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 Tc ∼ 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/kBTc ∼ 4. 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.
Figure 1: Temperature dependence of specific heat in zeromagnetic field. Dashed line is entropy conservation construction around critical temperature. Inset: exponential dependence of the electronic specific heat; full line represents the best fit of the exponential decay.
Figure 2: Temperature dependence of thermodynamic critical field as derived from electronic specific heat (line), left axis applies. Right axis applies to the μ0Hc(T) as calculated from Hc1 found in Ref. 16 and Hc2 from Fig. 3 (circles). Inset: Difference of entropy between normal and superconducting state.
Figure 3: Temperature dependence of the upper critical field in MgCNi3 from specific-heat measurements (circles) and point-contact spectroscopy (stars). Dashed line is corresponding temperature dependence of Hc2 from WHH theory.