  Results of our study on the superconducting and normal state properties of
the recently discovered superconductor MgCNi_3, 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 T_c 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 T_c. The
observed variation of T_c may be explained in terms of a competition between
increase in T_c 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 T_c (and the normal state resistance) followed by an
increase is observed on application of pressure. The decrease in T_c 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 T_c 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
T_c at higher pressures.
