Muon User Meeting 2023: celebrating the work of Pabitra Biswas

11 Sept 2023, 10:00 → 12 Sept 2023, 15:30 Europe/London

John Wilkinson (STFC), Rhea Stewart (ISIS Muon Group, STFC, UK)

The ISIS muon user meeting this year will be held jointly with PSI at The Queen's College, Oxford, a 14th-century college right in the heart of the historic city.

This meeting will celebrate the work of the late Pabi Biswas, who developed an extensive research programme in the fields of magnetism and superconductivity, making use of pressure techniques in his work. Pabi started his career as a PhD student at Warwick University, before moving to work as an Instrument Scientist first at PSI and then ISIS; he continued to exploit the complementarity of both sources as he developed his scientific career. A series of talks are planned with speakers sharing their latest results in these subject areas, with the aim of fostering scientific collaborations across the muon community.

We will also have an afternoon of facility updates from both ISIS and PSI, including short talks on the latest projects taking place at both facilities. There will also be the opportunity for the user community to provide feedback on these developments. 

We are keen to encourage participation from early career researchers and to facilitate this we invite PhD students and postdocs to submit abstracts about their work. More information can be found here.

Local expenses will be covered for all delegates, and travel expenses will be reimbursed for those coming from UK-based institutions.

(Photo credit: John Cairns)

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Monday, 11 September

10:15 → 10:30 Welcome

Speaker: Prof. Roger Eccleston

10:30 → 10:50 In memoriam: Pabi's life and career

Speaker: Dr Adrian Hillier (STFC)

Pabi.pptx

10:50 → 11:20 Welcome Refreshments

11:20 → 12:40 Supeconductivity I

Convener: Toni Shiroka

11:20 Noncentrosymmetric, nonsymmorphic, triplet and singlet: the complex soup giving rise to superconducting magnetism

Superconductivity and magnetism have been traditionally viewed as antagonistic states of matter. On the other hand over the last decade or so evidence has accumulated for not just coexistence of superconductivity and magnetism, but even intrinsically magnetic superconducting states. I will discuss the complex and sometimes subtle interactions between lack of inversion symmetry, non-symmorphic crystal structure, and singlet and triplet pairing that may bring about such remarkable behaviour.

Speaker: Dr Jorge Quintanilla (University of Kent)

11:40 Pabitra, Superconductivity, Muons and the University of Warwick

While at Warwick, studying for his PhD, Pabitra was prolific in his work studying superconductivity with Prof. Martin Lees and Prof. Don Paul. His thesis captures perfectly the vast array of experimental work he did during his time at the University of Warwick. He discovered new information on the superconducting properties of FeSe0.5Te0.5, FeTe1-xSx, Lu2Fe3Si5, CaAlSi, ZrB12, and two different superconducting phases of Re3W. Here, I aim to summarise the contents of Pabitra’s thesis work studying superconductivity using muons and neutrons, as well as some of the work I had the pleasure of collaborating with Pabitra on.

Speaker: Dr Daniel Mayoh (University of Warwick)

12:00 Muon stopping states in superconductors with time-reversal symmetry breaking

A crucial issue in resolving the mechanism for unconventional superconductivity is the presence or absence of time-reversal symmetry breaking (TRSB), a property that can provide a tight constraint on the symmetry of the superconducting gap. Spontaneous magnetic fields have been measured in a collection of superconductors in µSR experiments, which has been used as evidence for TRSB in these materials. However, the fact that, in many of these systems, the spontaneous magnetic fields have been observed only by implanted muons might lead one to wonder whether these could be the result of a muon-induced effect. In this talk, I will present the results of a systematic investigation of muon-stopping states in several superconductor that have been found to exhibit spontaneous magnetic field in µSR experiments [1]. These calculations demonstrate that the presence of the muon leads to only a limited and relatively localised perturbation to the local crystal structure, while any small changes to the electronic structure occur several electron volts below the Fermi energy, leading to only minimal changes in the charge density on ions close to the muon. Our results imply that the muon-induced perturbation alone is unlikely to lead to the observed spontaneous fields in these materials, whose origin is more likely intrinsic to the time-reversal symmetry-broken superconducting state.

References
[1] B. M. Huddart et al., Phys. Rev. Lett. 127, 237002 (2021).

Speaker: Dr Benjamin Huddart (University of Oxford)

12:20 Intrinsic magnetism in superconducting infinite-layer nickelates

The discovery of superconductivity in Nd0.8Sr0.2NiO2 introduced a new family of layered nickelate superconductors that has now been extended to include a range of strontium doping, praseodymium or lanthanum in place of neodymium. A number of studies have indicated that electron correlations are strong in these materials, a feature that often leads to the emergence of magnetism. Here we report muon spin rotation/relaxation studies of a series of superconducting infinite-layer nickelates. Regardless of the rare earth ion or doping, we observe an intrinsic magnetic ground state arising from local moments on the nickel sublattice. The coexistence of magnetism—which is likely to be antiferromagnetic and short-range ordered—with superconductivity is reminiscent of some iron pnictides and heavy fermion compounds, and qualitatively distinct from the doped cuprates.

Speaker: Dr Andreas Suter (Paul Scherrer Institut)

12:40 → 13:40 Lunch

13:40 → 15:00 User Meeting: Facility Updates

Convener: Stephen Blundell (University of Oxford)

13:40 ISIS Update

TBA

Speaker: Adrian Hillier (STFC)

14:10 PSI Update

TBA

Speaker: Thomas Prokscha (Paul Scherrer Institute)

14:40 Computing Update

TBA

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

15:00 → 15:10 Conference Photo

15:10 → 15:40 Refreshments

15:40 → 17:05 User Meeting: Facility Developments

Convener: Joseph Wright (University of East Anglia)

15:40 Super-MuSR Update

TBA

Speaker: Peter Baker (STFC)

15:55 Digital Muons

TBA

Speaker: Daniel Pooley (STFC)

16:10 WiMDA Project

TBA

Speaker: Dr Francis Pratt (STFC)

16:25 Leg stretcher

16:35 RIKEN Refurbishment

TBA

Speaker: Adam Berlie (Science and Technology Facilities Council)

16:50 Silicon Pixel Detectors at PSI

TBA

Speaker: Thomas Prokscha (Paul Scherrer Institute)

17:05 → 17:20 Discussion

Speaker: Tom Lancaster (Durham University)

17:20 → 17:30 Leg stretcher

17:30 → 18:30 Early Career Presentations

Convener: Mark Telling (STFC)

17:30 Magnetic behaviour in two-dimensional molecule-based magnets with S=2 spins

Molecular magnets consisting of metal ions surrounded by organic ligands offer a playground for investigating and manipulating the strength and nature of magnetic interactions present in a material. One possibility is to adjust the value of the magnetic spin centres in the molecular magnet, which means the crossover between quantum and classical regimes can be examined. This is realised by MnF3(pyz): a quasi-two-dimensional material consisting of S=2 Mn(lll) spin centres in which the presence of reduced dimensionality and a large value of S results in magnetic behaviour within this classical/quantum crossover regime. A combination of muon-spin spectroscopy, magnetometry and electronic structure calculations have been utilised in order to provide insight into this magnetism, which provides an overall picture not accessible by any technique in isolation.

Speaker: Nathan Bentley (University of Durham)

NPBentley_MuonUserMeeting23.pptx

17:40 Tunable magnetic frustration in PbM2Ni6Te3O18 (M = Mn, Fe, Co, Zn)

The pentanary oxides PbM2Ni6Te3O18, where M = Mn, Fe, Co, Zn, allow magnetic frustration to be tuned by changing the transition metal ion M. These compounds contain Ni2+ zigzag chains along the c- axis which order antiferromagnetically below TN, in addition to a kagome-like interchain structure in the a-b plane which becomes magnetically frustrated when coupled ferromagnetically. The competition between the ferromagnetic interchain exchanges J3 and J5 turns out to be crucial in determining the magnetic structures. By direct comparison of the muon-spin rotation (μSR) asymmetry, we demonstrate that when M = Mn, the larger M moment allows the M–Ni exchange J5 to dominate over the interchain (Ni–Ni) exchange J3 and suppresses magnetic frustration (see Figure 1b). But as J5 weakens (M = Fe, Co) and vanishes (M = Zn), J3 becomes increasingly significant and turns the system into a strongly frustrated one within the kagome-like structure. These results demonstrate beautifully how the nature of the magnetic ground state, whether fully ordered or strongly frustrated, can be constructed by the choice of a single magnetic ion in an isostructural family of materials containing zigzag chains.

Speaker: Hank Wu (University of Oxford)

17:50 Quantum tunnelling between muon sites in methylated benzene crystals

We present simulations of the effect of quantum tunnelling between muonium sites in benzene crystals that contain methyl groups. These are added to different carbon atoms, such that they alter the available transition pathways for tunnelling between candidate muon sites. The calculations are based on density functional theory, combining phonon computations (of the zero point energy of the muon) and transition state computations (of the height of the potential barrier between sites) in order to estimate the transition rates between sites and so determine which site pairs the muon can tunnel between during its lifetime. Our results are then used to simulate avoided level crossing (ALC) spectra, allowing us to assess the effectiveness of this methodology.

Speaker: Alberto Hernandez-Melian (Durham University)

18:00 Universal method to extract the average electron spin relaxation in organic semiconductors from muonium ALC resonances

"Muon spin spectroscopy and in particular the avoid level crossing (ALC) technique is a sensitive probe of electron spin relaxation (eSR) in organic semiconductors. In complex ALC spectra, eSR can be challenging to extract, as it requires the modelling of overlapping ALCs, where covariance between parameters can result in significant uncertainties. Here we demonstrate a general method
to extract eSR rate, which is independent on the number of ALCs resonances present, whether they overlap or not, and what the muonium hyperfine (isotropic and anisotropic) parameters are. This
can then be used as guidance for undertaking experiments efficiently. We will show this method used in TADF material and some organic molecules."

Speaker: Licheng Zhang (Queen Mary University of London)

18:10 Muon in biology: fundamentals and applications in cancer research

We apply quantum beam of muon to understand life phenomena like electron transfer in protein/DNA, detection of O2 in tissues, etc. Using muon, we propose a new noninvasive muon method to detect hypoxia in tumor/cancer tissue which will help to manage the treatment and early-stage diagnosis of cancer. We have tested the sensitivity of muon method for hypoxia detection and successfully detected the low O2 in dilute aqueous biological solutions (hemoglobin, albumin, serum, and tris-buffered saline solutions). For further systematic study, we perform MuSR measurements and calculations (DFT and PIMD) to understand muon and muonium behavior in water and buffer at different O2 concentrations. Recently, we have observed temperature dependent muonium oscillation at zero-field in ice. In the program, I will present new insight into MuSR in water and recent progress of muon in biology.

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

18:20 Muon Simulation Project - Software, Optimisation Methods and Online Training for Muon Users

Optimising the sample mounting to stop the muon beam is helpful when planning experiments with limited sample quantity. Using the instrument range curve is a good start in simple cases but computational tools can be used to help for more complex cases. We have established a design, search, and optimisation (DSO) process using musrSim and SRIM software tools to help in planning complex experiments. This has been tested against experimental data to confirm the comparison with the simulations. Comparisons have also been made with the methods previous proposed by Ziegler [1] and Elson et al. [2]. To explain the method to new muon users we are also preparing an online training course that will be available soon.

[1] Ziegler, J. F.; Biersack, J. P.; Littmark, U. (1985). The Stopping and Range of Ions in Matter. New York: Pergamon Press. ISBN 978-0-08-021607-2.
[2] Frank Elson et al 2023 J. Phys.: Conf. Ser. 2462 012024 doi:10.1088/1742-6596/2462/1/012024

Speaker: Debjyoti Sengupta (STFC)

18:45 → 19:30 Pre dinner drinks reception

19:30 → 21:30 Conference dinner

Tuesday, 12 September

09:30 → 10:50 Quantum Materials Under Pressure

Convener: Francis Pratt (STFC)

09:30 Designing the stripe-ordered cuprate phase diagram through uniaxial-stress

Cuprate high-temperature superconductors have complex phase diagrams with multiple competing ordered phases. Understanding to which degree charge, spin, and superconducting orders compete or coexist is paramount for elucidating the microscopic pairing mechanism in the cuprate HTSs. In this talk, I will report some novel results of muon-spin rotation (μSR), AC susceptibility and X-ray diffraction experiments on uniaxial stress effect on the static spin-stripe order and superconductivity in the La214 cuprates [1,2]. We find that in the cuprate system La2-xBaxCuO4 with x = 0.115 and 0.135, an extremely low uniaxial stress of 0.05 GPa induces a substantial decrease in the magnetic volume fraction and a dramatic rise in the onset of 3D superconductivity, from 10 to 32 K; however, the onset of at-least-2D superconductivity is much less sensitive to stress [1]. These results show not only that large-volume-fraction spin-stripe order is anti-correlated with 3D superconducting (SC) coherence, but also that these states are energetically very finely balanced. Moreover, the onset temperatures of 3D superconductivity and spin-stripe order are very similar in the large stress regime. These results strongly suggest a similar pairing mechanism for spin-stripe order, the spatially-modulated 2D and uniform 3D SC orders, imposing an important constraint on theoretical models.

References
[1] Z. Guguchia et. al., Phys. Rev. Lett. 125, 097005 (2020).
[2] Z. Guguchia et. al., arXiv:2302.07015 (2023).

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

09:50 Charge Order Stabilized Quantum Spin Liquid realised in Hollandite K2V8O16

Low dimensional magnetism is a field that has developed tremendously over the last decades. In search for model compounds, the use of high-pressure synthesis is an effective route for stabilizing otherwise inaccessible crystal structures and materials. Among such compounds we find K2V8O16, which is a quasi 1D magnet that belongs to the Hollandite family. K2V8O16 undergoes TMIT = 160 K, driven by a charge order (CO) formation, which is also visible in the bulk magnetic susceptibility that display a ~ 50 % reduction at TMIT = 160 K. Single crystal X-ray diffraction (XRD) explains the sudden decrease due to a dimerization in half of the V chains and the formation of spin singlets. Consequently, the remaining 50 % of the chains can be considered as isolated spin−1/2 chains, sustained within a ’sea’ of spin-dimerized chains. Muon spin relaxation measurement confirm that this CO stabilised isolated spins chains do not order down to lowest measured temperature of 100 mK. The longitudinal field dependence of the longitudinal field relaxation rate suggest that the ground state is a Tomonaga-Luttinger liquid. Additional hydrostatic and chemical pressure studies confirm the onset of highly fluctuating AF and short-range FM orders, in line with the predicted phase diagram of XXZ-Hamiltonian. Ambient and pressure dependent studies support a scenario in which the ground state at ambient pressure of K2V8O16 is a TLL, which here uniquely stabilised due to a peculiar form of CO.

Speaker: Ola Kenji Forslund (KTH Royal Institute of Technology)

10:10 TBA

Speaker: Dr Christopher Ridley (ISIS Neutron and Muon Source)

10:30 How spin states change under the pressure

TBA

Speaker: Isao watanabe (RIKEN)

10:50 → 11:20 Refreshments

11:20 → 12:40 Quantum Magnetism

Convener: Peter Baker (STFC)

11:20 Quantum spin liquids on triangular spin lattices

Magnetic materials usually undergo magnetic ordering at low temperatures, however, in certain instances they may remain quantum disordered even at zero temperature. Such a case is found in a quantum spin liquid (QSL), a state that is highly entangled, yet it lacks magnetic ordering. QSL states have been experimentally suggested for a variety of geometrically frustrated materials, including those featuring two-dimensional triangular spin lattice. For this lattice, however the simplest isotropic Heisenberg exchange leads to magnetic ordering, therefore, perturbations to the isotropic model are required to stabilize a QSL.
In my talk, I will focus on our recent discovery of a novel QSL candidate with perfect triangular symmetry neodymium heptatantalate, NdTa7O19, in which magnetic anisotropy is extremely large [1]. In-depth experimental studies including muon spin relaxation, neutron scattering, and electron spin resonance all suggested lack of magnetic ordering even at temperatures of only a few tens of millikelvins, much below the exchange-interaction scale of this compound. Furthermore, sizable Ising-like spin correlations between the nearest neighbors on the triangular lattice and persistent spin dynamics were found in the ground state. Recently, progress has been made also in theoretical understanding of this intriguing magnetic ground state of neodymium heptatantalate, which critically depends on small transverse magnetic exchange in addition to the dominant Ising exchange [2]. As the magnetic anisotropy of rare earths can greatly change from one ion to another, rare-earth heptatantalates provide a new generic framework for QSLs and other intriguing quantum states. Parallels will be drawn to some other promising members of the family.

References
[1] T. Arh, B. Sana, M. Pregelj, P. Khuntia, Z. Jagličić, M. D. Le, P. K. Biswas, P. Manuel, L. Mangin-Thro, A. Ozarowski, and A. ZorkoThe Ising triangular-lattice antiferromagnet neodymium heptatantalate as a quantum spin liquid candidate, Nat. Mater. 21, 416 (2022).
[2] M. Ulaga, J. Kokalj, A. Zorko, and P. Prelovšek, Quantum spin liquid in the easy-axis Heisenberg model on frustrated lattices, arXiv:2307.03545.

Speaker: Prof. Andrej Zorko (Josef Stefan Institute)

11:40 Quantum disorder in low-dimensional molecule-based magnets

The search for new states of matter such as quantum spin-liquids involves the characterization of disordered magnetic ground states, and spinons or topological excitations, along with signatures of entanglement. Model examples of these features are found in systems with reduced dimensionality, especially one-dimensional spin chains. I will describe our recent investigations of low-dimensional systems built from metal centres linked by molecules. These include muon measurements on staggered and chiral spin-chain materials and also on materials that host spin dimers. Each of these is supported by electronic structure calculations that reveal not only muon sites, but also the underlying interactions at play in the materials.

Speaker: Tom Lancaster (Durham University)

12:00 Organic Superconductivity Nearby Quantum Criticality of a Magnetic Frustration

The hole-doped organic superconductor kappa-(ET)4Hg3-dBr8, (k-HgBr), where d=11% and ET=bis(ethylenedithio)tetrathiafulvalene, has been the key to bridge the knowledge gap between half-filled organics and doped cuprate systems. Nonetheless, the isotropic triangular lattice of ET dimers of k-HgBr, organics, unlike the square lattice in cuprates, is suspectedly responsible for its provides extensive geometrically control through nearest, t, and next-nearest, t', transfer integrals between sites. In k-HgBr the temperature dependence of susceptibility which is well scaled with the organic spin liquid insulator k-(ET)2Cu(CN)3. follows isotropic triangular lattice model t ~ t', i. e., candidate of a doped Mott quantum spin liquid. However, both k-HgBr and cuprate superconductors have a wide region at high-temperature and high-pressure corresponding to a strange metallic state where resistivity exhibits a linear temperature dependence which is a non-Fermi-liquid (FL) behavior. In k-HgBr this non-FL region gradually changed to FL state by pressure [1] like the change of metallic state from optimal to overdoped cuprates. The 13C-NMR and heat capacity study suggested that the enhanced antiferromagnetic fluctuations towards low-temperature originates the non-FL k-HgBr [2]. This evidence may locate superconducting k-HgBr nearby quantum critical point (QCP) in between FL and localized states, where in its non-FL state the incoherent conductivity was observed [1,3]. Our zero-field muSR experiment showed the relaxation rate from temperature around 10 K down to 0.3 K is temperature-independent. This is a high possibility of the superconducting state that preserved time-reversal symmetry [4]. Furthermore, we will present the result of temperature dependence of penetration depth from transverse field muSR measurement, showing a peculiar estimation of strong coupling superconductivity with small superfluid density. We discuss the comparation with other organic superconductors which showed a typical and a deviation from traditional d-wave symmetry [5].

References
[1] H. Taniguchi, et al., J. Phys. Soc. Jpn. 11, 113709 (2007)
[2] Y. Eto, et al., Phys. Rev. B 81, 212503 (2010)
[3] H. Oike, et al., Nat. Commun. 8, 756 (2017)
[4] D. P. Sari, et al., J. Phys.: Conf. Ser. 2462, 012061 (2023)
[5] D. P. Sari, et al., Phys Rev B. 104, 224506 (2021)

Speaker: Dita Puspita Sari (Shibaura Institute of Technology)

12:20 Do spin dynamics persist at low temperature in frustrated magnets?

Speaker: Stephen Blundell (University of Oxford)

12:40 → 14:00 Lunch

14:00 → 15:20 Superconductivity II

Convener: Adrian Hillier (STFC)

14:00 Time-reversal symmetry breaking in the superconducting low carrier density quasi-skutterudites

Low carrier density superconductors have emerged as intriguing subjects of study, as they challenge our existing understanding of superconductivity while offering valuable insights into its underlying mechanisms. The complex structure of the Remeika phases, coupled with the superconductivity and low carrier concentrations, presents a unique opportunity to investigate the superconducting ground state in materials having low carrier densities.

In this presentation, I will discuss recent investigations using muon spin relaxation and rotation (μSR) techniques on these compounds. These studies have revealed the presence of a complex superconducting gap and indications of time reversal symmetry breaking. These findings indicates that electron-electron interactions play a crucial role in stabilizing the pairing mechanism, highlighting the unconventional nature of these low carrier density superconductors.

References:
A. Kataria, J. A. T. Verezhak, O. Prakash, R. K. Kushwaha, A. Thamizhavel, S. Ramakrishnan, M. S. Scheurer, A. D. Hillier, and R. P. Singh, Phys. Rev. B 107, L100506 (2023)

Speaker: Dr Ravi Prakash Singh (Indian Institute of Science Education and Research (IISER) Bhopal)

14:20 Probing the Superconducting Gap Structure in the Noncentro symmetric Topological Superconductor ZrRuAs and HfRuP

The superconducting gap structure of the topological superconductor candidates ZrRuAs and HfRuP with a noncentrosymmetric crystal structure has been investigated using muon-spin rotation/relaxation (μSR) measurements in transverse-field (TF) and zero field (ZF) geometries [1, 2]. Magnetization and electrical resistivity measurements reveal bulk superconductivity below a superconducting transition temperature Tc = 7.9(1) K in ZrRuAs and below 9.0(1) K in HfRuP. The temperature dependence of the effective penetration depth obtained from the TF-μSR spectra, and the electronic heat capacity in the superconducting state of ZrRuAs, are well described by an isotropic s-wave gap model. Our TF-μSR study of HfRuP also confirms the s-wave gap symmetry in HfRuP. Comparison of the electronic mean free path with the superconducting coherence length suggests superconductivity in the dirty limit in ZrRuAs. Our ZF μSR data of both the compounds show that there is no significant change in the muon-spin relaxation rate above and below Tc, indicating that time-reversal symmetry is preserved in the superconducting state. The results of the present study will be important to understand the superconductivity in other topological superconductors.

References
[1] D. Das et al, Phys. Rev. B 103, 144516 (2021).
[2] D. Das et al, Magnetochemistry, 8, 135, (2023);
https://doi.org/10.3390/magnetochemistry9050135

Speaker: Dr Devashibhai Adroja (ISIS Neutron and Muon Source)

14:40 Superconductors without inversion symmetry: expectations and surprises

In the recent search for unconventional- and topological superconductivity, noncentrosymmetric superconductors (NCSCs) are among the most promising candidate materials. Surprisingly, some of them - especially those containing rhenium - seem to exhibit also time-reversal symmetry (TRS) breaking in their superconducting state, while TRS is preserved in many other isostructural NCSCs. To date, a satisfactory explanation for such discrepant behavior is still missing. More in general, the reasons behind the unconventional superconductivity of these materials are not quite clear.

In our recent work, we consider the consequences of the lack of inversion symmetry, the breaking of time-reversal symmetry, and the role of spin-orbit coupling on selected materials mainly through the use of local probes, such as the nuclear magnetic resonance (NMR) and muon-spin rotation (µSR). Detailed NMR, µSR, magnetometry, and heat-capacity studies show that highly anticipated unconventional superconductors often show only ordinary BCS features. Although these results disprove the widespread belief that attributes to noncentrosymmetric superconductors almost invariably also an unconventional character, we still gain useful insight into the physics of NCSCs and about the implications of the lack of inversion symmetry.

References
[1] T. Shang, et al., T. Shiroka, Phys. Rev. B, 97, 020502 (2018).
[2] T. Shang, et al., T. Shiroka, npj Quantum Mater. 5, 76 (2020).

Speaker: Dr Toni Shiroka

15:00 Muon-spin relaxation studies of time-reversal symmetry breaking in superconductors

Time-reversal symmetry breaking (TRSB) in superconductors is manifested by the spontaneous appearance of small magnetic fields in the superconducting state, and can be detected using techniques such as muon-spin relaxation (μSR) or measurements of the Kerr effect. The most notable examples of such superconductors are a handful of strongly correlated magnetic materials, such as Sr2RuO4 and some U-based heavy fermions, where unconventional superconducting pairing states are readily anticipated. However, in recent years TRSB has been found in a number of weakly-correlated superconductors which otherwise appear to have conventional properties, such as a fully open superconducting gap.

Here I will discuss two studies examining TRSB in superconductors using μSR. We recently found that CaPtAs is a superconductor below Tc = 1.5 K [2], which has a noncentrosymmetric tetragonal structure with 3D honeycomb networks of Pt-As. Our μSR measurements show evidence for time-reversal symmetry breaking below Tc, while the penetration depth derived from muon-spin rotation and the tunnel-diode-oscillator based method suggest a nodal superconducting gap [3]. Nodal superconductivity with TRSB is an unusual combination in weakly correlated noncentrosymmetric superconductors, suggesting that CaPtAs may bridge the gap between different classes of superconductor with TRSB.

The kagome lattice superconductors (K, Rb, Cs)V3Sb5 have attracted tremendous attention, in part due them hosting unconventional charge-ordered states that may exhibit TRSB among other unusual phenomena. Our zero-field μSR measurements of CsV3Sb5 reveal an increased relaxation in the charge-ordered state [4]. However, an increase in the relaxation rate persists in longitudinal-field measurements, suggesting a dynamic nature of the internal fields, and therefore they cannot be readily associated with TRSB.

References
[1] Sudeep Kumar Ghosh, Michael Smidman, Tian Shang James F Annett, Adrian D Hillier, Jorge Quintanilla and Huiqiu Yuan, J. Phys.: Condens. Matter 33 033001 (2021).
[2] Wu Xie, PeiRan Zhang, Bin Shen, WenBing Jiang, GuiMing Pang, Tian Shang, Chao Cao, Michael Smidman and HuiQiu Yuan, Science China Physics, Mechanics & Astronomy, 63, 237412 (2020).
[3] T. Shang, M. Smidman, A. Wang, L.-J. Chang, C. Baines, M. K. Lee, Z. Y. Nie, G. M. Pang, W. Xie, W. B. Jiang, M. Shi, M. Medarde, T. Shiroka and H. Q. Yuan, Phys. Rev. Lett. 124, 207001 (2020)
[4] Zhaoyang Shan, Pabitra K. Biswas, Sudeep K. Ghosh, T. Tula, Adrian D. Hillier, Devashibhai Adroja, Stephen Cottrell, Guang-Han Cao, Yi Liu, Xiaofeng Xu, Yu Song, Huiqiu Yuan, and Michael Smidman, Phys. Rev. Research 4, 033145 (2022)

Speaker: Dr Michael Smidman

15:20 → 15:30 Closing Remarks

Speaker: Dr Russell Ewings