In the present study, we investigate the influence of the hafnium (Hf) impurities on the magnetoresistance of YBa 2 Cu 3 O 7−δ ceramic samples in the temperature interval of the transition to the superconducting state in constant magnetic field up to 12 T. The cause of the appearance of low-temperature "tails" (paracoherent transitions) on the resistive transitions, corresponding to different phase regimes of the vortex matter state is discussed. At temperatures higher than the critical temperature (T > T c ), the temperature dependence of the excess paraconductivity can be described within the Aslamazov-Larkin theoretical model of the fluctuation conductivity for layered superconductors.
In this work, we studied the fixation of magnetic vortices (pinning effect) on “inelastic” and “elastic” twins, whose wedge-shaped peaks in the direction of twinning do not reach the crystal surface. The effectiveness of the “elastic” twins effect as pinning centers was studied by measuring of the critical transport current Jc in single crystals YBa2Cu3O7– x. The presented experimental data indicate that the “elastic” twins boundaries are effective pinning centers of the magnetic flux lines in single crystals of the compound under study. The main pinning centers of the vortices are dislocation clusters forming at the vertices of the “elastic” twins. The dislocation lines are located in the twin plane and, if the orientation of the magnetic vortices and dislocation lines coincide, the energy of the vortices decreases, the pinning force appears, fixing their location, which ultimately leads to an increase in the critical density of the transport electric current.
We study the temperature dependence of the in-plane magneto-resistance ρab(T) in the untwined YBa2Cu3O7−δ single crystals after irradiation by fast electrons (energy 0.5–2.5 MeV, dose 1018 cm−2), also with a small oxygen hypostoichiometry at different angles between the external magnetic field 15 kOe and the ab-planes α. We found that at high temperatures in the pseudogap region external magnetic field does not affect the ρab(T), but it broadens transitional region Tc – Tc0 from 0.3 K at zero field and α = 0 to approximately 6 K at α = 90° in the field. In the case of an unirradiated sample, ρab(T) display a 3D to 2D dimensional crossover when temperature decreases from Tc to Tc0 and scaling near the Tc0 which we relate to the flux-flow and vortex-lattice melting. The reasons for the appearance of low-temperature “tails” (paracoherent transitions) on resistive transitions corresponding to different phase regimes of the vortex matter are discussed.
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