Evidence of $d$-wave superconductivity in K${}_{1-x}$Na${}_x$Fe${}_2$As${}_2$ ($x=0,0.1$) single crystals from low-temperature specific-heat measurements

From the measurement and analysis of the specific heat of high-quality K${}_{1-x}$Na${}_x$Fe${}_2$As${}_2$ single crystals we establish the presence of large $T^2$ contributions with coefficients ${\alpha }_{\mathrm{sc}}\approx 30$ mJ/mol K${}^3$ at low $T$ for both $x=0$ and $x=0.1$. Together with the observed $\sqrt{B}$ behavior of the specific heat in the superconducting state both findings give evidence of $d$-wave superconductivity on almost all Fermi-surface sheets with an average gap amplitude of ${\Delta }_0$ in the range of 0.4–0.8 meV. The derived ${\Delta }_0$ and observed $T_c$ agree well with the values calculated within Eliashberg theory, adopting a spin-fluctuation mediated pairing in the intermediate coupling regime.