SPECIFIC HEAT OF YxLu1−xNi2B2C IN THE MIXED STATE

The temperature and magnetic field dependences of the electronic specific heat cp in the superconducting mixed state as well as the upper critical field Hc2(T ) have been measured on a series of polycrystalline YxLu1−xNi2B2C samples with the aim to study the influence of disorder. The electronic specific heat contribution cesm = γ(H) · T in the mixed (vortex) state exhibits significant deviations from the usual γ(H) ∝ H-law for conventional superconductors for all compositions resulting in a disorder dependent negative curvature of γ ∝ H. For the samples with substitutional disorder at the rare earth site β and the positive curvature exponent α of Hc2(T ) are reduced but α, β > 0 holds for all x, as distinct from the case of much stronger disorder due to isoelectronic partial Pt substitutions at the Ni site where α and β rapidly vanish (Nohara et al. (1999) J. Phys. Soc. Jpn. 68 1078). α exhibits a clear minimum near x = 0.5. The magnitudes of Hc2(T ) and Tc, the curvatures of Hc2(T ) and γ(H), as well as the Sommerfeld constant γN in the normal state are reduced in a similar fashion for intermediate composition.