Thermally activated flux movement and critical transport current density in epitaxial ${\mathrm{Bi}}_2$${\mathrm{Sr}}_2$${\mathrm{CaCu}}_2$${\mathrm{O}}_{8+\mathrm{\delta }}$ films

In this article we report on thermally excited flux creep and the critical transport current density ${\mathit{j}}_{\mathit{c}}$ in high-quality epitaxial ${\mathrm{Bi}}_2$ ${\mathrm{Sr}}_2$ ${\mathrm{CaCu}}_2$ ${\mathrm{O}}_{8+\mathrm{\delta }}$ thin films. Both dissipative mechanisms are governed by the highly anisotropic behavior of this compound which was investigated by means of the angular dependence of the magnetoresistivity (γ≥150). The activation energy U for thermally excited flux creep was evaluated with respect to temperature, magnetic field, and applied current density j. U(T,B,j) is essentially increasing linearly with falling temperature, power-law dependent on the field (U∝${\mathit{B}}^{\mathrm{-}\mathrm{\alpha }}$ with α≊0.5), and almost independent of current density for j≤${10}^5$ A/${\mathrm{cm}}^2$. These experimental results are consistent with the concept of plastic flux creep. The critical current density exhibits high absolute values [${\mathit{j}}_{\mathit{c}}$(77 K, B=0)=4×${10}^5$ A/${\mathrm{cm}}^2$] and was measured for magnetic fields in the configuration B∥c up to 10 T and also with respect to various Θ angles between the c axis and field direction. The decrease of ${\mathit{j}}_{\mathit{c}}$ with increasing B was found to be significantly reduced in comparison to single crystals and prior results on thin films. A further enhancement in the ${\mathit{j}}_{\mathit{c}}$(B,T) behavior could not be achieved by chemical doping through partial substitution of copper by zinc.