Charge Orderings and Phase Separations in Itinerant Fermion Systems at Half Filling

Abstract

We analyse the ground state phase diagrams of the charge orderings in narrow band materials using two e ective models: (1) the spinless fermion model (t−W ) with repulsive intersite interaction (Wij > 0) and (2) the molecular crystal model with the coupling of electrons to intramolecular (crystal eld) vibrations. We present results for the case of half lled bands for d = 2 square lattice. The calculations are performed within the (broken symmetry) Hartree-Fock approximation. The study takes into consideration the e ects of frustrating next-nearest-neighbour hopping (t2) on the charge ordered states in these systems. We focus on the two cases: (i) homogeneous phases and phase separations involving checkerboard charge ordering with the nesting vector Q = (π, π) only and (ii) homogeneous phases and phase separations involving two types of charge ordering: (a) checkerboard charge ordering with the nesting vector Q = (π, π), and (b) collinear (CL) charge ordering with Q = (0, π) or Q = (π, 0).