Effects of post-annealing and cobalt co-doping on superconducting properties of (Ca,Pr)Fe2As2 single crystals

TL;DR

This study investigates how post-annealing and cobalt co-doping influence the superconducting properties of (Ca,Pr)Fe2As2 single crystals, revealing two-step superconductivity and the effects of strain release on superconducting volume and critical current.

Contribution

It demonstrates that post-annealing at ~400°C enhances bulk superconductivity and clarifies the origin of high-temperature superconductivity as local rather than bulk in (Ca,RE)Fe2As2.

Findings

Post-annealing at ~400°C increases c-axis length.

Superconducting volume fraction improves after annealing.

High Jc values indicate bulk superconductivity below Tc2.

Abstract

In order to clarify the origin of anomalous superconductivity in (Ca,RE)Fe2As2 system, Pr doped and Pr,Co co-doped CaFe2As2 single crystals were grown by the FeAs flux method. These samples showed two-step superconducting transition with Tc1 = 25~42 K, and Tc2 < 16 K, suggesting that (Ca,RE)Fe2As2 system has two superconducting components. Post-annealing performed for these crystals in evacuated quartz ampoules at various temperatures revealed that post-annealing at ~400{\deg}C increased the c-axis length for all samples. This indicates that as-grown crystals have a certain level of strain, which is released by post-annealing at ~400{\deg}C. Superconducting properties also changed dramatically by post-annealing. After annealing at 400{\deg}C, some of the co-doped samples showed large superconducting volume fraction corresponding to the perfect diamagnetism below Tc2 and high Jc values of 104~105 Acm-2 at 2 K in low field, indicating the bulk superconductivity of (Ca,RE)Fe2As2 phase occurred below Tc2. On the contrary, the superconducting volume fraction above Tc2 was always very small, suggesting that 40 K-class superconductivity observed in this system is originating in the local superconductivity in the crystal.