Electron Transport in Cryocrystals

We review our recent experimental studies of the excess electron transport in cryocrystals and cryoliquids. We use a muon spin relaxation technique to explore the phenomenon of delayed muonium formation: excess electrons liberated in the μ+ ionization track converge upon the positive muons and form Mu (μ+e−) atoms in which the μ+ polarization is partially lost. The spatial distribution of such electrons with respect to the moon is shown to be highly anisotropic: the μ+ thermalizes well “downstream” from the center of the electron distribution. Measurements in electric fields up to 30 kV/cm allow one to estimate the characteristic muon-electron distance in different insulators: the results range from 10−6 cm to 10−4 cm. This circumstance makes the basis of a recently developed new technique for electron transport studies on microscopic scale: electron mobility can be extracted when both the characteristic muon-electron distance and characteristic time for muonium atom formation are determined. The microscopic length scale enables the electron to sometimes spend its entire free lifetime in a state which may not be detected by conventional macroscopic techniques. The muonium formation process in condensed matter is shown to depend critically upon whether the excess electron forms a polaron or remains in a delocalized state. Different mechanisms of electron transport in insulators are discussed.