VORTEX PHASE DIAGRAM OF UNDERDOPED YBa2Cu3Oy IN PARALLEL MAGNETIC FIELDS

Abstract

In order to study the Josephson vortex phase diagram in high-Tc superconductors, we have prepared untwinned single crystals of underdoped YBa 2 Cu 3 O y and measured the c-axis resistivity ρc(T) in magnetic fields H parallel to the ab plane. In YBa 2 Cu 3 O y (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 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 T L (H) corresponds to the commensurability between the vortex spacing and the period of the CuO 2 planes. The lower critical point μ0H lcp ~ 5 T of T L (H) and the vortex slush phase result from the moderate disorder due to the oxygen vacancy in the CuO chain.