Low temperature tunnelling and quantum diffusion of hydrogen Nb(NH)x

We studied by neutron spectroscopy the tunnelling and the local jump diffusion of hydrogen interstitials in niobium that were trapped by nitrogen. The experiments were performed on three different Nb(NH) x samples with x ⩽ 0.004 1. (i) at 1.5 K and 2. (ii) in the range between 10 and 160 K. In the limit T → 0 K (superconducting electronic state), the tunnel splitting of the hydrogen was found to be J = 0.165 ± 0.004 meV. This value is about 30% smaller than that reported for the previously investigated system Nb(OH) x . The jump rates of the trapped hydrogen were determined in the range between 10 and 160 K. The results demonstrate that the jump diffusion behaviour of the hydrogen is, up to about 60 K, dominated by a non-adiabatic interaction with the conduction electrons.