CHARGE-FLUCTUATION EFFECT ON THE CRITICAL-TEMPERATURE OF LAYERED HIGH-TC SUPERCONDUCTORS

The value of the superconducting critical temperature (T(c)) of artificially layered superconductors made out of alternating Y-Ba-Cu-O and Pr-Ba-Cu-O layers is calculated as a function of both the number of layers of pure Y-Ba-Cu-O within a unit cell of the superlattice and the thickness of the insulating (Pr-Ba-Cu-O) layer. The calculation is performed within a model of electrostatically coupled two-dimensional (2D) arrays of ultrasmall Josephson junctions. The underlying mechanism is assumed to be the depression of the Beresinskii-Kosterlitz-Thouless transition temperature (T(BKT)) by quantum phase fluctuations due to charging effects. The T(c) value of the entire structure can then be tuned by varying the charging energy that depends on the average neighborhood of a typical site in Cu-O planes of Y-Ba-Cu-O layers. Recent experiments on such structures are described very accurately by the model. Furthermore, the ratio (T(c)(n)-T(c)(1))/(T(c)(2)-T(c)(1)), where T(c)(n) is the critical temperature of a thin film made up of n Y-Ba-Cu-O unit cells, is found to be independent of the fitting parameters. This prediction is well confirmed by available experimental data. Furthermore, the model also applies for the series of Bi- and Tl-based cuprates of general formulas Bi2Sr2Ca(n-l)Cu(n)O(y) and Tl2(1)Ba2Ca(n-l)Cu(n)O(y) containing n Cu-O planes per unit cell, for which the observed values of the ratio p = (T(c)(3)-T(c)(1))/(T(c)(2)-T(c)(1)) agree within 10% with the predicted ones.