Phase diagram of interlayer Josephson vortices in underdoped YBa₂Cu₃O_y

In order to study the phase diagram of interlayer vortices in high-T_c superconductors, we have prepared untwinned single crystals of YBa₂Cu₃O_y with the various oxygen content. We also measured the c-axis resistivity ρ_c(T) in magnetic fields H parallel to the ab plane. In YBa₂Cu₃O_y (T_c ∼ 60 K), the vortex liquid phase freezes into the Josephson vortex glass through two-stage processes with decreasing temperature in the high-H region above 5 T. Since the two phase transition lines consist of the first-order transition line T_L(H) and the second-order transition line T_g(H), the intermediated phase in the region of T_g(H) < T < T_L(H) is considered to be the vortex slush phase. We find that the oscillatory field dependence of the T_L(H) line and ρ_c(T) under high current densities corresponds to the commensurability between the vortex spacing and the period of the CuO₂ planes