Insulator-metal transition on the triangular lattice

TL;DR

This paper develops a theoretical framework for insulator-metal transitions on the triangular lattice, emphasizing the role of spin liquids, excitonic condensates, and superconductivity in organic compounds.

Contribution

It introduces a novel theory connecting Z_2 spin liquids, excitonic condensates, and superconductivity in triangular lattice Mott insulators.

Findings

Proposes excitonic condensate formation near the transition.

Explains low-temperature anomalies in organic compounds.

Describes routes to metallic states with Fermi surfaces.

Abstract

Mott insulators with a half-filled band of electrons on the triangular lattice have been recently studied in a variety of organic compounds. All of these compounds undergo transitions to metallic/superconducting states under moderate hydrostatic pressure. We describe the Mott insulator using its hypothetical proximity to a Z_2 spin liquid of bosonic spinons. This spin liquid has quantum phase transitions to descendant confining states with Neel or valence bond solid order, and the insulator can be on either side of one of these transitions. We present a theory of fermionic charged excitations in these states, and describe the route to metallic states with Fermi surfaces. We argue that an excitonic condensate can form near this insulator-metal transition, due to the formation of charge neutral pairs of charge +e and charge -e fermions. This condensate breaks the lattice space group symmetry, and we propose its onset as an explanation of a low temperature anomaly in kappa-(ET)2Cu2(CN)3. We also describe the separate BCS instability of the metallic states to the pairing of like-charge fermions and the onset of superconductivity.