s-wave superconductivity probed by measuring magnetic penetration depth and lower critical field of MgCNi3 single crystals

The magnetic penetration depth lambda has been measured in MgCNi3 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, delta lambda(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 lambda(0)=230 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.