Charge ordering and coexistence of charge fluctuations in quasi-two-dimensional organic conductors $\theta$-(BEDT-TTF)$_2X$

TL;DR

This paper proposes a theoretical explanation for the coexistence of different charge fluctuations in quasi-2D organic conductors near quantum criticality, highlighting two distinct mechanisms without phase competition.

Contribution

It introduces a scenario based on an extended Hubbard model with electron-phonon coupling that accounts for coexisting charge fluctuations from separate instabilities.

Findings

Coexisting fluctuations originate from Wigner crystallization and Fermi surface nesting.

The mechanism explains experimental temperature dependence of charge fluctuations.

No phase competition or inhomogeneity involved in coexistence.

Abstract

We present a scenario for the peculiar coexistence of charge fluctuations observed in quasi-2D 1/4-filled organic conductors $\theta$-(BEDT-TTF)$_2X$ in the quantum critical regime where the charge ordering is suppressed down to zero temperature. The scenario is explored in the extended Hubbard model including electron-phonon couplings on an anisotropic triangular lattice. We find that the coexisting fluctuations emerge from two different instabilities, the ``Wigner crystallization on lattice'' driven by the off-site Coulomb repulsion and the charge-density-wave formation due to the nesting of the Fermi surface, not from phase competition or real-space inhomogeneity. This mechanism explains the contrastive temperature dependence of two fluctuations in experiments.