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\ensuremath{\simeq}1.5, for pure and alloyed ${\mathrm{YBa}}_{2}$(${\mathrm{Cu}}_{1\mathrm{\ensuremath{-}}\mathit{x}}$${\mathrm{M}}_{\mathit{x}}$${)}_{3}$${\mathrm{O}}_{7+\mathrm{\ensuremath{\delta}}}$ with x=0 and 0.02, where M=Al, Fe, Ni, and Zn, measured for an applied field parallel to the c axis. However, for M=Ni and x=0.04 and 0.06, n\ensuremath{\simeq}2.0. This relationship is not applicable for either ${\mathrm{Bi}}_{2}$${\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 \ensuremath{\Delta}M for pure and alloyed ${\mathrm{YBa}}_{2}$${\mathrm{Cu}}_{3}$${\mathrm{O}}_{7}$. These results and the measurements of the flux creep \ensuremath{\Delta}M(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 \ensuremath{\Delta}M(t).