S-wave superconductivity probed by measuring magnetic penetration depth and lower critical field of MgCNi$_{3}$ single crystals

TL;DR

This study measures the magnetic penetration depth in MgCNi$_{3}$ single crystals, revealing a fully open superconducting gap and providing insights into superfluid density through advanced measurement techniques.

Contribution

It introduces precise measurements of magnetic penetration depth and critical field in MgCNi$_{3}$ single crystals, clarifying the nature of its superconducting gap.

Findings

Exponential increase of $\delta\lambda$(T) at low temperatures indicating a full superconducting gap.

Zero-temperature penetration depth $\lambda(0)=230$ nm.

Difference in superfluid density measurements suggests surface effects on critical temperature.

Abstract

The magnetic penetration depth $\lambda$ has been measured in MgCNi$_{3}$ single crystals using both a high precision Tunnel Diode Oscillator technique (TDO) 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 of the critical temperature at the sample surface.