Origin of the insulating and superconducting phases in molecular solid $\kappa$-(BEDT-TTF)$_2$Cu$_2$(CN)$_3$

TL;DR

This study uses an advanced computational method to clarify the electronic origins of insulating and superconducting phases in a complex organic solid, aligning theoretical predictions with experimental results and offering a new model for further many-body physics research.

Contribution

The paper introduces the DFT+GOU method to accurately determine the electronic structure of ${ ext{kappa}}$-(BEDT-TTF)$_2$Cu$_2$(CN)$_3$, resolving previous discrepancies and reproducing key experimental phenomena.

Findings

Corrected the energy gap between molecular orbitals explains insulating behavior.

Reproduced the superconducting dome from flat band states at the Fermi level.

Constructed a low-energy lattice model for many-body physics applications.

Abstract

Recent studies of organic molecular solids are highlighted by their complex phase diagram and light-induced phenomena, such as Mott insulator, spin liquid phase, and superconductivity. However, a discrepancy between experimental observation and first-principle calculation on the $\kappa$-(BEDT-TTF)$_2$X family inhibits understanding their properties. Here, we revisit the electronic structure of $\kappa$-(BEDT-TTF)$_2$Cu$_2$(CN)$_3$ with the recently developed DFT+GOU method to correct the energy level of molecular orbital states in the molecular solid. Our work reveals that the insulating electronic structure of $\kappa$-(BEDT-TTF)$_2$Cu$_2$(CN)$_3$ originates from the energy gap between the highest occupied and the lowest unoccupied molecular orbital states of the BEDT-TTF dimers, that are the periodic unit of the molecular solid. We verify that our calculation result provides consistent band gap, optical conductivity, and evolution of the metal-insulator transition as a function of pressure with experimental observations. Especially, the superconducting dome of $\kappa$-(BEDT-TTF)$_2$Cu$_2$(CN)$_3$, which originates from the flat band state at the Fermi level, is reproduced. Additionally, we constructed a new low-energy lattice model based on the ability of electronic structure data that can be used to address many-body physics, such as quantum spin liquid and double-holon dynamics. Our provides a deeper understanding of the complex phase diagram and various light-induced phenomena in the $\kappa$-(BEDT-TTF)$_2$X family and the other complex organic molecular solids.