Imaging Correlated Electron States in Single-layer Field-Effect Transistors

非技术摘要：这个项目的目的是探索存在于电子之间的排斥力(物理学家称之为“电子-电子相互作用”)很强并导致有趣的新行为的材料。这种现象在将电子限制在二维(2D)的材料中扮演着重要的角色，例如只有一个原子厚度的平面材料。该项目的一个中心活动是使用一种名为扫描隧道显微镜(STM)的仪器来表征2D材料中的这种行为，扫描隧道显微镜本质上是一种锋利的金属针，用于以非常高的空间分辨率(甚至低至单个原子的大小)直接成像电子的行为。这种感兴趣的现象被称为“多体”行为，因为如果一个电子在具有强烈电子-电子相互作用的材料中运动，那么所有其他电子都必须移动才能为它让路(就像人们在拥挤的房间里移动一样)。这导致了新的电磁行为，可能会被利用来创造有用的新技术。该项目的一个新方面是以一种新的方式合成了精致的2D材料，允许使用称为栅极的电接触连续改变材料中的电子密度，同时使用在极低温度下保持的STM对电子进行成像。这使研究小组能够对有关电子在不同条件下在2D材料中的行为的各种科学假说进行实验测试。该团队通过期刊出版物、国际会议演讲、新闻发布和面向广大受众的教育视频来传播项目成果。该项目还通过不同的外展计划为博士后、研究生、本科生以及高中生提供跨学科领域的高水平科学培训，从而为他们在STEM领域的职业生涯做好准备。技术摘要：该项目专注于通过结合栅极可调谐场效应管(FET)和扫描隧道显微镜(STM)来表征和操纵高度关联的单层二维(2D)材料的局部电子结构。探索了小带隙单层系统，其性质不容易被STM在剥离的样品中探测到，而必须通过分子束外延(MBE)生长的样品来访问。一种新开发的实验技术被用来将单层分子束外延生长的材料结合到可以用低温扫描隧道显微镜成像的栅调谐场效应晶体管中。这些样品的额外栅极可调谐载流子密度使得能够探索STM通常无法访问的新的物理区域，从而能够对高度相关的低维电子行为进行独特的原子分辨研究。这些材料系统中杂质的可控添加和操纵为探索低维杂质物理和测试为高度相互作用的材料开发的长期理论创造了新的机会。这些任务的成功将为基本问题提供重要的新的物理见解，例如相关性和拓扑学、磁相互作用和一维近藤物理之间的相互作用，以及降维系统中的莫特相变和量子自旋液体行为。该奖项反映了NSF的法定使命，并通过使用基金会的智力优势和更广泛的影响审查标准进行评估，被认为值得支持。

Nontechnical abstract: This project is aimed at exploring materials where the repulsion that exists between electrons (referred to by physicists as “electron-electron interactions”) is strong and leads to interesting new behavior. Such phenomena play an important role in materials that confine electrons to two dimensions (2D), such as planar materials only a single atom thick. A central activity of this project is the characterization of such behavior in 2D materials using an instrument called a scanning tunneling microscope (STM), which is essentially a sharp metal needle used to directly image the behavior of electrons at very high spatial resolution (even down to the size of single atoms). The phenomena of interest are referred to as “many-body” behavior because if one electron moves in a material with strong electron-electron interactions then all the other electrons have to move to make way for it (like people moving in a crowded room). This leads to new electronic and magnetic behavior that could potentially be exploited to create useful new technologies. A novel aspect of this project is that delicate 2D materials are synthesized in a new way that allows the density of electrons in the materials to be varied continuously using an electrical contact called a gate while simultaneously imaging the electrons with an STM held at very low temperature. This enables the research team to experimentally test various scientific hypotheses regarding how electrons behave in 2D materials under different conditions. The team disseminates the project results through journal publications, international conference talks, news releases, and educational videos targeting a broad audience. The project also provides high-level scientific training in a strongly interdisciplinary area to postdocs, graduate students, and undergraduates, as well as high school students through different outreach programs, thus preparing them for careers in STEM fields.Technical abstract: This project focuses on the characterization and manipulation of the local electronic structure of highly correlated single-layer two-dimensional (2D) materials by combining gate-tunable field effect transistors (FETs) with scanning tunneling microscopy (STM). Small-bandgap single-layer systems are explored whose properties cannot easily be probed by STM in exfoliated samples, but rather must be accessed via samples grown by molecular beam epitaxy (MBE). A newly developed experimental technique is used to incorporate single-layer MBE-grown materials into gate-tunable field effect transistors that can be imaged by cryogenic STM. The additional gate-tunability of carrier density for these samples enables the exploration of new physical regimes that are typically not accessible to STM, thus enabling unique atomically-resolved studies of highly correlated low-dimensional electronic behavior. The controllable addition and manipulation of impurities in these material systems creates new opportunities for exploring low-dimensional impurity physics and for testing long-standing theories developed for highly interacting materials. Success at these tasks will provide important new physical insight into fundamental questions, such as the interplay between correlation and topology, magnetic interactions and Kondo physics in 1D, as well as the Mott transition and quantum spin liquid behavior in systems of reduced dimensionality.This award reflects NSF's statutory mission and has been deemed worthy of support through evaluation using the Foundation's intellectual merit and broader impacts review criteria.