Tunneling of Hydrogen in the Transition Metals Nb, Ta and V at Liquid Helium Temperatures

Low temperature effects on sound absOl'ption, specific heat and neutron scattering related to tunnel states of trapped H in NbJ Ta, V and Nb-Ti alloys are discussed In the most investigated Nb-D(N)-H(D) alloy two relaxation processes are well resolved by acoustic measurements, and are attributed to two different configur'ations of the O(N)H(D) pair, each giving rise to a tunnel system Acoustic measurements reveal tunnel states of the trapped H(D) also in Tao-D(N) and Nb-Ti In all cases the tunneling systems seem to interact mainly with the conduction electronsJ and the measurements performed on Nb-O(N)-H(D) and Ta-O(N)-H(D) can be explained in the fl:'ameWOIk of the two-level tunnel system (TLS) model; for the Nb-Ti-H system, due to the higher symmetry of the Ti trapping centre, a fom-Ievel tunnel system should be more appr'Opriate. In V-O(N)-H, IF measurements indicate the presence of tunnel states of the trapped HJ but the process is at too low temperature to be resolved.