Surfaces and Interfaces of Layered Transition Metal Oxides

本研究项目解决了与层状过渡金属氧化物（TMOs），特别是表面和界面的对称性破坏物理相关的挑战和机遇。在TMOs中，电子的电荷和自旋与晶格自由度之间的强相互耦合导致了诸如电荷有序、轨道有序和自旋有序、巨大磁电阻和非常规超导性等显著效应。从概念上讲，创建一个表面或接口是一种干扰耦合系统的可控方式。这种独特的环境将产生新的现象，同时为研究这些复杂材料中的自旋-电荷-晶格耦合提供了新的途径。利用新获得的NEC光学浮区炉，制备天然层状TMOs的大单晶，并利用激光分子束外延（MBE）制备人工层状结构。变温扫描隧道显微镜（STM）将用于绘制表面原子和电子分布的空间图像。表面结构和晶格动力学将通过弹性和非弹性电子散射来确定。通过磁光克尔效应和变温磁力显微镜的结合，在宏观和微观层面上确定表面磁性，并使用非线性克尔旋转来观察埋藏界面。基于同步加速器的x射线散射将用于测量界面结构。理论程序将使用最先进的第一性原理计算机代码，在并行机器上运行，计算表面结构和电子和磁性，包括化学计量效应。本研究将与研究生和本科生一起进行，并有博士后研究助理的参与。在过去的五十年里，材料物理学的进步极大地改善了我们的生活。从计算机芯片到高透明光纤的发展，从国防应用到更好的滑雪板、网球拍和自行车，新材料的可用性推动了科学技术的进步。该项目是田纳西大学和橡树岭国家实验室（ORNL）的合作项目，旨在为一系列被称为过渡金属氧化物（TMOs）的材料开发世界级的合成、表征和教育设施。对这类材料的兴趣源于其丰富的物理性质（例如，包括不寻常的超导磁性和光学性质，高温超导性，巨磁阻），其基础物理的复杂性以及技术应用的前景。大型单晶和人工分层结构将被合成和研究，使用一套非凡的实验工具（包括ORNL的中子散射）和最先进的第一性原理理论技术。由于薄膜在技术应用中的重要性和对新出现现象的展望，本文将重点讨论表面和界面现象。目标有两个：(1)学习如何在这些复杂的TMOs中定制电子、磁性和结构表面（或界面）相变，用于传感器和传感器的应用；(2)创建一个新的跨学科课程和观点，说明合成和制造新材料对材料物理学进步的重要性。该研究将与研究生和本科生以及博士后研究人员一起进行。因此，他们将接受前沿研究领域的培训，并为进入21世纪的科技劳动力做好准备

This research project addresses the challenges and opportunities associated with the physics of broken symmetry in layered transition-metal oxides (TMOs), specifically at surfaces and interfaces. In TMOs, the strong mutual coupling between charge and spin of the electrons and the lattice degrees of freedom results in dramatic effects such as charge-, orbital-, and spin-ordering, colossal magnetoresistance, and unconventional superconductivity. Conceptually, creating a surface or interface is a controlled way to perturb the coupled system. This unique environment will produce new phenomena, while providing a fresh approach to the study of the spin-charge-lattice coupling in these complex materials. Large single crystals of naturally layered TMOs will be grown using a newly acquired NEC optical floating zone furnace, and artificially layered structures will be fabricated using laser molecular beam epitaxy (MBE). Variable-temperature scanning tunneling microscopy (STM) will be used to render spatial images of the atomic and electronic distributions at the surface. Surface structure and lattice dynamics will be determined with elastic and inelastic electron scattering. Surface magnetism will be determined both at a macroscopic and microscopic level by a combination of magneto-optical Kerr effect and variable-temperature magnetic force microscopy, with non-linear Kerr rotation used to observe buried interfaces. Synchrotron-based x-ray scattering will be utilized to measure interfacial structure. The theoretical program will use state-of-the-art first-principles computer codes, running on parallel machines, to calculate the surface structure and the electronic and magnetic properties, including stoichiometry effects. This research will be performed with graduate and undergraduate students, with the participation of postdoctoral research associates. %%%Advances in materials physics have dramatically improved our lives over the past fifty years. From the computer chip to the development of highly transparent optical fibers, from defense applications to better skis, tennis rackets, and bicycles, the availability of new materials has enabled progress in science and technology. This project, a partnership between The University of Tennessee and Oak Ridge National Laboratory (ORNL), is aimed at the development of a world-class synthesis, characterization, and educational facility for a family of materials known as transition-metal oxides (TMOs). Interest in this general class of materials stems from the richness of their physical properties (e.g., including unusual superconducting magnetic and optical properties, high temperature superconductivity, colossal magnetoresistance), complexity of their underlying physics, and the promise of technological applications. Large single crystals as well as artificially layered structures will be synthesized and studied using an extraordinary set of experimental tools (including neutron scattering at ORNL) and state-of-the-art first-principles theoretical techniques. The emphasis will be on surface and interface phenomena because of the importance of thin films in technological applications and the prospect on new emergent phenomena. The objectives are twofold: (1) to learn to tailor the electronic, magnetic, and structural surface (or interface) phase transitions in these complex TMOs for applications as sensors and transducers, and (2) to create a new, interdisciplinary curriculum and perspective on the importance of synthesis and fabrication of new materials for the advance of materials physics. The research will be conducted with graduate and undergraduate students as well as with postdoctoral research associates. They will thereby receive training in forefront research areas and be prepared to enter the scientific- technological workforce of the 21st century.***