TÍTULO

MAGNETOTRANSPORT PROPERTIES IN HIGH TEMPERATURE SUPERCONDUCTORS

TIPO

Tesis doctoral (Efeitos de Flutuações Termodinâmicas nas Propriedades de Magnetotransporte de Supercondutores de Alta Temperatura Crítica)

AUTOR

DIRECCIÓN PARA CONTACTO

Instituto de Física,
Universidade Federal do Rio Grande do Sul
Porto Alegre, Brasil

PALAVRAS CHAVE

Superconductivity, Magnetoconductivity, Fluctuations, Hall Effect

ENTIDADES QUE FINANCIARON LA INVESTIGACIÓN

COLCIENCIAS y Conselho Nacional de Pesquisas (CNPq)

RESUMO

In this work an experimental study is presented on the magnetotransport properties of the granular superconductors RBa₂Cu₃O_7-d , (R=Gd, Dy) and Hg_0.82Re_0.18Ba₂Ca₂Cu₃O_8+d . Thin films of RBa₂Cu₃O_7-d , (R=Y, Gd, Ho) are also investigated. Three experimental techniques were utilized to investigate the magnetotransport properties: magnetoresistivity ac, Hall Effect and magnetic susceptibility dc. Emphasis is given to the study of thermal fluctuation effects on the magnetoconductivity and to the characterization of the Hall effect near the normal-superconductor transition. The major contribution of this investigation consists on the original results related with the coherence transition and the thermodynamic fluctuations in the Hall conductivity. In the measured polycrystalline superconductors, the resistive transition is a two-step process. When the temperature is decreased, one first observes the pairing transition. Close to the zero resistance state, the coherence transition occurs. In this transition, the phases of the order parameter of individual grains become long-range ordered. This behavior shows that in granular systems it is possible to observe separately the regimes dominated by fluctuations of the amplitude (pairing transition) and phase (coherence transition) of the order parameter. In the normal phase of all samples, low-dimensional Gaussian regimes are detected in the fluctuation conductivity well above T_c. Close to T_c a crossover occurs to a 3D-Gaussian regime, which becomes unstable in magnetic fields above 10 kOe. Still closer to T_c, the critical exponent predicted by the 3D-XY universality class is obtained. This regime is rounded-off by small applied fields. In one of the GdBa₂Cu₃O_7-d samples, a critical regime beyond 3D-XY, with exponent l =0.14, is observed for the first time and is interpreted as precursory to a weakly-first-order transition. In the paracoherent phase above the coherence transition, 3D-Gaussian and genuine critical fluctuations are evidenced. The critical exponent is the one expected from the 3D-XY model with correlated disorder and spin-glass-like dynamics. In high magnetic fields and in the epitaxially grown thin films, the coherence transition is not observed. The irreversibility line in the polycrystalline systems RBa₂Cu₃O_7-d , (R=Gd, Dy) indicates that the upper limit for the paracoherent phase is related to broken ergodicity effects resulting from the stabilization of closed loops of weakly coupled grains.

The Hall resistivity in the normal phase of ceramics DyBa₂Cu₃O_7-d and Hg_0.82Re_0.18Ba₂Ca₂Cu₃O_8+d , and in the YBa₂Cu₃O_7-d thin film is positive and inversely proportional to T. Near T_c, thermal fluctuation effects are observed in the Hall conductivity. The analysis of the fluctuations effects in the Hall conductivity is done by using the FET approach for the Aslamazov-Larkin theory. Below T_c, the Hall resistivity becomes negative, goes through a minimum and vanishes near the zero diagonal resistance temperature. In this regime, the Hall effect is dominated by fluctuations of the order parameter’s phase.