Transient regimes of flux creep in high-Tcsuperconductors

Initial nonlogarithmic (t<t) stages of flux creep in superconductors have been studied both theoretically and experimentally. Measurements of the time constant τ in grain-oriented ${\mathrm{YBa}}_2$ ${\mathrm{Cu}}_3$ ${\mathrm{O}}_{7\mathrm{-}\mathit{x}}$ at T=20 K and B=6 T and various sweep rates ${\mathit{dB}}_{\mathit{e}}$/dt are presented. The experimental data as well as an analysis of nonlinear flux diffusion for different flux-creep models indicate that the constant τ is a macroscopic quantity essentially depending upon initial conditions and sample geometry. The value τ is shown to be inversely proportional to the sweep rate with τ reaching 5×${10}^3$ s at ${\mathit{dB}}_{\mathit{e}}$/dt=5×${10}^{\mathrm{-}6}$ Ts.