Superconductivity in Intermetallic Compound IRG 1: K.A. Regan, R. Cava, Y. Wang, N. Rogado, N.P. Ong, H.W. Zandbergen
As part of our research, we have discovered superconductivity at 8K in the perovskite structure intermetallic compound MgCNi3 ["Superconductivity in the non-oxide perovskite MgCNi3", T. He, K.A. Regan, M.A. Hayward, A.P. Ramirez, Y. Wang, P. Khalifah, T. He, J.S. Slusky, N. Rogado, K. Inumaru, M.K. Haas, H.W. Zandbergen, N.P. Ong, and R.J. Cava, Nature, 411 54 (2001) 2002]. The crystal structure of MgCNi3 (figure) is structurally fully analogous to familiar oxide perovskites like CaTiO3, and to superconducting perovskite oxides like (Ba,K)BiO3.
Figure: The crystal structure of the new perovskite intermetallic Superconductor MgCNi3 discovered in our group.
MgCNi3 resents a completely surprising analogy to the oxide perovskites. It is based on the combination of a light electropositive metal - Mg - with another light element - C, with the addition of a transition element, Ni. Ni was selected as the ternary element for a specific reason. The Ni in the MgCNi3 perovskite, which is structurally equivalent to O in the oxide perovskites, is likely to have holes in its electronic orbitals at the Fermi energy. Due to the nearly filled d orbitals in elemental nickel, intermetallic compounds of Ni, especially in the presence of electropositive elements such as Mg, often have filled or nearly completely filled Ni d states. If this is indeed the case for MgCNi3 then the conduction will involve holes in Ni d-states, in a strict electronic analogy to the holes in the O p-states in the perovskite oxide superconductors. The structural and electronic equivalence of the oxides and intermetallic perovskite superconductor MgCNi3 would then be complete. Ni metal is an important component of many ferromagnetic materials. The fact that superconductivity rather than ferromagnetism occurs in MgCNi3, where so much nickel is present, suggests that MgCNi3 is an excellent candidate for unconventional superconductivity. This new materials directly addresses an important topic in current condensed matter physics: the possible connection between ferromagnetism and superconductivity in a very special class of novel materials. Considerable work is now in progress in exploring its properties.