A method for distinguishing between propagons, diffusions, and locons

• Published in: Journal of applied physics Vol. 120; no. 2
• Main Authors: Seyf, Hamid Reza, Henry, Asegun
• Format: Journal Article
• Language: English
• Published: Melville American Institute of Physics 14.07.2016
• Subjects: ABUNDANCE; Amorphous silicon; Applied physics; Atomic structure; CLASSICAL AND QUANTUM MECHANICS, GENERAL PHYSICS; CLASSIFICATION; CONCENTRATION RATIO; CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND SUPERFLUIDITY; Crystal structure; Crystallinity; DEFECTS; DIFFUSION; EQUATIONS OF MOTION; GERMANIUM; LIMITING VALUES; Periodic variations; PERIODICITY; PHONONS; Plane waves; Propagation modes; SILICA; SILICON; Silicon dioxide; TRANSPORT THEORY; Velocity distribution; Vibration mode; WAVE PROPAGATION
• DOI: 10.1063/1.4955420
• ISSN: 0021-8979

The majority of intuition on phonon transport has been derived from studies of homogenous crystalline solids, where the atomic composition and structure are periodic. For this specific class of materials, the solutions to the equations of motions for the atoms (in the harmonic limit) result in plane wave modulated velocity fields for the normal modes of vibration. However, it has been known for several decades that whenever a system lacks periodicity, either compositional or structural, the normal modes of vibration can still be determined (in the harmonic limit), but the solutions take on different characteristics and many modes may not be plane wave modulated. Previous work has classified the types of vibrations into three primary categories, namely, propagons, diffusions, and locons. One can use the participation ratio to distinguish locons, from propagons and diffusons, which measures the extent to which a mode is localized. However, distinguishing between propagons and diffusons has remained a challenge, since both are spatially delocalized. Here, we present a new method that quantifies the extent to which a mode's character corresponds to a propagating mode, e.g., exhibits plane wave modulation. This then allows for clear and quantitative distinctions between propagons and diffusons. By resolving this issue quantitatively, one can now automate the classification of modes for any arbitrary material or structure, subject to a single constraint that the atoms must vibrate stably around their respective equilibrium sites. Several example test cases are studied including crystalline silicon and germanium, crystalline silicon with different defect concentrations, as well as amorphous silicon, germanium, and silica.