The Magnetic-Field Phase Diagram of High-Temperature Superconductors

The magnetic-field-temperature (H-T) phase diagrams of YBa_2Cu_3O _7 (YBCO) and Tl_2 Ba_2CaCu_2 O_8 (Tl-2212) high-temperature superconductor (HTS) films, were constructed using magnetotransport, dc magnetization, and ac susceptibility measurements. Correlation of these measurements on a YBCO sample demonstrates that there is one irreversibility line (IRL), which is a continuous vortex-glass phase transition (VGT). Critical behavior of the VGT is investigated in YBCO and Tl-2212 under various conditions. Studies of YBCO as a function of oxygen content suggest that stoichiometric alteration of HTS materials can move the vortex-glass phase boundary with respect to the upper critical field.