Quantum Behaviour of Trapped H and D in Nb and Ta

Internal friction measurements at liquid helium temperature on Nb and Ta specimens containing oxygen (nitrogen) and doped with hydrogen (deuterium) show peaks due to the tunnelling of H(D) trapped by O(N). The peak at higher temperature in Nb-O(N)-H(D) and that observed in Ta-O-D are interpreted as due to a two-level system (TLS), and fits to the relaxation curves have been made. In Nb-O(N)-H(D) both 2-phonon and electron transistions gave nearly equivalent fits, while in Ta-O-D the predominance of the TLS-electrons interaction is revealed both by a sharp discontinuity of the slope of the relaxation curve at the normal-superconductor transition temperature and by the fit of the exprimental points. The parameters characterizing the TLS and their coupling with phonons and electrons have been estimated.