Coulomb drag in ultra-clean and strongly interacting van der Waals materials: toward exciton condensation

非技术摘要本项目的目的是实验研究两个耦合的二维(2D)薄片中电子之间的相互作用。当2D层足够接近时，层间库仑相互作用会导致动量转移，因此在一层中运动的电子会导致第二层中的电子也随之移动，这种现象称为库仑阻力。这项工作将研究原子薄材料的分层异质结构--包括石墨烯和绝缘氮化硼--以实现对导电层之间的间距的原子控制。这将允许在强耦合极限和高流动性设备中探索库仑阻力，在这些设备中，电子传输是弹道的。这些结构是由原理研究人员开发的通过2D材料的机械分层来制造超清洁多层异质结构的技术实现的。主要工作将是系统地表征单层石墨烯中的阻力响应与温度、密度、层间距和磁场之间的关系。此外，阻力和层间隧道效应将被用来追踪理论上预测的激子凝聚相的特征，在该相中，由成对电子和限制在不同层中的空穴组成的空间间接激子凝聚成超流体基态。对库仑阻力响应的仔细研究为研究介观系统中的电子-电子相互作用提供了一个独特的工具，由于电子-电子相互作用是相关材料丰富而复杂的物理学的基础，因此有望产生超出2D系统研究的重大影响。如果成功，这项研究还可能使革命性的低功率电子设备成为可能。合作的跨学科工作将为博士后研究人员提供培训，并为高中和初中本科生提供研究经验。外展工作的重点将是扩大与两所附属公立学校教师的长期关系。技术摘要：本项目的目的是在小的层间分离的强相互作用极限下，实验研究由石墨烯和相关的van der Waals材料制成的高迁移率双层量子阱中的库仑阻力。主要目标将是通过输运测量，系统地描述单层石墨烯异质结构中的阻力响应与温度、密度和层间分离之间的关系，在零磁场和有限磁场下。一些悬而未决的问题将被解决，如先前报道的反常密度和温度依赖关系，双中性点的反常阻力响应的来源，以及有限磁场区域中霍尔响应的性质。此外，还将利用阻力和层间隧道效应来追踪激子凝聚相在两个区域(I)零磁场下的电子-空穴石墨烯层和(Ii)量子霍尔区半填充朗道能级的电子-电子石墨烯层中的特征。这项实验工作将包括由双层石墨烯和单层和少数层过渡金属双卤化物制备的异质结构的研究，其中带隙对激子结合的影响到目前为止还没有得到实验关注。石墨烯中的库仑阻力响应在最基本的水平上还没有被很好地理解。对这个系统进行建模的理论努力产生了相互矛盾的结果，没有一个能很好地与迄今报道的少数实验研究相匹配。在这方面，这里提出的系统研究有望为未来对这一系统的理解奠定重要的基础，更广泛地说，为准确模拟石墨烯中的电子传输所需的关键物理参数提供定量的界限，如电子屏蔽强度与密度的关系以及介电环境的具体作用。

Non-technical AbstractThe aim of this project is to experimentally study the interactions between electrons in two coupled two-dimensional (2D) sheets. When the 2D layers are sufficiently close, interlayer Coulomb interactions result in momentum transfer so that electrons moving in one layer cause those in the second sheet to move in response, a phenomenon known as Coulomb drag. This work will study layered heterostructures of atomically thin materials -- including graphene and insulating boron nitride --to achieve atomic control over the spacing between the conducting layers. This will allow the exploration of Coulomb drag in the strong-coupling limit, and in high-mobility devices where electrical transport is ballistic. These structures are made possible by techniques developed by the Principle Investigators to fabricate ultraclean multi-layered heterostructures by mechanical layering of 2D materials. The primary effort will be a systematic characterization of the drag response in monolayer graphene versus temperature, density, layer separation, and magnetic field. Drag resistance together with inter-layer tunneling will additionally be used to pursue signatures of a theoretically-predicted exciton condensate phase in which spatially indirect excitons consisting of paired electrons and holes confined to separate layers condensed into a superfluid ground state. Careful studies of the Coulomb drag response provides a unique tool in which to study electron-electron interactions in mesoscopic systems, which is expected to have significant impact beyond the study of 2D systems, since electron-electron interactions underlie the rich and complex physics of correlated materials. If successful, this research could also enable revolutionary new low power electronic devices. The collaborative interdisciplinary work will provide training to a postdoctoral researcher as well as providing research experience to high school and junior level undergraduate students. Outreach efforts will focus on expanding long-term relationships with teachers at two affiliated public schools. Technical Abstract: The aim of this project is to experimentally study Coulomb drag in high mobility double layer quantum wells fabricated from 2D materials, such as graphene and related van der Waals materials, in the strongly interacting limit of small interlayer separation. The primary goal will be a systematic characterization of the drag response in monolayer graphene heterostructures versus temperature, density and interlayer separation, under both zero and finite magnetic field, through transport measurements. Several outstanding questions will be addressed such as the anomalous density and temperature dependences reported previously, origin of the anomalous drag response at the double neutrality point, and the nature of the Hall response in the finite magnetic field regime. Drag resistance together with inter-layer tunneling will additionally be used to pursue signatures of the exciton condensate phase in two regimes (i) electron-hole graphene layers at zero magnetic field, and (ii) electron-electron graphene layers at half filled Landau levels in the quantum Hall regime. The experimental effort will include studies of heterostructures fabricated from bilayer graphene, and mono and few-layer transition metal dichalcogenides where the effect of a bandgap on the exciton binding has so far received no experimental attention. The Coulomb drag response in graphene is not well understood at the most basic level. Theoretical efforts to model this system have yielded conflicting results, none of which well match the few experimental studies that have been reported so far. In this regard the systematic study proposed here promises to lay important groundwork for future understanding of this system, and more generally to provide quantitative boundaries on key physical parameters necessary to accurately model electron transport in graphene such as the strength of electron screening versus density and the specific role of the dielectric environment.