Theory of rigid-plane phonon modes in layered crystals

TL;DR

This paper models the low-frequency rigid-plane phonon modes in layered crystals like graphene multilayers and boron nitride, revealing how their frequencies depend on layer number and interactions, with implications for Raman spectroscopy and friction studies.

Contribution

It provides a theoretical framework for understanding rigid-plane phonon modes in layered crystals, including fan diagrams and relations to bulk phonon dispersions, based on nearest-neighbor interactions.

Findings

Frequencies depend on layer number and are described by fan diagrams.

Interactions are mainly between nearest-neighbor layers, with screening of distant interactions.

Derived master curves connect phonon frequencies across different layer counts.

Abstract

The lattice dynamics of low-frequency rigid-plane modes in metallic (graphene multilayers, GML) and in insulating (hexagonal boron-nitride multilayers, BNML) layered crystals is investigated. The frequencies of shearing and compression (stretching) modes depend on the layer number ${\EuScript N}$ and are presented in the form of fan diagrams. The results for GML and BNML are very similar. In both cases only the interactions (van der Waals and Coulomb) between nearest-neighbor planes are effective, while the interactions between more distant planes are screened. A comparison with recent Raman scattering results on low-frequency shear modes in GML [Tan {\it et al.}, arXiv:1106.1146v1 (2011)] is made. Relations with the low-lying rigid-plane phonon dispersions in the bulk materials are established. Master curves which connect the fan diagram frequencies for any given ${\EuScript N}$ are derived. Static and dynamic thermal correlation functions for rigid-layer shear and compression modes are calculated. The results might be of use for the interpretation of friction force experiments on multilayer crystals.