Critical Current Density in the Heterogeneous High-T_c Superconductor Ca_1−xLa_xFe₂As₂

We investigate the critical current density of La-doped CaFe₂As₂ (LaCa122) single crystals via transport and magnetization measurements. A large piece of the LaCa122 shows a sharp superconducting (SC) phase transition at a temperature of T_c ~ 45 K with a transition width of ΔT_c = 3.3 K. Small pieces exfoliated from the large crystal, however, show multiple SC phase transitions: two disparate SC phases sequentially occurred at ~ 15 (low T_c) and ~ 45 K (high Tc). The temperature dependence of transport critical current density also shows two disparate curves, and a kink appears at ~ 8 K. The magnetization critical current density shows a rapid suppression under an applied magnetic field for T CloseSPigtSPi 10 K. These results indicate that the low-T_c phase exists globally in the La-doped CaFe₂As₂, while the high-T_c phase has a small volume fraction and locally resides within small domains. The disparate critical current density determined from transport and magnetization measurements, therefore, reflects the heterogeneous superconducting phases in La-doped CaFe₂As₂.