Correlation between the Josephson coupling energy and the condensation energy in bilayer cuprate superconductors - art. no. 024523

We review some previous studies concerning the intrabilayer Josephson plasm ons and present ellipsometric data of the c-axis infrared response of almos t optimally doped Bi2Sr2CaCu2O8. The c-axis conductivity of this compound e xhibits the same kind of anomalies as that of underdoped YBa2Cu3O7-delta We analyze these anomalies in detail and show that they can be explained with in a model involving the intrabilayer Josephson effect and variations of th e electric field inside the unit cell. The Josephson coupling energies of d ifferent bilayer compounds obtained from the optical data are compared with the condensation energies and it is shown that there is a reasonable agree ment between the values of the two quantities, We argue that the Josephson coupling energy, as determined by the frequency of the intrabilayer Josephs on plasmon, represents a reasonable estimate of the change of the effective c-axis kinetic energy upon entering the superconducting state. It is furth er explained that this is not the case for the estimate based on the use of the simplest "tight-binding" sum rule. We discuss possible interpretations of the remarkable agreement between the Josephson coupling energies and th e condensation energies. The most plausible interpretation is that the inte rlayer tunneling of the Cooper pairs provides the dominant contribution to the condensation energy of the bilayer compounds: in other words that the c ondensation energy of these compounds can be accounted for by the interlaye r tunneling theory. We suggest an extension of this theory, which may also explain the high values of T-c in the single-layer compounds Tl2Ba2CuO6 and HgBa2CuO4, and we make several experimentally verifiable predictions.