Screening of a proton moving through the electron gas

The problem is considered of a positively charged particle moving through the electron gas with a velocity comparable to the Fermi velocity v/sub F/ or smaller. The problem is solved in the static RPA approximation. It is found that up to a velocity of about 0.8v/sub F/ the positive particle is completely screened. For higher velocities oscillations appear in the screening charge distribution - different from the Friedel oscillations - and complete screening does not occur. For velocities lower than about 0.8v/sub F/ the anisotropy of the screening is studied. The relevance of this problem to positron annihilation in metals is pointed out. (author)