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。纳米粒子已经在从医学到消费电子产品的一系列应用中得到了应用。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。为了使量子计算机工作，比特（称为量子比特）必须保持在它们准备的状态。不幸的是，今天存在的大多数量子位与环境有很强的相互作用，导致量子位的状态以不受欢迎的方式改变。在这个项目中，我们将研究称为拓扑绝缘体的新材料。当拓扑绝缘体被制成纳米粒子时，理论预测它们可以充当良好的量子比特，只受到环境的微弱扰动。本计画的目标是合成拓扑绝缘体奈米粒子并研究其性质。纳米粒子将使用最先进的工具合成和表征，如特拉华州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分辨光电子能谱。该项目的成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。到目前为止，大多数对量子限制的研究都使用了基于半导体的材料。最近，人们对一类称为拓扑绝缘体（TI）的新材料产生了浓厚的兴趣。这些材料包含二维表面状态，其中包含无质量电子，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，当被限制在纳米尺度时，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。该奖学金的首要目标是了解拓扑绝缘体纳米粒子中离散态的性质。为了实现这一目标，特拉华州大学将使用分子束外延合成具有一系列尺寸的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分辨光电子能谱进行表征。该奖学金的目标是：1）确定量子化能级间距如何取决于纳米颗粒的尺寸和温度; 2）了解能量-动量色散关系如何作为颗粒尺寸和温度的函数变化; 3）测量量子化TI态的自旋极化程度; 4）映射TI NP中的波函数以确定其空间对称性。该项目的预期成果是不同尺寸的TI纳米颗粒的实验自旋分辨能带结构图。这将是首次对TI NP中的量子态进行实验测量，并将为科学研究和新设备的可能性开辟新途径。该奖项反映了NSF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.