Magnetization and resistivity measurements on untwinned YBa2Cu3Oy single crystals are performed in high magnetic fields up to 30T. The upper critical point Hucp at. which three different phases (the vortex liquid, the vortex glass, and the Bragg glass) meet strongly depends on the oxygen content y. For fully oxidized YRa2Cu3Oy (y ≃ 7, Tc = 87.5 K), the first-order vortex lattice melting line Hm(T) is observed up to 30 T. In the low field region below 5 T, on the other hand, Hm(T) changes to the second peak and the entropy jump disappears: the results imply the existence of the lower critical point. For optimally doped YBa2Cu3Oy (y ≃ 6.92, Tc ≃ 93 K), the vortex glass transition line Hg(T) approaches to the first-order transition line well below H, indicating the new vortex state such as a vortex slush (a pinned liquid) regime.
ASJC Scopus subject areas
- Electronic, Optical and Magnetic Materials
- Condensed Matter Physics
- Energy Engineering and Power Technology
- Electrical and Electronic Engineering