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

Abstract

62.Dh -rare earth and transition metal substitution. PACS. 74.25.Bt -specific heat.Abstract. -The temperature and magnetic field dependence of the specific heat cp(T, H) in the superconducting mixed state as well as the upper critical field H c2 (T ) have been measured for polycrystalline YxLu 1−x Ni 2 B 2 C and Y(Ni 1−y Pty) 2 B 2 C samples. The linear-in-T electronic specific heat contribution γ(H) · T exhibits significant deviations from the usual linear-in-H law for all x and y the transition metal site (T ) resulting in a disorder dependent negative curvature of γ(H). The deviations from that linear behaviour of our unsubstituted samples are the largest reported so far for any superconductor. 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 γ(H) dependence is discussed in the unitary d-wave as well as in the quasi-clean s-wave limits. From a consideration of γ(H) data only, d-wave pairing cannot be ruled out.