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The 2024 Muon User Meeting will be held at The Cosener's House, Abingdon, Thursday 5th and Friday 6th September, 2024. This year marks 40 years since first neutrons at ISIS, but also 40 years since planning for muons started in earnest, after funding for the new facility was secured from the EC and a number of participating countries.
The meeting will have the combined themes of 'Computational Techniques' and 'Techniques for Pulsed Beams' - hopefully something in the programme to interest everyone! There will also be facility news, including a report on the new SuperMuSR instrument, a forward look to ISIS-II, and the chance for young researchers to contribute to the meeting.
Further details will follow.
MnO has been the skeleton in the closet for the numerous scientists that have been trying to model and understand the μSR results of magnetic oxides [1-8]. In zero field, this material shows a single precession frequency whose origin was hard to reconcile with the proposed muon sites and the possible formation of muonium-like states [4-6]. The picture is further complicated by a time-window dependent Knight shift discovered by Uemura and co-workes [3], where different Knight shifts are obtained considering the asymmetry in the first μs as compared to that for the second μs, etc. The puzzle is eventually solved by highlighting the role of symmetry, magnetostriction, and muon diffusion in the system. In this talk I will describe how first principles simulations and molecular dynamics exploiting machine learning force fields allow to verify or disprove various proposals that have been discussed over the years [3-8] on the microscopic description of the muon life in MnO. This finally allows to easily explain both the zero-field data and the unusual time-dependent Knight shift in MnO, solving, possibly for the second time [9], an old puzzle.
[1] R. S. Hayano et al, Phys. Rev. Lett. 41 421 (1978)
[2] Y. J. Uemura et al., Hyperfine Interactions 6, 127 (1979)
[3] Y. J. Uemura et al., Hyperfine Interactions 8, 725 (1981)
[4] Y. J. Uemura et al., Hyperfine Interactions 17-19, 339 (1984)
[5] K. Ishida et al., Hyperfine Interactions 17-19, 927 (1984)
[6] C. Boekema, hyperfine interactions 17-19, 305 (1984)
[7] K. Nishiyama et al., Hyperfine Interactions 104 349 (1997)
[8] Erik Lidstrom and Ola Hartmann, J. Phys.: Condens. Matter 12 4969 (2000)
[9] Theoretical investigation of hyperfine fields in fluoromethanes and transition metal oxides Gopalakrishnan, Gowri, State University of New York at Albany Ph.D. Theses, 1998.
We are carrying out DFT investigations in conjunction with muSR measurements in order to reveal electronic states of strongly corelated systems, organic molecular systems and battery materials. As our first step, we tried to creproduce muSR results on the mother material of the La-based high-Tc superconducting oxide, La2CuO4 (LCO), which is a typical strongly correlated system. There were some early muSR results showing the appearance of the muon-spin precession due to the formation of a long-range ordered state of Cu spins. However, all of DFT and computational studies have failed to reproduce the muSR results on LCO and we could not achieve realistic feature of Cu spins from muSR for a long time. We tackled this problem by using DFT calculations with the electronic-correlation functional GGA+U assuming a supercell model with only one muon. Since our computational model was too big for a standard PC, we needed to use a supercomputing system with GPU units and many core numbers. I am going to report how to manage our DFT calculations to reproduce muSR results on LCO and how we reveal Cu-spin states. Through this study, we found some new problems regarding how to handle DFT calculations. I will also report those new problems to be considered to use DFT to understand muSR results.
Muons are part of natural cosmic radiation but can also be generated at spallation sources for material science and particle physics applications. Recently, pulsed muons have been used to characterize the density of free charge carriers in semiconductors and their recombination lifetime. Muon beam irradiation can also result in the formation of dilute levels of crystal defects in silicon. We have investigated the characteristics of these defects in terms of their formation, recombination activity, and deactivation. Charge carrier lifetime assessments and photoluminescence imaging has a great sensitivity to measure the generated defects in high-quality silicon samples exposed to ~4 MeV (anti)muons and their recombination activity despite the extremely low concentration. The defects reduce the effective charge carrier lifetime of both p- and n-type silicon and appear to be more detrimental to n-type silicon. Defects are created by transmission of muons through the wafer and there are indications that slowed or implanted muons may create additional defects. In a post-exposure isochronal annealing study, we observe that annealing at temperatures of up to 450 °C does not by itself fully deactivate the defects. A recovery of charge carrier lifetime was observed when the annealing was combined with Al2O3 surface passivation, probably due to passivation of the bulk defects from hydrogen from the dielectric film.
"The Kagome lattice, characterized by a 2D arrangement of corner-sharing triangles, exemplifies frustration's pivotal role in the emergence of intriguing magnetic phases and quantum spin liquids. The pursuit of materials realizing such quantum states recently led to the discovery of a new family of kagome compounds. These compounds arise from substituting Zn²⁺ with Y³⁺ in the renowned spin liquid candidate, herbertsmithite ( ZnCu(OH)₆Cl₂ ), initially aiming to charge-dope a gapless quantum spin liquid. Despite not achieving charge doping, the resulting insulating materials, named Y-kapellasite ( YCu3(OH)₆₊ₓCl₃₋ₓ, x=0 or ⅓ ), persist as intriguing frustrated magnets with well-isolated kagome layers and minimal magnetic lattice dilution.
Focusing on the anisotropic x=1/3 counterpart, Y₃Cu₉(OH) ₁₉Cl₈, this variant materializes in an anisotropic kagome lattice with three different nearest neighbor interactions, showing significant development in theoretical [1] and experimental [2],[3] realms in recent years. The corresponding theoretical model [1] hosts a rich classical phase diagram, including an exotic spin liquid phase and long-range orders. Phase-pure single crystals of Y₃Cu₉(OH) ₁₉Cl₈ exhibit a magnetic transition at 2.1 K, as confirmed by complementary experimental methods [2], aligning with the theoretical prediction [1] of a (1/3,1/3) long-range ordering with a notably reduced ordered moment for Cu^{+2} [3].
These observations naturally instigate further investigation, with a focus on perturbing the system and exploring the phase diagram [1]. Specifically,hydrostatic pressure effects has been emphasized in this abstract, which can potentially influence the exchange couplings within the anisotropic kagome network. Due to its extreme sensitivity to even small magnetic moments, μSR experiment stands out as a particularly well-suited technique for monitoring the system's evolution under pressure. A gradual decrease in T_N has been observed with an increase in pressure within the Kbar range in our μSR experiment. The system exhibits a strong suppression of magnetism under pressure, and beyond the pressure of 18 Kbar, no magnetic ordering has been observed, indicating a potential dynamic ground state.
References:
[1] M.Hering et al , npj Comput Mater 8, 10 (2022).
[2] Q.Barthelemy et al , Phys. Rev. Materials, 3 (7), 074401 (2019).
[3] D.Chatterjee et al , Phys. Rev. B 107, 125156 (2023)"
"Muonic X-ray Emission Spectroscopy is a non-destructive method of elemental analysis, which has recently been used in cultural heritage [1] for finding the chemical composition of artifacts. It is desirable to have a robust way of computationally modelling these experiments, to allow for simpler and more systematic identification of elemental X-ray intensities. When a muon is captured by an atom, it may transition down the energy levels via two routes: radiative transitions, and Auger electron emission. Muonic cascade calculations are performed to calculate both of these contributions. Throughout the literature, this has often been done by using the cascade code developed by Akylas [2].
An important feature of cascade calculations is the initial angular momentum distribution of the muon when it is captured by the atom. Different parameterised distributions have previously been investigated [3], but newer computational techniques now allow for more systematic investigations. This talk will show the effect of different l-distributions on calculated X-ray intensities, as well as detailing the theory behind the Akylas cascade code. I will also outline the path forward to updating the cascade code with more modern theoretical and computational techniques.
(1) Sturniolo, S.; Hillier, A. Mudirac: A Dirac Equation Solver for Elemental Analysis with Muonic X-Rays. X-Ray Spectrometry 2021, 50 (3), 180–196. https://doi.org/10.1002/xrs.3212.
(2) Akylas, V. R.; Vogel, P. Muonic Atom Cascade Program. Computer Physics Communications 1978, 15 (3), 291–302. https://doi.org/10.1016/0010-4655(78)90099-1.
(3) Hartmann, F. J.; Bergmann, R.; Daniel, H.; Pfeiffer, H.-J.; von Egidy, T.; Wilhelm, W. Measurement of the Muonic X-Ray Cascade in Mg, AI, In, Ho, and Au. Z Physik A 1982, 305 (3), 189–204. https://doi.org/10.1007/BF01417434."
"V_{1/3}NbS_2, an intercalated transition-metal dichalcogenide, has been investigated previously using a variety of techniques, resulting in different conclusions about its magnetic properties.
We present muon-spin relaxation (μSR) and susceptibility measurements which examine both the static and dynamic magnetic behaviour of V_{1/3}NbS_2. A transition to long-range magnetic order
has been identified at 52.5(2) K and a further magnetic transition around 10 K has been observed in the magnetic dynamics via ZF measurement and also in wTF measurements. Our μSR measurements are supported by density functional theory (DFT) calculations, which allow for the determination of a single muon stopping site that experiences a dipolar field consistent with a previously-suggested double-Q magnetic state."
For deeper understanding of exotic magnetic phenomena of condensed matters, the muon spin relaxation (µSR) measurement is a powerful experimental technique to probe local magnetic properties of condensed matters. In order to understand SR results, we are approaching to magnetic phenomena of by using the density functional theory (DFT) calculations. I am going to report our recent results of DFT investigations, especially on La2CuO4 and Nd2Ir2O7. Our challenges to those typical condensed matters are to investigate the DFT calculation results by changing the functional and compare with µSR results. We tested the local spin density functionals (LSDA), the generalized gradient approximation (GGA) including U as an adjustable parameter (DFT+U), and the strongly constrained and appropriately normed meta-GGA (SCAN) to discuss what electronic orbitals contribute to magnetic phenomena of La2CuO4 and Nd2Ir2O7 and which functional is the most suitable for each system.
In this talk, I will present concrete examples of applications of the MSCP's software tools for the interpretation of a set of concrete muon experiments. The examples will comprise applications in catalysis, magnetic materials and superconductors.
The metal-organic-framework (MOF) compound Cu3(HOTP)2 is a small-gap semiconductor containing a kagome lattice of antiferromagnetically coupled S = 1/2 CuII spins with exchange coupling J~2 K. The J calculated using DFT+U matches experiment and structural modelling shows frustrated layer stacking without long-range order. Muon spin relaxation confirms no magnetic ordering down to 50 mK and sees spin fluctuations diffusing on a 2D lattice, consistent with a quantum spin liquid (QSL) ground state. Reduction of the diffusion rate on cooling from the paramagnetic region to the low temperature QSL region is assigned to the effects of quantum entanglement. Combined analysis of the spin diffusion, magnetic susceptibility and specific heat in the QSL region suggests proximity to a quantum critical point and a large density of low energy spinless electronic excitations. A Z2-linear Dirac model for the spin excitations of the QSL is found to provide the best match with experiment.
An important point of applying the muon to battery studies is understanding the muon dynamics within battery materials. To address this issue, our first target system is LiFePO4. The diffusion of Li-ion directly corresponds to the fluctuation rate of the muon. On the other hand, due to the zero-vibration motion of the muon, the implanted muon also has the probability to diffuse inside the system, which also contributes to the fluctuation rate of the muon. The diffusion of muon and Li-ion inside the system is very sensitive to temperature. How to distinguish the muon and Li-ion diffusion is one of the fundamental problems in battery studies using muon. We are now approaching this problem using density functional theory (DFT) Calculations.
Deoxyribonucleic acid (DNA) is a complex molecule composed of nucleotides that store genetic information used in the development of all living things. Nitrogenous bases can be either guanine (G), adenine (A), cytosine (C), or thymine (T). The properties of DNA have attracted considerable interest from physicists, chemists, and material scientists due to its capability to act as a medium for electron transport and its potential in industrial applications as microdevices. Muon Spin Resonance (μSR) experimental technique has been successfully applied to study the microscopic properties and processes of different materials including molecules of biological importance. The hyperfine interaction between the muon’s dipole moment and the surrounding unpaired electron can be measured using ALC-μSR spectroscopic technique and can be calculated using DFT quantum mechanical procedure. In DFT calculation, a basis set is an important factor in determining the quality of results, especially for hyperfine interaction where changes in the local geometry and electronic structure can have a big impact. The use of extensive basis sets is always desirable and is expected to produce better results. However, for DNA molecules of more than 10 base-pairs, all-electron geometry optimization calculation using large basis sets such is prohibitively difficult even on a supercomputer. An alternative is to use smaller basis sets. Combination of various basis set and functional were used to determine several properties of short DNA molecules. Some of the studied properties are optimized geometry, HOMO-LUMO gap, atomic charge, and muon trapping sites. The effect of different basis set and functional on the associated muon hyperfine interactions was also investigated. The ranking of muon stable sites is not affected by the different choice of basis set or functional. Other properties are affected at varying degrees.
The FAMU experiment aims to measure the hyperfine splitting (HFS) of the muonic hydrogen in the ground state to get insight into the proton magnetic structure. The FAMU is located at RIKEN-RAL muon facility. Here, the muon beam is directed onto a gas target to form muonic hydrogen. Then, a mid-infrared pulsed laser, specifically developed by the collaboration, is injected in the target. The laser wavelength is tuned in a window around 6.8 μm to search for the hyperfine splitting resonance. The X-rays that signal the HFS transition are detected by 34 LaBr3:Ce detectors; a fibre hodoscope monitors the muon beam profile and is used to estimate the muon current. In 2023, the experiment successfully collected data for the first time using the complete set of equipment. Effective data collecting is still going on in 2024, while the data analysis continues to progress. This contribution will focus on the status of the experiment and the performances during beam time.
I will give an accessible introduction to unsupervised machine learning and dimensionality reduction, analysing the role they can play in condensed matter physics research. I will then focus on principal component analysis (PCA) as a particularly simple and effective way to achieve this and explain, in practical terms, how one can deploy this technique for the analysis of muon spectroscopy data.
A setup for microwave excitation around 4 GHz has been constructed at the Swiss muon source at PSI. Two classes of experiments are shown, where μSR with such microwave excitation has the potential to obtain new insights. The first example is microwave spectroscopy of muonium centres that are formed after a delay upon muon implantation, which causes dephasing in transverse-field μSR. The second example is μSR of a ferromagnet with conical spin texture that is driven into ferromagnetic resonance (FMR). While both FMR and μSR of such helimagnets are well established, the combined approach could reveal low-frequency MHz dynamics that emerge from the GHz drive via non-linear interactions, such as a recently proposed magnetic Archimedean screw.
Crystalline silicon is used for >90% of photovoltaic solar cells. Electron-hole pairs are produced when light of certain frequencies is absorbed. Some of these charge carriers are lost due to recombination processes in the silicon before they are collected, and the lifetime (or diffusion length) of the carriers defines the quality of a silicon wafer. Photoexcited muon spin spectroscopy (photo-μSR), developed at ISIS, is a means to measure depth-dependent excess charge carrier density – distinguishing surface- and bulk- recombination of charge carriers. Our objective is to further develop the photo-μSR approach by defining the range and upper limits of surface recombination velocities distinguishable.