Effect of doping on structural and superconducting properties in Ca1−xNaxFe2As2single crystals (x=0.5, 0.6, 0.75)

Abstract

We study the correlation between crystalline structure and superconducting properties in Na-doped Ca${}_{1\ensuremath{-}x}$Na${}_{x}$Fe${}_{2}$As${}_{2}$ single crystals for three chemical compositions ($x$ = 0.5, 0.6, 0.75). We find the maximum superconducting transition temperature ${T}_{c}$ \ensuremath{\sim} 33.4 K at $x$ \ensuremath{\sim} 0.75. The Na substitution causes the decrease of the a-b crystallographic axes and the increase of the $c$ axis in the tetragonal phase. The single crystals show perfect diamagnetism, indicating full superconducting volume. The anisotropy ratio for the upper critical field near the superconducting transition temperature is \ensuremath{\gamma} = 1.85 \ifmmode\pm\else\textpm\fi{} 0.05, independently of the Na content. A narrow vortex liquid phase was detected in the sample with highest ${T}_{c}$ ($x$ = 0.75), consistent with the expectations based on a Lindemann criterion. The analysis of the critical currents shows no evidence of correlated pinning and indicates that the pinning arises from a combination of several mechanisms. At low fields, pinning by random nanoparticles dominates. At higher fields, a small and field independent ${J}_{\mathrm{c}}$ in the optimally doped crystal may originate in the simultaneous presence of sparse large nanoparticles and a much denser distribution of smaller particles, with the sparse pins producing a caging effect that constrains the volume of the vortex bundle associated with the denser and weaker defects.