Probing phonon hydrodynamics in 2D materials

奇怪的是，尽管热与每一个物理过程都有着不可分割的联系，但科学家和工程师们仍然无法描绘出热物理的全貌。推动我们对热物理的基本理解对于提高清洁能源技术（即热电和光伏）的能量转换效率，改善电子设备（即cpu）的冷却以及解决量子计算机中量子比特退相干的挑战至关重要。典型的教科书对固体中热输运的理解是用扩散方程或傅立叶定律在现象学上描述的。然而，当系统的大小缩小到微观热载体（即声子）之间散射的长度尺度的数量级时，这幅图就会崩溃，需要一个原子的描述。我们开发了一种自下而上的理论方法，将从第一性原理量子力学计算（即密度函数理论）中获得的材料特性作为玻尔兹曼输运方程的输入，以预测实验观察（即温度）。这个框架被扩展，并发现能够捕获什么是以前被认为是一个外来的热输运制度：声子流体动力学。这种状态的一个特征是温度不按照扩散方程演变，而是服从波动方程。应用这一理论，我们报道了在100 K以上温度下石墨中第二声的实验证实，创造了高温声子流体力学的当前记录。在此结果之前，第二声只在少数温度低于20 K的材料中被观察到，活跃的研究在20世纪70年代停止。我们的研究结果重振了一个几乎被遗忘的想法，并为基础研究和应用研究开辟了多条道路，这将是拟议的发现基金研究计划的重点。我们将首先扩展我们的数值框架，该框架将从第一性原理和机器学习计算中获得的数据作为输入，并预测实验流体动力学特征。然后，我们将使用这个框架来指导我们的实验工作，以观察二维材料和范德华异质结构中的声子流体动力学。利用我们验证的理论和实验工具，我们将设计微观特性材料来控制特定技术应用的声子流体动力学。为了突破我们的清洁能源转换和储存技术以及计算能力的极限，我们必须推动我们对伴随的热过程的理解。该研究项目将为声子流体动力学的未来研究建立一个最先进的平台，这将有利于加拿大采用清洁能源技术的努力，并培养下一代世界级的科学家和工程师。

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.