Abstract
Motivated by recent experimental suggestions of charge-order-driven ferroelectricity in organic charge-transfer salts, such as κ-(BEDT-TTF)2Cu[N(CN)2]Cl, we investigate magnetic and charge-ordered phases that emerge in an extended two-orbital Hubbard model on the anisotropic triangular lattice at 3/4 filling. This model takes into account the presence of two organic BEDT-TTF molecules, which form a dimer on each site of the lattice, and includes short-range intramolecular and intermolecular interactions and hoppings. By using variational wave functions and quantum Monte Carlo techniques, we find two polar states with charge disproportionation inside the dimer, hinting to ferroelectricity. These charge-ordered insulating phases are stabilized in the strongly correlated limit and their actual charge pattern is determined by the relative strength of intradimer to interdimer couplings. Our results suggest that ferroelectricity is not driven by magnetism, since these polar phases can be stabilized also without antiferromagnetic order and provide a possible microscopic explanation of the experimental observations. In addition, a conventional dimer-Mott state (with uniform density and antiferromagnetic order) and a nonpolar charge-ordered state (with charge-rich and charge-poor dimers forming a checkerboard pattern) can be stabilized in the strong-coupling regime. Finally, when electron–electron interactions are weak, metallic states appear, with either uniform charge distribution or a peculiar 12-site periodicity that generates honeycomb-like charge order.
Highlights
Orbital, charge, and spin degrees of freedom are intertwined in correlated electron systems and the search for unconventional quantum phases emerging from the interplay of these degrees of freedom is a very active field of research in condensed-matter physics
When magnetism is included in the variational wave functions, we find that it coexists with charge order
By using variational wave functions and quantum Monte Carlo techniques, we have investigated the ground-state phase diagram of an extended two-orbital Hubbard model at 3/4 filling on the anisotropic triangular lattice, which is relevant for the κ-(ET)2X family of organic charge-transfer salts
Summary
Charge, and spin degrees of freedom are intertwined in correlated electron systems and the search for unconventional quantum phases emerging from the interplay of these degrees of freedom is a very active field of research in condensed-matter physics. Because of the strong hybridization between ET molecules belonging to the same dimer, the gap between the bonding and anti-bonding orbitals is large; the former one is fully occupied, while the latter one is half filled, justifying a single-band picture This coarse-grained approach cannot explain the possible emergence of ferroelectricity (or multiferroicity) in these materials, which has been suggested to arise from a charge disproportionation within each dimer. When magnetism is included in the variational wave functions, we find that it coexists with charge order These results could explain the observed behavior in κ-(ET)2Cu[N(CN)2]Cl. On the contrary, magnetism is crucial to stabilize the uniform dimer-Mott insulator, which appears in a narrow region between the two polar phases.
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