Orbital Frustration and Emergent Flat Bands

TL;DR

This paper explores how orbital frustration in multiband systems with strong spin-orbit coupling can lead to the formation of narrow, flat electronic bands, offering new pathways to study correlated metallic phases beyond existing models.

Contribution

It introduces a mechanism where inter-orbital hopping dominates, creating flat bands in metals with strong spin-orbit coupling, expanding the understanding of flat band formation beyond traditional systems.

Findings

Narrow bandwidths emerge when inter-orbital hopping dominates

Berry phase effects influence optical properties in these systems

Flat bands are positioned near instabilities for correlated phases

Abstract

We expand the concept of frustration in Mott insulators and quantum spin liquids to metals with flat bands. We show that when inter-orbital hopping $t_2$ dominates over intra-orbital hopping $t_1$, in a multiband system with strong spin-orbit coupling $\lambda$, electronic states with a narrow bandwidth $W\sim t_2^2/\lambda$ are formed compared to a bandwidth of order $t_1$ for intra-orbital hopping. We demonstrate the evolution of the electronic structure, Berry phase distributions for time-reversal and inversion breaking cases, and their imprint on the optical absorption, in a tight binding model of $d$-orbital hopping on a honeycomb lattice. Going beyond quantum Hall effect and twisted bilayer graphene, we provide an alternative mechanism and a richer materials platform for achieving flat bands poised at the brink of instabilities toward novel correlated and fractionalized metallic phases.