Reversible depression in the Tc of thin Nb films due to enhanced hydrogen adsorption

The effect of large hydrogen concentrations (∼30% atomic), on the superconducting transition temperature Tc of Nb films is discussed. The hydrogen is incorporated in solutionlike phases which occur at these concentrations because the (α to β) hydride transition in these films is suppressed. X-ray-diffraction data show an asymmetric expansion of the [110] Nb interplanar spacing. This can be as high as a 6 endash 8thinsp% expansion perpendicular to the plane of the film for the addition of ∼70% hydrogen, with only a 1 endash 2thinsp% change in the plane of the film. Tc is depressed to a value near 50% of that of the undoped film with about 30% atomic hydrogen, and returns to its initial value when the hydrogen is removed. A discussion is given of how both disorder and changes in the electronic structure can affect Tc. copyright 1998 The American Physical Society