Mott transition and phase diagram of κ-(BEDT-TTF) ₂Cu(NCS) ₂ studied by two-dimensional model derived from ab initio method

We present an ab initio analysis for the ground-state properties of a correlated organic compound κ-(BEDT-TTF) ₂Cu(NCS) ₂. First, we derive an effective two-dimensional low-energy model from first principles, having shortranged 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 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 κ-BEDT-TTF family compounds is also pointed out.