Phase Diagram and Weak-link Behavior in Nd-doped CaFe_2As_2

TL;DR

This study investigates the phase diagram, superconducting phases, and weak-link behavior in Nd-doped CaFe_2As_2, revealing coexistence of two SC phases with distinct T_c values and their relation to structural transitions.

Contribution

It provides a detailed phase diagram of Nd-doped CaFe_2As_2, identifying two superconducting phases and their connection to structural transitions, along with insights into weak-link behavior.

Findings

Coexistence of two superconducting phases with different T_c values.

High-T_c phase exhibits Josephson weak-link behavior.

Structural transition from tetragonal to collapsed tetragonal phase correlates with doping levels.

Abstract

The transport properties, phase diagram, and dopant distribution are investigated in systematically Nd doped CaFe_2As_2 single crystals. Coexistence of two superconducting (SC) phases with different critical transition temperature (T_c) is observed. The low-T_c phase emerges as x >= 0.031, and the T_c value increases to its maximum value of about 20 K at x = 0.083, the maximum doping level in our study. As x >= 0.060, the high-T_c phase with a T_c value of about 40 K is observed. The structure transition (STr) from tetragonal to orthorhombic phase vanishes suddenly around x = 0.060, where a new STr from tetragonal to collapsed tetragonal phase begins to turn up. Compared to the low-T_c phase, the end point of SC transition of the high-T_c phase is more sensitive to the magnetic field, showing a characteristic of Josephson weak-link behavior. Possible scenarios about this system are discussed based on our observations. We also find that the non-uniform SC properties cannot be attributed to the heterogeneous Nd distribution on the micro scale, as revealed by the detailed energy dispersive X-ray spectroscopy (EDS) measurements.