Anharmonic, dimensionality and size effects in phonon transport

Area of Science:

• Condensed Matter Physics
• Materials Science
• Computational Physics

Background:

• Understanding phonon transport is crucial for thermal management in solids.
• Anharmonicity, dimensionality, and size effects significantly influence thermal conductivity.
• Existing methods may lack efficiency or accuracy in capturing these complex phenomena.

Purpose of the Study:

• To develop and apply a numerically efficient semi-ab initio theory for studying phonon transport.
• To investigate anharmonic, dimensionality, and size effects in 2D and 3D solids.
• To quantify the contribution of four-phonon processes to thermal resistivity.

Main Methods:

• Employed a semi-ab initio theory combining density-functional theory (DFT) and relaxation-time approximations.
• Utilized third- and fourth-order terms of the crystal Hamiltonian.
• Generated inputs from DFT-based phonon calculations and density-functional perturbation theory.

Main Results:

• Four-phonon processes contribute measurably to lattice thermal resistivity above the Debye temperature.
• Phonon conductivity exhibits significantly stronger sample length dependence in low-dimensional solids (2D) compared to bulk (3D).
• Results were validated for bulk Si, Ge, MoS2, and monolayer MoS2.

Conclusions:

• The developed theory accurately captures complex phonon transport phenomena.
• Four-phonon scattering is a key mechanism affecting thermal transport, especially at higher temperatures.
• Engineering thermal properties in low-dimensional materials requires careful consideration of size effects.