Probing phonon hydrodynamics in 2D materials

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It is strange that, given heat's inseparable connection with every physical process, a complete picture of thermal physics has continued to elude scientists and engineers. Pushing our fundamental understanding of thermal physics is critical to increasing energy conversion efficiency in clean energy technologies (i.e., thermoelectrics and photovoltaics), improving the cooling of electronic devices (i.e., CPUs) and addressing the challenges of qubit decoherence in quantum computers. The typical textbook understanding of thermal transport in a solid is described phenomenologically by the diffusive equation or, equivalently, Fourier's Law. However, as the size of the system shrinks to the order of the length scale of the scattering between the microscopic carriers of heat (i.e., phonons), this picture breaks down and an atomistic description is required. We developed a bottom-up theoretical approach that took material properties obtained from first principles quantum mechanical calculations (i.e., density functional theory) as input to the Boltzmann transport equation to predict the experimental observable (i.e., temperature). This framework was extended and found to be capable of capturing what was previously considered to be an exotic thermal transport regime: phonon hydrodynamics. A signature of this regime is that temperature does not evolve according to the diffusion equation, but rather obeys the wave equation. Applying this theory, we reported the experimental confirmation of second sound in graphite at temperatures above 100 K, setting the current record for high temperature phonon hydrodynamics. Prior to this result, second sound had only been observed in a handful of materials at temperatures below 20 K, with active research ceasing in the 1970s. Our result revitalizes a nearly forgotten idea and opens up multiple paths forward for fundamental and applied research, which will be the focus of the proposed Discovery Grant research program. We will begin by extending our numerical framework that will take data obtained from first principles and machine learning calculations as input and predict experimental hydrodynamic signatures. We will then use this framework to guide our experimental efforts to observe phonon hydrodynamics in 2D materials and van der Waals heterostructures. Using our validated theoretical and experimental tools, we will engineer the microscopic properties materials to control phonon hydrodynamics for specific technological applications. To push the limits of our clean energy conversion and storage technologies as well as our computing power, we must push our understanding of the concomitant thermal processes. This research program will establish a state of the art platform for the future study of phonon hydrodynamics that will benefit Canada's efforts to adopt clean energy technologies and train the next generation of world class scientists and engineers.