Irreversibility temperatures in superconducting oxides: The flux-line-lattice melting, the glass-liquid transition, or the depinning temperatures

Abstract

The magnetic-field dependence of the irreversibility temperatures follows an H=a[1-${\mathit{T}}_{\mathit{r}}$(H)/${\mathit{T}}_{\mathit{c}}$(0)${]}^{\mathit{n}}$ relationship with n≃1.5, for pure and alloyed ${\mathrm{YBa}}_2$(${\mathrm{Cu}}_{1\mathrm{-}\mathit{x}}$ ${\mathrm{M}}_{\mathit{x}}$ ${)}_3$ ${\mathrm{O}}_{7+\mathrm{\delta }}$ with x=0 and 0.02, where M=Al, Fe, Ni, and Zn, measured for an applied field parallel to the c axis. However, for M=Ni and x=0.04 and 0.06, n≃2.0. This relationship is not applicable for either ${\mathrm{Bi}}_2$ ${\mathrm{Sr}}_2$ ${\mathrm{CaCu}}_2$ ${\mathrm{O}}_8$ or (Bi,Pb${)}_2$ ${\mathrm{Sr}}_2$ ${\mathrm{Ca}}_2$ ${\mathrm{Cu}}_3$ ${\mathrm{O}}_{10}$ powders. It is also shown that the irreversibility temperature is a strong function of the magnetic hysteresis width ΔM for pure and alloyed ${\mathrm{YBa}}_2$ ${\mathrm{Cu}}_3$ ${\mathrm{O}}_7$. These results and the measurements of the flux creep ΔM(t) for these specimens suggest that ${\mathit{T}}_{\mathit{r}}$(H) is a depinning line rather than a lattice melting or glass-to-liquid phase-transition temperature. However, the conventional flux-creep model cannot account for all of the observed temporal dependences of ΔM(t).