Josephson-vortex states at higher magnetic fields in Bi2Sr2CaCu2O8+y

Josephson-vortex (JV) states in Bi2Sr2CaCu2O8+y (Bi-2212) have been studied by measuring the flow resistance of JVs and I-V characteristics with a current along the c-axis and a magnetic field parallel to the superconducting layers. In the flow resistance, periodic oscillations have been found as a function of the parallel field. As this phenomenon is related to the formation of a triangular lattice of JVs as the ground state, the 3D-ordered state of JVs can be determined in the magnetic phase diagram. With increasing the doping level of Bi-2212, the 3D-ordered phase is shifted towards a higher magnetic field and a higher temperature, which is well explained by the theoretical analyses. Above the 3D-ordered phase, JV state has been studied with I-V characteristic measurements, in which the voltage shows a distinct change in the exponent of the power-law dependence to the current between two phases. This suggests that the 3D-ordered phase changes to the disordered phase along the c-axis.