Recent progress in the understanding of light particle tunneling in metals

Abstract Light positive particles can be introduced in metallic lattices where they take up interstitial positions. The motion of particles between these sites is usually described as a phonon‐assisted tunneling process, which at high temperatures approaches a classical over‐barrier jump motion. The present paper reviews briefly the study of such phenomena at very low temperatures where phonon assistance is no longer effective. Experiments on proton transfer between certain sites in niobium metal and on positive muons which diffuse in copper or aluminium lattices show unusual temperature dependencies, which have been explained quantitatively with recent tunneling theories. These approaches take into account the simultaneous dissipation of energy to the electron bath, a phenomenon which was shown by J. Kondo to be determined by the screening electrons following the particle. The temperature dependence is essentially an effect of the Fermi distribution of the conduction electrons. The interaction of the particle with the itinerant electrons will also determine whether the particle wavefunction will be localized or form an extended (Bloch‐like) state.