Superconductivity and disorder in Y xLu 1− xNi 2B 2C

Abstract

The superconducting transition temperature T c, the temperature dependence of the upper critical field H c2( T), and the lattice structure of polycrystalline Y x Lu 1− x Ni 2B 2C samples have been studied by susceptibility, resistivity, and X-ray measurements, respectively. All samples exhibit the same LuNi 2B 2C-type structure and the lattice parameters depend linearly on the yttrium content. The residual resistivity ratio (RRR) decreases steeply first for slight deviations from both pure stoichiometric limiting contents and then saturates at a low level in a broad plateau in between 0.1< x<0.8, whereas T c( x) and H c2( x) at fixed T change nearly parabolically. The H c2( T)-curves show a positive curvature for all samples. This dependence can be described within a broad temperature region 0.3 T c< T≤0.95 T c by a simple and convenient empirical expression H c2( T)∝(1− T/ T c) α . The parameter α>1 describing the positive curvature of H c2( T) characterizes the sample quality and decreases with increasing disorder in the Y x Lu 1− x Ni 2B 2C samples. However, the case of complete disorder (dirty limit) where α=1 is not reached yet in the investigated samples. The nonmonotonous dependence of T c and H c2 upon x is attributed to partial disorder within the Y–Lu basis layers. Together with the RRR-saturation they reflect the complex multiband character of the materials under consideration.