Novel Electron Fluids in Quantum Materials

石墨烯和其他原子薄量子材料的发现定义了纳米科学的新范式。这些材料中的电子表现为光线透过窗户玻璃，以弹道方式传播，不受无序和缺陷的阻碍。这导致了创纪录的高导电性和其他独特的特性，这些特性为设备工程提供了新的方向，并有可能从根本上改变电子设备的性能。值得注意的是，这些优秀电子特性背后的量子现象即使在室温下也会持续存在，改变了信号处理的规则，为量子电子学开辟了新的途径，并呼吁对纳米电子学采取创新的方法，利用新的物理概念，而不是传统的方案。电子流体是一种新的物质状态，可能有助于解决这一挑战。在电子流体中，电荷的流动模拟了粘性液体的流动，如水、蜂蜜或空气，这与教科书上在传统金属和半导体中看到的欧姆定律完全不同。在20世纪，粘性流体被用来设计流体回路，甚至是在低频下运行的简单但功能齐全的液压计算机。(例如，这些设备在车辆的自动变速器系统中得到了应用。)量子材料中的电子流体，特别是石墨烯，一种只有一个原子那么薄的碳层，移动得更快，规模也更短。将流体设计扩展到电子流体将导致逻辑门和集成电路的运行速度快10亿倍，体积小10000倍。流体电路的性能能超过传统的半导体晶体管吗？我们认为这个问题的答案是肯定的：在新材料中使用电子流体的流体电路元件可以运行得更快，规模更小，并提供新的功能。电子流体将使超高速低功率晶体管、低电阻率互连和直流变压器成为可能。流体结构将通过实现模拟纳米设备在超高频下的能效操作，为机器学习等现代技术提供支持。为了将这些想法建立在坚实的基础上，并释放电子流体的潜力，必须对电子流体的物理学有更深的理解。这个项目是凝聚态理论家对纳米电子学需求的回答。在PI的初步工作中，一个有趣的和潜在有用的制度，流动性的开始被确定。我们将把我们的努力集中在流动性的开始上，通过数学模型来描述它，并利用这些模型来提出应用于纳米电子学的设计思想。当载流子之间的碰撞频率达到某一阈值时，流体就会开始流动，这样电流就可以拖曳周围的粒子。在这种情况下，非局部效应和电流之间的非线性耦合有望达到最大。后者对于潜在的应用非常有利：电流可以用来控制另一种电流的流动。最近更详细的分析表明，起始点不仅是流体力学行为的阈值，而且是一种全新的制度，在这种制度下，注入的电流通过由电子和空穴组成的定向射流在流体中传播。我们将从理论上研究石墨烯流动性开始时的关键现象：电荷流、射流的形成、电流之间的非线性耦合、能量传输、对外部磁场的敏感性、对快速电场的响应。这项研究将与项目合作伙伴所做的实验工作联系起来。对流动性物理的洞察最终将有助于通过与其他团队的互动，提出基于电子流体学原理的纳米电子电路元件的新颖设计。

The discovery of graphene and other atomically thin quantum materials has defined a new paradigm in nanoscience. Electrons in these materials behave as light shining through window glass, propagating ballistically, unimpeded by disorder and defects. This leads to record-high electric conduction and other unique properties, which enable new directions for device engineering and have the potential to radically transform the performance of electronic devices. Remarkably, the quantum phenomena underlying these excellent electronic properties persist even at room temperature, changing the rules for signal processing and opening new avenues for quantum electronics and calling for innovative approaches to nanoelectronics that exploit new physical ideas rather than the conventional schemes. Electron fluid (e-fluid) is a new state of matter that may help to address this challenge. In e-fluids, the flow of electric charge mimics that of viscous fluids, such as water, honey, or air, in a radical departure from textbook Ohm's law seen in conventional metals and semiconductors. In 20th century, viscous fluids were employed to engineer fluidic circuits and even simple but fully functional hydraulic computers operating at low frequencies. (For example, such devices found their use in automatic transmissions systems in vehicles.) E-fluids in quantum materials, in particular graphene, a one-atom-thin layer of carbon, move much faster and on much shorter scales. Extending fluidic designs to e-fluids will lead to logic gates and integrated circuits that operate a billion times faster, and are 10000 times smaller. Can the performance of fluidic circuits surpass that of conventional semiconductor transistors?We believe that the answer to this question is in the affirmative: fluidic circuit components employing e-fluids in novel materials may operate faster, on a smaller scale, and provide novel functionalities. E-fluids will enable ultrafast low-power transistors, low-resistivity interconnects, and direct-current transformers. The fluidic architectures will provide support to modern technologies such as machine learning through achieving energy-efficient operation of analogue nanoscale devices at ultrahigh frequencies. To put these ideas on a firm ground and to unleash the potential of e-fluidics, a deeper understanding of the physics of e-fluids must be developed. This project is a condensed-matter theorist's answer to the demands of nanoscale electronics. In the PI's preliminary work, an interesting and potentially useful regime, the onset of fluidity was identified. We shall focus our efforts on the onset of fluidity, describing it via mathematical models, and employing these to suggest design ideas for applications in nanoscale electronics. The onset of fluidity occurs when the frequency of collisions between charge carriers reaches a certain threshold such that a current flow can drag ambient particles. In this regime, nonlocal effects and nonlinear couplings between currents are expected to be maximal. The latter is very beneficial for potential applications: electric current can be employed to manipulate the flow of another current. More detailed recent analysis demonstrate that the onset is not just a threshold for fluid-mechanical behaviour but an entirely new regime, in which injected currents propagate through the fluid via directed jets comprised of electrons and holes. We will study theoretically the key phenomena occurring at the fluidity onset in graphene: charge flows, formation of jets, nonlinear coupling between the currents, energy transport, sensitivity to external magnetic field, response to fast electric fields. The research will be linked to experimental efforts done by project partners. The insights into the physics of fluidity will eventually help, via interaction with other teams, to propose novel designs of elements of nanoelectronic circuits based on the principles of e-fluidics.