Phase Diagram and Calorimetric Properties of NaFe$_{1-x}$Co$_x$As

TL;DR

This study maps the phase diagram of NaFe$_{1-x}$Co$_x$As, revealing how Co doping suppresses structural and magnetic transitions while enhancing superconductivity, with insights into the superconducting gap structure and magnetic susceptibility behavior.

Contribution

It provides detailed phase diagram data, magnetic susceptibility analysis, and evidence for a two-band s-wave superconducting gap in NaFe$_{1-x}$Co$_x$As.

Findings

Co doping suppresses structural and magnetic transitions.

Superconducting transition temperature increases with Co doping.

Superconducting gap is consistent with a two-band s-wave symmetry.

Abstract

We measured the resistivity and magnetic susceptibility to map out the phase diagram of single crystalline NaFe$_{1-x}$Co$_x$As. Replacement of Fe by Co suppresses both the structural and magnetic transition, while enhances the superconducting transition temperature ($T_{\rm c}$) and superconducting component fraction. Magnetic susceptibility exhibits temperature-linear dependence in the high temperatures up to 500 K for all the superconducting samples, but such behavior suddenly breaks down for the non-superconducting overdoped crystal, suggesting that the superconductivity is closely related to the T-linear dependence of susceptibility. Analysis on the superconducting-state specific heat for the optimally doped crystal provides strong evidence for a two-band s-wave order parameter with gap amplitudes of $\Delta_1(0)/k_{\rm B}T_{\rm c}$= 1.78 and $\Delta_2(0)/k_{\rm B}T_{\rm c}$=3.11, being consistent with the nodeless gap symmetry revealed by angle-resolved photoemission spectroscopy experiment.