REVERSIBLE DEPRESSION IN THE T-C OF THIN NB FILMS DUE TO ENHANCED HYDROGEN ADSORPTION

The effect of large hydrogen concentrations (approximate to 30% atomic ), on the superconducting transition temperature T-c of Nb films is di scussed. The hydrogen is incorporated in solutionlike phases which occ ur at these concentrations because the (alpha to beta) hydride transit ion in these films is suppressed. X-ray-diffraction data show an asymm etric expansion of the [110] Nb interplanar spacing. This can be as hi gh as a 6-8 % expansion perpendicular to the plane of the film for the addition of similar to 70% hydrogen, with only a 1-2 % change in the plane of the film. T-c is depressed to a value near 50% of that of the undoped film with about 30% atomic hydrogen, and returns to its initi al value when the hydrogen is removed. A discussion is given of how bo th disorder and changes in the electronic structure can affect T-c.