The positive muon implanted in metals - a story full of surprises

During the period 1975-1990 there was an intense development, at some accelerator laboratories, of a new spectroscopy based on the interaction of an elementary particle, the positive muon, with local magnetic fields present in magnets and superconductors. It was called μSR (or in some applications MuSR), which stands for Muon Spin Rotation or Muon Spin Relaxation. At the present time this spectroscopy has given access to detailed information (often unobtainable with other means), on a large number of magnetic, semiconducting and superconducting systems, but the present article deals with another aspect of μSR, namely the information it could provide on how a light positive particle behaves when placed in a metallic environment. This behaviour has much in common with that of the technologically important "hydrogen in metal"-problem, but could now be studied in a very direct way since the positive muon is radioactive and sends out an easily detectable signal. This signal contains information on the particle's local environment, its motion from one lattice position to another, its trapping and release from other impurity atoms in the metal and also on the character of the motion itself, particularly at low temperatures where quantum effects dominate. It took nearly 15 years to fully interpret and understand these phenomena which is the topic of the present historic presentation.