Program & Abstracts


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Day 1—February 1, 2021

Yasutomo Uemra

8:15 am - 9:05 am ET

Yasutomo J. Uemura

Columbia University
 
Muon Spin Relaxation (MuSR) and Neutron Scattering Studies in Quantum Materials »

In studies of quantum materials, Muon Spin Relaxation (MuSR) measurements can provide important information complementary to neutron scattering studies in the following aspects:  

(1) Different time windows: MuSR has a sensitivity to the spin fluctuation rate ranging between 106 to 1011 [/sec], covering regions with slower fluctuations (lower energy transfers) than neutrons. 
Examples: dilute alloy spin glasses [1], NiGa2S4 [2], MnSi “partial order” [3]

(2) MuSR can detect a very small static magnetic moment, of the size of nuclear dipolar moments even in highly disordered / random spin configurations. 
Examples: time reversal symmetry breaking in Sr2RuO4 [4], UPt3 [5]; details of spin glasses [1]  

(3) Neutron scattering Bragg peak intensity is proportional to the ordered moment S squared multiplied by the ordered volume fraction VM.  MuSR can provide independent information on the local ordered moment size and VM.  This feature helps detection of phase separation and first order magnetic transitions.
Examples: Mott transition systems V2O3, RENiO3 [6]; MnSi tuned by hydrostatic pressure [3]; Phase boundaries between parent AFM and SC states in unconventional superconductors [7]

(4) Absence of static magnetic order can be confirmed with much better accuracy by MuSR as compared to neutron scattering.  MuSR’s sensitivity to slow spin fluctuations helps this.
Examples:  Quantum spin liquids [8,9]; low dimensional spin systems [10], frustrated magnets

(5) The magnetic field penetration depth of superconductors can be determined by MuSR.  The energy scales inferred from the superfluid density from MuSR can be combined with the energy scales of the magnetic resonance mode from neutron scattering
Examples:  High-Tc cuprate, FeAs, heavy-fermion superconductors [11]

(6) MuSR can be applied to thin films with the thickness of 200 Angstroms or more, as will be discussed in a presentation of Prokscha in this meeting.

(7) MuSR can provide essential information even with polycrystalline or powder samples.  The amount of specimens required is about 100 mg, which is significantly less than in neutron scattering. 
Examples:  Organic conductors [12,13], C60 systems [14]

When an unknown magnetic material is synthesized, it would be most sensible to perform MuSR first, followed by more detailed studies of spin structures and spin excitations by neutron scattering.   In this presentation, I would like to point out significant merits of performing MuSR and neutron scattering on the same materials and compare and combine their results.  Having the both capabilities at the same facility would lead to quite productive studies of novel magnetic / superconducting quantum systems

[1] Y.J. Uemura et al., Phys. Rev. B31, 546-563 (1985).
[2] Y. Nambu et al., PRL 115, 127202 (2015)
[3] Y.J. Uemura et al., Nature Physics 3 (2007) 29-35.
[4] G.M. Luke et al., Nature 394 (1998) 558 - 561.
[5] G.M. Luke et al., Phys. Rev. Lett. 71, 1466-1469 (1993).
[6] B.A. Frandsen et al., Nature Communications, 7 (2016) 12519.
[7] Y.J. Uemura, Nature Materials 8 (2009) 253-255 and references therein.
[8] Y.J. Uemura et al., Phys. Rev. Lett. 73, 3306-3309 (1994).
[9] P. Mendels et al., Phys. Rev. Lett. 98, 077204 (2002)
[10] K. Kojima et al., Phys. Rev. Lett. 78, 1787-1790 (1997).
[11] Y.J. Uemura, Phys. Rev. Materials 3 (2019) 104801, and references therein.
[12] L.P. Le et al., Phys. Rev. B48, 7284-7296 (1993).
[13] F.L. Pratt et al., Nature 471, 612-616 (2011).
[14] Y. Takabayashi and K. Prassides, Philos. Trans. R. Soc. A374, 20150320 (2016), and references therein.

Jun Sugiyama9:05 am - 9:55 am ET

Jun Sugiyama

CROSS-Tokai
 
Battery Materials with Muons »

Due to complete muon spin polarization, muon spin rotation and relaxation (μ+SR) is a very powerful and sensitive tool to study local magnetic environments in condensed matters, even in a zero magnetic field [1]. This feature is naturally attractive for magnetic materials research, but also is essential for energy materials research, such as battery materials and hydrogen storage materials research, through the observation of a nuclear magnetic field. By using this feature, μ+SR distinguishes a nuclear magnetic field from an electron magnetic field in a paramagnetic state of a magnetic material. In fact, jump diffusion of Li+ ion in LixCoO2 was successfully measured with μ+SR [2], despite the presence of magnetic Co ions in the lattice. Note that Li-NMR is unable to detect Li diffusion in materials containing magnetic ions. Since then, many battery materials have been investigated with μ+SR in order to determine the intrinsic jump diffusion coefficient (DJ) of Li+, Na+, and K+ ions [3,4].

Since the mass of muon is about 1/9 of the mass of proton, one has a naive question that muon is more mobile than Li+ and/or Na+ in the lattice. In order to reply the question, the dynamics of a nuclear magnetic field in LiMnPO4 was studied with both positive and negative muons, i.e., with m±SR. Muon diffusion can only occur in μ+SR, because the implanted μ- forms a stable muonic atom at the lattice site, and therefore any dynamics of a nuclear magnetic field measured with μ-SR must be due to Li diffusion. As a result, it was confirmed that muons sense Li diffusion in LiMnPO4 [5].

One of the future directions of μSR work on battery materials is expected to be in situ measurements [6] during an electrochemical reaction using a high flux muon beam provided in the advanced proton accelerator like SNS. The recently developed multi-detector counting system has so increased the counting rate that the μ+SR spectrum is measured per one muon pulse, i.e., 25 Hz in J-PARC [7]. This would enable us to measure a chemical diffusion coefficient (DC) of ions in battery materials in the near future.   

[1] A. Yaouanc and P. D. de Reotier, in Muon Spin Rotation, Relaxation, and Resonance: Applications to Condensed Matter (Oxford, Oxford, 2011).
[2] J. Sugiyama et al., Phys. Rev. Lett. 103, 147601 (2009).
[3] M. Månsson and J. Sugiyama, Phys. Scr. 88, 068509 (2013).
[4] N. Matsubara, … J. Sugiyama et al., Scientific Reports 10, 18305 (2020).
[5] J. Sugiyama et al., Phys. Rev. Research 2, 033161 (2020).
[6] J. Sugiyama et al., Sustainable Energy & Fuels 3, 956 (2019).
[7] S. Nishimura, … J. Sugiyama et al., in the Abstract of the 2020 Autumn Meeting of Phys. Soc. Jpn. (2020).

∗Electronic address: juns@triumf.ca or j_sugiyama@cross.or.jp


    

10:10 am - 11:50 am ET

James Lord

ISIS Neutron and Muon Source
 
Muon Spin Relaxation in Semiconductors and Oxides »

Hydrogen is a ubiquitous impurity in most semiconductors, as a result of processing, and is usually electrically active. The low concentration makes direct measurements difficult. Many technologically important oxide materials and insulators also contain hydrogen as an impurity. The positive muon can be considered as a light isotope of hydrogen, and is used as a model for hydrogen to determine its site and electrical behaviour. The measurement uses the hyperfine coupling between the muon spin and an unpaired electron or hole bound to it, so revealing the details and symmetry of the site.

Muons and hydrogen can behave as deep compensating centres with a compact electron wavefunction, or as a shallow donor or acceptor. Varying the doping or temperature, or illuminating the sample, will change the carrier concentrations and the resulting muon site charge changes can be detected.

If the muon response is well characterised, it can then be used to probe the excess carrier density. This allows measurements of the carrier lifetime following injection of excess carriers with a short laser pulse. Bulk and surface recombination mechanisms can be separated.

Light can also interact directly with the muon centre, ionising the bound electron into the conduction band. This spectroscopic technique gives another method to measure the energy levels of acceptors or donors.

Peter Baker

11:00 am - 11:50 am ET

Peter Baker

ISIS Neutron and Muon Source
 
Ionic Diffusion »

Over the last decade muons have developed into a significant probe of lithium diffusion in battery materials at the atomic scale [1]. The method itself is of course more general. It has long been applied to the motion of muons in materials [2] and more recently to less commonly studied ions like Mg2+ and I- [3,4]. These also have significant and increasing technological applications. Some of these ions are just as easy as Li+ to study, with similar moment sizes and abundances, but others have either low abundances or small moments that make muon measurements far more challenging.

With the data rates currently available it is possible to make detailed temperature-dependent studies in reasonable amounts of time. These are now revealing details of multiple diffusion processes in some materials [5]. What has already been learned about modelling the motion of muons in simple materials [2], may in future be applied to analysing data from these more complicated materials.

Another area of recent progress has been in operando measurements of battery materials as electrochemical cells are cycled on the beamline [6,7]. This provides an additional challenge in making the cells and because the material of interest forms a smaller proportion of the sample. What can be learned about how the materials change in the way they are really is nevertheless far greater than can be obtained from ex situ samples.

A future muon source that is more intense offers the prospect of extending the capabilities of the technique in all these areas. Data collection at higher rates opens the possibility of studying more difficult nuclei and connecting with more advanced data modelling. Smaller beam sizes could allow the use of isotopically enriched samples of far smaller mass, make using electrochemical cells far easier, and allow smaller volumes within the cells to be probed.

[1] J. Sugiyama et al., Phys. Rev. Lett. 103, 147601 (2009).
[2] G. M. Luke et al., Phys. Rev. B 43, 3284 (1991).
[3] R.D. Bayliss et al., Chem. Mater. 32, 663 (2020).
[4] D.W. Ferdani et al., Energy Environ. Sci. 12, 2264 (2019).
[5] T.E. Ashton et al., J. Mater. Chem. A 8, 11545 (2020).
[6] I. McClelland et al., (manuscript in review).
[7] I. McClelland et al., (manuscript in preparation).

     

Day 2—February 2, 2021

Thomas Prokscha

8:20 am - 8:50 am ET

Thomas Prokscha

Paul Scherrer Institute
 
Low-Energy Muons »

Low-energy, positively charged spin polarized muons (LE-m+) with tuneable energies between 1 keV and 30 keV allow the extension of the powerful muon spin rotation technique (mSR) to the investigation of thin-films, heterostructure interfaces, and near surface regions with nanometer depth resolution at mean implantation depths between 5 nm and about 200 nm. With the implementation of the low-energy muons facility LEM at PSI in 2006, a new era has started for mSR in the characterization of nanometer-thin samples, including the extension of mSR applications to new research fields (e.g. proximity effects between layers with diverse physical properties including topological materials, crossover from 3D to 2D systems, artificial spin systems, defects at semiconductor interfaces, controlled manipulation of charge carrier profiles in semiconductors etc.).

In addition to condensed matter applications in thin films and technologically relevant device structures, a beam of LE-m+ can be employed for fundamental questions in particle physics by performing high-precision spectroscopy experiments on muonium (Mu 1s-2s transition and the Mu 2s Lamb-shift) to test QED and for providing a more precise value of the muon mass, an important standard model parameter for solving the muon g-2 mystery.

Moreover, a LE-m+ beam with small initial phase space can be re-accelerated to hundreds of keV or MeV energies. Such a small phase-space beam can be focused on small beam spot sizes of order 1 mm2, which would give mSR a new direction of applications on small samples, lateral scanning of samples, and filling the “range gap” between the present LE-m+ beams (range < 200 nm) and conventional surface muon beams (range > 100 mm). At J-PARC, such a re-accelerated beam is planned for a future muon g-2 and muon electric dipole moment (m-EDM) experiment, and for a future “muon microscope”.

Various methods are currently pursued to generate “epithermal” or “ultra-slow” muons. At the LEM facility, a moderation technique based on solid-rare gas moderators is used to generate a continuous beam of “epithermal” muons in the 10 – 20 eV range, which are post-accelerated electrostatically up to 30 keV. At the pulsed muon beam facility (MUSE) at J-PARC, “ultra-slow” muons (USM) with about 0.2 eV energy are generated by laser ionization of a cloud of thermal muonium emitted from a hot tungsten foil. The USM facility is currently under development. At PSI, the “muCool” projects aims at a higher efficiency and better phase space parameters compared to the “classical” moderation technique. The project demonstrated successfully the cooling procedure, but extraction of the compressed beam from the cooling region still needs to be achieved.

After presenting an overview of these methods, I will discuss some of the recent research highlights of LE-mSR applications, and the very recent preliminary Mu 2s Lamb-shift result with an already factor two improvement in precision compared to the hitherto only measurements published in 1984.

Stephen Blundell8:50 am - 9:40 am ET

Stephen J. Blundell

The University of Oxford
 
The Muon-fluorine Interaction: A Model Quantum System for Exploring Decoherence »

In non-magnetic fluorides, an implanted muon will stop very close to the highly electronegative fluoride ion, or very often stop between two of them. The dipolar interaction between the fluorine nuclei spins and the muon spin gives rise to a characteristic signature which characterises the state [1]. The remaining fluorine nuclei are more distant and usually ignored, since their coupling to the muon is weaker. We show that taking them properly into account allows one to model the data in much greater quantitative detail, understand the stopping site more accurately, and to explore how the quantum information held by the state evolves with time [2].  Modelling these rather subtle decoherent effects is only possible due to advances in high-rate beamlines which allow for counting statistics ~109 muons per run in reasonable time.  The possibilities opened up by high-rate muon experiments will be discussed and provide a strong motivation for high-rate beamlines in future muon sources.

[work carried out with John M. Wilkinson]Blundell Muon Abstract

[1] J. H. Brewer, S. R. Kreitzman, D. R. Noakes, E. J. Ansaldo, D.R. Harshman, and R. Keitel, Phys. Rev. B 33, 7813 (1986).
[2] J. M. Wilkinson and S. J. Blundell, Phys. Rev. Lett. 125, 087201 (2020).

Sarah Dunsiger10:00 am - 10:50 am ET

Sarah Dunsiger

TRIUMF/Simon Fraser University
 
From Correlations to Functionality Using Depth Resolved Spin Resonance Techniques »

Variants of conventional nuclear magnetic resonance (NMR), μSR and β detected NMR techniques offer a sensitive probe of the local magnetic environment in condensed matter.  Using examples of compounds which are of fundamental interest and may also have practical applications for data storage and transfer, I describe how radioactive species like muons or 8Li nuclei may be used to probe the nature of the magnetic excitations:

Europium chalcogenides have long been recognised as experimental realisations of classic 3D Heisenberg spin systems, with simple rock salt crystalline structures. The compounds EuO and EuS are also rare examples of ferromagnetic semiconductors, where strong indirect exchange interactions between the localised Eu2+ ions are mediated by charge carriers rather than superexchange. More recently, artificial magnetic semiconducting heterostructures have generated tremendous interest, due to their potential for spintronics applications.  One candidate, EuO1−x / n-Si:As combines a model ferromagnet with the dominant semiconductor used for practical devices.

The controlled manipulation of a nonequilbrium spin population is central to the field of spintronics. Typical devices making use of giant magnetoresistance (like hard drive read heads) or spin transfer torque effects are based on spin polarised electron currents. As such, the spin and electron transport are not separated and devices are subject to the constraints of circuit capacitance, heat generation and electron migration. Recently, attention has turned to developing ideas and systems based on pure spin or angular momentum currents, which may be generated through spin pumping, which relies on the precession of the magnetisation induced in a ferromagnet excited by a radio-frequency magnetic field on resonance (FMR). The magnetisation precession is damped via the emission of a polarised spin current into the neighbouring normal metal or “spin sink”.  Widely used in microwave applications, ferrimagnetic Y3Fe5O12 (TC=550 K) has very low magnetic losses and is the protypical material for the generation of pure spin currents. 

Crystalline materials without inversion symmetry may exhibit a variety of exotic magnetically ordered states with canted spin structures like helices, conical states or a periodic array of “Skyrmions”, a hedgehog-like spin structure. The presence of interfaces and confined geometries like thin films leads to the greatly enhanced stability of Skyrmion phases, although the mechanism is not fully understood. While thermal fluctuations are thought to stabilise the Skyrmion phase in bulk samples,  the evolution of the magnetic excitation spectrum with film thickness is largely unexplored. Due to the ability to move Skyrmions easily, their topological stability, their small size (possibly down to 1 nm) and the ability to write and erase individual Skyrmions, these materials are promising for information processing.

The low frequency spin dynamics may be investigated locally both within the film and also in a proximal manner using depth resolved low energy μSR.  Complementary β-NMR measurements, sensitive to spin fluctuations on much longer millisecond timescales, enable the investigation of the dynamical response of the underlying substrate.  Within a wider context, the spin resonance phenomena are compared with extensive neutron scattering investigations. As such, a comprehensive account is presented, using techniques sensitive to both frequency and reciprocal space.