Superconducting energy gap in MgCNi₃ single crystals: Point-contact spectroscopy and specific-heat measurements

Specific heat has been measured down to 600 mK and up to 8 T by highly sensitive ac microcalorimetry on MgCNi₃ single crystals with T _c 7 K. Exponential decay of the electronic specific heat at low temperatures proved that a superconducting energy gap is fully open on the whole Fermi surface, in agreement with our previous magnetic penetration depth measurements on the same crystals. The specific-heat data analysis shows consistently the strong-coupling strength 2Δ/k_BT_c4. This scenario is supported by the direct measurements of the gap via the point-contact spectroscopy. Moreover, the spectroscopy measurements show a decrease in the critical temperature at the sample surface, which accounts for the observed differences of the superfluid density deduced from the measurements by different techniques.