THE NEW CLASS OF SUPERCONDUCTING QUATERNARY R-T-B-C SYSTEMS (R=RARE EARTH, T=NI, PD)

Our discovery of superconductivity (SC) at an elevated temperature (similar to 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 mu-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 Te of YNi2B2C. Superconductivity is quenched in Y0.9U0.1Ni2B2C. In multiphase Y-Pd-B-C system (obtained by replacing Ni by Pd), we observe two SC-transitions, T-c similar to 22K and 10K. LuPd4BC0.2 has a minority SC-phase with T-c similar to 10 K. Microwave absorption has been used to study the temperature dependence of the lower critical field, H-c1, in YPd5B3C0.35. ThNi2B2C, a multiphase material, exhibits SC at similar to 5K whereas two SC transitions are observed in ThPd2B2C (T-c similar to 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.