Electronic structure, charge transfer excitations, and high-temperature superconducting oxides (invited)

We present high-precision results on the electronic band structure and properties of YBa₂Cu₃O_7-δ, YB ₂Cu₃O₆, GdBa₂Cu₃O _7-δ, and La_2-xM_xCuO₄ as obtained from highly precise state-of-the-art local density calculations. The results obtained demonstrate the close relation of the band structure to the structural arrangements of the constituent atoms and provide an integrated chemical and physical picture of the interactions and their possible relation to superconductivity. The ionic character of the Y is proven by similar detailed highly precise local density calculations for high T_C GdBa ₂Cu₃O₇, and explains the coexistence of magnetism and superconductivity in the high T_C rare-earth superconductors. Surprising features are the low density of states (DOS) at E_F, especially for δ≥0.1 which is lower per Cu atom than that in La_2-xSr_xCuO₄ - in agreement with experiment and a relatively large magnetic Stoner factor. Strong indications are demonstrated for the inadequacy of a conventional phonon mechanism for obtaining the higher T_C. Charge transfer excitations of occupied Cu1-O dpπ antibonding orbitals into their empty Cu1-O dpσ antibonding orbital partners, result in poorly screened "Cu^{3 +}-Cu⁴⁺" -like charge fluctuations. These charge transfer excitations (excitons) thus lead to strong polarization effects in these poorly screened (highly ionic) materials and induce attractive interactions among the 2D electrons. Thus, these interactions via exchange of excitons enhance the electron pairing and serve to enhance the T_C proposed for the quasi-2D superconductors.