The new class of superconducting quaternary R-T-B-C systems (R= rare earth, T = Ni, Pd)

Abstract

Our discovery of superconductivity (SC) at an elevated temperature (~12K) in the system Y-Ni-B-C [1-4] has opened up an entirely new area of research in superconductivity in intermetallics. Our specific heat measurements show that YNi₂B₂C is strongly-coupled superconductor. SC and magnetism coexist, below 7K, in ErNi₂B₂C. From μ-SR studies, carried out on LuNi₂B₂C and TmNi₂B₂C, Ni-atoms are inferred to carry local moment. Substituents such as Th and Gd (at the Y-sites) and Fe and Co (at the Ni-sites) depress T_c of YNi₂B₂C. Superconductivity is quenched in Y_0.9U_0.1Ni₂B₂C. In multiphase Y-Pd-B-C system (obtained by replacing Ni by Pd), we observe two SC-transitions, T_c ~22K and 10K. LuPd₄BC_0.2 has a minority SC-phase with T_c ~10 K. Microwave absorption has been used to study the temperature dependence of the lower critical field, H_cl, in YPd₅B₃C_0.35. ThNi₂B₂C, a multiphase material, exhibits SC at ~5K whereas two SC transitions are observed in ThPd₂B₂C (T_c~20K and 14K) and in ThPd₄BC. No SC is observed in U-based Ni-and Pd-borocarbides and in YT₂B₂C (T = Co, Rh, Ir and Os) down to 4.2K.