A study of electrical magnetoconductivity paracoherent transition of granular YBCO doped superconductors: a possible evidence of an inverted XY transition

This communication reports an electrical magnetoconductivity fluctuation study in polycrystalline (granular) samples of YBa2Cu2.985Fe0.015O $_{\mathrm {7-\delta}}$ and YBa1.75Sr0.25Cu3O $_{\mathrm {7-\delta }}$ superconductors. The measurements were performed in magnetic fields $H \le1000$ Oe applied parallel to the measurement density current. The samples were prepared by a standard solid-state reaction technique. The results were analyzed in terms of the temperature derivative of the electrical resistivity, $d\rho $ /dT , and of the inverse of the logarithmic derivative of the electrical conductivity with respect to temperature, $\chi _{\sigma }^{-1}(T)$ . The results allowed the identification of power law divergences of the magnetoconductivity and show that the superconductivity transition in our samples is a two-stage process. The genuine superconductor state with zero resistance stabilizes at the coherence transition temperature, $T_{\mathrm {C0}}$ , in which a long-range-ordered state for the phase of the order parameter is established in the whole granular array. The approach to the zero-resistance state is characterized by the critical exponent $\lambda \sim ~3.5$ for our Sr doped sample and $\lambda \sim ~4$ for our Fe doped sample. It indicates that the universality class for the coherence transition for our samples is that of the 3-D XY model where non-trivial disorder is relevant. At a temperature called $T_{\mathrm {C}}^{\mathrm {IN}}$ ( $T_{\mathrm {C}}^{\mathrm {IN}} > T_{\mathrm {C0}}$ ), we observed another exponent, $\lambda ^{\mathrm {IN}} ~ \sim ~1.4$ , which suggests a crossover in the critical phenomenology dynamic nearby the coherence transition. The origin of this crossover may be related to a change in the universality class of the transition and the value of $\lambda ^{\mathrm {IN}}$ is consistent with a true asymptotic inverted XY critical regime. The chemical doping generally enhances the YBa2Cu3O $_{\mathrm {7-\delta }}$ superconductor granular character; however, its physical contribution to the $\lambda ^{\mathrm {IN}}$ exponent is an open question.