Muons as Light Hydrogen Probes-Diffusion and Trapping
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MUONS AS LIGHT HYDROGEN PROBES-DIFFUSION AND TRAPPING
D. RICHTER Institut fur Festkbrperforschung, Kernforschungsanlage
Jblich, West Germany
ABSTRACT Contributions of the muon spin rotation technique (iSR) to metal physics problems are surveyed. The similarity between the muon and the proton constitutes wSR as a new tool to investigate the physics of H-like interstitials on an atomic scale. Experiments on the lattice location and on local properties like the Knight shift as well as diffusion measurements are reviewed. In particular the extreme sensitivity of muons toward impurities and lattice defects is emphasized. Results on the trapping of muons by substitutional impurities or vacancies in the ppm range are displayed.
INTRODUCTION During the last few years, muon spin rotation (IiSR) has emerged as a new kind of spectroscopy for microscopic investigation of condensed matter. Although USR as a technique to study solid state phenomence dates back less than 10 years, its application ranges already from fields like kinetics of chemical reaction to semiconductors and in particular to metal physics which will be the concern of this paper /l/. Muon sources, which require intense primary medium energy proton beams, at present are available at the so called meson factories: SIN in Switzerland, TRIUMF in Canada, LAMPF in USA and KEK in Japan, and at the New synchrocyclotrons at CERN, Dubna and Gatchina the latter both in the USSR. facilities at Brookhaven (USA), IKO (Amsterdan) and Rutherford Laboratory (UK) are under construction. 2
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With a mean lifetime of T = . ljs the muon decays into a positron and 2 neutrinos. The correlation between the direction of the emitted positron and the spin direction of the muon allows one to measure the spin precession frequency and/or the decay of the muon polarization of an ensemble of muons implanted in a solid. Static as well as dynamic properties can be investigated. In magnetic materials the precession frequency reveals information on the local field at the interstitial muon site similarly to Mdebauer and NMR experiments which yield the local fields on substitutional sites. In nonmagnetic materials Knight shift measurements cast light on the local electronic structure around the muon. The investigation of the field dependent decoupling of electronic and magnetic interactions allows the determination of the electric field gradient (EFG) created by the muon on its neighboring atoms, as well as the assignment of the muon lattice location. The time scale HSR can cover is related to the occuring magnetic field fluc2 tuations and the lifetime of the muon. In nonmagnetic substances, 2 where is caused by nuclear dipolar fields, the resulting time scale amounts to 10-7 _< t < 10-5 s, in magnetic materials it can be extended down to 10-1S. Field fluctuations may occur either due to dynamic processes in the host material like spin relaxation in spin glasses /2/ or by motion of the muon itself. Concerning muon diffusion two aspects are of importance: (i) Since the
234 muon can be c
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