The technique of muon spin rotation has become a major tool for the investigation of structure of all kinds of condensed matter and has even developed its own research communities. A recent major conference highlighted progress to date.
Some 40 years since it was first recognized that the positively charged muon could be used as a local microscopic probe of condensed matter, the application of muons in this field has grown from the exotic hobby of some particle physicists in the late 1950s and early 1960s into an established and mature technique.
Polarized positive muons, brought to a stop inside materials, precess in the local magnetic fields. This muon spin rotation (mSR) method competes with and complements approaches like neutron scattering, Mössbauer spectroscopy, nuclear magnetic resonance (NMR) and electron paramagnetic resonance, to form the arsenal of modern experimental tools in condensed matter research.
With interest continually growing, the mSR community has become the largest user group at the meson factories of PSI in Switzerland and TRIUMF in Canada, and it now shares equal status with the neutron scatterers at the ISIS facility of the UK’s Rutherford Appleton Laboratory.
Continuing in the tradition that was established in 1978 in Switzerland, and followed up in Vancouver (1980), Shimoda (1983), Uppsala (1986), Oxford (1990), Maui (1993) and Nikko (1996), the 8th International Conference on Muon Spin Rotation, Relaxation and Resonance (mSR 1999) was held in its country of origin. Close to 180 physicists and physical chemists gathered in the beautiful alpine setting of Les Diablerets to discuss and learn of the latest applications of positive and negative muons in condensed matter research and physical chemistry.
Condensed matter contributions
More than 200 original contributions werepresented (orally and in poster form). Each session was opened by one of eight invited plenary speakers from outside the mSR community, thereby providing a link to the condensed matter community as a whole.
Superconductivity, and in particular high-temperature super-conductors and their discovery, were a key feature of a talk given by Nobel laureate K A Müller (Zürich), who emphasized that, rather than pure luck, it was the result of well focused year-long research into material properties and their understanding that led to their discovery.
Techniques such as NMR, closely related to mSR, are now being employed to study high-Tcsuperconductors. These exciting studies were outlined by C P Slichter (Urbana-Champaign).
New results from mSR studies of non-superconducting materials, such as the cuprate La1-xSrxCuO4, revealed a huge isotope effect. This oxygen isotope effect manifests itself by dramatically increasing, by up to 80%, the spin glass transition temperature (phase transition) when oxygen-16 is replaced by oxygen-18, thus pointing to a strong electron-phonon coupling in cuprates that in turn is also expected to govern the cooper-pairing mechanism in superconducting compounds.
A second new result was the first unambiguous discovery of a spontaneous magnetic field signalling the onset of superconductivity in Sr2RuO4. This result implies that time-reversal invariance is broken, as in the case of a ferromagnet, and that the Cooper pairs are in a different spin state compared with conventional or high-Tcsuperconductors.
Magnetic moments
Magnetism featured high on the list of popularity, with nearly half of the contributed papers focusing on the subject, undoubtedly reflecting the fact that the positive muon is itself a magnetic probe. X-rays and neutrons are also now used as complementary probes to muons in magnetism, as demonstrated in a talk by G H Lander (EC-JRS-ITE, Karlsruhe).
A new class of magnetic materials – molecular magnets – which are made up of either purely organic or inorganic molecules or a mixture of both, together with molecular clusters and magnetic nanoparticles, are currently under study with muons. Molecular clusters exhibit large spins and their magnetization relaxation may be governed by magnetic quantum tunnelling. A first observation of this, in CrNi6 and CrMn6, was presented at the conference and was seen as a highlight in this field. New opportunities for mSR in studying such systems were presented by D Gatteschi (Florence).
Another subject of intense study concerns lower-dimensional magnetic systems. Some materials, such as SrCu2O3, have their magnetic moments aligned in parallel running chains, forming a ladder-like structure, where the spins combine into non magnetic spin singlets. By creating couplings between the so-called ladders, or doping these materials with non-magnetic species, one can drive these systems into a long-range magnetically ordered state, and this has indeed been verified. In one example it even appears as if the presence of the muon can break up the spin-singlet pairs.
Probing particles
The muon can also be thought of as a light proton isotope and it has been used as such to study the structural and dynamic features of materials, including the determination of the site of the implanted muon. Muonium, which is a hydrogen-like quasi atom (m+e–), has been used extensively as a probe in mSR studies of the behaviour of hydrogen in semiconductors, hydrogen being a common impurity affecting the electronic properties (e.g. the passivation of donors or acceptors) in these substances. A session devoted to muons in semiconductors was opened with a talk by B Bech-Nielsen (Aarhus) on vacancy-hydrogen defects in silicon.
As far as the role of muons as light proton isotopes in metal hosts is concerned, current investigations are part of the extensive and technologically relevant research on hydrogen-metal systems, as pointed out in a talk by P Vajda (Palaiseau) on hydrogen ordering and magnetic phenomena in metal-hydrogen systems.
Two new highlights in this area, both concerning the quantum nature of muon or muonium diffusion in matter, were reported. The first, which has been found independently by two groups, consisted of the first observation of a local muon tunnelling state seen in two different materials. The second showed that muonium can travel in the form of a Bloch wave, through propagation in a band-like state at very low temperatures (below 10 mK) – something that many had considered impossible.
New results on the formation of muonium in liquids and solids seem to suggest that the formation happens mainly after the thermalization of the implanted muon, followed by capture of a free electron created by ionization near the end of the muon track, also termed as delayed muonium formation. The liberated electrons were found to be located downstream of the stopped muon, indicating that the initial momentum direction is largely conserved during the slowing-down phase.
The step from muonium formation to muonium chemistry is only a short one. Here the muon can, for example, be used as a polarized spin label in physical chemistry. Muonic radicals (muon-containing molecules, each with an unpaired electron) have also been used to investigate materials, and studies have been made of molecular dynamics and intermolecular electron transfer in systems of biological interest.
A session on instrumentation and techniques, devoted to new ways of using muons, showed that the development of low-energy muon sources in the 10 eV – 20 keV range, and in particular the successful implementation of such a source at PSI together with the first applications, opens up new possibilities for the study of thin films and multilayers.
The complementary use of spin-polarized beta-radioactive nuclei, as produced at ISOLDE (CERN) and soon to be produced at ISAC (TRIUMF), were described by R F Kiefl (Vancouver). Positron spin relaxation, inspired by mSR, was presented by J Major (Stuttgart).
Future facilities
New developments also bring a push for new facilities. An evening session devoted to further developments and new facilities gave an overview as well as allowing discussion. In particular, plans for muon sources at the Japanese KEK-JAERI accelerator complex (most likely to be realized), the neutron spallation source at Oak Ridge and the second planned spallation target of the Rutherford Appleton Laboratory ISIS facility were presented.
Three devil’s advocates – Yuri Kagan (Moscow), John Mydosch (Leiden) and Roderick Wasylishen (Halifax) – provided friendly and sometimes critical feedback throughout the sessions, culminating in their summaries at the end of the meeting.
In recognition of his important role in the development of mSR at the venerable 184 inch cyclotron at the Lawrence Berkeley Laboratory in around 1970, Kenneth M Crowe was guest of honour. Many groups can trace their roots back to those exciting days in Berkeley.
The conference, under the patronage of the European Physical Society, was organized by A Schenck, conference chairman (ETH Zürich); E Roduner, chairman of the programme committee (Stuttgart); G Solt, conference secretary (PSI); and a local organizing committee. It was supported by PSI, the Swiss National Fund, ETH-Zürich, the University of Zürich, the European Science Foundation through the FERLIN Programme and industrial and private sponsors.
mSR 2002 will take place in Richmond, Virginia, with C Stronach (Virginia State) as conference chairman and Y J Uemura (Columbia) as programme committee chairman.
