Wavelet Analysis Unfolds Thermal Phonon Coherence

Coherence of thermal excitations is a long-standing and general problem in physics. The conventional theory of the phonon gas model indeed does not include coherence and its impact on thermal transport.<br/>We propose a general heat conduction formalism based on theoretical arguments and direct atomic simulations, which bridges conventional phonon gas model and the wave picture of thermal phonons.<br/> <br/>By naturally introducing wavepackets in the fundamental heat flux expression [1], we derive an original thermal conductivity formula where coherence times and life-times appear [2]. We apply the theory to a complex crystal where interatomic potentials are optimized by a Machine Learning procedure. The simulation reveals an intrinsic and a -previously investigated- mutual coherence appearing in two different temperature ranges.<br/> <br/>[1] Zhongwei Zhang, Yangyu Guo, Marc Bescond, Jie Chen, Masahiro Nomura, and Sebastian Volz, Generalized decay law for particlelike and wavelike thermal phonons, Physical Review B 103(18):184307.<br/>[2] Z. Zhang, Y. Guo, M. Bescond, J. Chen, M. Nomura and S. Volz, Heat conduction theory including phonon coherence. Physical Review Letters, accepted.