# Phase Competition and Superconductivity in $\kappa$-(BEDT-TTF)$_2$X: Importance of Intermolecular Coulomb Interactions

## TL;DR

This study uses variational Monte Carlo simulations to explore how intermolecular Coulomb interactions influence phase competition and superconductivity in $ppa$-(BEDT-TTF)$_2$X, revealing the significance of charge order instabilities over Mott transitions.

## Contribution

It demonstrates the crucial role of intermolecular Coulomb interactions and intradimer charge degrees of freedom in stabilizing superconductivity in molecular conductors.

## Key findings

- Superconductivity is stabilized near charge order instabilities.
- Multiple competing phases including Mott insulator and charge-ordered states are identified.
- Superconducting pairing symmetry is an extended-$s$+$d_{x^2-y^2}$-wave type.

## Abstract

We theoretically study the competition among different electronic phases in molecular conductors $\kappa$-(BEDT-TTF)$_2$X. The ground-state properties of a 3/4-filled extended Hubbard model with the $\kappa$-type geometry are investigated by a variational Monte Carlo method. We find various competing phases: dimer-Mott insulator, polar charge-ordered insulator, 3-fold charge-ordered metal, and superconductivity, whose pairing symmetry is an "extended-$s$+$d_{x^2-y^2}$"-wave type. Our results show that the superconducting phase is stabilized not on the verge of the Mott metal-insulator transition but near charge order instabilities, clearly indicating the importance of the intradimer charge degree of freedom and the intermolecular Coulomb interactions, beyond the simple description of the half-filled Hubbard model.