Phase diagram of interlayer Josephson vortices in underdoped YBa2Cu3Oy

In order to study the phase diagram of interlayer vortices in high-Tc superconductors, we have prepared untwinned single crystals of YBa2Cu3Oy with the various oxygen content. We also measured the c-axis resistivity ρc(T) in magnetic fields H parallel to the ab plane. In YBa2Cu3Oy (Tc ∼ 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 TL(H) and the second-order transition line Tg(H), the intermediated phase in the region of Tg(H) < T < TL(H) is considered to be the vortex slush phase. We find that the oscillatory field dependence of the TL(H) line and ρc(T) under high current densities corresponds to the commensurability between the vortex spacing and the period of the CuO2 planes.