Vortex state of Pb-substituted Bi2201 studied by in-plain resistivity measurements

Abstract In order to investigate the Pb concentration dependence of the irreversibility field H irr in the Pb-substituted Bi 2 Sr 2 CuO 6+ δ (Bi 2.12− x Pb x Sr 1.88 CuO 6+ δ ), we have measured the temperature dependence of the in-plane electrical resistivity in magnetic fields up to 15 T parallel to the c -axis. It is found that H irr increases for x  ⩽ 0.27 with increasing x , while it remarkably decreases for x  > 0.32. Striking feature is the enhancement of H irr around x  ∼ 0.27–0.32. A possible origin of the enhancement is the two-phase microstructures consisting of modulated and modulation-free domains. In such a case, the phase boundary of the microstructures works effectively as pinning centers for vortices, in contrast to the homogeneous modulated x  = 0.18 and the homogeneous modulation-free x  = 0.37. Our results strongly suggest the close relation between two-phase microstructures and the enhancement of H irr .