QuMat seminar

2024-11-27, 16:00 – BBG 7.12 Revisiting phonons: the mode-coupling theory of anharmonic lattice dynamics Speaker: Alois Castellano – University of Liege Host: Rembert Duine

Abstract:

In crystalline materials, the collective vibrations of atoms are described within the framework of lattice dynamics, where these motions are represented as phonons: a particle-like quantum of lattice vibrations. Phonons play a fundamental role in determining how material properties evolve with temperature. They govern heat conduction in insulators, influence electronic scattering in metals, and provide a key signature detected in spectroscopic techniques like Raman, X-ray, and neutron scattering. Consequently, the ability of phonons to accurately capture atomic motion is essential for understanding and predicting material behavior.

The standard lattice dynamics approach relies on the harmonic approximation, which defines phonons from the quadratic terms of the potential energy and treats anharmonic contributions as small perturbations. While effective for systems with weak anharmonicity, this approximation becomes inadequate when anharmonic effects are pronounced. Under these conditions, harmonic phonons fail to provide an accurate description of atomic motion, limiting their utility for predicting key material properties. This underscores the need for a more robust framework capable of capturing the full range of vibrational behaviors.

In this talk, I will present the mode-coupling theory of anharmonic lattice dynamics [1], a framework that goes beyond the harmonic approximation by incorporating temperature-dependent phonons and their interactions. Unlike conventional approaches, this theory directly addresses the dynamics of atomic motion, offering a more accurate and versatile description of strongly anharmonic systems. Through practical applications [2-4], I will illustrate how this theory enhances predictions of material properties, from heat transport to spectroscopy and beyond, opening pathways to a deeper understanding of anharmonic phenomena.

[1] A. Castellano et al, J. Chem. Phys, 159, 234501 (2023)
[2] D. Ourdani et al, Phys. Rev. B 110, 014427 (2024)
[3] A. Castellano et al, Phys. Rev. B 110, 014427 (2024)
[4] N. Girroto Erhardt et al, arXiv:2410.23791

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