DISSIPATIVE TUNNELING OF HYDROGEN TRAPPED IN NIOBIUM - ELECTRON SCREENING VS. PHONON DRESSING

Interstitial hydrogen trapped by an oxygen or nitrogen impurity in Nb is confined to two states and shows a rich dynamic behavior due to qua ntum tunneling: at temperatures below 10 K, the hydrogen impurity perf orms coherent oscillations about the trap atom. At higher T, adiabatic interaction with conduction electrons drives a cross-over to overdamp ed motion; the relaxation rate decreases with temperature according to the law Gamma proportional to T2K-1. At about 60 K, however, the rate shows a minimum as a function of T and increases on further rising th e temperature. We report strong evidence for the behavior above 60 K b eing due to coupling to lattice vibrations; the resulting rate is gove rned by multiphonon processes. (C) 1998 Elsevier Science Ltd.