Electron Spectroscopy of Emergent Quantum Structures

根据化学成分、温度或压力的轻微变化，强关联电子材料表现出显著的量子基态范围，例如绝缘、金属、磁性、超导。最近人们非常感兴趣的是处于两个稳定的零温度基态之间的系统的行为，即在一个量子临界点。这个单独的研究项目使用光电子发射和逆光电子发射光谱技术来测量显示出新的量子行为的选定强关联电子材料的单粒子电子结构，并试图构建这些行为从潜在的电子结构出现或从一种顺序转换到另一种顺序的路径。在下一个项目期间，该计划将集中于两个主要主题，(A)与(1)非费米液体(NFL)行为有关的量子临界性，(2)低维材料中的电子分馏，以及(B)某些新颖的或聚合的相变。如果成功，这项工作将证明需要一幅总体的理论图景，将量子临界性、分馏化和超越维度和材料类型的NFL行为联系起来。这些实验既在家庭实验室进行，也在各种国内和国际同步加速器设施中进行。实验数据通过与以不同方式处理库仑相互作用的理论进行比较来分析，这些理论提供了光谱与电或磁性质之间的联系。该计划依赖于与其他小组的密切合作，以获得表征良好的样品进行测量，确定新材料的电、热和磁性能，并提供专家理论建议和高级计算。因此，它建立了跨越地理、机构和学科边界的人与人之间的桥梁。它还让博士生与世界各地各种机构的各种专业角色的科学家密切接触，并对他们进行协作工作技术的培训。材料的电、磁和机械属性是其电子和组成原子的基本量子力学的“紧急集体行为”。固体物理学和材料科学的一个主要目标就是解释这种现象。实现这一目标将意味着有能力设计出对任何特定应用都是最佳的材料。目前对固体中电子的主要理解是“费米液体理论”。这一理论解释了为什么固体中的电子通常可以用一幅简单的图画来描述，而似乎忽略了已知的电子相互作用的巨大排斥力。越来越多的人认识到，这一理论可能不适用于整个材料类别，有人怀疑，这种失败与不同可能出现的行为之间尚未解决的竞争有关。这项个人研究人员拨款支持一项实验计划，该计划旨在使用一种名为电子光谱学的技术来测量和量化材料中电子的潜在量子力学行为，这些材料以各种方式表现出这种尚未解决的竞争。如果成功，这项工作将确定基本的想法，这些想法必须结合在一起，将工程材料的目标带到下一个复杂的水平。这些实验既在家庭实验室进行，也在各种国内和国际同步加速器设施中进行。该计划依赖于与其他小组的密切合作，以获得表征良好的样品进行测量，确定新材料的电、热和磁性能，并提供专家理论建议和高级计算。因此，它建立了跨越地理、机构和学科边界的人与人之间的桥梁。它还让博士生与世界各地不同机构中担任各种专业角色的科学家密切接触，并培训他们合作工作的技术。

Strongly correlated electron materials exhibit a remarkable range of quantum ground states, e.g., insulating, metallic, magnetic, superconducting, depending on modest changes in chemical composition, temperature or pressure. Of great recent interest are the behaviors of a system poised between two stable zero temperature ground states, i.e. at a quantum critical point. This individual investigator project uses the techniques of photoemission and inverse photoemission spectroscopies to measure the single-particle electronic structures of selected strongly correlated electron materials displaying novel quantum behaviors, and seeks to construct the paths whereby these behaviors emerge from the underlying electronic structure or transform from one order to another. During the course of the next project period, the program will focus on two main themes, (A) quantum criticality in relation to (1) Non-Fermi Liquid (NFL) behavior generally and (2) electron fractionalization in low dimensional materials, and (B) certain novel or paradigmatic phase transitions. If successful the work will demonstrate the need for an overarching theoretical picture linking quantum criticality, fractionalization and NFL behavior that transcends dimensionality and material type. The experiments are performed both in a home laboratory, and at various national and international synchrotron facilities. The experimental data are analyzed by comparison to theories which treat the Coulomb interactions in different ways, and which provide a link between the spectra and the electrical or magnetic properties. The program relies on strong collaborations with other groups for well-characterized samples to measure, for determining electrical, thermal and magnetic properties of new materials, and for expert theoretical advice and advanced calculations. Thus, it builds human bridges across geographic, institutional and disciplinary boundaries. It also brings Ph.D. students into close contact with scientists in a variety of professional roles at a variety of institutions around the world, and trains them in the techniques of collaborative work.The electrical, magnetic and mechanical properties of materials are "emergent collective behaviors" of the underlying quantum mechanics of their electrons and constituent atoms. A principal goal of solid state physics and materials science is to elucidate this emergence. Achieving this goal would imply the ability to engineer a material that is optimum for any particular application. The mainstay of the current understanding of electrons in solids is known as the "Fermi liquid theory." This theory explains why electrons in solids can often be described in a simplified picture that appears to ignore the large repulsive forces electrons are known to exert on one another. There is a growing appreciation that this theory probably fails for entire classes of materials and there is the suspicion that the failure has to do with unresolved competition between different possible emergent behaviors. This individual investigator grant supports an experimental program aimed at using a technique called electron spectroscopy to measure and quantify the underlying quantum mechanical behaviors of electrons in materials that manifest this unresolved competition in various ways. If successful the work will pinpoint essential ideas that must be combined to take the goal of engineering materials to the next level of sophistication. The experiments are performed both in a home laboratory, and at various national and international synchrotron facilities. The program relies on strong collaborations with other groups for well-characterized samples to measure, for determining electrical, thermal and magnetic properties of new materials, and for expert theoretical advice and advanced calculations. Thus, it builds human bridges across geographic, institutional and disciplinary boundaries. It also brings Ph.D. students into close contact with scientists in a variety of professional roles at a variety of institutions around the world, and trains them in the techniques of collaborative work.