Trimer Formation and Metal-Insulator Transition in Orbital Degenerate Systems on a Triangular Lattice

TL;DR

This paper investigates the formation of orbital-ordered trimers and the associated metal-insulator transition in triangular lattice systems, revealing a new trimer state driven by Fermi surface nesting and electron interactions.

Contribution

It identifies a novel orbital-ordered trimer state in multiorbital Hubbard models, differing from previous orbital-ordered states, and explains its emergence in intermediate correlation regimes.

Findings

Discovery of a new orbital-ordered trimer phase.

Identification of Fermi surface nesting as the mechanism.

Comparison with experimental compounds like LiVX2 and NaVO2.

Abstract

As a prototypical self-organization in the system with orbital degeneracy, we theoretically investigate trimer formation on a triangular lattice, as observed in LiVO2. From the analysis of an effective spin-orbital coupled model in the strong correlation limit, we show that the previously-proposed orbital-ordered trimer state is not the lowest-energy state for a finite Hund's-rule coupling. Instead, exploring the ground state in a wide range of parameters for a multiorbital Hubbard model, we find an instability toward a different orbital-ordered trimer state in the intermediately correlated regime in the presence of trigonal crystal field. The trimer phase appears in the competing region among a paramagnetic metal, band insulator, and Mott insulator. The underlying mechanism is nesting instability of the Fermi surface by a synergetic effect of Coulomb interactions and trigonal-field splitting. The results are compared with experiments in triangularlattice compounds, LiVX2 (X=O, S, Se) and NaVO2.