Acoustic Absorption Due to Hydrogen Tunneling in NbN0.0015H0.0025
Some years ago it was found that doping of niobium with hydrogen gives rise to excess specific heat around and below 1 K . At corresponding temperatures the thermal phonon mean free path was reduced by an order of magnitude. This has been interpreted as resonant scattering of thermal phonons by the discrete set of energy levels associated with interstitial motion of H or D atoms . Meanwhile it could be demonstrated that these thermal anomalies induced by hydrogen are strongly intensified by the additional presence of N interstitials [3,4]. N (or 0) impurities act as trapping centers, and at low temperatures the hydrogen is still able to tunnel between adjacent interstitial sites around the trapping atoms. This tunneling has directly been observed in inelastic neutron scattering experiments on Nb doped with 0 and H .
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