Bonds, bands, charge transfer excitations and superconductivity of YBa₂Cu₃O_7-δ

The different energy dispersion and Fermi surfaces arising from the bands which dominate the electronic band structure near E_F in YBa₂Cu₃O_7-δ are presented and related to their orbital charge density distributions for the Cu2 plane and Cu1 chain arrangements thereby relating their physical and chemical descriptions. Magnetic isolation of the Y ions is found to explain the existence of high T_c superconductivity of the RBa₂Cu₃O_7-δ compounds (where R = magnetic heavy lanthanides). A conventional phonon mechanism is found to be inadequate for obtaining high T_c. Charge transfer excitations ("excitons") of occupied Cu1-O dpπ bonding orbitals into their empty Cu1-O dpσ antibonding orbital partners result in poorly screened Cu³⁺-Cu⁴⁺-like charge fluctuations which induce attractive interactions (-U centers) both in the chains and to the 2D (Cu2) bands-thereby promoting the high T_c via exchange of these "excitons".