Ingredients for Generalized Models of $\kappa$-Phase Organic Charge-Transfer Salts: A Review

TL;DR

This review discusses the complex physics of $$-phase organic charge-transfer salts, emphasizing the need for generalized models beyond the simple Hubbard model to explain their diverse magnetic and electronic phenomena.

Contribution

It highlights recent experimental and theoretical findings that incorporate additional ingredients like spin-orbit coupling and disorder into models of these materials.

Findings

Identification of phenomena beyond the simple Hubbard model.

Relevance of additional ingredients like spin-orbit coupling.

Implications for other strongly correlated systems.

Abstract

The families of organic charge-transfer salts $\kappa$-(BEDT-TTF)$_2X$ and $\kappa$-(BETS)$_2X$ have proven to serve as a powerful playground for the investigation of the physics of frustrated Mott insulators. These materials have been ascribed model character, since dimerization of the organic molecules allows to map these materials onto a single band Hubbard model, in which the dimers reside on an anisotropic triangular lattice. By changing the inorganic unit $X$ or applying physical pressure, the correlation strength and anisotropy of the triangular lattice can be varied. This has lead to the discovery of a variety of exotic phenomena, including quantum spin liquid states, a plethora of long-range magnetic orders in proximity to a Mott metal-insulator transition, and unconventional superconductivity. While many of these phenomena can be described within this effective one-band Hubbard model on a triangular lattice, it became evident in recent years that this simplified description is insufficient to capture all observed magnetic and electronic properties. The ingredients for generalized models that are relevant include, but are not limited to, spin-orbit coupling, intra-dimer charge and spin degrees of freedom, electron-lattice coupling, as well as disorder effects. Here, we review selected theoretical and experimental discoveries that clearly demonstrate the relevance thereof. At the same time, we outline that these aspects are not only relevant to this class of organic charge-transfer salts, but are also receiving increasing attention in other classes of inorganic strongly correlated electron systems. This reinforces the model character that the $\kappa$-phase organic charge-transfer salts have for understanding and discovering novel phenomena in strongly correlated electron systems from a theoretical and experimental point of view.