ELECTRONIC PROPERTIES OF STRONGLY CORRELATED SYSTEMS

Abstract

The observation that the energy scale of the magnetic excitations determined by the Heisenberg coupling constant ( J ≈ 0.1eV ) is much smaller than the charge excitation energies (≳ 2eV ) places the stoichiomatic Cu-oxides with formal valence Cu 2+ in the class of Mott insulators. Holes introduced into the CuO 2 layers can therefore be described by an effective Hamiltonian which contains a hopping term for holes between nearest neighbor CuO 4-squares (matrix element, t ) in addition to the Heisenberg term1). This effective Hamiltonian is restricted to the Hilbert subspace with one or less electrons in the Wannier orbital on each CuO 4 square. The Wannier orbital is made up from the [Formula: see text] Cu-orbital and a combination of the 2p O-orbitals with the same symmetry. The hybridization energy is maximized for a hole by forming a spin singlet combination of these orbitals so that the form of the effective Hamiltonian does not differ in form2) from that of a single band Hubbard model in the strongly correlated limit. The inclusion of O-O hopping does not change this conclusion3). Estimates of the parameter t , give a value t ≈ 0.5eV so that the ratio J/t ≪ l .