R. Menegotto Costa

Fluctuation conductivity and the dynamical universality class of the superconducting transition in the high-Tc cuprates

Systematic measurements of the fluctuation conductivity in several high-temperature superconducting cuprates show the occurrence of a genuine critical regime. The conductivity exponent is consistent with the predictions of the 3D-XY universality class and yields a dynamical critical exponent z = 1.5.

Splitting of the bulk resistive transition in high-Tc superconductors : evidence for unconventional pairing

Abstract We report on measurements of the resistive transition in several samples of polycrystalline YBa 2 Cu 3 O 7− y and Bi 2 Sr 2 Ca 2 Cu 3 O 10+ x , and single crystal Bi 2 Sr 2 CaCu 2 O 8+ x . The results are analyzed in terms of the temperature derivative of the resistivity, d ϱ /d T . In the ceramic samples, we systematically find that the transition proceeds as a two-stage process. Initially, superconductivity establishes in some disconnected regions of the homogeneous phase. Long range ordering is obtained at lower temperatures as a consequence of a paracoherent-coherent transition involving the granular array. A new, reproducible and general feature is also revealed in our results. The principal maximum of d ϱ /d T , which denotes the intragranular transition, shows a two-peak splitting. This characteristic, which is observed in all investigated samples strongly suggests the occurrence of a double bulk superconducting transition, which is in itself a signature of unconventional order parameter symmetry.

Critical and Gaussian conductivity fluctuations in YBa2Cu3O7−δ and Bi2Sr2CaCu2O8☆

Abstract Systematic conductivity fluctuation measurements are reported for polycrystalline YBa 2 Cu 3 O 7−° and single-crystal Bi 2 Sr 2 CaCu 2 O 8 . Gaussian and critical regimes are observed. For both systems, the critical exponents are consistent with s-wave pairing.

Pairing and coherence transitions in granular GdBa2Cu3O7−δ

Abstract We report on fluctuation magnetoconductivity measurements in a granular sample of GdBa 2 Cu 3 O 7− δ . The experiments show that the resistive transition is a two-stage process in this sample. The pairing transition occurs at a higher temperature, where superconductivity is established in the grains. The critical thermodynamics for this transition is that of the 3D–XY universality class, with dynamic exponent z =1.5. At a lower temperature occurs the coherence transition, where the fluctuating phases of individual grains become long-range ordered and a zero resistance state sets in.

Low-Field Fluctuation Magnetoconductivity in Bi2Sr2CaCu2O8 and YBa2Cu3O7: Gaussian, Critical and LLL Scalings

Systematic measurements of low-field magnetoconductivity in single crystals of Bi2Sr2CaCu2O8 and YBa2Cu3O7 in the vicinity of Tc reveal effects of thermal fluctuations. Gaussian, critical and lowest Landau level scalings are studied. Critical regimes with exponent λcr = 0.33 predicted by the full dynamic 3D-XY universality class are present in both systems. Closer to Tc, two other critical scaling regimes are evidenced in YBa2Cu3O7, with exponents \(\lambda _{cr}^{\left( 2 \right)}\) = 0.23 and \(\lambda _{cr}^{\left( 3 \right)}\) = 0.17. Data are condensed on H-T diagrams showing the regions of stability of the various fluctuation regimes in fields applied parallel or perpendicular to the Cu-O atomic planes.

Low field fluctuation magnetoconductivity in Bi2Sr2CaCu2O8: critical, Gaussian and LLL scalings

We report measurements of the conductivity transition of a Bi2Sr2CaCu2O8 (Bi-2212) single crystal. Several magnetic fields up to 200 Oe were applied either parallel or perpendicular to the measuring current. We derive H-T phase diagrams showing the regions of dominance of 3D-XY, Gaussian and LLL fluctuation regimes above the critical temperature.