Electronic and magnetic properties of the ionic Hubbard model on the striped triangular lattice at 3 4 filling

Abstract

We report a detailed study of a model Hamiltonian which exhibits a rich interplay of geometrical spin frustration, strong electronic correlations, and charge ordering. The character of the insulating phase depends on the magnitude of the onsite energy Δ/|t| and on the sign of the hopping amplitude t. We find a Mott insulator for Δ □ U □ |t|; a charge-transfer insulator for U □ Δ □ |t|; and a correlated covalent insulator for U □ Δ □ |t|, with U the onsite Coulomb repulsion energy. The charge-transfer insulating state is investigated using a strong-coupling expansion. The frustration of the triangular lattice can lead to antiferromagnetism or ferromagnetism depending on the sign of t. We identify the "ring" exchange process around a triangular plaquette which determines the sign of the magnetic interactions. Exact diagonalization calculations are performed on the model for a wide range of parameters and compared to the strong-coupling expansion. The regime U □ Δ |t| and t < 0 is relevant to Na0.5 CoO2. The calculated optical conductivity and the spectral density are discussed in the light of recent experiments on Na0.5 CoO2. © 2009 The American Physical Society.