Influence of Disorder on Superconductivity in Rare-Earth Nickel Borocarbides

Abstract

The superconducting properties of pseudo-quaternary Ho_x R ₁__xNi₂B₂C compounds with R = Y and Lu and of nonmagnetic Y_xLu_1-xNi₂B₂C compounds have been investigated. The superconducting transition temperature T _c of the Ho-based compounds is mainly controlled by magnetic pair-breaking and, for R = Lu, additionally influenced by local lattice distortions due to the large difference of the ionic radii between the Lu and the Ho ion. The effect of disorder-induced local lattice distortions is studied in nonmagnetic Y_xLu_1-xNi₂B₂C compounds. Disorder is found to reduce several relevant quantities as T _c , the upper critical field H _c2 , a characteristic positive curvature of H _c2 (T) observed for these compounds, the Sommerfeld parameter y _N in the normal state as well as the curvature of γ (H) with γ as the electronic specific heat coefficient in the mixed state. These quantities have their highest values for the pure compounds and show a minimum for the most disordered compounds near x = 0.5. The results for H _c2 (T) are explained within a two-band model for clean limit superconductors. It is shown that growing degree of substitutional disorder results in a reduction of the relevant properties. The case of dirty limit is not yet reached even for the maximum disordered samples, whereas disorder on the transition metal site in Y(Ni_0.8Pt_0.2)₂B₂C reported in literature points to a dirty limit superconductor.