Implanted muon studies in condensed matter science

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Abstract
The positive muon is a unique microscopic probe. When it is implanted in condensed matter, the evolution of its spin polarisation may be readily monitored to give information on the crystallographic or molecular sites it occupies, the local fields it experiences and the time dependence of these fields. This article describes the experimental techniques in current use, and the manner in which the muon behaviour is characterised. The implications for solid state and chemical physics are discussed. The comparison between muon and proton behaviour in similar circumstances is emphasised. For chemical aspects, the equivalent comparison is between muonium and hydrogen, with muonium considered as a radioactive light isotope. Examples of studies are reviewed for magnetic materials, for metals and metal hydrides, for semiconductors and ionic crystals, and for certain organic materials and liquids. Novel information is obtained on topics including spin dynamics, dynamic correlations and critical phenomena, the localisation and quantum mobility of light interstitial particles, charge screening, the local electronic structure of hydrogen-like defect centres, hyperfine interactions, various kinetic and dynamic isotope effects and the role of vibrational effects, including zero-point motion, in determining molecular structure. Comparisons are drawn with studies by other techniques, notably magnetic resonance. Techniques and applications appropriate to pulsed muon sources are considered. The major achievements to date and the potential for future developments and new science are identified.