Os efeitos da desordem química na magnetocondutividade e flutuações termodinâmicas do supercondutor YBa2Cu3O7-δ

Abstract

At this dissertation we have studied by magnetoresistivity measurements the influence of the low Sr and Ca chemical doping on the thermodynamic fluctuation regimes (TFR) of [  (T,H)] -1 plots in a Y 0.99 Ca 0.01 Ba 2 Cu 3 O 7- and a YBa 1.98 Sr 0.02 Cu 3 O single crystals. The samples were prepared by self-flux growth process. The low-current and low-frequency magnetoresistivity, ρ(T,H) measurements were performed with a PPMS while DC H  50kOe were simultaneously applied perpendicular to the ab plane of samples and to their measurement current density. The T of samples decreases smoothly as compared to it in a no doped sample. Their X-ray diffraction showed only the pure orthorhombic phase. The dρ(T,H)/dT data of samples highlights that their ρ(T,H) superconducting transition is a signature of a granular superconductor. Accordingly to this scenario at the T C the superconductivity sets in within the grains and at the T c0 (H) [T C (H)

> T (H)] a long range superconducting order takes place over the whole sample. The superconducting granularity contribution to the TFR behavior of samples is evaluated to paraconducting [T > T c0 (H)] and paracoherent [T c (H) > T > T c0 (H)] temperature regions of the [  (T,H)] data. The field dependence of the TFR at the paraconducting region as the temperature is lower from above is characterized by the occurrence of Gaussian, critical and supercritical regimes. The c axis coherence length,  (0) of the doped samples is estimated from the 3D Gaussian TFR. The critical TFR are consistent with the predictions of the dynamic of 3D XY university class and its variations. Otherwise the field dependence of the TFR in the paracoherent region is characterized by a power law represented by a critical exponent S c (H) ≈ 3 to H  2.5kOe. This result is an indicative that the coherent transition of the samples belongs to the 3D-XY universality class with relevant disorder. The analysis of TFR behavior on the [ 0 data suggests that the low Ca and Sr doping in the YBa 2 Cu 3 O structure does not supports a radical modification of the YBCO TFR dynamics to T > T 7- (H). By other hand the granular coherence transition of samples is probably induced by the field activation of point defect structure originated from segregate Sr and Ca atoms in the superconducting YBCO matrix. The Sr and Ca doping produces distinct effects on the  c c (0) of the YBCO superconductor.