Study of Normal and Superconducting States of MgCNi₃ upon Fe and Co Substitution and External Pressure

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Results of our study on the superconducting and normal state properties of the recently discovered superconductor MgCNi₃, the effect of Fe and Co substitution at the Ni site and the effect of pressure are reported. It is shown that a two band model provides a consistent interpretation of the temperature dependence of the normal state resistance and the Hall constant. Whereas band structure calculations suggest an increase in Tc upon partial substitution of Ni with Fe and Co, Co substitution quenches superconductivity and Fe substitution leads to an increase followed by a decrease in Tc. The observed variation of Tc may be explained in terms of a competition between increase in Tc due to increase in density of states and a decrease due to spin fluctuations. Based on these results, it is suggested that the spin fluctuations are weaker in Fe doped samples as compared to the Co doped ones. An initial decrease in Tc (and the normal state resistance) followed by an increase is observed on application of pressure. The decrease in Tc for small applied pressures can be understood in terms of the decrease in the density of states at the Fermi level. The subsequent increase in Tc with pressure is due to a lattice softening or a structural phase transition, consistent with the band structure calculations. It is conjectured that suppression of spin fluctuations by pressure may also be responsible for the observed increase in Tc at higher pressures.