Magnetic phase diagram of Josephson vortices in strongly anisotropic superconductor Bi-2212

Magnetic phase diagram of Josephson vortices in strongly anisotropic superconductor Bi2Sr2CaCu2O8+δ has been studied by measuring the Josephson-vortex flow resistance. A triangular-lattice phase, in which periodic oscillations of the flow resistance have been observed, was found to start from about 6 kOe, independent of temperature. Although the triangular lattice phase exists over 7 T below 30 K, at higher temperatures the boundary to the others (glass phase) decreases with increasing temperature, and drops suddenly at 80 K. The flow resistance also decreases close to the boundary. These facts indicate that the thermal excitation of pancake vortex-antivortex pairs or missing a long-range order in the triangular lattice along the c-axis causes the reduction of the flow resistance. Furthermore, above the triangular lattice phase there finds some change in the flow resistance, which suggests the existence of a different kind of glass phases in the Josephson-vortex system