Josephson-Coupling Origin for the Upward Curvature of the Pseudo-Upper-Critical Field inBi2Sr2−xLaxCuO6+δCrystals

By measuring the temperature dependence of the diamagnetic moment for La-doped $\mathrm{Bi}{}_2\mathrm{Sr}{}_{2-x}\mathrm{La}{}_x\mathrm{CuO}{}_{6+\delta }$ singe crystals, we observed two superconducting transitions on each sample. With increasing magnetic field, the transition at the high temperature $T_{c1\mathrm{}}$ shifts very slowly, while the transition at the low temperature $T_{c2\mathrm{}}$ shifts dramatically, showing a clear upward curvature of the critical field. It is found that the transition corresponding to $T_{c1\mathrm{}}$ is described very well by the critical fluctuation theory with a very steep ${-dH}_{c2\mathrm{}}/dT$ near $T_{c1\mathrm{}}$, while that corresponding to $T_{c2\mathrm{}}$ can be described by a recent theory by Geshkenbein, Ioffe, and Millis [Phys. Rev. Lett. 80, 5778 (1998)] indicating strong evidence of a Josephson-coupling origin.