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 YBa
₂
Cu
_2.985
Fe
_0.015
O
$_{\mathrm {7-\delta}}$
and YBa
_1.75
Sr
_0.25
Cu
₃
O
$_{\mathrm {7-\delta }}$
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 YBa
₂
Cu
₃
O
$_{\mathrm {7-\delta }}$
superconductor granular character; however, its physical contribution to the
$\lambda ^{\mathrm {IN}}$
exponent is an open question.