGalaxy, a web-based implementation of MuSpinSim that is based on the Galaxy platform.

Speaker: Leandro Liborio (Scientific Computing Department, Rutherford Appleton)

11:00 Entanglement between muon and I>1/2 nuclear spins as a probe of charge environment

Quantum coherence between an implanted positively-charged muon and nuclei in a solid was first conclusively demonstrated using muon-spin spectroscopy experiments on simple ionic fluorides [1]. In this case the nuclear spin $I=\frac 1 2$ of the $^{19}$F nuclei couples to the muon spin through the dipolar interaction.
Here we identify the first example of muon spin quantum coherence in systems with nuclear spin larger than $\frac 1 2$. The effect is shown for vanadium intermetallic compounds which adopt the A15 crystal structure, and whose members include all technologically dominant superconductors.

The presence of $I\ge 1$ nearest neighbours (nn) nuclei implies the inclusion of quadrupolar interactions. The muon embedding in the crystal drastically alters the electric field gradient (EFG) at the nuclei nearest neighbours of the muon. Nevertheless, this perturbation can be effectively described with Density Functional Theory based simulations [2]. Once the muon site, the structural distortion and the charge perturbation induced by the muon are established through cost effective ab initio simulations, our modelling of the coherence is extremely accurate.

This case-study demonstrates that high-statistics measurements of systems in which the muon spin becomes entangled with nearby nuclear spins can yield information about small changes in local structure and charge order, even in the absence of magnetic ground states.

[1] J. H. Brewer, et al.,Phys. Rev. B 33, 7813R (1986)
[2] P. Blaha, et al., Phys. Rev. Lett. 54, 1192 (1985)

Speaker: Pietro Bonfà

11:20 What can we learn from muon-stopping site analysis?

Two of the most fundamental limitations of the muon-spin spectroscopy ($\mu^+$SR) technique are the lack of knowledge of the muon stopping site, and the uncertainty surrounding the degree to which the muon distorts its local environment. Over the past decade there has been significant progress in calculating muon stopping sites using ab initio methods, particularly density functional theory (DFT). These methods can provide significant insight into how the muon probes the system, thereby enhancing the information that can be extracted from a $\mu^+$SR experiment.

Establishing the degree to which the muon perturbs it environment can be crucial for confirming that the phenomena observed by the muon are intrinsic to the system under study. Here we investigate the muon stopping states in a range of correlated electron systems. 1) In superconductors that exhibit time-reversal symmetry breaking, where spontaneous magnetic fields have been observed using $\mu^+$SR, we show how knowledge of the muon stopping site shows how the muon is a faithful probe that provides sensitivity to the intrinsic magnetism in the system [1]. 2) By calculating the muon site and its associated hyperfine interactions in the quantum spin-liquid candidate 1T-TaS$_2$ we can model how the muon couples to diffusing spinon excitations [2]. Here we are also able to compute details of the muon's own diffusion between sites. 3) Calculating the distribution of magnetic fields at the muon site allows us to link the $\mu^+$SR spectra directly to the underlying magnetic structure. We discuss the use of this approach in skyrmion-hosting systems, whose phase diagrams comprise several complicated incommensurate magnetic structures as a function of magnetic field and temperature.

[1] B. M. Huddart et al. Phys. Rev. Lett. 127, 237002 (2021).
[2] S. Mañas-Valero et al., npj Quantum Mater. 6, 69 (2021).

Speaker: Dr Benjamin Huddart (Durham University)

11:40 → 12:20 Invited Talks: Invited Talk V (Hybrid)

11:40 Dipolar-Octupolar Quantum Spin Liquids in Ce-based Pyrochlores

In geometrically-frustrated Ce-based pyrochlores, such as Ce$_2$Zr$_2$O$_7$, the effective S=1/2 of the Ce3+ crystal field ground state doublet is known to act both as a conventional dipole magnetic moment, and as an octupole. This constrains the form of its near-neighbour Hamiltonian, and allows for different ordered or quantum disordered ground states in this family of materials, where either the dipolar or octupolar nature of the S=1/2 degree of freedom dominates. I will describe recent experiments [1,2], mostly neutron scattering and heat capacity, which show how the nature of the Ce3+ ground state doublet can be revealed, and how a particular form of quantum spin liquid can be identified as the likely ground state in Ce$_2$Zr$_2$O$_7$.

Speaker: Bruce Gaulin (McMaster University)

12:20 → 13:00 Oral contributions: Spin liquids and related phenomena II (Hybrid)

12:20 Quantum critical spin-liquid behavior in $S = 1/2$ quasikagome lattice CeRh$_{1-x}$Pd$_{x}$Sn investigated using muon spin relaxation

We present the results of muon spin relaxation ($\mu$SR) on the Ce-based quasikagome lattice CeRh$_{1-x}$Pd$_{x}$Sn ($x$ = 0.1 to 0.5). Our zero-field (ZF) $\mu$SR results reveal the absence of both static long-range magnetic order and spin freezing down to 0.05 K in the single crystal sample of $x = 0.1$. The weak temperature-dependent plateaus of the dynamic spin fluctuations below 0.2 K in ZF-$\mu$SR together with its longitudinal-field (LF) dependence between 0 and 3 kG indicate the presence of dynamic spin fluctuations persisting even at $T$ = 0.05 K without static magnetic order. On the other hand, the magnetic specific heat divided by temperature $C_{\text{4f}}$/$T$ increases as -log $T$ on cooling below 0.9 K, passes through a broad maximum at 0.13 K and slightly decreases on further cooling. The ac-susceptibility ($\chi_{\text{ac}}$) also exhibits a frequency independent broad peak at 0.16 K, which is prominent with an applied field $H$ along the $c$-direction. We, therefore, argue that such behavior for $x=0.1$ (namely, a plateau in the spin relaxation rate ($\lambda$) below 0.2 K and a linear $T$-dependence in $C_{\text{4f}}$ below 0.13 K) can be attributed to a metallic spin-liquid (SL) ground state near the quantum critical point (QCP) in the frustrated Kondo lattice. The LF-$\mu$SR study suggests that the out of kagome plane spin fluctuations are responsible for the SL behavior. The ZF-$\mu$SR results for the $x = 0.2$ polycrystalline sample exhibits similar behavior to that of $x = 0.1$. A saturation of $\lambda$ below 0.2 K suggests a spin-fluctuating SL ground state down to 0.05 K. The ZF-$\mu$SR results for the $x = 0.5$ sample are interpreted as a long-range antiferromagnetic (AFM) ground state below $T_{\text{N}}$ = 0.8 K, in which the AFM interaction of the enlarged moments probably overcomes the frustration effect.

Speaker: Rajesh Tripathi (Rutherford Appleton Laboratory ISIS Facility, UK & Jawaharlal Nehru Center for Advanced Scientific Research Bangalore India )

12:40 Quantum spin liquid behavior in geometrically frustrated Mo pyrochlore antiferromagnet Lu$_2$Mo$_2$O$_{5-y}$N$_2$

The magnetic ground state of a quantum spin liquid (QSL) candidate compound, Lu$_2$Mo$_2$O$_{5-y}$N$_2$ oxynitride pyrochlore ($S=1/2$, Mo$^{5+}$), was investigated by muon spin rotation/relaxation experiment. In contrast to Lu$_2$Mo$_2$O$_7$ ($S=1$, Mo$^{4+}$) which exhibits a spin glass-like freezing of Mo moments below $T_g\simeq16$ K, no such spin freezing or long range magnetic order was observed down to 0.3 K. More interestingly, two distinct magnetic domains discerned by spin dynamics were observed below $\sim$13 K; one showing the ``sporadic'' spin fluctuation similar to that observed in other QSL candidate compounds including the kagome antiferromagnets, and the other showing the fast paramagnetic fluctuation that is only weakly suppressed with decreasing temperature. Their origins are discussed in terms of the bond randomness induced by the partial substitution of O with N and the inhomogeneous Mo valency due to O deficiency ($y>0$) [1].

References
[1] S. K. Day et al., arXiv:2206.13049.

Speakers: Dr Sourav K. Dey (IMSS, KEK) , Prof. Ryosuke Kadono (IMSS, KEK)

13:00 → 13:10 Conference Event: Conference Photo

13:10 → 14:00 Lunch

14:00 → 19:00 Social Programme: Conference Excursion

20:00 → 23:00 Social Programme: Conference Banquet

Thursday, 1 September

09:00 → 09:40 Invited Talks: Invited Talk VI

09:00 Low-energy Excitations in Quantum Spin Liquid Derived from Molecular Mott Insulator

A molecular Mott insulator ß'-EtMe$_3$Sb[Pd(dmit)$_2$]$_2$ (dmit = 1,3-Dithiol-2-thione-4,5-dithiolate) is a quantum spin liquid (QSL) candidate. In the crystal with the space group $C2/c$, Pd(dmit)$_2$ anion radicals are strongly dimerized to form a dimer with spin 1/2. The dimers are arranged in an approximately isosceles-triangular lattice, which leads to a frustrated S = 1/2 Heisenberg spin system.
The system shows no magnetic order down to a very low temperature (~19 mK) that corresponds to $J$/12,000, where $J$ (~250 K) is the nearest-neighbor spin interaction energy. The HOMO-LUMO mixing in the dimer unit induces fragmentation of S=1/2 electron spin with strong quantum fluctuation.
Low-energy excitations in the QSL state are open to debate even now. Heat capacity and magnetization indicate gapless fermion-like excitations, while $^{13}$C-NMR indicates an existence of a nodal gap. ESR and µSR probed the spinons, revealing their gapless character and an unexpectedly large degree of in-plane anisotropy in the spin dynamics. This anisotropic spin dynamics indicates quasi-one-dimensional diffusive motion in the direction of the weakest magnetic coupling in the triangular lattice.
In 2010, it was reported that thermal conductivity is characterized by its large value and gapless behavior (a finite temperature-linear term). In 2019, however, two other research groups reported opposite data (much smaller value and a vanishingly small temperature-linear term) and the discrepancy in the thermal conductivity measurement data emerges as a serious problem concerning the ground state of QSL. An origin of the discrepancy will be discussed.

I deeply thank all my collaborators, especially, H. Cui, M. Uebe, S. Fujiyama, Y. Oshima, I. Watanabe (RIKEN), Y. Nakazawa (Osaka Univ.), Y. Ishii (Shibaura Institute of Technology) and F. L. Pratt (STFC).

Speaker: Reizo Kato

09:40 → 10:20 Oral contributions: Spin liquids and related phenomena III

09:40 Universal fluctuating regime in triangular chromate pure Heisenberg S=3/2 antiferromagnets

The series of triangular compounds ACrO$_2$ is a model series for studying the Heisenberg model on S=3/2 (Cr$^{3+}$: half-filled t$_{2g}$ orbitals) triangular antiferromagnets and the impact of interlayer couplings on the dynamics. For this, we report µSR measurements on α-HCrO$_2$ and KCrO$_2$ [1] which complete former studies on the series of triangular compounds ACrO$_2$, A = Li , Na [2, 3]. Coupled to $^1$H and $^{39}$K nuclear magnetic resonance (NMR), we establish the static character at low-T, as expected for a near neighbour Heisenberg model, yet displaying a broad and remarkable regime with slow fluctuations extending from $T_N$ down to 0.2 $T_N$. This regime is marked by a maximum in the μSR relaxation rate occuring at 0.7 $T_N$, associated with an NMR wipe-out .
The scaling of the NMR and μSR data with respect to J or T$_N$ supports a scenario where a crossover from 2D to 3D correlations sets in around 0.7 T$_N$ preceded by a typical 2D regime of the TLHAF which appears to be a hallmark of the TLHAF with ABC stacking. We discuss the role of interlayer frustration which may bear implications to recent spin-liquid candidates with the triangular geometry and exclude a scenario à la Berezinskii-Kostelitz-Touless of vortex-antivortex topological excitations in that regime. In turn, this underlines the crucial need of further neighbour interactions, anisotropy typical of rare earth or even disorder to stabilize a quantum spin liquid state in triangular antiferromagnets such as YbMgGaO$_4$.
[1] K. Somesh et al. Phys. Rev. B 104, 104422 (2021).
[2] A. Olariu et al. Phys. Rev. Lett. 97, 167203 (2006).
[3] A. Olariu et al. Phys. Rev. B 79, 224401 (2009).

Speaker: Philippe Mendels (Université Paris(Sacaly)

10:00 Searching for Spin Liquids in Buckled Compounds

The phrase ‘quantum spin liquid’ (QSL) refers to a system in which strong quantum fluctuations prevent long-range magnetic order from being established, even at temperatures well below any interaction energy scale. No spontaneous symmetry breaking is involved, nor a conventional local order parameter. Thus, it is not described using the Landau theory of phase transitions and constitutes a novel phase of matter. These systems exhibit a wealth of exotic phenomena like long-range entanglement and fractional quantum excitations, which are of fundamental interest but also hold great potential for quantum communication and computation.

Magnetic species decorating a two dimensional kagome lattice constitute the most heavily studied QSL candidates. Quantum fluctuations are prevalent due to geometrical magnetic frustration, low coordination number and quasi low dimensionality. Two particularly well-studied experimental realisations are volborthite, where it is believed spatial anisotropy plays an important role and herbertsmithite ZnCu$_3$(OH)$_6$Cl$_2$. However, the presence of excess Cu$^{2+}$ replacing the nonmagnetic Zn$^{2+}$ induces randomness in the magnetic exchange coupling, complicating explanations of the experimental observations.

Our focus is the investigation of a series of newly synthesised QSL candidates. The insulating materials YCu$_3$(OH)$_6$O$_x$Cl$_{3−x}$ (x = 0, 1/3) display a kapellasite-like structure and no sign of Cu/Y mixing from single crystal x-ray refinements. In the x = 0 compound, the kagome lattice is perfect; in the x = 1/3 compound, it is slightly buckled.
In Ba$_4$Ir$_3$O$_{10}$, Ir$^{4+}$(5d$^5$) ions form Ir$_3$O$_{12}$ trimers of three dimensional face-sharing IrO$_6$ octahedra, which are vertex-linked, forming wavelike 2D sheets. However, it is proposed that intra-trimer exchange is reduced and the lattice recombines into an array of coupled 1D chains with additional spins. As such, the compound is a candidate Tomonaga-Luttinger liquid (TLL) and presents a novel route to exploring quantum liquid behaviour. A muon spin relaxation investigation of these novel compounds is discussed.

Speaker: Sarah Dunsiger (TRIUMF / Simon Fraser University)

10:20 → 10:40 Break

10:40 → 11:20 Invited Talks: Invited Talk VII

10:40 Muons and Quantum Computing Hardware: Challenges and Opportunities

One might wonder: what do muons have to do with quantum computing? I will argue that environmental muons and ionizing radiation in general represent a source of noise and dissipation which until recently has been underestimated in the quantum devices community. I will present measurements performed in the deep-underground laboratory of Gran Sasso [1] which show a significant improvement in the performance of superconducting quantum hardware thanks to the shielding provided by 1.6 Km of granite. On the other hand, low energy muon beams engineered at dedicated large-scale facilities represent a powerful materials characterization tool, and as such might play a role in the understanding and mitigation of material defects in superconducting and semiconducting quantum hardware.

[1] Cardani, Valenti et al., Nature Comm. 12, 2733 (2021)

Speaker: Ioan Pop (Karlsruhe Institute of Technology)

11:20 → 13:00 Oral contributions: Superconductivity

11:20 Phase transition from a magnetic-field-free stiffness meter and LEM viewpoints

A new method to measure the superconducting stiffness tensor ${\bar \rho _s}$, without subjecting the sample to magnetic field, is applied to La$_{2-x}$Sr$_x$CuO$_4$ (LSCO) [1]. The method is based on the London equation $\mathbf{ J } = - \bar{{ \rho}}_\mathbf{s}\mathbf{ A}$, where $\mathbf{ J}$ is the current density and $\mathbf{ A}$ is the vector potential. Using rotor free $\mathbf{ A}$ and measuring $\mathbf{ J}$ via the magnetic moment of superconducting rings, we extract ${\bar \rho _s}$ at $T \to {T_c}$. The technique, named Stiffnessometer is sensitive to very small stiffness, which translates to penetration depth on the order of a few millimeters. We apply this method to two different LSCO rings: one with the current running only in the CuO$_2$ planes, and another where the current must cross planes. We find different transition temperatures for the two rings, namely, there is a temperature range with two-dimensional stiffness. The same method is also used to measure the coherence length $\xi_0$, by increasing $A$ to a point where linear response break. Finally, we compare our result with a LEM experiment performed on the same samples and discuss the advantage and disadvantage of each technique.

Speaker: Prof. Amit Keren (Technion Israel Institute of Technology)

11:40 Unconventional pressure dependence of the superfluid density in topological superconductor α−PdBi2

The Pd-Bi family of compounds has become quite popular system to explore topological superconductivity due to their intrinsic capability to maintain strong spin orbit coupling (SOC). Amongst various members of this family, $\alpha$-PdBi$_2$ turns out to be very promising due to its superconducting ($T_c$ = 1.7 K) as well as topological properties such as Dirac point at 1.26 eV below the Fermi energy at the zone center, Rashba state near the Fermi energy etc. Notably, the ARPES data display multiple band crossings at the Fermi energy which signals a possible multiple gap superconducting gap structure in this compound. To explore this interesting aspect, we investigated the superconducting properties of the topological superconductor α-PdBi2 at ambient and external pressures up to 1.77 GPa using muon spin rotation ($\mu$SR) experiments. The ambient pressure $\mu$SR measurements demonstrate a fully gapped $s$-wave superconducting state in the bulk. The observation of $s$-wave superconductivity in $\alpha$-PdBi$_2$ is quite crucial in search for Majorana fermions as it is theoretically predicted that in presence of an in-plane magnetic field, the Majorana zero mode can be realized utilizing the coupling of an $s$-wave superconductor with a material exhibiting Rashba states. Further, AC magnetic susceptibility and $\mu$SR measurements under hydrostatic pressure manifest a continuous suppression of $T_c$ with increasing pressure. We observed a considerable decrease of superfluid density by ~20% upon application of external pressure. Remarkably, the superfluid density follows a linear relation with $T_c$ which was found before in some unconventional topological superconductors and hole doped cuprates. This finding indicates a possible crossover from Bose-Einstein to Bardeen-Cooper-Schrieffer like condensation in $\alpha$-PdBi$_2$.

Reference
Debarchan Das, R. Gupta, C. Baines, H. Luetkens, D. Kaczorowski, Z. Guguchia, and R. Khasanov, Phys. Rev. Lett. 127, 217002 (2022).

Speaker: Debarchan Das (Paul Scherrer Institute)

12:00 $\mu$SR Study of the Relationship between the Magnetism, Superconductivity and Electronic Nematicity in Iron-Chalcogenide Thin Films

The iron-chalcogenide FeSe exhibits various electronic states such as superconductivity, the so-called electronic nematicity, as well as a magnetic order under hydrostatic pressure. Therefore, this system attracts considerable research attention in an effort to understand the interplay between the different electronic states. In S-substituted thin films of FeSe$_{1-x}$S$_x$ in which positive chemical pressure is induced by the smaller S substitution for larger Se, we formerly found a kink in the temperature dependence of the electrical resistivity at highly S-substituted thin films of $x \ge 0.18$ without the nematic state [1]. The kink has been observed around the magnetic transition temperature $T_N$ in bulk FeSe under pressure [2]. To investigate the possible magnetism in FeSe$_{1-x}$S$_x$ and compare with Te-substituted FeSe$_{1-y}$Te$_y$ in which negative chemical pressure is induced, we performed muon-spin-relaxation ($\mu$SR) measurements [3].
Zero-field $\mu$SR time spectra of FeSe$_{1-x}$S$_x$ with $x=0.3$ and $0.4$ revealed the formation of a short-range magnetic order at low temperatures. The value of $T_N$ is higher in $x=0.4$ than in $x=0.3$, suggesting a S-induced magnetic order in the FeSe$_{1-x}$S$_x$ thin films. For slightly S-substituted $x=0.1$ with the nematic state, on the other hand, it was found that a long-range magnetic order was formed at low temperatures. As the value of $T_N$ at $x=0.1$ is higher than that of $x=0.4$, distinct magnetic states would be formed in the slightly (with nematic) and highly (without nematic) S-substituted FeSe$_{1-x}$S$_x$.

[1] F. Nabeshima et al., J. Phys. Soc. Jpn. 87, 073704 (2018).
[2] T. Terashima et al., J. Phys. Soc. Jpn. 84, 063701 (2015).
[3] F. Nabeshima et al., Phys. Rev. B 103, 184504 (2021).

Speaker: Tadashi Adachi (Sophia University)

12:20 Complex nature of charge order and superconductivity interplay in correlated kagome superconductor CsV$_3$Sb$_5$

Recent family of Kagome superconductors AV3Sb5 (A = Rb, K, Cs) offers a natural playground to study the interplay between different electronic states such as non-trivial chiral charge order (CO) and unconventional superconductivity [1-5]. This is because of its unique crystal structure that results in flat bands across the Brillouin zone, crossing of linear bands at K-corner, appearance of van Hove singularities at M-edges of the Brillouin zone. CsV3Sb5 is of particular interest compared to Rb and K counterparts due to distinct M-dome shaped two peak behaviors in its superconducting transition temperature Tc vs. pressure phase diagram. The phase diagram is however drawn through transport measurements accessing only macroscopic nature of interplay between CO and SC [6]. Thus, microscopic nature and theoretical understanding of their correlation remains unanswered. We have carried out muon spin relaxation/rotation (μSR) experiments under hydrostatic pressure up to 1.9 GPa. Nearly threefold enhancement in Tc and superfluid density ns at 1.74 GPa compared to their respective ambient pressure values has been observed. Interestingly, ns also displays two peak like feature with pressure. Three different regions of phase diagram manifest distinct linear relationship between Tc and ns. The μSR results and DFT calculations conjointly suggest possible evolution of CO from a superimposed tri-hexagonal Star-of-David phase at low pressures to the staggered tri-hexagonal phase at intermediate pressures [7]. Our studies thus uncover different regions of phase diagram with CO showing varying degree of interplay with SC.

• contributed equally
  [1] Neupert et al., Nature Physics 18, 137 (2022).
  [2] Zhao et. al. , Nature 599, 216–221 (2021).
  [3] Mielke III et al., Nature 602, 245 (2022).
  [4 Guguchia et al., https://arxiv.org/abs/2202.07713.
  [5] Khasanov et al., Phys. Rev. Research 4, 023244 (2022).
  [6] K. Chen, et al., Physical Review Letters 126, 247001 (2021).
  [7] R. Gupta et al., https://arxiv.org/abs/2203.05055.

Speaker: Dr Ritu Gupta (Geneva University)

12:40 Is the Abrikosov’s vortex-model still valid in nematic superconductors?

Spontaneous rotational-symmetry breaking (RSB) in the amplitude of the superconducting gap is a necessary condition for “nematic” superconductivity. This was evidenced in the topological superconductor Cu$_x$Bi$_2$Se$_3$ where, despite the threefold symmetry of its lattice, a twofold symmetry of electronic properties emerged from nuclear magnetic resonance$^1$, transport$^2$, and specific-heat$^3$ measurements, when the applied magnetic field is rotated in the Se planes. This is also the case of CaSn$_3$ semimetal with the cubic AuCu$_3$-type structure: we prove a spontaneous RSB below Tc$^4$ by magnetotransport- and muon-spectroscopy (μSR) measurements.
Particularly meaningful are the transverse-field (TF)- μSR results in the mixed superconducting phase of CaSn$_3$, where the muon-depolarization rate depends on the magnetic field direction (here, applied along the [110] or [001] crystal directions). The absence of any additional muon depolarization along [110] suggests that an unconventional vortex lattice (VL) sets in. Conversely, in the [001] case, a VL encompassing at least 52% of the sample volume indicates the bulk nature of superconductivity.
Similarly, by scanning tunnelling spectroscopy in Cu$_x$Bi$_2$Se$_3$, vortices exhibit an elliptical shape within stretched VLs for applied fields H orthogonal to the Se planes, whereas “no obvious in-plane vortices” could be observed for H parallel to the Se layers$^5$.
Such evidence and our current experimental results on CaSn$_3$ seriously question the pertinence of the conventional Abrikosov model to the superconducting mixed state of nematic superconductors since multi-component order parameter superconductors may exhibit unusual vortex structures (fractional and/or non-axial vortices)$^6$.
Finally, the superfluid density in the (001) planes, extracted from TF-µSR data, shows a fully gapped low-temperature behaviour, with $\Delta$(0)=0.61(7) meV. Additional zero-field μSR results indicate that the superconducting state is time-reversal invariant. This fact and the RSB in a fully-gapped superconductor suggest CaSn$_3$ as nematic superconductor with an unconventional pairing state in a multidimensional representation.
$^1$https://doi.org/10.1038/nphys3781
$^2$https://doi.org/10.1038/s41467-019-14126-w
$^3$https://doi.org/10.1038/nphys3907
$^4$https://doi.org/10.1103/PhysRevB.105.094508
$^5$https://doi.org/10.1103/PhysRevX.8.041024
$^6$https://doi.org/10.1103/RevModPhys.63.239

Speaker: Dr Gianrico LAMURA (CNR-SPIN)

13:00 → 14:20 Lunch

14:20 → 16:00 Oral contributions: Semiconductors

14:20 Muonium reaction in semiconductors and insulators: the role of the transition state

The model describes the reaction of atom-like muonium with the host lattice at the end of the implantation trajectory. Reactions of the bare muon with the host or prompt formation of the final states are not covered by this model. Since these alternative processes are temperature independent, their maximum contribution can be estimated from the smallest value that occurs at any given temperature. They can be considered as a temperature-independent "background".
At the end of the trajectory, the muonium has just enough kinetic energy to jump across the potential barrier from one interstitial site to the next. At the top of the barrier, muonium is so slow that a strong inelastic interaction, e.g., the excitation of a local stretching mode, can occur and a weakly bound muon-electron configuration, the transition state, is formed (see Fig. 1).

Speaker: Rui Vilao (CFisUC, Department of Physics, University of Coimbra)

14:40 Muonium states in semiconducting transition metal dichalcogenides

The usual response of muonium to an external magnetic field is dominated by the hyperfine interaction, which causes the observed spectrum to show the transition frequencies between different muonium spin states. However, we have recently discovered an unconventional magnetic muonium state in 2H-MoTe$_2$ where the muonium acts a magnetic impurity, which polarizes the local electronic magnetic moments $[1]$. For sufficiently small externally applied fields, the "magnetic" muonium effectively behaves as a diamagnetic muon in a local magnetic field. Here, we show experimentally that in 2H-MoTe$_2$ the magnetic muonium coexists with another conventional, non-magnetic muonium state (Fig. 1b). The latter is axially symmetric with a hyperfine coupling of A$_\parallel$=1426(1) MHz and A$_\perp$=1368(3) MHz, corresponding to an effective Bohr radius of $\approx$ 0.82 Angstrom. The hyperfine coupling remains fairly constant, as a function of temperature, until the state disappears around the same temperature where the magnetic muonium disappears as well. We employ density functional theory calculations to reveal that this is linked to the presence of two muonium sites in the compound: one within the van der Waals gap that becomes magnetic, and a second one inside the layer, that is conventional. A similar behavior is also observed in 2H-WSe$_2$ (Fig. 1a), indicating that this is a more general feature of semiconducting transition metal dichalcogenides.

$[1]$ J. A. Krieger, et al., arXiv:2206.03051 (2022)

Speaker: Jonas A. Krieger (Max Planck Institute of Microstructure Physics)

15:00 Probing hydrogen sites and negative hyperfine parameter in semiconducting BaSi2 by muon spin rotation

Hydrogen passivation of defects is commonly used to reduce defects in semiconductors such as GaAs, diamond, and Si. We recently found by experiment that atomic hydrogen is also very effective in significantly increasing a minority-carrier lifetime (> 10 μs) in BaSi2, one of the emerging materials for thin-film solar cell applications. This means that defects no longer act as recombination centers in BaSi2 after hydrogen passivation [1-2]. But three has been no experimental data about the hydrogen site in BaSi2. We employed muons to study the hydrogen state in single-crystalline BaSi2. Distinct neutral muonium state was identified in the high transverse-field measurements. From the temperature dependence, negative hyperfine parameters was suggested. From the angle-dependence of the hyperfine parameter in the magnetic fields applied in the a x b, b x c, and c x a planes, and comparison to the calculations based on density-functional theory (DFT), the hydrogen site in the BaSi2 crystal is proposed.
[1] Z. Xu et al., Phys. Rev. Mater. 3, 065403 (2019).
[2] X. Xu et al., J. Appl. Phys. 127, 233104 (2020).

Speaker: Dr Kenji Kojima (TRIUMF and SBQMI, UBC)

15:20 Carrier lifetimes in high-lifetime silicon wafers and irradiation induced recombination centres

Photoexcited muon spin spectroscopy (photo-$\mu$SR) was used to measure excess charge carrier lifetimes in passivated silicon wafers. Optically generated excess carriers interact with muonium centres via carrier exchange interaction and induce relaxation in the $\mu$SR time spectrum. The photo-$\mu$SR technique utilises this additional relaxation rate as a measure of the excess carrier density, which in turn enables us to measure carrier lifetime spectra by controlling delays between a muon and laser pulse $[1]$. In addition, the depth-resolved measurement can characterise carrier kinetics at specific depths within a Si wafer and enables us to separate bulk and surface recombination rates $[2]$. Based on these developments, we recently applied the technique to passivated Si samples with extremely long effective lifetimes ($>$1 ms) and observed that prolonged muon irradiation resulted in significant degradation of a measured lifetime $[3]$. Follow-up characterisation measurements, including deep-level transient spectroscopy, strongly suggested that beam damage generated defect-related recombination centres in bulk. Our results demonstrate an extremely rare case in $\mu$SR applications, where beam damage to crystalline lattice was clearly detected by virtue of high-lifetime Si wafers and, in turn, low native defect densities.
$[1]$ K. Yokoyama, et al. Phys. Rev. Lett. 119, 226601 (2017); Appl. Phys. Lett. 115, 112101 (2019).
$[2]$ K. Yokoyama, et al. Appl. Phys. Lett. 118, 252105 (2021).
$[3]$ J. D. Murphy, et al. submitted to Journal of Applied Physics.

Speaker: Dr Koji Yokoyama (ISIS, STFC, UK)

15:40 Ambipolar Property of Isolated Hydrogen in Oxide Materials Revealed by Muon

The study on the electronic state of muon as pseudo-hydrogen (represented by the elemental symbol Mu) by muon spin rotation has long been appreciated as one of the few methods to experimentally access the electronic state of dilute hydrogen (H) in semiconductors and dielectrics. Meanwhile, theoretical predictions on the electronic state of H in these materials by first-principles calculations using density functional theory (DFT) do not always agree with the observed states of Mu. In order to address this long-standing issue, we have re-examined the vast results of previous Mu studies in insulating/semiconducting oxides with special attention to the non-equilibrium character and the ambipolarity of Mu. As a result, we established a semi-quantitative model that enables systematic understanding of the electronic states of Mu in most oxides.
First of all, Mu often occurs simultaneously in a neutral (Mu$^0$) and a diamagnetic state (Mu$^+$ or Mu$^-$) in wide-gap oxides, which is not explained by DFT calculations that predict only diamagnetic states with the polarity determined by the equilibrium charge-transition level ($E^{+/-}$). Our model considers that $\mu^+$ interacts with self-induced excitons upon implantation to form relaxed-excited states corresponding to a donor-like (Mu$_D$) and/or an acceptor-like (Mu$_A$) states. Moreover, these states are presumed to accompany the electronic level ($E^{+/0}$ or $E^{-/0}$) predicted by the DFT calculations for H. By considering that the stability of these two states including their valence is determined by i) the relative position of $E^{\pm/0}$ in the energy band structure of the host and ii) a potential barrier associated with the transition between Mu$_D$ and Mu$_A$, we find that the known experimental results can be explained systematically in accordance with $E^{\pm/0}$. The model also provides new insights into the polaron-like nature of the electronic states associated with shallow donor Mu complexes and the fast diffusion of Mu$^0_A$.

Speakers: Prof. Ryosuke Kadono (IMSS, KEK) , Dr Masatoshi Hiraishi (IMSS, KEK)

16:00 → 16:20 Break

16:20 → 17:20 Oral contributions: New techniques

16:20 Impact of Growth Conditions on the CH$_3$NH$_3$PbI$_3$ Perovskite Solar Cells, Studied by Low-Energy $\mu$SR

Metal halide perovskites (MHPs) have attracted great attention in recent years due to their enormous potential for application in optoelectronic devices. However, the defects at surface/interfaces and grain boundaries of perovskite films, which impede the further enhancement of power conversion efficiency (PCE) and long-term stability of halide perovskite solar cells (PSCs), still need to be fully understood. Here, we studied the impact of different growth conditions on the interface and grain boundaries of CH$_3$NH$_3$PbI$_3$ perovskite films by low-energy $\mu$SR. Our measurements show that low-energy $\mu$SR can become a powerful technique for studying the defect engineering of PSCs.

Speaker: Xiaojie Ni (Paul Scherrer Institute)

16:40 Coherent microwave control of muonium

We demonstrate the most fundamental coherent control techniques by excitation of microwave spin transitions in muonium, namely driven Rabi oscillations and Ramsey fringes upon free evolution. Unprecedented performance is achieved by triggering microwave pulses by a single implanted muon, which enables coherent spin manipulation of individual muonium atoms.

As a first example, we suppress extrinsic line broadening with the Ramsey experiment on strongly coupled muonium in SiO$_2$ (Fig. 1). As a second example, we retrieve the electron $g$-factor of bond-centered muonium in Si using the double electron-muon resonance (DEMUR) technique and decouple the system from its environment by strong driving of the electron-muon double quantum transition.

Overall, we expect that this capability will provide a powerful tool to investigate the effect of the environment on isolated coupled spins, uncover the details of coupled electron-muon systems in matter and validate quantum electrodynamics in the context of (vacuum) muonium spectroscopy.

Speaker: Dr Andrin Doll (Paul Scherrer Institute)

17:00 Status of the New Muonic Helium Atom HFS Measurements at J-PARC MUSE

We recently proposed new precision microwave spectroscopy measurements of the ground-state hyperfine structure (HFS) of muonic helium atom [1]. Muonic helium is a hydrogen-like atom composed of a helium atom with one of its electrons replaced by a negative muon. The ground-state HFS, resulting from the interaction of the remaining electron and the negative muon magnetic moment, is very similar to that of muonium but inverted, and the same technique can be used to precisely measure muonic helium HFS. It is a sensitive tool to test three-body atomic system, bound-state quantum electrodynamics theory, and determine fundamental constants of the negative muon magnetic moment and mass. The world most intense pulsed negative muon beam at J-PARC MUSE gives an opportunity to improve previous measurements, and to test further CPT invariance through comparison of the magnetic moments and masses of positive and negative muons.
Test measurements at D-line are in progress utilizing MuSEUM apparatus at zero field. Muonic helium HFS were measured at different helium pressures to determine the pressure shift using methane as an electron donor. The obtained results have already better accuracy than previous measurements [2,3]. Muonium HFS was also measured to investigate the isotopic effect on the pressure shift.
We also started investigating a new experimental approach to improve HFS measurements by repolarizing muonic helium atoms using a spin exchange optical pumping (SEOP) technique [4]. If successful, this would drastically improve the measurement accuracy.
An overview of the different aspects of these new muonic helium HFS measurements and the latest results will be presented.

[1] P. Strasser, et al., JPS Conf. Proc. 21 (2018) 011045.
[2] H. Orth, et al., Phys. Rev. Lett. 45 (1980) 1483.
[3] C.J. Gardner, et al., Phys. Rev. Lett. 48 (1982) 1168.
[4] A.S. Barton, et al., Phys. Rev. Lett. 70 (1993) 758.

Speaker: Patrick Strasser (KEK)

17:20 → 19:00 Posters: Poster Session III

17:20 → 19:00 Sponsor Event: Exhibition

18:00 → 19:30 Conference Event: Public Engagement Lecture (the event language is Italian)

Conveners: Dr Massimiliano Clemenza (INFN Sezione Milano Bicocca) , Dr Raffaele Silvani, Prof. Roberto De Renzi (Università di Parma)

18:00 Muons and cultural heritage - Quantum Magic

The event language is Italian

Speakers: Dr Massimiliano Clemenza (INFN Sezione Milano Bicocca) , Dr Raffaele Silvani

Locandina1settembre.jpg

Friday, 2 September

09:00 → 09:40 Invited Talks: Invited Talk VIII

09:00 Microscopic Dynamics of Structural Glasses Investigated by Quasielastic Neutron Scattering

In this presentation I will give a short introduction into quasielastic neutron scattering (QENS) and its application to glass-forming systems. QENS operates on time scales from picoseconds to a microsecond and at the same time has a spatial resolution in the Ångström range. Therefore, it is well suited for the study of molecular and polymeric glass-formers.

The dynamics of glass-formers is still poorly understood, but certain universal features can be found which a theory has to explain. Foremost, there is the α relaxation, which governs what is usually called ‘glass transition’. Its temperature-dependence is highly non-Arrhenius and the shape of correlation functions non-exponential. In addition, faster relaxations may be present, among which the universal ‘fast β relaxation’ in the picosecond range is strongly related to the α relaxation in mode-coupling theory. As the fastest universal process, glasses show an excess of the vibrational density of states above the Debye model in the low frequency range, the so-called ‘boson peak’.

All these phenomena can be observed by QENS with the additional information of a length scale. In addition, it is possible to study them in confined glass-formers in order to access their system-size-dependence. Selected QENS experiments will be presented and discussed.

Speaker: Dr Reiner Zorn (Forschungszentrum Juelich)

09:40 → 10:20 Oral contributions: Molecular chemistry and chemical physics II

09:40 First depth-resolved beta-NMR measurements of 1-ethyl-3-methylimidazolium acetate

Over the past decade, we have been using beta-detected NMR to examine the properties of amorphous materials. While this has typically focused on polymers,$^1$ we have recently been interested in ionic liquids (ILs). ILs are binary mixtures: they are composed of two oppositely charged molecular species. They are also liquid at room temperature. Their properties, determined by strong electrostatic forces, make them attractive candidates for the development of next-generation battery technology.

The long-range forces between ions also affect their dynamics, one of our primary interests in amorphous materials. This makes ILs a fascinating comparison to the relatively well-understood case of polymers. As with polymers, many ILs are extremely resistant to crystallization and will instead vitrify upon cooling. In our prior work, we showed that $\beta$-NMR was a good probe of bulk IL dynamics and dynamic heterogeneity.$^2$ In our present experiments, we turn to the question of how the surface modifies these properties, presenting the first depth-resolved $\beta$-NMR measurements in 1-ethyl-3-methylimidazolium acetate. This interfacial region is important for understanding how constrained dimensionality affects dynamics, which in turn may affect this IL's effectiveness as a potential electrolyte in batteries or capacitors.

We will show that both the surface and the glass transition have large effects on molecular dynamics, which in many aspects differs greatly from our expectations. In the glassy phase, the surface dynamics appear to be simultaneously faster (i.e., liquid-like) and yet still heterogeneous (i.e., glass-like), an apparent departure from our understanding of "normal" behaviour. Additionally, relaxation becomes faster below the glass transition temperature.

$^1$McKenzie, I. et al. J. Chem. Phys. 156, 084903 (2022).
$^2$Fujimoto, D. et al. Chem. Mater. 31, 9346–9353 (2019).

Speaker: Dr Derek Fujimoto (TRIUMF)

10:00 Probing the [FeFe]-hydrogenase subsite using muon spectroscopy

In the drive to replace fossil fuels with sustainable alternatives, achieving the reversible interconversion of protons and dihydrogen is a crucial target. The reaction can be carried out readily using platinum-based systems, but the cost and availability of this precious metal preclude scaling such approaches. In nature, the [FeFe]-hydrogenase enzymes have evolved to perform the very same task at rates that rival platinum electrodes. These systems feature a large protein component in addition to a core bioinorganic unit, the {2Fe2S} subsite. To enable us to produce practical catalysts we need to mimic the chemistry carried out by the enzyme: the natural system itself is too large and sensitive for wide-scale use. Thus understanding the chemistry of the {2Fe2S} subsite is vital.

Central to the hydrogen chemistry carried out by the subsite is its interaction with protons. Probing the solution kinetics and electrochemisty of model systems allows us to understand key reactivity of iron hydrides on a timescale as short as one second. However, much of the most interesting behaviour of these models occurs on much shorter timescale. For example, the location of the primary protonation sites is still an open question, with terminal and bridging hydrides possible candidates along with the sulphur, carbonyl and cyanide ligands. Muonium, as a ‘light’ analogue of H·, offers the means of studying the structure and dynamics of such chemistry on the nanosecond timescale. The use of the avoided level cross (ALC) technique has now allowed to identify two sites for primarily muonation in this model in the solid stat, with density functional theory (DFT) assignment strongly implicating competing bridging and terminal binding. This unique insight opens up the possibility of new reaction pathways in both models and the enzyme as well as demonstrating the wider importance of muon techniques in studying reactive organometallic systems.

Speaker: Joseph Wright (University of East Anglia)

10:20 → 10:40 Break

10:40 → 11:20 Oral contributions: Molecular chemistry and chemical physics III

10:40 Advances in biochemical applications of $\beta$-detected NMR

Since the implementation of $\beta$-detected NMR ($\beta$-NMR) at TRIUMF, it has mainly been used to study condensed matter systems ranging from metals to superconductors to topological insulators. In the last few years, there has been a desire to extend the applications of $\beta$-NMR to include the study of biochemical problems. For a number of metal ions in our body, such as Mg(II), Zn(II) and Cu(I), the absence of convenient physical and spectroscopic properties limits our ability to characterize their role in health and disease using conventional techniques, such as classical NMR. However, $\beta$-NMR has the possibility to help address these gaps in our knowledge by aiding in the elucidation of metal coordination in biomolecules.

In this presentation, I demonstrate that we are able to observe $^{31}$Mg binding to the biomolecule adenosine 5’-triphosphate (ATP) in solution. The resonance spectrum shows two distinct peaks which indicates that we observe not one, but two distinct complexes between Mg$^{2+}$ and ATP. We identify these complexes with $^{31}$Mg $\beta$-NMR complemented by $^{31}$P NMR and DFT calculations. This represents the first measurement of a $\beta$-NMR probe binding to a biomolecule and is an important milestone in applying $\beta$-NMR to the study of biochemical problems$^1$.

1. R. M. L. McFadden et al., Angew. Chem. Intl. Ed. e202207137 (2022)

Speaker: Victoria Karner (TRIUMF)

11:00 New insight into $\mu$SR in water

Over the past four decades, muon spin rotation and relaxation technique in water and ice has been reported by several groups [1-4]. Most of the previous studies were focused on muonium chemistry (detection, its relaxation, reaction and frequencies) in water and ice. To deepen the understanding of muon behavior in water and application of $\mu$SR to life sciences and hydrated samples, we performed temperature dependent $\mu$SR study in water. We found the temperature dependent oscillation in zero-field spectra in ice for the first time and proposed a new model – interaction between four spin-one-half system – to interpret the data. We found two stopping sites (proportion of 35% and 10% of incident muons) for muons in hexagonal ice in which the muons in larger fraction (35%) move towards optimized geometry site with temperature approaching the melting point. The distances of the muon and protons are successfully detected in subatomic scale. This study will be helpful to understand the charge dynamics in materials, for example, ion diffusion in battery materials, proton transfer in hydrated materials, proton transfer in biological membranes and in general transport of other spin-nuclei in solid state materials.

[1] P. W. Percival, et al., Chem. Phys. Lett. 39 (1976) 333; Hyperfine Interact. 8 (1981) 325; Hyperfine Interact. 18 (1984) 543; Chem. Phys. 32 (1978) 353; Chem. Phys. 95 (1985) 321.
[2] K. Nagamine, et al., Chem. Phys. Lett. 87 (1982) 186.
[3] S. Cox, et al., Hyperfine Interact. 65 (1991) 993; Physica Scripta 1992 (1992) 292; Hyperfine Interact. 86 (1994) 747.
[4] Y. Wang, et al., Physica B: Condensed Matter 350 (2004) E451.

Speaker: Dr Amba Datt Pant (IMSS, KEK, Japan)

11:20 → 12:00 Oral contributions: Strongly correlated electron systems III

11:20 Beta detected NMR of $^8$Li in 2H molybdenum ditelluride

Layered transition-metal dichalcogenides (TMDs) are proposed as building blocks for van der Waals (vdW) heterostructures. Semiconducting TMDs are further prone to host magnetic impurities, e.g. at defects or interstitials. Here we investigate the behavior of interstitial $^8$Li$^+$ implanted into 2H-MoTe$_2$ at depths of $\sim$110 nm with $\beta$-detected NMR. We find that unlike muons $[1]$, the $^8$Li$^+$ does not show any signature of induced magnetism. We confirm this result by density functional theory, which identifies the Li stopping site at the 2a Wyckoff position in the vdW gap and shows the absence of Li-induced electronic spin polarization. Both, the spin lattice relaxation (Fig. 1c) and the resonance lines (Fig. 1a) show evidence for Li diffusion or a site change above 200K. The line shape of $^8$Li$^+$ is found to consist of quadrupolar satellites on top of a broad central peak (Fig. 1a). Therefore, we employ a frequency comb measurement, where four frequencies, $\omega_0-3\omega_{\mathrm{comb}}$, $\omega_0-\omega_{\mathrm{comb}}$, $\omega_0+\omega_{\mathrm{comb}}$, and $\omega_0+3\omega_{\mathrm{comb}}$ corresponding to the first-order quadrupolar satellite transitions are excited simultaneously as a function of $\omega_{\mathrm{comb}}$. This offers an enhanced sensitivity to the quadrupolarly split portion of the line. Using this method, we find a small decrease of the quadrupolar frequency with increasing temperature (Fig. 1b), showing the typical behavior associated with thermally excited phonons.

$[1]$ J. A. Krieger, et al., arXiv:2206.03051 (2022)

Speaker: Jonas A. Krieger (Max Planck Institute of Microstructure Physics)

11:40 Intriguing Topological Kagome Magnetism of TbMn$_6$Sn$_6$

Magnetic topological phases of quantum matter are an emerging frontier in physics and material science [1-6], of which kagome magnets appear as a highly promising platform. Here, we explore magnetic correlations in the recently identified topological kagome system TbMn$_{6}$Sn$_{6}$ using $\mu$SR, combined with local field analysis and neutron diffraction [1,4]. Our studies identify an out-of-plane ferrimagnetic structure with slow magnetic fluctuations which exhibit a critical slowing down below T$^{*}_{C1}\simeq$ 120 K and finally freeze into static patches with ideal out-of-plane order below T$_{C1}\simeq$ 20 K. The appearance of the static patches sets in at a similar temperature as the appearance of topological transport behaviors. We further show that a hydrostatic pressure of 2.1 GPa stabilizes the static out-of-plane topological ferrimagnetic ground state in the whole volume of the sample. Therefore the exciting perspective arises of a magnetically-induced topological system whose magnetism can be controlled through external control parameters. The present results [4] will stimulate theoretical investigations to obtain a microscopic understanding of the relation between the low-temperature volume-wise magnetic evolution of the static $c$-axis ferrimagnetic patches and the topological electronic properties in TbMn$_{6}$Sn$_{6}$.

[1] J.-X. Yin et al., Nature $\textbf{583}$, 533-536 (2020).
[2] Z. Guguchia et al., Nature Comm. $\textbf{11}$, 559 (2020).
[3] N.J. Ghimire and I.I. Mazin, Nature Materials $\textbf{19}$, 137-138 (2020).
[4] C. Mielke III et al., arXiv:2101.05763 (2021).
[5] C. Mielke III et al., Phys. Rev. Materials $\textbf{5}$, 034803 (2021).
[6] C. Mielke III et al. … Z. Guguchia, Nature $\textbf{602}$, 245-250 (2022).

Speaker: Charles Mielke III (Paul Scherrer Institut)

12:00 → 12:20 Invited Talks: Conference Summary

12:20 → 12:50 Conference Event: Prize Giving and Closing Ceremony (with IEEE SC Chapter presentation)