Effect of the doping on structural and superconducting properties in Ca1-xNaxFe2As2 single crystals

N. HABERKORN; B. MAIOROV; M. JAIME; I. USOV; M. MIURA; G. F. CHEN; W. YU; L. CIVALE

PHYSICAL REVIEW B - CONDENSED MATTER AND MATERIALS PHYSICS

American Physical Society

Año: 2011 vol. 84 p. 64533 - 64543

0163-1829

We study the correlation between crystalline structure and superconducting properties in Na doped Ca1-xNaxFe2As2 single crystals for three chemical compositions (x = 0.5, 0.6, 0.75). We find the maximum superconducting transition  temperature Tc= 33.4 K at x=0.75. The Na substitution causes the decrease of the a-b crystallographic axis 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 g=1.85+/-0.05, independently of the Na content. A narrow vortex liquid phase was detected in the sample with highest Tc (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 Jc 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.