Magnetic orderings and phase separations in a simple model of insulating systems

Authors:

• Konrad Jerzy Kapcia,
• Szymon Murawski,
• Waldemar Kłobus,
• Stanisław Robaszkiewicz

Abstract

A simple effective model for a description of magnetically ordered narrow-band insulators is studied. The Hamiltonian considered consists of the effective on-site interaction (U) and intersite magnetic exchange interactions (J^z, J^xy) between nearest-neighbours. The phase diagrams and properties of this model for arbitrary chemical potential μ and arbitrary electron density n have been determined within several approaches: (i) the variational method (which treats the on-site interaction term exactly and the intersite interactions within the mean-field approximation) for any J^z,J^xy≠0 (exact in the limit of infinite dimensions), (ii) the Monte Carlo simulations on a square lattice with periodic boundary conditions for J^xy=0, and (iii) other approximate methods (inter alia: random phase approximation and spin-wave approximation) as well as (iv) rigorous treatment to obtain results concerning the ground state phase diagrams (the two last also for J^z,J^xy≠0). The investigations of the general case show that, depending on the values of interaction parameters and electron concentration n, the system can exhibit not only homogeneous phases: (anti-)ferromagnetic (F_α, α=z,xy) and nonordered (NO), but also phase separated states (PS_α: F_α/NO). For a fixed n one finds the following phase transitions (both continuous and discontinuous ones) and their sequences, which can occur with increasing temperature: F_α→NO, PS_α→NO, PS_α→F_α→NO, PS_α→F_α→PS_α→NO. The system analysed exhibits also tricritical behaviour.