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Superconductivity
Magnetoresistance of superconducting grains in ceramic YBa$_2$Cu$_3$O$_{7-\delta}$ high-temperature superconductors in weak magnetic fields
T. V. Sukhareva,
V. A. Finkel National Science Centre Kharkov Institute of Physics and Technology
Abstract:
The current-voltage characteristics
$E(j)_{H_{\mathrm{treat}}=\operatorname{const}}$ of ceramic (granular) YBa
$_2$Cu
$_3$O
$_{6.95}$ samples preliminarily magnetized in different transverse magnetic fields
$\mathbf{H}_{\mathrm{treat}}$ have been measured in a zero field (
$H_{\mathrm{ext}}$ = 0) at
$T$ = 77.3 K for elucidating specific features of dissipation in superconducting grains of high-temperature superconductors (HTSCs). The current-voltage curves measured in the range 0
$\le H_{\mathrm{trap}}\le H_{c2J}$ (where
$H_{\mathrm{trap}}$ is the magnetic field trapped as a result of the pretreatment in
$\mathbf{H}_{\mathrm{treat}}$ and
$H_{c2J}$ is the upper critical field of the Josephson weak links) have been used to construct the field dependences of the magnetoresistance
$\rho_A(H_{\mathrm{treat}})_{j=\operatorname{const}}$ of superconducting grains. It has been established that the magnetoresistance
$\rho_A$ of the superconducting grains is significantly lower than the magnetoresistance
$\rho_J$ for the Josephson medium. The dependence of
$\rho_A$ on
$H_{\mathrm{treat}}$ and on the transport current density
$j$ has been investigated. It has been shown that the dependences $\rho_A(H_{\mathrm{treat}})_{j=\operatorname{const}}$ exhibit a clearly pronounced tendency to saturation,
$\rho_{\mathrm{satur}}$, and the value of
$\rho_{\mathrm{satur}}$ increases with increasing
$j$. It has been found that the lower critical field
$H_{c1A}$ of the superconducting grains strongly depends on the transport current density.
Received: 14.09.2010
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