Charge Ordered Insulator without Magnetic Order Studied by Correlator Projection Method

The Hubbard model with additional intersite interaction ‘V ’ ( the extended Hubbard model) is investigated by the correlator projection method (CPM). CPM is a newly developed numerical method which combines the equation-of-motion approach and the dynamical mean-field theory. Using this method, properties of the extended Hubbard Model at quarter filling are discussed with special emphasis on the metal-insulator transition induced by electron-electron correlations. The result shows a metal-insulator transition to a charge ordered insulator with antiferromagnetic order at low temperatures, but a charge ordered insulator without magnetic symmetry breaking at intermediate temperatures. Here, the magnetic order is found to be suppressed to low temperatures because of the smallness of the exchange coupling Jeff . The present results are in sharp contrast with the Hatree-Fock approximation whereas in agreement with the experimental results on quarter-filled materials with strong correlations, such as organic conductors.