Phase transition of the Josephson vortices in YBa2Cu3O7−δ single crystals with various oxygen deficiencies

We measured the resistivity of optimally doped, heavily overdoped, and slightly underdoped YBa2Cu3O7−δ single crystals in magnetic fields parallel to the CuO2 planes, to study the phase transition of the Josephson vortices as a function of the doping condition. We found that a first-order phase transition occurs in the Josephson vortex system in the magnetic fields between 3 and 15 T for the optimally doped samples, while only a second-order phase transition is observed for both heavily overdoped and slightly underdoped samples. These results clearly mean that the phase transition of Josephson vortices does not depend on the number of point disorders and that a novel vortex pinning independent of the point disorders is weak in the optimally doped region.