CAREER: Fundamental Investigation of the Wave Nature of Lattice Thermal Transport

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Heat transfer is a limiting factor in nanoscale engineering: if the finely tuned elements such as microchips or lasers get too hot or conduct heat uncontrollably, the devices will not function as intended. Thus, understanding how to control heat transfer at the nanoscale is critical in many engineering applications. Heat conduction in semiconductor crystals is dominated by collective atomic vibrations known as phonons. Conventionally, phonons are treated as particles that can propagate through a material, thus carrying heat, and can be scattered by material defects or other phonons. However, the wave nature of phonons can emerge or even dominate heat conduction in structures composed of periodically or quasi-periodically arranged components, e.g., quantum cascade lasers made of alternating layers of materials and integrated circuits containing densely packed nano-transistors. In those structures, the propagation of phonons can be promoted by constructive interference or halted by destructive interference. This project aims to explore these intriguing wave behaviors and elucidate the transition between particle-like and wave-like phonons through complementary computational and experimental techniques. This research will lead to novel engineering strategies to maximize heat dissipation in modern devices like quantum cascade lasers and integrated circuits or minimize heat conduction in thermoelectric materials and thermal barriers. This project will tightly integrate research, education, and outreach—including themed science exhibits, new curriculum development, hands-on research mentoring, and online course and research tool sharing—to have a broad, long-term impact on STEM education for K-12, undergraduate, and graduate students. The research objectives of this project are to: 1) rigorously understand the transport, scattering, and localization of wave-like phonons in periodic, quasi-periodic, and aperiodic structures, leveraging complementary spectral phonon analysis and phonon spectroscopy techniques; and 2) minimize lattice thermal transport through decoupled suppression of wave-like and particle-like phonon transport, leveraging machine learning-aided spectral phonon analysis and structure optimization. The proposed research on the wave nature of phonons can greatly advance the state of knowledge of thermal transport in crystalline solids, which has until now been primarily based on the particle nature of phonons. Such knowledge will enable the development of novel engineering strategies based on the wave nature of phonons, breaking the bottleneck of the thermal design or management of various heat transfer-limited technologies.This award reflects NSF's statutory mission and has been deemed worthy of support through evaluation using the Foundation's intellectual merit and broader impacts review criteria.