Upper critical and irreversible fields of polycrystalline CeFeAsO_1-xF_x superconductors

Full text: We have investigated the upper critical field (H_c2), irreversible field (H_irr) and critical current density (J_c) of polycrystalline samples of Ce oxypnictide at different doping levels (x = 0.13 to 0.25). H_c2 was obtained from temperature dependent resistivity measurements with increasing applied magnetic field. Critical field values as high as 150 Tesla were observed with a decreasing trend as the doping level shifts from a slightly under-doped state to the highly over-doped region. The irreversible fields of as-prepared polycrystalline samples were lower in this superconductor compared with other rare-earth oxypnictides, with values below 3 Tesla at 20 K. However, H_irr in the magnetic-field aligned samples with the c-axis parallel to the applied magnetic field show a much higher H_irr when compared with non-aligned samples. Furthermore, H_irr and the coherence length, ξ(0) were found to increase with increasing doping. The origin of irreversibility was studied by determining the exponent 'n' extracted from plots of log₁₀(H_irr) versus log₁₀(1-T/T_c)ⁿ. We found that H_irr follows a 3D vortex lattice melting model similar to the other low anisotropic iron-based superconductors. Preliminary J_c measurements from field-loop magnetization measurements show intragrain self-field J_c values as high as 10⁶ A/cm² , which is consistent with that previously found in underdoped (x= 0.10) Ce-oxypnictide. (author)