Critical Current Density in the Heterogeneous High-T c Superconductor Ca1-x La x Fe2As2

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 Delta T (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 > 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.