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）片中电子之间的相互作用。 当2D层足够接近时，层间库仑相互作用导致动量转移，使得在一层中移动的电子引起第二层中的电子移动，这种现象称为库仑阻力。这项工作将研究原子级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间距的原子控制。 这将允许在强耦合极限下探索库仑阻力，以及在电传输为弹道的高迁移率器件中。 这些结构是由主要研究人员开发的技术通过2D材料的机械分层来制造超净多层异质结构。 主要的努力将是单层石墨烯的阻力响应与温度，密度，层分离和磁场的系统表征。阻力与层间隧穿将另外用于追求理论上预测的激子凝聚相的签名，其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基态。对库仑阻力响应的仔细研究提供了一种独特的工具，可以研究介观系统中的电子-电子相互作用，预计这将对2D系统的研究产生重大影响，因为电子-电子相互作用是相关材料丰富而复杂的物理学的基础。 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。 合作的跨学科工作将提供培训，博士后研究人员以及提供研究经验，高中和初中水平的本科生。 外联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：该项目的目的是实验研究在小层间分离的强相互作用极限下，由2D材料（例如石墨烯和相关的货车德瓦尔斯材料）制造的高迁移率双层量子威尔斯中的库仑阻力。 主要目标将是通过传输测量，在零磁场和有限磁场下，单层石墨烯异质结构中的阻力响应与温度、密度和层间分离的系统表征。几个悬而未决的问题将得到解决，如异常的密度和温度的依赖性，在双中性点的异常阻力响应的起源，和霍尔响应的性质在有限的磁场制度。 拖曳阻力与层间隧穿将另外用于在两种机制中追踪激子凝聚相的特征：（i）零磁场下的电子-空穴石墨烯层，和（ii）量子霍尔机制中半填充朗道能级下的电子-电子石墨烯层。实验工作将包括从双层石墨烯制造的异质结构的研究，以及单层和少层过渡金属二硫属化物，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。对这一系统进行理论建模的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很好地匹配。在这方面，本文提出的系统研究有望为未来理解该系统奠定重要基础，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边界，例如电子屏蔽强度与密度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.