Generation and applications of slow polarized muons

The muon is an elementary particle and, as such, forms a fundamental building block of the Standard Model of elementary particles. At the same time, the muon, especially the positively charged muon, when implanted in matter has established itself as a magnetic spin microprobe and hydrogen-like probe to study a variety of static and dynamic phenomena in superconductivity, magnetism, radical chemistry, semiconductor physics and many other fields. The limitation of this technique has been up until recently that it is restricted to investigation of bulk properties, because muons are generated from pion decay as high-energy particles with a broad stopping depth pro. le. This article describes recent developments of so-called 'slow muon' beams, where the energy is reduced down to the electronvolt to kiloelectronvolt range, thus making the probe depth sensitive on the nanometre scale. Some recent applications of this novel probe are described, such as the measurement of magnetic field profiles at the surface of superconductors and the measurement of the spatial variation in the polarization of conduction electrons in magnetic/non-magnetic multilayers. The generation of slow negative muons and possible applications of low-energy muons in the design of future muon colliders and neutrino sources are also briefly discussed.