Nanoscale Lattice Dynamics in Hexagonal Boron Nitride Moiré Superlattices*

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.