Thermal Phonon Physics Lab
1. Coherent thermal phonons
The legacy of transport physics establishes that thermal phonons of a given mode can be understood as quasiparticles having the same lifetime and coherence time. Their behaviors are modeled by Boltzmann transport theory and the phonon-gas model. On the other hand, phonons are defined in essence as vibrational waves. Experimental investigations demonstrated that the wave nature or the coherence of thermal phonons significantly contributes to thermal transport. However, the intrinsic coherence or wavelike behavior of thermal phonons still could not be revealed from the prevailing theories and simulation approaches.
Our research project is developing new approaches and theories to simultaneously describe the particlelike and wavelike pictures of thermal phonons. In addition, the effect of coherence or wave behaviors on the phonon decay and thermal transport is also expected to be demonstrated.
2. Phonon wave hybridization
Phonon hybridization is demonstrated as another important wave behavior of thermal phonons as their frequencies and dispersion can be stunned through the lattice dynamics. The phonon wave hybridization is widely reported in the pillar based systems and host-guest systems. Our research is focusing on the phonon hybridization in the host-guest systems, for instance, clathrates, perovskites, and carbon negative curvature crystals. Due to the complex interaction between the host main body and guest parts, phonon hybridization and its effects on thermal transport are still under debate. With the molecular dynamic simulations, we did several works on this project and there are still important problems that can be continued in the future.
3. Thermal transport in nanostructures
Due to the rapid development of nanotechnology, lots of novel nanostructures are proposed in the near years, such as nanoheterostructures, van der Waals stacking nanostructures, nanocapacitor, etc. In this direction, we studied several thermal transport phenomena in this group of nanostructures. For instance, we demonstrate a promising phonon nanocapacitor for storing and emitting phonons, built by the carbon schwarzite based host-guest system. This phonon nanocapacitor takes advantage of the inherently strong phonon confinement of the hybridized modes in host-guest system. The monochromaticity and coherence of the stored phonons are well demonstrated via the ultralong phonon lifetime and coherent time. More interestingly, the frequency of the nanocapacitor is widely tunable from gigahertz to several terahertz by engineering the host-guest interaction. Finally, the stored phonons with different polarizations can be emitted separately with the application of uniaxial strain along a particular direction. This work may provide new opportunities for studying the coherent wave effect of phonons.
4. Thermoelectrics in nanostructures
Modern society is hungry for electrical power. To improve the efficiency of energy harvesting from heat, extensive efforts seek high-performance thermoelectric materials that possess large differences between electronic and thermal conductance. The nanoscale engineering of thermal transport provides pathways to improve thermoelectric performance. Previously, we report a super high-performance material of consisting of MoS2/WS2 hybrid nanoribbons discovered from a theoretical investigation using nonequilibrium Green’s function methods combined with first-principles calculations and molecular dynamics simulations. The hybrid nanoribbons show higher efficiency of energy conversion than the MoS2 and WS2 nanoribbons due to the fact that the MoS2/WS2 interface reduces lattice thermal conductivity more than the electron transport. By tuning the number of the MoS2/WS2 interfaces, a figure of merit ZT as high as 5.5 is achieved at a temperature of 600 K. Our results imply that the MoS2/WS2 hybrid nanoribbons have promising applications in thermal energy harvesting.
