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

TL;DR

This study combines specific heat and point-contact spectroscopy to confirm a fully open superconducting energy gap in MgCNi3 single crystals, revealing strong coupling and surface effects influencing superconducting properties.

Contribution

It provides the first combined thermodynamic and spectroscopic evidence of a fully open gap and strong coupling in MgCNi3, highlighting surface effects on superconductivity.

Findings

Superconducting gap is fully open on the entire Fermi surface.

Strong coupling strength 2Δ/kTc ~ 4 observed.

Surface reduction of Tc affects superfluid density measurements.

Abstract

Specific heat has been measured down to 600 mK and up to 8 Tesla by the highly sensitive AC microcalorimetry on the 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 2D/kTc ~ 4. This scenario is supported by the direct gap measurements via the point-contact spectroscopy. Moreover, the spectroscopy measurements show a decrease in the critical temperature at the sample surface accounting for the observed differences of the superfluid density deduced from the measurements by different techniques.