Evidence for s-wave pairing from measurement on lower critical field in $MgCNi_3$

Magnetization measurements in the low field region have been carefully performed on a well-shaped cylindrical and an ellipsoidal sample of superconductor $MgCNi_3$. Data from both samples show almost the same results. The lower critical field $H_{c1}$ and the London penetration depth $lambda$ are thus derived. It is found that the result of normalized superfluid density $lambda^2(0)/lambda^2(T)$ of $MgCNi_3$ can be well described by BCS prediction with the expectation for an isotropic s-wave superconductivity.