Multi-gap superconductivity in single crystals of Ba${}_{0.65}$Na${}_{0.35}$Fe${}_2$As${}_2$: A calorimetric investigation

Abstract

We investigate the electronic properties and the superconducting gap characteristics of a single crystal of hole-doped 122 Fe-pnictide Ba${}_{0.65}$Na${}_{0.35}$Fe${}_2$As${}_2$ by means of specific heat measurements. The specific heat exhibits a pronounced anomaly around the superconducting transition temperature $T_c$ = 29.4 K, and a small residual part at low temperature. In a magnetic field of 90 kOe, the transition is broadened and $T_c$ is lowered insignificantly by an amount $\sim$ 1.5 K. We estimate a high electronic coefficient in the normal state with a value 57.5 mJ mol${}^{-1}$ K${}^2$, being consistent with hole-doped 122 compounds. The temperature-dependent superconducting electronic specific heat cannot be described with single-gap BCS theory under weak coupling approach. Instead, our analysis implies a presence of two s-wave like gaps with magnitudes ${\Delta }_1(0)/k_B{T}_c$ = 1.06 and ${\Delta }_2(0)/k_B{T}_c$ = 2.08 with their respective weights of 48% and 52%. While our results have qualitative similarities with other hole-doped 122 materials, the gap's magnitude and their ratio are quite different.

7 pages, 4 figures

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