Fluctuation specific heat in multilayered superconductors: Bilayered Gaussian-Ginzburg-Landau scenario for the thermal fluctuations of Cooper pairs around T-c in YBa2Cu3O7-delta single crystals

摘要

The effects around the superconducting transition of thermal fluctuations of Cooper pairs on the heat capacity in zero applied magnetic field (H=0) are explicitly calculated in bilayered superconductors, with two superconducting layers and tunneling couplings per layer periodicity length. The calculations are performed on the grounds of a generalization to multilayered superconductors of the Lawrence-Doniach Ginzburg-Landau functional, and assuming Gaussian fluctuations. In addition to the fluctuation heat capacity c(fl), we also obtain various useful relationships between c(fl) and other fluctuation-induced observables experimentally accessible in multilayered copper oxide superconductors. It is then shown that if the effects of the multilaminarity are taken into account, the mean-field-like Gaussian-Ginzburg-Landau approach may explain simultaneously and at a quantitative level the available experimental data, both the amplitude and the epsilon behavior, of the fluctuation specific heat, the in-plane paraconductivity Delta sigma(ab) and the fluctuation-induced diamagnetism Delta chi(ab) in YBa2Cu3O7-delta(Y-123) single crystals under zero or weak magnetic fields (H -->0), up to reduced temperatures of the order of epsilon=T-T-clT(c) similar to 10(-2). The corresponding coherence length amplitudes (at T = 0 K) are xi(ab)(0)= 1.1 nm and xi(c)(0) = 0.12 nm for the in-plane (ab) and transversal (c) directions, respectively. In contrast, the same data cannot be explained, in the same epsilon region, in terms of the 3DXY theory for full-critical fluctuations with a value of the dynamic critical exponent of z = 3/2, which corresponds to the same universality class as the superfluid-normal lambda transition of He-4 liquid, although these analyses do not exclude the applicability of such a scenario for epsilon less than or equal to 10(-2), as suggested by previous measurements of Delta sigma(ab) and Delta chi(ab) in the same Y-123 single crystals. However, another dynamic universality class, with z = 2, makes the 3DXY full-critical behavior compatible with the experimental data for 2 x 10(-2) less than or equal to epsilon less than or equal to 10(-1). [References: 54]