Dependence of Superconductivity and Its Weakly Linked Behavior in Bulk LaO_(1-x)F_xFeAs on F Doping

摘要

Samples of oxypnictide compound LaO_(1-x)F_x FeAs, with x = 0.15 and 0.2 corresponding to over- and highly over-doped compositions, respectively, were prepared by solid-state reaction. We present their characterization by XRD and HRTEM, as well as resistivity ρ(T), magnetization M(B) and microwave modulated absorption (MMA) response between 4.2-300 K and applied fields B = 0-8 T. With change in x, both the superconducting and magnetic behavior of the samples shows an interesting pattern. The "magnetic anomaly" at T ～ 130 K, observed in M(T) for x = 0, instead of getting totally suppressed shows a tendency to reappear in x = 0.2 sample. Both samples typically show ρ(300 K) > 2.8 × 10~(-3) Ω cm and critical current density J_c(5 K, 1 T) < 2 × 10~7 A/m~2. The superconducting transitions as measured by ρ(T) at B = 0 are found broad for both x = 0.15 and 0.2 samples with transition widths ～2.5 and 6 K, respectively. The slope |dB_(c2)/dT| (where B_(c2) is upper critical field) determined by resistive onsets, for the x = 0.15 and 0.2 samples, has values ～7.5 and 3.5 T/K, respectively. The superconducting state characteristics as reflected by ρ(T, B), M(T), magnetic J_c(B) and MMA response are typical of the presence of weakly linked inter-grain regions in both the samples. Our HRTEM images of the x = 0.15 sample show the presence of high angle (～43°) grain boundaries, which are well known to limit the J_c in cuprate-based high T_c bulk materials.