Heat capacity jump atTcand pressure derivatives of superconducting transition temperature in the Ba1−xNaxFe2As2(0.1≤x≤0.9) series

Abstract

We present the evolution of the initial (up to $\ensuremath{\sim}10$ kbar) hydrostatic pressure dependencies of ${T}_{c}$ and of the ambient pressure, and the jump in the heat capacity associated with the superconducting transition as a function of Na doping in the Ba${}_{1\ensuremath{-}x}$Na${}_{x}$Fe${}_{2}$As${}_{2}$ family of iron-based superconductors. For Na concentrations $0.15\ensuremath{\le}x\ensuremath{\le}0.9$, the jump in specific heat at ${T}_{c}$, $\ensuremath{\Delta}{C}_{p}{|}_{{T}_{c}}$, follows the $\ensuremath{\Delta}{C}_{p}\ensuremath{\propto}{T}^{3}$ (the so-called BNC scaling) found for most BaFe${}_{2}$As${}_{2}$-based superconductors. This finding suggests that, unlike the related Ba${}_{1\ensuremath{-}x}$K${}_{x}$Fe${}_{2}$As${}_{2}$ series, there is no significant modification of the superconducting state (e.g., change in superconducting gap symmetry) in the Ba${}_{1\ensuremath{-}x}$Na${}_{x}$Fe${}_{2}$As${}_{2}$ series over the whole studied Na concentration range. Pressure dependencies are nonmonotonic for $x=0.2$ and $0.24$. For other Na concentrations, ${T}_{c}$ decreases under pressure in an almost linear fashion. The anomalous behavior of the $x=0.2$ and $0.24$ samples under pressure is possibly due to the crossing of the phase boundaries of the narrow antiferromagnetic tetragonal phase, unique for the Ba${}_{1\ensuremath{-}x}$Na${}_{x}$Fe${}_{2}$As${}_{2}$ series, with the application of pressure. The negative sign of the pressure derivatives of ${T}_{c}$ across the whole superconducting dome (except for $x=0.2$) is a clear indication of the nonequivalence of substitution and pressure for the Ba${}_{1\ensuremath{-}x}$Na${}_{x}$Fe${}_{2}$As${}_{2}$ series.