Effective tight-binding models in optical moir\'e potentials

TL;DR

This paper investigates the bandstructure of twisted moiré potentials, demonstrating how tight-binding models can accurately describe the system's properties and relate to quantum phases and experimental conditions.

Contribution

It introduces a classification of bands by four types of tight-binding models and validates their accuracy in describing twisted moiré potentials.

Findings

Excellent agreement between continuous spectrum and tight-binding models

Identification of four distinct tight-binding band types

Mapping of energy gaps to strongly correlated insulating phases

Abstract

A twist between two systems offers the possibility to drastically change the underlying physical properties. To that end, we study the bandstructure of twisted moir\'e potentials in detail. At sets of commensurate twisting angles, the low energy single-particle spectrum of a twisted moir\'e potential will form into distinct bands and gaps. To a first approximation, energy bands can be qualitatively modelled by harmonic states, localised in different potential minima. The bands are intrinsically linked to the number of distinct minima and size of the moir\'e unit cell, with smaller cells producing larger gaps and vice versa. For shallower potential depths, degeneracies between harmonic states are lifted by virtue of anharmonic confinement and coupling between states. Depending on the exact geometry of potential minima, bands can then be classified by $4$ unique forms of tight-binding models. We find excellent agreement between the continuous spectrum and fitting to our tight-binding models, allowing for accurate tunnelling rates and onsite energies to be extracted. Our results are directly relevant to the bosonic, many-body problem, and thus provide further understanding on the relative stability of quantum phases both in theory and experiments. In particular, the prominence of gaps can be mapped to strongly correlated insulating phases. Furthermore, tunnelling rates of different bands serve as thresholds on temperature in which a phase can be either a normal fluid or superfluid.