Nanoscale lattice dynamics in hexagonal boron nitride moir\xe9 superlattices

Samuel L Moore ,B S Kim,S E Rossi,D N Basov,Prineha Narang ,Kenji Watanabe ,Abhay Pasupathy ,Kaiyuan Yao ,Yinming Shao ,Evan J Telford ,P James Schuck ,Cory Dean ,James Hone ,Christopher J Ciccarino ,Nathan Finney ,Dorri Halbertal ,Takashi Taniguchi ,Guangxin Ni ,Aaron Sternbach ,L J Mcgilly

doi:10.1038/s41467-021-26072-7

Abstract

Twisted two-dimensional van der Waals (vdW) heterostructures have unlocked a new means for manipulating the properties of quantum materials. The resulting mesoscopic moiré superlattices are accessible to a wide variety of scanning probes. To date, spatially-resolved techniques have prioritized electronic structure visualization, with lattice response experiments only in their infancy. Here, we therefore investigate lattice dynamics in twisted layers of hexagonal boron nitride (hBN), formed by a minute twist angle between two hBN monolayers assembled on a graphite substrate. Nano-infrared (nano-IR) spectroscopy reveals systematic variations of the in-plane optical phonon frequencies amongst the triangular domains and domain walls in the hBN moiré superlattices. Our first-principles calculations unveil a local and stacking-dependent interaction with the underlying graphite, prompting symmetry-breaking between the otherwise identical neighboring moiré domains of twisted hBN.

Highlights

Twisted two-dimensional van der Waals heterostructures have unlocked a new means for manipulating the properties of quantum materials
We report on the lattice dynamics in twisted layers of the prototypical van der Waals (vdW) polar insulator hexagonal boron nitride[17,18]
In a past study[12], the lattice dynamics of naturally stacked (AA′) hexagonal boron nitride (hBN) hexagonal moiré patterns demonstrated a small variation of the phonon linewidth within the 500 nm-sized domains

Summary

Introduction

Twisted two-dimensional van der Waals (vdW) heterostructures have unlocked a new means for manipulating the properties of quantum materials. Scanning probe measurements are being widely utilized to visualize the electronic structure and elementary excitations across the moiré landscapes[1,9,10]. Very recently, these efforts have extended to nanoscale studies of lattice dynamics[11,12]. Phonons serve as effective reporters of strain[15] and ferroelectricity[16] Pursuant to these goals, we report on the lattice dynamics in twisted layers of the prototypical vdW polar insulator hexagonal boron nitride (hBN)[17,18]. Our ab initio calculations implicate the underlying graphite substrate in the stark phonon contrast between the moiré domains, hinting at a role of atomic registry in the dielectric coupling between hBN and graphite

Methods

Results

Conclusion