RII Track-4: Probing the Electronic States of Quantum-Confined Topological Insulator Nanostructures

当材料非常小时，材料的属性可能会发生巨大变化。纳米粒子已经在从医学到消费电子产品的一系列应用中使用。纳米粒子最令人兴奋的应用之一是用作量子计算机中的比特。为了让量子计算机工作，比特(称为量子比特)必须保持在它们准备好的状态。不幸的是，今天存在的大多数量子比特与环境有很强的相互作用，这会导致量子比特的状态以一种不受欢迎的方式发生变化。在这个项目中，我们将研究被称为拓扑绝缘体的新材料。理论预测，当拓扑绝缘体被制成纳米粒子时，它们可能会充当只受环境微弱干扰的好量子比特。本项目的目标是合成拓扑绝缘子纳米颗粒并研究其性质。这些纳米粒子将使用最先进的工具进行合成和表征，如特拉华大学的分子束外延和布鲁克海文国家实验室的扫描隧道显微镜和角度分辨光电子能谱。该项目的成果将是深入了解拓扑绝缘子在极小尺寸范围内的行为，这对更广泛的科学界是有用的，并对其在各种应用中的适用性进行评估。当材料被限制在纳米尺度上时，它们的电子占据了量子化的离散能级。到目前为止，大多数关于量子限制的研究都是使用基于半导体的材料。最近，人们对一类名为拓扑绝缘体(TIS)的新材料产生了浓厚的兴趣。这些材料包含二维表面态，其中包含无质量的电子，这些电子在拓扑上受到保护，不受后向散射的影响。最近的理论建议表明，当限制在纳米尺度上时，拓扑绝缘体中的表面态应该表现出离散的、量子化的能级，这些能级保持拓扑保护。该研究会的首要目标是了解拓扑绝缘体纳米颗粒中离散状态的性质。为了实现这一目标，特拉华大学将使用分子束外延技术合成各种尺寸的钛纳米颗粒。布鲁克海文国家实验室将使用扫描隧道显微镜和角度分辨光电子能谱来表征纳米粒子的电子结构。这项研究的目标是1)确定量子化的能级间距如何依赖于纳米粒子的尺寸和温度；2)了解能量-动量色散关系如何随着粒子尺寸和温度的变化而变化；3)测量量子化的TI态的自旋极化程度；4)绘制TINPs中的波函数图，以确定它们的空间对称性。该项目的预期结果是不同维度的TI纳米粒子的实验自旋分辨能带结构图。这将是第一次对TI NPs中的量子化状态进行实验测量，并将为科学研究和新设备的可能性打开大门。这一奖项反映了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。

The properties of materials can change dramatically when the materials are extremely small. Nanoparticles are already in use in a range of applications ranging from medicine to consumer electronics. One of the most exciting applications for nanoparticles is for use as the bit in a quantum computer. For a quantum computer to work, the bits (called qubits) must remain in the state that they are prepared in. Unfortunately, most qubits that exist today have strong interactions with the environment that cause the state of the qubit to change in an undesirable way. In this project, we will study new materials called topological insulators. When topological insulators are made into nanoparticles, theory predicts that they may act as good qubits that are only weakly perturbed by their environment. The goal of this project is to synthesize topological insulator nanoparticles and study their properties. The nanoparticles will be synthesized and characterized using state-of-the-art tools like molecular beam epitaxy at the University of Delaware and scanning tunneling microscopy and angle-resolved photoemission spectroscopy at Brookhaven National Laboratory. The outcome of this project will be a deep understanding of how topological insulators behave at extremely small size scales which is of use to the broader scientific community and an evaluation of their suitability for a variety of applications. When materials are confined to nanoscale dimensions, their electrons occupy quantized discrete energy levels. To date, most of the research into quantum confinement has used semiconductor-based materials. Recently, there has been substantial interest in a new class of materials called topological insulators (TIs). These materials contain two-dimensional surface states that house massless electrons that are topologically-protected from backscattering. Recent theoretical proposals indicate that, when confined to nanoscale dimensions, the surface states in topological insulators should exhibit discrete, quantized energy levels that remain topologically-protected. The overarching goal of this fellowship is to understand the properties of discrete states in topological insulator nanoparticles. To accomplish this goal, TI nanoparticles with a range of sizes will be synthesized using molecular beam epitaxy at the University of Delaware. The electronic structure of the nanoparticles will be characterized at Brookhaven National Laboratory using scanning tunneling microscopy and angle-resolved photoemission spectroscopy. The objectives for this fellowship are to 1) determine how the quantized energy level spacing depends on nanoparticle size and temperature; 2) understand how the energy-momentum dispersion relationship changes as a function of particle size and temperature; 3) measure the degree of spin-polarization of the quantized TI states; 4) map the wavefunctions in TI NPs to determine their spatial symmetry. The expected outcome of this project is an experimental spin-resolved band structure diagram for TI nanoparticles of varying dimensions. This will be the first experimental measurement of quantized states in TI NPs and will open the door to new avenues of scientific research and new device possibilities.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.