Vortex matter phase diagram of untwinned YBa₂Cu₃O_y single crystals with different oxygen content

Resistivity, current-voltage characteristic and magnetization are measured on optimally doped and slightly underdoped untwinned YBa₂Cu₃O_y (YBCO) single crystals. Resistivity behaves quite differently from that of slightly overdoped ones and shows successive phase transitions; the first-order vortex liquid-liquid transition and the second-order vortex glass transition. The current-voltage characteristic on such a two-step transition shows S-like shape in extremely wide temperature range. Hysteresis curves of the magnetization also show the remarkable change depending on the different oxygen content. All these results suggest that the pinned liquid state such as the `vortex slush' regime exists on the vortex matter phase diagram for untwinned YBCO with the limited oxygen concentration.