Mesoscopic mechanisms of the photovoltaic effect and microwave absorption in granular metals

A theory for the photovoltaic current and microwave absorption in mesoscopic samples is developed. The photovoltaic current is determined by the displacement of the center of mass of the electronic wave function produced by inelastic electron transitions. At low frequencies the main contribution to the microwave conductivity of isolated mesoscopic samples is shown to be due to a relaxation mechanism which resembles the Mandelstam-Leontovich sound-absorption mechanism in gases and the Pollak-Geballe conductivity mechanism in doped semiconductors. A giant positive magnetoresistance in granular metals is predicted.