The physics of the non-oxide perovskite superconductor MgCNi₃

The present review article discusses the physics of the non-oxide perovskite superconductor MgCNi₃ on the basis of theoretical and experimental results available on the material up to July 2004. It was discovered following on from the breakthrough of the finding of the MgB₂ superconductor at the beginning of 2001, which has subsequently been intensively studied; however, less attention has been paid to it due to its much lower superconducting transition temperature, T_c ( K), as compared to that of MgB₂ ( K). But it has many interesting properties which need to be focused on to obtain an understanding of its complicated physics. Energy band calculations show that the density of states (DOS) at the Fermi level, N(E_F), is dominated by Ni d states and there is a von Hove singularity in the DOS just below E_F (<50–120 meV). It is surprising that the conduction electrons in it are derived from partially filled Ni d states, which typically lead to ferromagnetism in metallic Ni and many Ni-based binary alloys. MgC_xNi₃ has a simple cubic perovskite structure with space group and the lattice parameter a is for at ambient temperature and pressure. However, the Ni₆(O₆) octahedron is locally distorted from those expected in the perfect cubic form. The carbon atom of MgCNi₃ at the body centre is surrounded by six Ni atoms at the face centred positions and eight Mg atoms at the cube corners. The carriers in it are of electron type in the normal state, although theoretically they were predicted to be of hole type. T_c increases with increase of x in MgC_xNi₃, but generally decreases with Ni site doping with Co, Fe, Mn, Cu etc. Theoretically, the DOS peak should be greatly reduced by doping at the Mg or Ni site, which accounts for the reduced T_c. The T_c is found to increase with increase of the external pressure (P) at a rate of , which is the same as that for the intermetallic RNi₂B₂C (R = rare earth) superconductors but about one order lower than that for MgB₂. The T_c(P) result focuses our attention on the feature that N(E_F) should increase with pressure due to the broadening of the energy level. Also, a controversial magnetoresistance is reported. It has been observed that the electronic contribution is slightly higher than the lattice one in the normal state thermal conductivity. Specific heat and tunnelling spectroscopic studies indicate that this is an s-wave BCS-type weak/moderate coupling type-II superconductor, but this needs further confirmation as the penetration depth distinctly exhibits a non-s-wave BCS low temperature behaviour which theoretically suggests a d-wave superconductor.