Vortex phases and dissipation in high-temperature superconducting oxides

An (H,T) phase diagram is proposed to illustrate various vortex phases in the high-temperature superconducting oxides. Two types of vortices (thermally and magnetic field induced) are studied phenomenologically as a function of the temperature (T), magnetic field (H,Hc axis), and current density (J). It is concluded that the quasi-two-dimensional nature of the high-temperature superconductors results in a vortex plasma phase above Hc1 and below a crossover field Hx. Between Hx and Hc2 and for T<TM, where TM is the two-dimensional melting temperature, the superconductors are in the flux-solid phase. The irreversible line Tirr(H) is attributed to the melting of a flux-solid to a flux-liquid phase. Dissipation occurs in the flux-liquid phase, and the (H,T) dependence of the dissipation is determined by the pinning mechanism. Above TKT and below Tc0, flux entanglement may take place near Hc1, and the entangled-flux-liquid phase proposed by Nelson is compatible with the flux-liquid phase near Hc2. The proposed phase diagram provides a consistent picture for the experimental results in both YBa2Cu3O7 and Bi2.2Sr2Ca0.8Cu2O8 single crystals. © 1989 The American Physical Society.