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

当材料非常小的时候，材料的性质会发生巨大的变化。纳米粒子已经应用于从医药到消费电子等一系列领域。纳米粒子最令人兴奋的应用之一是在量子计算机中用作比特。为了使量子计算机工作，比特（称为量子位）必须保持在它们准备好的状态。不幸的是，目前存在的大多数量子比特都与环境有很强的相互作用，导致量子比特的状态以一种不希望的方式改变。在这个项目中，我们将研究一种叫做拓扑绝缘体的新材料。当拓扑绝缘体被制成纳米粒子时，理论预测它们可以作为良好的量子比特，只受环境的微弱干扰。本课题的目标是合成拓扑绝缘体纳米粒子并研究其性能。纳米粒子的合成和表征将使用最先进的工具，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发射光谱。该项目的结果将是深入了解拓扑绝缘体在极小尺寸尺度下的行为，这对更广泛的科学界有用，并评估其对各种应用的适用性。当材料被限制在纳米尺度时，它们的电子占据量子化的离散能级。迄今为止，大多数关于量子约束的研究都使用了半导体材料。最近，人们对一种叫做拓扑绝缘体（TIs）的新材料产生了浓厚的兴趣。这些材料包含二维表面状态，可以容纳无质量的电子，这些电子在拓扑上受到保护，不受反向散射的影响。最近的理论建议表明，当局限于纳米尺度时，拓扑绝缘体的表面状态应该表现出离散的、量子化的能级，并保持拓扑保护。本研究的首要目标是了解拓扑绝缘体纳米颗粒中离散态的性质。为了实现这一目标，特拉华大学将利用分子束外延技术合成各种尺寸的TI纳米颗粒。纳米粒子的电子结构将在布鲁克海文国家实验室使用扫描隧道显微镜和角度分辨光发射光谱进行表征。本研究的目标是：1)确定量子化能级间距如何取决于纳米颗粒的大小和温度；2)了解能量-动量色散关系随粒径和温度的变化规律；3)测量量子化TI态的自旋极化程度；4)绘制TI np中的波函数，确定它们的空间对称性。该项目的预期结果是不同尺寸的钛纳米粒子的实验自旋分辨能带结构图。这将是首次在TI纳米粒子中进行量子化态的实验测量，并将为科学研究和新设备的可能性打开大门。该奖项反映了美国国家科学基金会的法定使命，并通过使用基金会的知识价值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.