PHASE SEPARATION AND JOSEPHSON-COUPLING ORIGIN FOR THE ANOMALOUS H_c2 BEHAVIOR IN OVERDOPED Bi₂Sr_2-xLa_xCuO_6+\delta CRYSTALS

Hole-overdoped Bi₂Sr_2-xLa_xCuO_6+\delta single crystals with two coexistent superconducting transitions have been intensively studied by measuring the dc magnetic susceptibility. It is found that the coexistence of these two phases cannot be attributed to any chemical or vortex dynamical effect but to some intrinsic driving force, such as the electronic-driven phase separation. Furthermore, the upper critical fields of the two phases behave in a rather different way; that is, one of them shows a "normal H_c2 behavior" which can be described by the critical fluctuation theory, while the other exhibits an "anomalous H_c2 behavior" and can be well explained by a recent theory based on an assumption of a Josephson-coupling origin, indicating a strong evidence of Josephson-coupling origin for the upward curvature of the so-called H_c2 in high T_c superconductors.