  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.
