First-principles method is a powerful approach to study atomic scale physics and deals with fundamental physical processes. With its introduction into thermal transport area, the quantum description of quantized lattice vibrations, phonons, achieved great success in predicting/explaining thermal transport properties in the past two decades. However, frontier technologies based on solid-state systems and devices will soon meet the boundary of our usual understanding of energy transport. Our pursuit of faster computing, higher energy efficiency and better sustainability requires major advancements in the study of quantum energy transport. Our research on phonon dynamics and thermal transport then has both technological and scientific importance that could lead to discovery of novel transport phenomena and disruptive technology strategies.

Challenges

My PhD research is motivated by the following challenges in our community:

• The lowest-order perturbation theory is found to be insufficient to describe emerging solid-state materials;
• High temperature holds promise for next-generation power technology but knowledge on high temperature thermal transport is limited;
• Nonequilibrium physics may prevail in quantum thermal transport and alter our interpretation of thermal metrology.

Line of Recent Research

My research aims to tackle the challenges above along these directions:

Thermal transport in graphene

Nonequilibrium carrier transport

High temperature thermal transport

Temperature-dependent optical responses

[Sustained community software: FourPhonon](https://zrhan.notion.site/Sustained-community-software-FourPhonon-4d813e9773774925bc83d7ac4cd56214)

Thermal transport in graphene

Graphene has been studied as an emerging atomically thin electronic and optoelectronic material and for thermal management. As a longstanding question and despite extensive theoretical and experimental studies, its thermal transport is still not well understood and many aspects remain inconclusive. In this two-phase study, we start from optical phonon dynamics and seek to extend the methodology to the full phonon spectrum:

• The Raman peak shift and linewidth depend on the coupling of the Raman-active optical phonon mode with electrons and other phonon polarizations. The intrinsic linewidth of a zone-center optical phonon in graphene can be expressed as $\gamma^{\rm in}= \gamma^{\rm e-ph}+\gamma^{\rm ph-ph}$. Prior study predicted that $\gamma^{\rm e-ph}$ dominates the linewidth which should decrease with temperature, contradicting the experiments. We add four-phonon scattering to this picture and find leading contribution from $\gamma^{\rm ph-ph}$: for the first time, temperature trend is corrected to be increasing. Further inclusion of phonon renormalization gives us good quantitive agreement with Raman experiments from literature and our collaborators at UT Austin (Shi group).

  <aside> 📃 Related article: Z. Han et al., Phys Rev Lett 128, 045901 (2022).

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The out-of-plane vibration, ZA mode, plays a special role in the thermal transport of graphene due to its quadratic dispersion.