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, Tc (?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(EF), is dominated by Ni d states and there is a von Hove singularity in the DOS just below EF (<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. MgCxNi3 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. Tc increases with increase of x in MgCxNi3, 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 Tc. The Tc 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 Tc(P) result focuses our attention on the feature that N(EF) 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.