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Critical Current Density in the Heterogeneous High-Tc Superconductor Ca1−xLaxFe2As2

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 Tc ~ 45 K with a transition width of ΔTc = 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 Tc) 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-Tc phase exists globally in the La-doped CaFe2As2, while the high-Tc 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.