Superconductivity and magnetism in quaternary-F (F-Y or F-elements) transition-metal borocarbides and magnetic-properties of RNI(4)B (R-Y or RARE-EARTH)

Abstract

Our recent pioneering discovery of bulk superconductivity (SC) in Y-Ni-B-C at an elevated temperature (T-c approximate to 12 K) has triggered intense research activity in borocarbide materials containing 4f and 5f elements. This was an offshoot of our studies of RNi(4)B (R=Y or rare earth) series of materials. Replacing Ni by Pd produces a superconducting phase in Y-Pd-B-C with record T-c (approximate to 23 K) in bulk intermetallics. Here we present the results of some of our investigations in borocarbide materials. Our specific heat studies suggest that YNi2B2C is a strong-coupling type II superconductor. The temperature dependence of H-c2(T) is rather unusual. mu-SR measurements indicate the possibility that Ni atoms carry a magnetic moment. SC and magnetism coexist in RNi(2)B(2)C (R=Ho, Er or Tm). CeNi2B2C is found to be a mixed-valence material. NdNi2B2C and GdNi2B2C, which we have been able to synthesize as a single phase, do not superconduct but order magnetically at T-m approximate to 4.5 K and T-m approximate to 19 K respectively. We have observed SC in Th-Ni-B-C and Th-Pd-B-C systems. There are at least two superconducting transitions in the Pd-containing systems Th-Pd-B-C and Y-Pd-B-C. YT2B2C (T=Co, Ir, Rh or Os) crystallize in a YNi2B2C-type structure but do not superconduct down to 4.2 K. Substitution studies show that Th (non-magnetic) and Gd (magnetic) suppress T-c by nearly the same extent. We also summarize our results on RNi(4)B series of materials which show remarkable physical properties.