Tight-binding parameters and exchange integrals ofBa2Cu3O4Cl2

Abstract

Band-structure calculations of ${\mathrm{Ba}}_{2}{\mathrm{Cu}}_{3}{\mathrm{O}}_{4}{\mathrm{Cl}}_{2}$ within the local-density approximation (LDA) are presented. The investigated compound is similar to the antiferromagnetic parent compounds of the cuprate superconductors but contains additional ${\mathrm{Cu}}_{B}$ atoms in the Cu-O planes. Within the LDA, metallic behavior is found with two bands crossing the Fermi surface. These bands are built mainly from Cu ${3d}_{{x}^{2}\ensuremath{-}{y}^{2}}$ and O ${2p}_{x,y}$ orbitals, and a corresponding tight-binding (TB) model for these bands has been parameterized. All orbitals can be subdivided in two sets corresponding to the $A$ and $B$ subsystems, respectively, the coupling between which is found to be small. To describe the experimentally observed antiferromagnetic insulating state, we propose an extended Hubbard model using the parameters derived from the TB fit and local correlation terms characteristic for cuprates. Using this parameter set we calculate the exchange integrals for the ${\mathrm{Cu}\mathrm{}}_{3}{\mathrm{O}}_{4}$ plane, the results being in quite reasonable agreement with the experimental values for the isostructural compound ${\mathrm{Sr}}_{2}{\mathrm{Cu}}_{3}{\mathrm{O}}_{4}{\mathrm{Cl}}_{2}$.