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

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 $\hat{I}”C/T{\mid }_{T_c}$ = 11.6 $\pm$ 1.0 $mJ/mol{K}^2$. The residual SH coefficient ${\hat{I}}^3(0)$ in the superconducting state at T$\to$ 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 ${\hat{I}}_n^3$ is about 6 $\pm$ 0.5 $mJ/mol{K}^2$ leading to $\hat{I}”C/{\hat{I}}_n^{3}T{\mid }_{T_c}$ = 1.93 and placing this new superconductor in the strong coupling region.

4 pages, 4 figures