Phonons, i.e., quanta of vibrational waves, are commonly considered as one of the fundamental quasiparticles, simultaneously exhibiting wave- and particlelike characteristics in nanostructured crystals or bulk materials. The wavelike behavior of phonons impacts thermal properties via coherence mechanisms, as highlighted by several pioneering and recent works. The particlelike behavior has been treated by the Boltzmann transport equation (BTE) and the phonon-gas model in most solids. Experiments have revealed, however, that the wave nature of thermal phonons plays a substantial role in thermal transport, as, for example, in the observations of coherent thermal transport in nanophononic crystals. Later, theoretical and simulation studies were devoted to the understanding of phonon coherence, such as the one producing band folding, but missing the particle behavior. Recently, the theoretical study revealed that the realistic phonon dynamics can only be manifested if both intrinsic coherence relevant to the extension of phonon wave packets and the particlelike behavior of thermal phonons are taken into account.
(a) The conventional lattice of Tl3VSe4. (b) The phonon dispersion of Tl3VSe4 from lattice dynamic calculations (white lines) and room-temperature spectral energy density calcula tions (contour) for the conventional cell . (c) Evolution-time- and coherence-time dependent phonon number (contour) of Tl3VSe4 for the 0.93 THz mode at 100 and 300 K. (d) Phonon decay (correlation) versus correlation time in Tl3VSe4 for the 0.25 and 0.93 THz modes at 100 and 300 K. The dashed-dotted line shows the theoretical fit of this work, and the dotted line shows the classical exponential decay.
Understanding and quantifying the fundamental physical property of coherence of thermal excitations is a long-standing and general problem in physics. The conventional theory, i.e., the phonon gas model, fails to describe coherence and its impact on thermal transport. In this Letter, we propose a general heat conduction formalism supported by theoretical arguments and direct atomic simulations, which takes into account both the conventional phonon gas model and the wave nature of thermal phonons. By naturally introducing wave packets in the heat flux from fundamental concepts, we derive an original thermal conductivity expression including coherence times and lifetimes. Our theory and simulations reveal two distinct types of coherence, i.e., intrinsic and mutual, appearing in two different temperature ranges. This contribution establishes a fundamental frame for understanding and quantifying the coherence of thermal phonons, which should have a general impact on the estimation of the thermal properties of solids.
Prof. Zhang Zhongwei, a PhD graduate of Prof. Chen Jie's group and a postdoctoral fellow at the University of Tokyo, Japan, is the first author of the paper, and Prof. Chen Jie and Prof. Sebastian Volz of CNRS are the co-corresponding authors. He has published more than 10 SCI papers with his group.
This project is partially supported by the grants from the National Natural Science Foundation of China and Science and Technology Commission of Shanghai Municipality .
Link：https://link.aps.org/doi/10.1103/PhysRevLett.128.015901
