Electron transport in solids and liquids

We review our recent experimental studies of the excess electron stales in insulating solids and liquids. We use a muon spin relaxation technique to explore the phenomenon of delayed muonium formation: excess electrons liberated in the mu(+) ionization track converge upon the positive muons and form Mu (mu(+)e(-)) atoms in which the mu(+) polarization is partially lost. The spatial distribution of such electrons with respect to the muon is shown to be highly anisotropic: the mu(+) 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) to 10(-4) cm. Such a 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. This circumstance illustrates the potential of mu(+)SR techniques in studies of electron transport in matter 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.