Specific heat and disorder in the mixed state of non-magnetic borocarbides

The temperature and magnetic-field dependence of the specific heat c(p)(T, H) in the superconducting (sc) mixed state as well as the upper critical field H-c2(T) have been measured for polycrystalline YxLu1-xNi2B2C and Y(Ni(1-y)Pty)(2)B2C samples. The linear-in-T electronic specific-heat contribution gamma(H) . T exhibits significant deviations from the usual linear-in-H law resulting in a disorder-dependent negative curvature of gamma(H). The H-c2(T) data point to the quasi-clean limit for (Y, Lu)-substitutions and to a transition to the quasi-dirty limit for (Ni, Pt)-substitutions. The gamma(H)-dependence is discussed in the unitary d-wave as well as in the quasi-clean s-wave limits. From a consideration of gamma(H) data only, d-wave pairing cannot be ruled out.