THE TEMPERATURE-DEPENDENCE OF THE IRREVERSIBILITY LINE OF BI2SR2CACU2OX SUPERCONDUCTORS

The irreversibility line has been determined down to 15 K for both a Bi2Sr2CaCu2Ox (Bi-2212) single crystal and a melt-processed polycrystalline sample. It is shown that the scaling law H(T(r)) is-proportional-to (1-T(r)/T(c))n is only valid close to T(c). A kink is observed at about 30 K for the single crystal in a semi-logarithmic plot of the irreversibility line H(T(r)), similar to the kink found in the temperature dependence of the critical current density J(cr)(T). Theoretically, a thermally activated flux depinning model valid for arbitrary current dependence of the effective pinning potential is developed for the irreversibility line. This yields that the irreversibility line can be described as H(T(r)) is-proportional-to (1 - T(r)/T(c))n/T(m) over the whole temperature region by using an empirical field dependence of the pinning potential U(H) is-proportional-to H-1/m. The experimental data are analyzed in the light of the theoretical result.