Mott Transition and Phase Diagram of $\kappa$-(BEDT-TTF)2Cu(NCS)2 Studied by Two-Dimensional Model Derived from Ab initio Method

TL;DR

This study derives an ab initio two-dimensional model for $$-(BEDT-TTF)2Cu(NCS)2 and uses variational Monte Carlo to map its phase diagram, revealing the effects of off-site interactions on Mott transition and charge fluctuations.

Contribution

It introduces a first-principles derived low-energy model and explores the impact of off-site Coulomb interactions on the phase diagram of the organic compound.

Findings

Parameters are close to the first-order Mott transition, consistent with experiments.

Off-site Coulomb and exchange interactions stabilize metallic phase and enhance charge fluctuations.

Arc-like Fermi surface structures are observed near charge fluctuation regions.

Abstract

We present an ab initio analysis for the ground-state properties of a correlated organic compound $\kappa$-(BEDT-TTF)2Cu(NCS)2. First, we derive an effective two-dimensional low-energy model from first principles, having short-ranged transfers and short-ranged Coulomb and exchange interactions. Then, we perform many-variable variational Monte Carlo calculations for this model and draw a ground-state phase diagram as functions of scaling parameters for the onsite and off-site interactions. The phase diagram consists of three phases; a paramagnetic metallic phase, an antiferromagnetic (Mott) insulating phase, and a charge-ordered insulating phase. In the phase diagram, the parameters for the real compound are close to the first-order Mott transition, being consistent with experiments. We show that the off-site Coulomb and exchange interactions affect the phase boundary; (i) they appreciably stabilize the metallic state against the Mott insulating phase and (ii) enhance charge fluctuations in a wide parameter region in the metallic phase. We observe arc-like structure in Fermi surface around the region where the charge fluctuations are enhanced. Possible relevance of the charge fluctuations to the experimentally observed dielectric anomaly in the $\kappa$-BEDT-TTF family compounds is also pointed out.