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

摘要本项目的目的是实验研究两个耦合二维（2D）薄片中电子之间的相互作用。当二维层足够接近时，层间的库仑相互作用会导致动量转移，因此在一层中移动的电子会导致第二层中的电子相应移动，这种现象被称为库仑阻力。这项工作将研究原子薄材料（包括石墨烯和绝缘氮化硼）的层状异质结构，以实现对导电层间距的原子控制。这将允许探索在强耦合极限下的库仑阻力，以及在电传输为弹道的高迁移率设备中。这些结构是由主要研究人员开发的技术通过机械分层二维材料来制造超净多层异质结构而成为可能。主要的工作将是系统地表征单层石墨烯的阻力响应与温度、密度、层分离和磁场的关系。阻力和层间隧道效应还将用于寻找理论预测的激子凝聚相的特征，在该相中，由成对电子和空穴组成的空间间接激子被限制在单独的层中，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种独特的工具，由于电子-电子相互作用是相关材料丰富而复杂的物理基础，因此预计这将对二维系统的研究产生重大影响。如果成功，这项研究还可以实现革命性的新型低功耗电子设备。这项跨学科的合作工作将为博士后研究员提供培训，并为高中和初中的本科生提供研究经验。拓展工作将侧重于扩大与两所附属公立学校教师的长期关系。技术摘要：本项目旨在实验研究由二维材料（如石墨烯和相关范德华材料）制成的高迁移率双层量子阱在小层间分离的强相互作用极限下的库仑阻力。主要目标是通过输运测量，系统表征单层石墨烯异质结构的阻力响应与温度、密度和层间分离的关系，包括零磁场和有限磁场。几个悬而未决的问题将得到解决，如先前报道的异常密度和温度依赖性，在双中性点的异常阻力响应的起源，以及有限磁场下霍尔响应的性质。阻力和层间隧道效应将被用于研究两种状态下激子凝聚相的特征(i)零磁场下的电子-空穴石墨烯层，以及（ii）量子霍尔状态下半填充朗道能级的电子-电子石墨烯层。实验工作将包括研究由双层石墨烯、单层和少数层过渡金属二硫族化合物制成的异质结构，其中带隙对激子结合的影响迄今尚未得到实验关注。石墨烯中的库仑阻力响应在最基本的层面上还没有得到很好的理解。为这一系统建模的理论努力产生了相互矛盾的结果，其中没有一个与迄今为止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理解该系统奠定重要基础，并更广泛地提供准确模拟石墨烯中电子传输所需的关键物理参数的定量边界，例如电子筛选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.