Muons as a probe in solid state and surface physics and in materials research

Abstract

Facilities providing muons are coming up more and more. Correspondingly, muons, positive (μ+) as well as negative (μ−), are increasingly being used as tools in pure and applied research. μ+ are needed for muon spin rotation (μsr) where one measures mainly internal magnetic fields, but also for channeling; in both cases the site where the muon or pion decays is of interest. In chemistry the μ+ acts as a marked light “proton”. μsr with μ− is also of some importance. Muonic X-rays, emitted when the μ− cascades down the atomic ladder, provide the unique opportunity to investigate quantitatively the elemental composition of a sample in a fully nondestructive way. Imaging with μ+ and μ− using the incoming pion or muon track and the outgoing decay electron track is possible and has been demonstrated. Slow muons (μ+ and μ−) are stopped in the surface layer (down to monomolecular layers) of a sample and are, hence, excellent thin-film and surface probes. Muonic hydrogen transfers the μ− to heavier atoms; this transfer can be used to study surfaces and the spatial structure of gaseous molecules. Present techniques and techniques envisaged for the future are reviewed; emphasis will be put on phase space compression of μ beams.