Imaging electron hydrodynamics in graphene

对电子如何在材料中运动的理解是基于简单的半经典方程，其中所有电子相互作用-与晶格，与无序以及彼此之间-都是通过将电子的描述改变为解释相互作用的不同粒子来完成的。然而，在具有小无序或强电子-电子相互作用的实际材料中，连续流体动力学描述是一个更好的模型，其中电子被称为像液体一样。最近，只有少数实验发现了这种奇怪的电子液体的存在，并提出了许多新的理论来解释它，这意味着这是一个全新领域的开始，这个领域充满了新奇而有用的技术。UCSB提供了一个独特的机会，将一种新的超清洁材料系统（石墨门控单层石墨烯）与一种新开发的高灵敏度扫描探针技术相结合，该技术可以直接探测纳米级的新型电子流。新的电子现象将被直接观察到，如粘度驱动的漩涡和电子流湍流的开始。也有机会将这些微小的电子流模式与大规模的电输运测量相匹配。流体力学制度提供了一个机会，建立基于操纵电子流体现象的新设备，这是以前从未想象过的。这项研究还与一个强大的教育计划紧密结合，该计划旨在向下一代科学家灌输对新型电子材料、量子传感器及其应用的兴奋。技术摘要：低维系统中的电子流通常由半经典运动方程描述，其中所有电子相互作用-与晶格，与无序以及彼此之间-只导致裸电子被修饰成电子样准粒子，其动力学决定电流。然而，在量子临界系统中，强烈的电子间碰撞可以冲走单个电子的自由度——电子-电子碰撞的长度比其他所有维度都短得多。宏观的可观察性，如电导率和导热性，然后由流体力学方程和宏观参数——密度、粘度和平均速度——描述，这些参数来自于电子之间的微观碰撞。最近，几个研究小组报道了石墨烯中流体动力学的特征——石墨烯是一种能带结构模仿量子临界点的无间隙色散的材料。然而，这些实验都集中在宏观输运性质上。因此，他们无法直接探测所有相关长度尺度上的水动力流动的出现。在这里，主要研究人员建议结合最先进的石墨烯器件，其迁移率比以前实现的要高一个数量级，以及最先进的局部磁强计，利用金刚石中的氮空位中心缺陷，来探测相关长度尺度上流体动力流动的出现。该奖项反映了美国国家科学基金会的法定使命，并通过使用基金会的知识价值和更广泛的影响审查标准进行评估，被认为值得支持。

Non-Technical AbstractOur understanding of how electrons move in materials is based on simple, semiclassical, equations where all electron interactions - with the lattice, with disorder, and with each other - is done by changing the description of the electron into a different particle that accounts for the interactions. However, in real materials with small disorder or strong electron-electron interactions, a continuous hydrodynamic description is a better model where the electrons are said to act like a liquid. Only a handful of experiments have recently detected the presence of such a strange electron liquid with many new theories to explain it, which means this is the beginning of a whole new field of novel and useful technologies. This proposal is driven by a unique opportunity at UCSB to combine a new, ultra-clean materials system (graphite gated monolayer graphene) with a newly developed, high sensitivity scanned probe technique that can directly probe the novel electron flows down to the nanoscale. New electron phenomena will be directly observed such as viscosity-driven whirlpools and the onset of turbulence of electron flows. There is also an opportunity to match these tiny electron flow patterns with large scale electrical transport measurements. The hydrodynamic regime presents an opportunity to build novel devices based on manipulating electron fluid phenomena that have never before been imagined. This research is also tightly coupled with a strong educational plan that aims to imbue the next generation of scientists with an excitement for novel electronic materials, quantum sensors, and their applications. Technical AbstractElectron flow in low dimensional systems is typically described by semiclassical equations of motion, in which all electron interactions - with the lattice, with disorder, and with each other - leads only to a dressing of the bare electron into an electron-like quasiparticle whose dynamics determine current flow. However, in quantum critical systems, strong inter-electron collisions can wash away the individual electronic degrees of freedom - the electron-electron collision length is much shorter than all other dimensions. Macroscopic observables such as electrical and thermal conductivity are then described by the equations of fluid mechanics, and the macroscopic parameters-density, viscosity, and mean velocity-which emerge from the microscopic collisions between electrons. Recently, several groups have reported signatures of the hydrodynamic regime in graphene - a material whose band structure mimics the gapless dispersion of a quantum critical point. However, these experiments have all focused on macroscopic transport properties. As such they are unable to directly probe the emergence of hydrodynamic flow on all the relevant length scales. Here, the principal investigators propose to combine state-of-the-art graphene devices with an order of magnitude higher mobility than previously realized along with state-of-the art local magnetometry, utilizing nitrogen-vacancy center defects in diamond, to probe the emergence of hydrodynamic flow across the relevant length scales.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.