Correlation Effects on Charge Order and Zero-Gap State in the Organic Conductor \alpha-(BEDT-TTF)2I3

TL;DR

This study uses a theoretical extended Hubbard model and variational Monte Carlo methods to explore how electron correlations influence charge order and the zero-gap state in the organic conductor -(BEDT-TTF)2I3, revealing a pressure-induced phase transition.

Contribution

It provides a detailed theoretical analysis of the interplay between electron correlation, charge order, and zero-gap states under pressure in -(BEDT-TTF)2I3, highlighting a first-order transition and contrasting spin and charge behaviors.

Findings

Presence of a nonmagnetic horizontal-stripe charge order.

First-order transition to a zero-gap state under uniaxial pressure.

Spin gap remains unaffected despite charge order suppression.

Abstract

The effects of electron correlation in the quasi-two-dimensional organic conductor \alpha-(BEDT-TTF)2I3 are investigated theoretically by using an extended Hubbard model with on-site and nearest-neighbor Coulomb interactions. A variational Monte Carlo method is applied to study its ground-state properties. We show that there appears a nonmagnetic horizontal-stripe charge order in which nearest-neighbor correlation functions indicate a tendency toward a spin-singlet formation on the bonds with large transfer integrals along the charge-rich stripe. Under uniaxial pressure, a first-order transition from the nonmagnetic charge order to a zero-gap state occurs. Our results on a spin correlation length in the charge-ordered state suggest that a spin gap is almost unaffected by the uniaxial pressure in spite of the suppression of the charge disproportionation. The relevance of these contrasting behaviors in spin and charge degrees of freedom to recent experimental observations is discussed.