Phase diagram and calorimetric properties of NaFe1−xCoxAs

Abstract

We measured the resistivity and magnetic susceptibility to map out the phase diagram of single crystalline NaFe${}_{1\ensuremath{-}x}$Co${}_{x}$As. Replacement of Fe by Co suppresses both the structural and magnetic transitions; however, it also enhances the superconducting transition temperature (${T}_{\mathrm{c}}$) and superconducting component fraction. Magnetic susceptibility exhibits temperature-linear dependence in high temperatures up to 500 K for all the superconducting samples, but such behavior suddenly breaks down for the nonsuperconducting overdoped crystal, suggesting that the superconductivity is closely related to the $T$-linear dependence of susceptibility. Analysis on the superconducting-state specific heat of the optimally doped crystal provides strong evidence for a two-band $s$-wave order parameter with gap amplitudes of ${\ensuremath{\Delta}}_{1}(0)/{k}_{B}{T}_{c}=1.78$ and ${\ensuremath{\Delta}}_{2}(0)/{k}_{B}{T}_{c}=3.11$, being consistent with the nodeless gap symmetry revealed by angle-resolved photoemission spectroscopy experiment.