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

The effect of large hydrogen concentrations (≈30% atomic), on the superconducting transition temperature $T_c$ 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 6–8 % expansion perpendicular to the plane of the film for the addition of ∼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 initial value when the hydrogen is removed. A discussion is given of how both disorder and changes in the electronic structure can affect $T_c.$