Sandwich Excitons as the Mechanism of High-T C Superconductivity
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SANDWICH EXCITONS AS THE MECHANISM OF HIGH-Tc SUPERCONDUCTIVITY BRENT A. RICHERT,* EWALD SCHACHINGER,** AND ROLAND E. ALLEN*** *Department gf Physics, U.S. Air Force Academy, Colorado,,Springs, CO 80840 **Institut fur Theoretische Physik, Technische Universitat Graz, Austria ***Center for Theoretical Physics, Texas A&M University, College Station, TX 77843 ABSTRACT One potential objection to virtual "sandwich excitons" as a pairing mechanism is that the transition densities a(X)h(x) may not be large enough to overcome the inherent disadvantage of a large energy denominator NO" In model calculations, we find that some of the transition densities and matrix elements can be large even if the dielectric constant is sizeable for, e.g., LaO excitons in La 2 xSr xCuO4'
If one views high-Tc superconductors as "sandwich structures", in which metallic CuO2 or BiO2 planes are always adjacent to insulating LaO, BaO, etc. layers, then it is natural to conjecture that the bosons which mediate the pairing interaction are "sandwich excitons"--i.e., virtual 0 y to metal d excitons in the insulating layers [1]. This is, of course, a specific version of the original excitonic mechanism of Little [2], Ginzburg [3], and Allender, Bray, and Bardeen [4], in which the charge carriers and excitons are spatially separated. There are several potential objections to this simplistic mechanism, which is depicted in Fig. l.: (i) The attractive carrier-exciton-carrier interaction is nonretarded, and may thus be dominated by the repulsive Coulomb interaction. (ii) The existence of the effect may imply that e(q",O)
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