QUANTUM TUNNELING OF TRAPPED HYDROGEN IN Nb

Abstract
Quantum tunneling of hydrogen trapped by oxygen in a Nb lattice is found at low temperatures. Information obtained by various measurement techniques will be reviewed and compared. Ultrasonic measurements of attenuation and velocity as a function of polarization, temperature, frequency, defect concentration, isotope and cooling rate provide detailed quantitative information particularly concerning the symmetry and dynamics of isolated systems in Nb where the normal to superconducting transition is used to establish the effect of conduction electrons on the tunneling rate. In the OH system a peak at 2.25 K at 10 MHz in the superconducting state disappears in the normal state. In a second system produced by rapid cooling, a peak at 6 K at 10 MHz also moves dramatically, but in this case the response can be fully measured in the normal state. It is found that a two level system (TLS) formalism which takes into account the relaxation stimulated by inelastic scattering of electrons gives a good quantitative description of the quantum behavior of the OH system. The theory used for analyzing the data is similar to that for metallic glasses but simpler. Ultrasonic experiments, particularly high accuracy velocity measurements, are sufficient for the complete evaluation of all three parameters of the TLS ; Ɗ₀ - the minimum gap, α - the coupling to the strain field, and ε₀ - the average absolute strain magnitude. For the OH system, the relaxation rate in the normal state cannot be measured. For the quenching peak, the relaxation rate as well as the quantum depletion of the relaxation strength, are directly accessible.