Electron and Lattice Dynamics Across Phase Transitions in Triangular Lattices and Atom Chains on Surfaces

****非技术摘要****具有层状或链状晶体结构的低维电子材料是现代材料研究中一些最令人兴奋的发现的核心。诺贝尔奖得主发现人工结构半导体材料中的新型量子力学效应（1985年，1998年），堆积金属层中的巨磁电阻（由于磁场存在而导致的电阻变化）（2007年），在由铜和氧制成的低维化合物中存在无能量损失的导电性（“高温超导性”）（1987年），这只是定义凝聚态科学前沿的变革性发现的几个例子。特别是，技术上重要的氧化物材料的迷人特性植根于电子的相关运动，这是由电子电荷之间的微妙相互作用引起的，它的磁性被称为“自旋”或“磁矩”，以及原子振动。该项目旨在研究低维模型系统中这些微妙的相互作用，例如表面上的原子链或单原子层。这些极低维系统也表现出与相关电子运动相关的丰富物理特性，但它们比大块氧化物材料更容易控制和分析。该项目的关键方面将是教育和培训两名博士研究生和一名博士后研究助理使用先进的科学仪器和分析解决问题。这些品质和经验为在学术界和高科技行业的职业生涯提供了良好的准备。该项目培育和扩展了材料创新的基础知识基础和劳动力，最终可能推动技术和经济发展。****技术摘要****表面和界面是研究低维电子系统和相变的理想平台。本项目将重点研究IV族元素在奇数电子数的Si（111）和Ge（111）表面的三角形表面相；以及具有嵌套费米面和强自旋轨道耦合的硅表面上的准一维原子链。这些表面相表现出由竞争性电子-电子相互作用、电子-声子耦合、磁相互作用、对称性破缺和几何挫折引起的丰富的物理性质，并且适用于多体相互作用的第一性原理计算和理论建模。利用高分辨率角分辨光发射和氦原子散射的独特组合，将研究这些系统中电子相变的性质和驱动力。电子和声子激发之间的强耦合将使用一种新颖的时间分辨光发射泵浦-探针方案来解开。最后，将利用化学掺杂和外延应变压力调谐来探索多体相互作用和电子相图的性质和强度。该项目为凝聚态物理前沿的两名博士生和一名博士后的教育和培训提供了一个极好的环境。这些学习机会将通过直接访问先进的仪器和理论合作者的国际网络而得到加强。成功的实施不仅推进了现代凝聚态科学的前沿，而且还培养了基础知识，最终将促进通过设计创造新的功能材料。

****NON-TECHNICAL ABSTRACT****Low-dimensional electronic materials with layered or chain-like crystal structures are at the core of some of the most exciting discoveries in modern materials research. Nobel prize winning discoveries of novel quantum-mechanical effects in artificially structured semiconducting materials (1985, 1998), giant magneto-resistance (the change in electrical resistance due to the presence of a magnetic field) in stacked metal layers (2007), and the existence of electrical conductivity with no energy loss ("high-temperature superconductivity") in low-dimensional compounds made from copper and oxygen (1987) are just a few examples of transformational discoveries that define the frontiers of condensed matter science. In particular, the fascinating properties of technologically important oxide materials are rooted in the correlated motion of electrons, arising from the delicate interplay between the electron's charge, its magnetic property known as "spin" or "magnetic moment", and atomic vibrations. This project aims to study these delicate interactions in low-dimensional model systems, such as atom chains or single-atom layers on surfaces. These extreme low-dimensional systems also exhibit the rich physics associated with correlated electron motion but they are easier to control and analyze than bulk oxide materials. The key aspect of this project will be the education and training of two PhD students and a postdoctoral research associate in the use of advanced scientific instrumentation and analytical problem solving. These are qualities and experience that provide an excellent preparation for careers in academia and high-tech industry. The project both nurtures and expands the fundamental knowledge base and workforce for materials innovations that may ultimately drive technological and economic development.****TECHNICAL ABSTRACT****Surfaces and interfaces function as ideal platforms for studying low-dimensional electron systems and phase transitions. This project will focus on the triangular surface phases of group IV elements on Si(111) and Ge(111) surfaces with odd electron counts; and on quasi one-dimensional atom chains on silicon surfaces with nested Fermi surfaces and strong spin-orbit coupling. These surface phases exhibit the rich physics arising from competing electron-electron interactions, electron-phonon coupling, magnetic interactions, broken symmetries and geometrical frustration, and are amenable to first principles calculations and theoretical modeling of the many-body interactions. The nature and driving forces of the electronic phase transitions in these systems will be studied using the unique combination of high-resolution angle-resolved photoemission and helium atom scattering. The strong coupling between the electronic and phonon excitations will be disentangled using a novel pump-probe scheme for time-resolved photoemission. Finally, the nature and strength of the many-body interactions and electronic phase diagram will be explored using chemical doping and pressure tuning via epitaxial strain. This program provides an excellent setting for the education and training of two PhD students and a postdoc at the frontiers of condensed matter physics. These learning opportunities will be enhanced by direct access to sophisticated instrumentation and an international network of theory collaborators. Successful implementation not only advances the frontiers of modern condensed matter science but also fosters fundamental knowledge that will eventually facilitate the creation of novel functional materials by design.