Electronic Nematic Phases in Correlated Electron Systems

水冷却时就会结成冰。冰和水的性质截然不同。从水到冰的状态变化是“相变”的常见例子。当冰形成时，水分子有序，系统的对称性降低。在这个项目中，我们将研究一些具有“电子向列相”(ENP)的系统的性质。要研究的材料是层状Ru酸盐类和铁基超导体。这些材料的共同特点是在不改变晶体对称性的情况下，电子性质(如电阻)在低温下变得各向异性。这表明，载流电子(传导电子)在低温下形成了一种新的有序状态或电子向列相。在过去的几年里，电子向列相在凝聚物质中的重要性已经被认识到。它们存在于二维半导体材料和强磁场下的Ru3Ru2O7材料、高温(铜酸盐)超导体以及最近发现的铁基超导体中。本项目的目的是将两种类型的ENP(铁基超导体和铬酸盐)的各向异性电子行为与它们的初始磁性质联系起来。我们将使用中子和x射线散射来研究这些材料中的集体磁激发。实验将在国际设施中进行，如英国的ISIS散裂源和法国的ILL。我们的初步结果表明，集体磁激发是理解这两个系统中ENP行为的关键。新的仪器进展将使我们能够通过施加单轴应力或强磁场首次在原位创建和研究ENP相。

When water is cooled it forms ice. The properties of ice and water are dramatically different. The change of state from water to ice is a common example of a "phase transition". When ice forms, the water molecules order and the symmetry of the system is lowered. In this project, we will investigate the properties of a number of systems which have "electronic nematic phases" (ENP's). The materials to be investigated are layered ruthenates and iron-based superconductors. The common feature of the these materials is that the electronic properties (such as the electrical resistance) become anisotropic at low temperatures without a change of crystal symmetry. This suggests that the current carrying electrons (conduction electrons) form a new "ordered" state or electronic nematic phase at low temperature.The importance of electronic nematic phases in condensed mater has been recognized in the last few years. They occur in two dimensional semiconductor materials and the ruthenate material Sr3Ru2O7 at high magnetic field, high-temperature (cuprate) superconductors, and the recently discovered iron-based superconductors. The objective of the present project is to relate the anisotropic electronic behaviour of two types of ENP (iron-based superconductors and ruthenates) to their incipient magnetic properties. We will use neutron and x-ray scattering to investigate the collective magnetic excitations in these materials. Experiments will be performed at international facilities such as the ISIS spallation source in the UK and ILL in France. Our preliminary results suggest that the collective magnetic excitations are key to understanding the ENP behavior in both these systems. New instrumental advances will enable us to create and study the ENP phases in situ for the first time by applying a uniaxial stress or high magnetic field.