H Tunneling and Local Diffusion in NbOxHy
The presently best understood example of low-temperature H tunneling in metals is represented by H interstitials in Nb that are trapped (below ~150 K /1,2/) by impurity atoms such as 0 or N. The tunneling behavior has been investigated in a great number of specific heat, ultrasonic (anelastic) and neutron spectroscopic measurements (see e.g. the studies of /3–11/ and references therein). The relevant tunneling process observed in the experiments is most likely to occur between two nearest-neighbor tetrahedral trap sites located in the neighborhood of an 0 or N trap center (for a detailed discussion, see /2,5/). In the temperature range below ~ 5 K, the tunneling matrix element was found to be ~ 0. 2 meV for the isotope H /3,5–7,9/ and ~ 0.02 meV for D /9/.
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