Charge-transfer excitations in the cuprate superconductors

First-principles calculations of the dielectric matrix of the key structural element of the copper-oxide super-conductors show a weakly damped resonance at 3.5 eV which contains most of the oscillator strength in charge-transfer excitations. Such excitations have been proposed as a mediating boson for superconductive pairing. Although this mode is a well-developed and robust excitation, we find its energy to be too large to give high superconducting transition temperatures.