Charge Order with Structural Distortion in Organic Conductors: Comparison between \theta-(ET)2RbZn(SCN)4 and \alpha-(ET)2I3

TL;DR

This paper theoretically investigates charge ordering with structural distortion in two organic conductors, highlighting the importance of lattice effects and band structure in stabilizing charge order.

Contribution

It demonstrates the crucial role of lattice distortions and band structure differences in stabilizing charge order in heta-(ET)2RbZn(SCN)4 and eta-(ET)2I3 using a Hartree-Fock extended Hubbard model.

Findings

Lattice distortion stabilizes horizontal charge order in both compounds.

Lattice effects are essential for charge order in heta-(ET)2RbZn(SCN)4.

Band structure influences charge order in eta-(ET)2I3.

Abstract

Charge ordering with structural distortion in quasi-two-dimensional organic conductors \theta-(ET)2RbZn(SCN)4 (ET=BEDT-TTF) and \alpha-(ET)2I3 is investigated theoretically. By using the Hartree-Fock approximation for an extended Hubbard model which includes both on-site and intersite Coulomb interactions together with Peierls-type electron-lattice couplings, we examine the role of lattice degrees of freedom on charge order. It is found that the experimentally observed, horizontal charge order is stabilized by lattice distortion in both compounds. In particular, the lattice effect is crucial to the realization of the charge order in \theta-(ET)2RbZn(SCN)4, while the peculiar band structure whose symmetry is lower than that of \theta-(ET)2RbZn(SCN)4 in the metallic phase is also an important factor in \alpha-(ET)2I3 together with the lattice distortion. For \alpha-(ET)2I3, we obtain a phase transition from a charge-disproportionated metallic phase to the horizontal charge order with lattice modulations, which is consistent with the latest X-ray experimental result.