Phase diagram and weak-link behavior in Nd-doped CaFe2As2

The transport properties, phase diagram, and dopant distribution are investigated in systematically Nd-doped CaFe2As2 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 measurements.