Superconducting properties and structure of ion bombarded Nb layers

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Abstract
The influence of ion bombardment on the superconducting transition temperature Tc and the structure of thin evaporated niobium layers has been investigated as a function of ion species and layer purity. Irradiation through pure layers with neon ions and fluences of typically 1016 ions/cm2 leads to relatively small Tc decreases (δTc × 0.5 K), while in oxygen contaminated layers larger effects depending on oxygen concentration are observed. Homogeneous implantation of chemically active impurities (nitrogen, oxygen) also drastically depresses Tc reaching the detection limit of 1.2K at a concentration of 15 at. %N. The Tc depressions correlate with a lattice parameter expansion of the Nb bcc structure at a rate of about 0.1 %per 1 at. % impurity.