Nodeless superconductivity in K$_x$Fe$_{2-y}$Se$_2$ single crystals revealed by low temperature specific heat

TL;DR

This study investigates the low-temperature specific heat of K$_x$Fe$_{2-y}$Se$_2$ single crystals, revealing nodeless superconductivity with strong coupling characteristics through specific heat measurements and magnetic field analysis.

Contribution

The paper provides evidence for nodeless superconductivity in K$_x$Fe$_{2-y}$Se$_2$ using specific heat measurements, highlighting its strong coupling nature.

Findings

Residual SH coefficient is very small, indicating high-quality superconducting state.

Magnetic field dependence suggests a nodeless superconducting gap.

The superconductor exhibits strong coupling with a high ΔC/γnT ratio.

Abstract

Low temperature specific heat (SH) has been measured in K$_x$Fe$_{2-y}$Se$_2$ single crystals with T$_c$ = 32 K. The SH anomaly associated with the superconducting transition is moderate and sharp yielding a value of $\Delta C/T \mid_{T_c}$ = 11.6 $\pm$ 1.0 $mJ/mol K^2$. The residual SH coefficient $\gamma(0)$ in the superconducting state at T$\rightarrow$ 0 is very small with a value of about 0.39 $mJ/mol K^2$. The magnetic field induced enhancement of the low-T SH exhibits a rough linear feature indicating a nodeless gap. This is further supported by the scaling based on the s-wave approach of the low-T data at different magnetic fields. A rough estimate tells that the normal state SH coefficient $\gamma_n$ is about 6 $\pm$ 0.5 $mJ/mol K^2$ leading to $\Delta C/\gamma_nT\mid_{T_c}$ = 1.93 and placing this new superconductor in the strong coupling region.