CAREER: Nanoscale Phonon Spectrometer to Quantitatively Characterize Low-Dimensional Heat Transfer

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****TECHNICAL ABSTRACT****The experiments in this Faculty Early Career Development (CAREER) project aim to study the fundamental science of heat flow through nanoscale geometries. The project intends to elucidate key model predictions of phonon behavior, such as the wavelength dependence of surface scattering and the ability of complex nanoscale geometries to selectively block, enhance, or direct heat flow. This will be accomplished by developing a device for probing single-frequency phonon transmission in nanostructures. Phonons of controlled energy will be generated by using superconducting tunnel junctions (STJs). Injection of excited-state electrons through the junction causes emission of phonons during the subsequent relaxation and recombination of the electrons into the Cooper-pair ground state. The same principle will be used for phonon detection by STJs: incident phonons break Cooper pairs in the detector, creating a measurable tunnel current. Successful measurements will be of broad interest to researchers in the fields of nanomaterials, thermal transport, and superconducting devices. The project will support the training of a graduate student in nanofabrication, cryogenics, and sensitive measurement techniques. It will educate the student in a broad range of contemporary physics research areas, including superconducting devices and phonon transport. The research will also be incorporated into a new outreach module promoting science learning to students and a broader understanding of nanotechnology. ****NON-TECHNICAL ABSTRACT****In insulating materials, heat is transmitted by atomic vibrations ('phonons'), which move through a solid like ripples in water or sound waves in air, but much more rapidly. Existing physical theories explain heat transport well in bulk materials; however, these laws break down when the same material is reduced to only a few thousand atoms wide. Theoretical modeling predicts remarkable behaviors for nanostructures - such as the selective transmission of heat waves based solely on the nanostructure's shape - but these ideas have not been experimentally quantified. This Faculty Early Career Development (CAREER) project will study nanoscale heat transport by building a nano-sized device to create and probe atomic vibrational heat transfer. This work will inform improved engineering of devices like thermoelectrics and microelectronic cooling modules through the exploitation of nanomaterials' unusual thermal properties. This work has only recently been made possible due to newly available advanced nanofabrication techniques. The project will support the training of a graduate student in related cutting-edge nanofabrication, cryogenics, and sensitive measurement techniques. It will educate the student in a broad range of contemporary physics research areas, including superconducting devices and phonon transport. In addition, the research will be incorporated into a new outreach module promoting science learning to students and a broader understanding of nanotechnology.