Fluctuation-induced diamagnetism in biperiodic layered superconductors in the weak magnetic field regime

Abstract

By using a generalization of the Lawrence-Doniach free-energy functional, we calculate in this work the fluctuation-induced diamagnetism above the superconducting transition, Δ χ, in a layered superconductor having two different interlayer distances, d 1 and d 2, in the superconducting layer periodicity length, s= d 1+ d 2, and two different Josephson-coupling strengths, γ 1 and γ 2, between adjacent superconducting layers. The calculations are performed in the weak magnetic field limit. We include in our treatment the possibility of an unconventional superconducting pairing by considering in each plane a superconducting wave function of g complex components. For the first time, Δ χ in a multiperiodic layered superconductor is obtained explicitly, as a function of the coherence length amplitudes, of g and of γ 1 and γ 2. Our results show that by introducing an effective number, N e, of independent fluctuating layers in the layer periodicity length, it is possible to express Δ χ in terms of the fluctuation-induced diamagnetism for one-single periodicity layered superconductor, with the same periodicity length s= d 1+ d 2. As an example of their interest, these theoretical results for the superconducting-order parameter fluctuations above T c are used to quantitatively analyze the existing experimental Δ χ( T) data in the mean-field0like region, measured in the weak magnetic field limit in single crystals of two different copper oxide systems having two Josephson coupled superconducting CuO 2 planes in s: The Y 1 Ba 2 Cu 3 O 7− δ and the Bi 2Sr 2Ca 1Cu 2O 8 compounds.