Symmetry breakdown in franckeite: spontaneous strain, rippling and interlayer moir\'e

TL;DR

This study reveals that franckeite's layered structure spontaneously ripples and becomes anisotropic due to incommensurate lattice interactions, affecting its electrical, vibrational, and optical properties.

Contribution

It uncovers the mechanism of symmetry breakdown in franckeite caused by interlayer lattice incommensurability and van der Waals interaction modulation.

Findings

Spontaneous rippling induces structural anisotropy.

Inhomogeneous in-plane strain correlates with rippling.

Anisotropic electrical, vibrational, and optical responses observed.

Abstract

Franckeite is a naturally occurring layered mineral with a structure composed of alternating stacks of SnS2-like and PbS-like layers. Although this superlattice is composed of a sequence of isotropic two-dimensional layers, it exhibits a spontaneous rippling that makes the material structurally anisotropic. We demonstrate that this rippling comes hand in hand with an inhomogeneous in-plane strain profile and anisotropic electrical, vibrational and optical properties. We argue that this symmetry breakdown results from a spatial modulation of the van der Waals interaction between layers due to the SnS2-like and PbS-like lattices incommensurability.