# Mapping Phonon Modes from Reduced-Dimensional to Bulk Systems

## TL;DR

This paper introduces an algorithm to map phonon modes from 2D film systems to their 3D bulk counterparts, analyzing its effectiveness across different materials and film thicknesses.

## Contribution

The paper presents a novel normal mode decomposition algorithm for mapping phonon modes from reduced-dimensional films to bulk systems, accounting for anisotropy and thickness effects.

## Key findings

- The algorithm performs well for thick LJ argon films but less so for thinner ones.
- For graphene, the mapping remains accurate across all film thicknesses due to inherent anisotropy.
- Phonon density of states shifts with film thickness, affecting mode correspondence.

## Abstract

An algorithm for mapping the true phonon modes of a film, which are defined by a two-dimensional (2D) Brillouin zone, to the modes of the corresponding bulk material, which are defined by a three-dimensional (3D) Brillouin zone, is proposed. The algorithm is based on normal mode decomposition and is inspired by the observation that the atomic motions generated by the 2D eigenvectors lead to standing-wave-like behaviors in the cross-plane direction. It is applied to films between two and ten unit cells thick built from Lennard-Jones (LJ) argon, whose bulk is isotropic, and graphene, whose bulk (graphite) is anisotropic. For LJ argon, the density of states deviates from that of the bulk as the film gets thinner due to phonon frequencies that shift to lower values. This shift is a result of transverse branch splitting due to the film's anisotropy and the emergence of a quadratic acoustic branch. As such, while the mapping algorithm works well for the thicker LJ argon films, it does not perform as well for the thinner films as there is a weaker correspondence between the 2D and 3D modes. For graphene, the density of states of even the thinnest films closely matches that of graphite due to the inherent anisotropy, except for a small shift at low frequency. As a result, the mapping algorithm works well for all thicknesses of the graphene films, indicating a strong correspondence between the 2D and 3D modes.