In-plane paraconductivity and fluctuation-induced magnetoconductivity in biperiodic layered superconductors: Application to YBa2Cu3O7−δ

Abstract

The existing mean-field-like calculations of the different direct and indirect order parameter fluctuation (OPF) contributions to the in-plane paraconductivity Δσab and fluctuation-induced magnetoconductivity Δσ̃̃̃ab are extended here to layered superconductors with two different interlayer distances and different strengths of the tunneling couplings between adjacent layers (the so-called bilayered, or biperiodic layered, superconductors). The calculations are performed for magnetic fields H in the weak limit, applied perpendicular to the superconducting layers, and at temperatures near but above the H=0 mean-field transition temperature, Tc0. We obtain final explicit expressions and find that the effects of the layer biperiodicity may be summarized through an effective number Ne of independent fluctuating superconducting layers per unit cell length, already encountered also in our recent calculations of the fluctuation-induced diamagnetism Δχab in biperiodic layered superconductors. Our study includes some limiting cases of the indirect contributions associated with the density of states (DOS) fluctuations, which have been recently proposed for Δσab and Δσ̃̃̃ab for single periodic layered superconductors. As an application, we use then our theoretical results to analyze the paraconductivity and the fluctuation-induced magnetoconductivity recently measured in the a direction (nonaffected by the presence of CuO chains) of untwinned YBa2Cu3O7−δ crystals. This analysis shows that the approaches based on the conventional Lawrence-Doniach (i.e., single layered) model cannot explain simultaneously and quantitatively the intrinsic Δσa, Δσ̃̃̃a, and Δχab in YBa2Cu3O7−δ crystals, even when the DOS contributions are considered. In contrast, when the double periodicity of this layered superconductor is taken into account, it is possible to explain consistently and at a quantitative level all such experimental data in the reduced temperature region above Tc0 bounded by, approximately, 2×10−2 and 10−1, which is expected to correspond to the mean-field region without high temperature and nonlocal effects. In the resulting biperiodic scenario, the indirect (i.e., Maki-Thompson and DOS) contributions to the in-plane Δσa and Δσ̃̃̃a are negligible, confirming our earlier findings which suggested unconventional, pair-breaking, wave pairing. Moreover, the direct OPF effects have a dimensionality (two-dimensional–three-dimensional) crossover in the mean-field region. Our results strongly suggest also that to understand at a quantitative level the OPF effects on any in-plane or bulk physical observable in layered superconductors with various superconducting layers per unit cell length, it is crucial to take into account the influence of such a multiperiodicity. © 1996 The American Physical Society.

Source-reported materials — not catalogue approval

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