Probing phonon hydrodynamics in 2D materials

奇怪的是，考虑到热与每个物理过程密不可分的联系，科学家和工程师们仍然无法对热物理有一个完整的了解。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、改善电子器件(即CPU)的冷却以及解决量子计算机中量子比特消相干的挑战至关重要。教科书上对固体中热输运的典型理解是由扩散方程或相当于傅里叶定律现象学地描述的。然而，当系统的大小缩小到热的微观载流子(即声子)之间散射的长度尺度的数量级时，这幅图就分解了，需要原子描述。我们发展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(即密度泛函理论)获得的材料性质作为玻尔兹曼输运方程的输入来预测实验可观量(即温度)。对这个框架进行了扩展，发现它能够捕捉到以前被认为是一种奇异的热传输机制：声子流体动力学。这一体系的一个特征是温度不是根据扩散方程演化的，而是服从波动方程。应用这一理论，我们报道了在100K以上温度下在石墨中产生第二声的实验证实，创造了目前高温声子流体动力学的记录。在此之前，在温度低于20K的少数物质中只观察到了第二声，活跃的研究在20世纪70年代停止了。我们的结果重振了一个几乎被遗忘的想法，并为基础和应用研究开辟了多条前进道路，这将是拟议的探索拨款研究计划的重点。我们将首先扩展我们的数值框架，将从第一性原理和机器学习计算中获得的数据作为输入，并预测实验的水动力特征。然后我们将使用这个框架来指导我们的实验工作，以观察2D材料和范德华异质结构中的声子流体动力学。使用我们经过验证的理论和实验工具，我们将为特定的技术应用设计微观性能材料来控制声子流体动力学。为了推动我们清洁能源转换和存储技术以及我们的计算能力的极限，我们必须推动我们对伴随而来的热过程的理解。这项研究计划将为未来声子流体力学的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和培养下一代世界级科学家和工程师的努力。

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.