Ab initio Derivation of Low-Energy Model for $\kappa$-ET Type Organic Conductors

TL;DR

This paper presents the first ab initio derivation of low-energy Hubbard models for $ppa$-(ET)$_2X$ organic conductors, revealing stronger onsite interactions and longer-range Coulomb effects than previous estimates, prompting a reexamination of their physics.

Contribution

It introduces a novel ab initio downfolding method to derive realistic Hubbard models for organic conductors, highlighting significant differences from traditional approaches.

Findings

Onsite Coulomb interaction U is approximately 0.8 eV, much stronger than previous estimates.

Nearest neighbor transfer integral t is around 0.055-0.067 eV.

Reveals longer-ranged interactions and discrepancies from conventional extended Hckel results.

Abstract

We derive effective Hubbard-type Hamiltonians of $\kappa$-(ET)$_2X$, using an {\em ab initio} downfolding technique, for the first time for organic conductors. They contain dispersions of the highest occupied Wannier-type molecular orbitals with the nearest neighbor transfer $t$$\sim$0.067 eV for a metal $X$=Cu(NCS)$_2$ and 0.055 eV for a Mott insulator $X$=Cu$_2$(CN)$_3$, as well as screened Coulomb interactions. It shows unexpected differences from the conventional extended H\"uckel results, especially much stronger onsite interaction $U$$\sim$0.8 eV ($U/t$$\sim$12-15) than the H\"uckel estimates ($U/t$$\sim$7-8) as well as an appreciable longer-ranged interaction. Reexamination on physics of this family of materials is required from this realistic basis.