s-wave superconductivity probed by measuring magnetic penetration depth and lower critical field ofMgCNi3single crystals

The magnetic penetration depth $\ensuremath{\lambda}$ has been measured in ${\text{MgCNi}}_{3}$ single crystals using both a high-precision tunnel diode oscillator (TDO) technique and Hall probe magnetization (HPM). In striking contrast to previous measurements in powders, $\ensuremath{\delta}\ensuremath{\lambda}(\text{T})$ deduced from TDO measurements increases exponentially at low temperature, clearly showing that the superconducting gap is fully open over the whole Fermi surface. An absolute value at zero temperature $\ensuremath{\lambda}(0)=230\text{ }\text{nm}$ is found from the lower critical field measured by HPM. We also discuss the observed difference of the superfluid density deduced from both techniques. A possible explanation could be due to a systematic decrease in the critical temperature at the sample surface.