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

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 $ΔC/T \mid_{T_c}$ = 11.6 $\pm$ 1.0 $mJ/mol K^2$. The residual SH coefficient $γ(0)$ 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 $γ_n$ is about 6 $\pm$ 0.5 $mJ/mol K^2$ leading to $ΔC/γ_nT\mid_{T_c}$ = 1.93 and placing this new superconductor in the strong coupling region.

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