Superconductivity
Supravodivosť

    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_c ∼ 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.

Z. Pribulová, J. Kačmarčík, C. Marcenat, P. Szabó, T. Klein, A. Demuer, P. Rodiere, D. J. Jang, H. S. Lee, H. G. Lee, S.-I. Lee, and P. Samuely:
Superconducting energy gap in MgCNi₃ single crystals: Point-contact spectroscopy and specific-heat measurements;
Physical Review B 83, 104511 (2011).

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 μ₀H_c(T) as calculated from H_c1 found in Ref. 16 and H_c2 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 H_c2 from WHH theory.