Specific Heat of YxLu1-xNi2B2C in the Mixed State

Abstract

The temperature and magnetic field dependences of the electronic specific heat c p in the superconducting mixed state as well as the upper critical field H C2 (T) have been measured on a series of polycrystalline Y x Lu1-x Ni2B2C samples with the aim to study the influence of disorder. The electronic specific heat contribution c esm = γ(H) · T in the mixed (vortex) state exhibits significant deviations from the usual γ(H) α H-law for conventional superconductors for all compositions resulting in a disorder dependent negative curvature of γ α H 1- β(x) For the samples with sub-stitutional disorder at the rare earth site β and the positive curvature exponent α of H C2 (T) are reduced but α, β > 0 holds for all x, as distinct from the case of much stronger disorder due to iso-electronic partial Pt substitutions at the Ni site where α and β rapidly vanish (Nohara et al. (1999) J. Phys. Soc. Jpn. 68 1078). β exhibits a clear minimum near x = 0.5. The magnitudes of H C2 (T) and T c , the curvatures of H C2 (T)and γ(H), as well as the Sommerfeld constant βN in the normal state are reduced in a similar fashion for intermediate composition.