Mapping phonon modes from reduced-dimensional to bulk systems

Abstract

A lattice-dynamics-based algorithm for mapping the true phonon modes of a film, defined by a two-dimensional (2D) Brillouin zone, to the modes of the corresponding bulk material, defined by a three-dimensional (3D) Brillouin zone, is proposed. The mapping allows for an assessment of the common assumption that film phonon modes have the same harmonic properties as the corresponding bulk phonon modes. The algorithm is based on normal mode decomposition and is inspired by the observation that the atomic trajectories generated by the 2D eigenvectors lead to standing-wave-like behaviors in the cross-plane direction. This behavior enables an unfolding scheme that does not require an assumption of cross-plane periodicity, as used in previous approaches for mapping phonon modes. The algorithm 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 frequencies. 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.