The Microscopic Electronic Structure of Iron Superconductors Under Strain: New Frontiers in Scanning Probe Microscopy

非技术摘要：当一种材料被机械拉伸时，其性能会发生戏剧性的变化。例如，我们熟悉橡胶在拉伸时的弹性如何变化。不太为人所知但同样重要的是材料电子性质的变化。当拉伸时，材料可以改变它们的电阻、光吸收和许多其他电子特性。电子性质发生这种变化的原因通常在微观层面上很难理解。在这个项目中，研究小组将使用扫描隧道显微镜(STM)来研究材料在拉伸时的电子性质。扫描隧道显微镜是一种以单原子的精度测量材料电子性质的技术。在这项研究中，一种专门设计的装置被用来在扫描隧道显微镜中安装和拉伸晶体材料，同时监测它们的响应。该设备由首席研究人员的研究团队设计、制造、组装和运行。它用于向高中、本科和研究生水平的学生传授培训，并用于与行业合作伙伴联系。该项目的目标将是发展对材料中应变带来的变化的微观理解，特别是那些属于铁粒子类超导体的材料。技术摘要：施加在材料上的外部应变可以通过改变能带结构、相互作用强度甚至相变而引起电子变化。了解应变的应用带来的微观变化是该项目感兴趣的关键科学问题。研究的主要范围是研究铁质辉石的电子向列相。在这一阶段，电子性质表现为向列性，即晶格的基本离散旋转对称性自发破缺。发展对向列相电子相及其与这些材料的其他相的耦合的理解是该项目的关键科学目标。使用的主要实验技术将是原子分辨率、低温扫描隧道显微镜测量。该项目实现了一项技术突破，首次允许对样品施加单轴或双轴面内应变，同时在相同的原子分辨表面区域连续进行微观测量。使用该装置，扫描隧道显微镜被用来测量(A)光谱的能隙，(B)来自地形的磁区结构和(C)准粒子干涉的散射率，所有这些都是在应变的影响下进行的。该装置在NaFe(Co)As、Fe(Te)Se和LiF(Co)三个不同的物系相图中使用，表现出非常不同的向列相行为。该项目的最终目标是确定向列性的驱动力及其与小潮中的超导电性的联系。

Non-technical Abstract:When a material is mechanically stretched, its properties can undergo dramatic changes. For example, we are familiar with how the elasticity of rubber changes as it is stretched. Less familiar but equally important are changes to the electronic properties of materials. When stretched, materials can change their electrical resistance, light absorption and a host of other electronic properties. The causes for such changes in the electronic properties are often poorly understood at the microscopic level. In this project, the research team will use scanning tunneling microscopy (STM) to study the electronic properties of materials as they are stretched. STM is a technique to measure the electronic properties of materials with the precision of single atoms. In this research, a specially designed apparatus is used to mount and stretch crystal materials in the STM while their response is monitored. This equipment is designed, manufactured, assembled and run by the principal investigator's research team. It is used to impart training to students at the high school, undergraduate and graduate levels, and is used to liaison with industrial partners. The goal of the project will be to develop a microscopic understanding of the changes brought about by strain in materials, especially those belonging to the iron pnictide class of superconductors. Technical abstract:External strain when applied to materials can cause electronic changes via modifying band structure, interaction strengths and even changes in phase. Understanding the microscopic changes brought about by the application of strain is the key scientific problem of interest in this project. The main scope of the research is to study the electronic nematic phase of the iron pnictides. In this phase, electronic properties display nematicity, the spontaneous breaking of the underlying discrete rotational symmetry of the lattice. Developing an understanding of the nematic electronic phase and its coupling to the other phases of these materials is the key scientific goal of this project. The main experimental technique used will be atomic-resolution, cryogenic STM measurements. The project is enabled by a technical breakthrough that allows for the very first time the application of uniaxial or biaxial in-plane strains to a sample, while continuously performing microscopic measurements on the same atomically resolved area of the surface. Using this apparatus, STM is used to measure (a) gaps from spectroscopy (b) domain structure from topography and (c) scattering rates from quasiparticle interference, all under the influence of strain. The apparatus is used across the phase diagram in three different pnictide families of NaFe(Co)As, Fe(Te)Se and LiFe(Co)As systems that display very different nematic behaviors. The final goal of the project is to determine the driving forces for nematicity and its connection to superconductivity in the pnictides.