Quaternary borocarbide superconductors: Some perspectives

Discovery of quaternary borocarbide superconductors is an exciting event in superconductivity. Coexistence of superconductivity and magnetism in the newly discovered quaternary borocarbides takes place at temperatures much higher than observed before, which makes these materials ideal for investigating phenomena associated with interplay of magnetism and superconductivity. T(c) of RE Ni(2)B(2)C (RE = Dy, Er, Ho and Tm) follows nearly the de Gennes scaling despite the fact that RE-moments are correlated at T > similar to T(c). Superconductivity sets in a magnetically ordered lattice in DyNi(2)B(2)C, T(c) similar to 6 K and T(N) similar to 11 K. It is an ideal material to look for gapless superconductivity in a superconducting antiferromagnet, a phenomenon not observed so far. We suggest some other materials of this kind. In YbNi(2)B(2)C no superconductivity is observed down to 2 K, though T(c) is expected to be high ( > 10 K). gamma similar to 200 mJ/mol K(2) suggests the formation of a heavy fermion state in YbNi(2)B(2)C. We comment on the possibility of Ni-atoms carrying a moment. A conjecture that the borocarbides may be electron-doped Mott insulators is relevant in this regard.