Phase separation and Josephson-coupling origin for the anomalous Hc2 behavior in overdoped Bi2Sr2-xLaxCuO6+δ crystals

Hole-overdoped Bi2Sr2-xLaxCuO6+delta single crystals with two coexistent superconducting transitions have been intensively studied by measuring the de 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 tan "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.