Microscopic model for high-Tcoxide superconductors

We report results of a BCS-pairing theory of high-$T_c$ superconductors based on a model using interlayer charge-transfer excitation as the intermediate boson. We apply the model specifically to ${\mathrm{YBa}}_2$ ${\mathrm{Cu}}_3$ ${\mathrm{O}}_7$. The relevant electronic band structure is calculated analytically using the tight-binding approximation, which is in agreement with numerical results. A specific interlayer charge-transfer excitation is identified. The Eliashberg equations are solved in the Kirzhnits-Maksimov-Khomskii approximation to obtain $T_c$ and the gap Δ. We find 2Δ/${\mathrm{KT}}_c$∼10–14, and $T_c$ increasing exponentially with a decrease in the Cu(2)-O(4) bond length. These results are in general agreement with some recent experiments.