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,H∥c 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 $H_{c1}$ and below a crossover field $H_x$. Between $H_x$ and $H_{c2}$ and for T<$T_M$, where $T_M$ is the two-dimensional melting temperature, the superconductors are in the flux-solid phase. The irreversible line $T_{\mathrm{irr}(\mathrm{H}}$) 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 $T_{\mathrm{KT}}$ and below $T_{c0}$, flux entanglement may take place near $H_{c1}$, and the ‘‘entangled-flux-liquid phase’’ proposed by Nelson is compatible with the flux-liquid phase near $H_{c2}$. The proposed phase diagram provides a consistent picture for the experimental results in both ${\mathrm{YBa}}_2$${\mathrm{Cu}}_3$${\mathrm{O}}_7$ and ${\mathrm{Bi}}_{2.2}$${\mathrm{Sr}}_2$${\mathrm{Ca}}_{0.8}$${\mathrm{Cu}}_2$${\mathrm{O}}_8$ single crystals.