Phase transitions and vortex structure evolution in two-level high-temperature granular superconductor YBa$_{2}$Cu$_{3}$O$_{7-\delta}$ under temperature and magnetic field
Abstract:
Temperature dependences of the resistivity $\rho(T)$ of samples of granular high-temperature superconductor YBa$_{2}$Cu$_{3}$O$_{7-\delta}$ are measured at various transverse external magnetic fields at 0 $\le\mathbf H_{\operatorname{ext}}\le$ 1900 Îe in the temperature range from the upper Josephson critical temperature of “weak bonds” $T_{c2J}$ to temperatures slightly exceeding the superconducting transition temperature $T_c$. Based on the data obtained, the behavior of the field dependences of the critical temperatures of superconducting grains and “weak bonds”, and temperature and field dependences of the magnetic contribution to the resistivity $[\Delta\rho(T,H)=\rho(T)_{H_{\operatorname{ext}}=\operatorname{const}}-\rho(T)_{H_{\operatorname{ext}}=0}]$. It is shown that the behavior of the magnetic contribution to the resistivity $\Delta\rho$ along the line of the phase transition related to the onset of the magnetic field penetration in the form of Abrikosov vortices into the subsystem of superconducting grains $T_{c1g}(H_{\operatorname{ext}})$ is anomalous. The concepts on the magnetic flux redistribution between both subsystems of two-level HTSC near in the vicinity of $T_{c1g}$ : the Josephson vortex decreases, and the Abrikosov vortex density increases.
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