Spatiotemporal Measurements of the Kondo Cloud

现代物理学研究的焦点材料通常以电子的几种组织方式之间的竞争为特征。这既是一种祝福，也是一种诅咒，开启了丰富的可能性，但也阻碍了对这些材料的基本理解。如果人们可以建立一个实验系统，显示被认为是驱动感兴趣的材料的行为的关键特征，同时放弃可能掩盖基本机制的其他效应，那会怎么样？晶体管和激光器等现代技术依赖于这样一个事实，即向材料中添加即使是少量的杂质也会大大改变其特性。这种敏感性的另一个例子是近藤效应：在金属中加入百万分之几的磁性杂质，会大大改变其低温电学行为。这样的杂质会在其周围产生一个电子“云”，从而屏蔽杂质的磁矩。该项目旨在纳米制造一种设备来测量这种云的空间范围，并研究当两种杂质聚集在一起时杂质-云和杂质-杂质相互作用之间的竞争。通过在一个简化和高度可控的系统中实现竞争，这将揭示一类被称为重费米子金属的材料，其中正是这种竞争被怀疑是关键。除了研究部分，首席研究员和学生都有积极参与纳米科学推广的记录和计划，包括中学教师的短期课程，技术摘要本项目旨在确定近藤屏蔽云的空间范围，并使用精心设计的基于GaAs/AlGaAs异质结构中栅极定义的量子点的模型系统研究近藤-RKKY竞争。虽然近藤效应已经在许多不同的系统中被发现和研究，但仍然缺乏对其空间范围的明确测量。在所提出的器件中，杂质间相互作用和杂质浴相互作用之间的竞争，被怀疑是几个重费米子化合物的行为的关键，是门可调的。在更可控的纳米结构中实现这种效应可能会阐明这些材料中发生的丰富行为，包括可能发挥关键作用的量子相变。作为精确输运研究的补充，扫描门实验，其中金属尖端在纳米图案化双层异质结构的表面上扫描，可以在受控位置扰动近藤云，提供量子多体状态的空间范围的直接映射。

Non-Technical AbstractMaterials at the focus of modern physics research often feature competition among several ways electrons can organize. This is both a blessing and a curse, opening rich possibilities but also hindering basic understanding of these materials. What if one could build an experimental system which displays the key features believed to drive behavior of a material of interest, while discarding other effects which might obscure the fundamental mechanism?Modern technology such as transistors and lasers depends on the fact that adding even a small amount of impurities to a material can drastically alter its properties. Another example of such sensitivity is the Kondo effect: a few parts per million of magnetic impurities added to a metal drastically change its low-temperature electrical behavior. Such an impurity creates a "cloud" of electrons around it, which shields the magnetic moment of the impurity. This project aims to nanofabricate a device to measure the spatial extent of this cloud, and to study the competition between impurity-cloud and impurity-impurity interactions when two such impurities are brought together. This will shed light on a class of materials known as heavy fermion metals, in which exactly this competition is suspected to be key, by realizing the competition in a simplified and highly-controllable system.In addition to the research component, both the principal investigator and the students have a track record and plans for active involvement in outreach regarding nanoscience, including short programs for middle school teachers, and involving high school and college students in research.Technical AbstractThis project aims to determine the spatial extent of the Kondo screening cloud and study the Kondo-RKKY competition using a carefully-engineered model system based on gate-defined quantum dots in a GaAs/AlGaAs heterostructure. While the Kondo effect has been seen and studied in many different systems, a clear measurement of its spatial extent is still lacking. In the proposed device, the competition between the inter-impurity interaction and the impurity-bath interaction, suspected to be key to the behavior of several heavy-fermion compounds, is gate-tunable. Realizing the effect in a much more controllable nanostructure may elucidate the rich behavior occurring in those materials, including a quantum phase transition which could play a key role.As a complement to precision transport studies, a scanning gate experiment, where a metallic tip is scanned over the surface of a nanopatterned bilayer heterostructure, can perturb the Kondo cloud at a controlled location, providing a direct mapping of the spatial extent of a quantum many-body state.