From Single Ion to Correlation: Probing the Physics of Kondo Topological Insulator with Lattice Constant Engineering

非技术摘要：想知道当你拉伸材料时，材料的电子特性会发生什么变化吗？在电子材料中，原子的价态告诉我们它与其他原子的结合力的强度，并且通常不随时间而变化。在一些非常特殊的材料中，称为“混合价化合物”，化合价在多个值之间快速波动。主要研究人员最近发现，拉伸某些混合价化合物可以减缓或加快化合价的涨落。因此，电子性能被有效地调整，以满足器件应用的要求。在这个项目中，研究小组研究了拉伸对几种所谓的“强相关材料”中混合价的影响。这些材料的替代品已经发现了技术上的重要性：用于MRI诊断的高温超导体，iPad内部的巨磁阻材料，仅举两例。该项目致力于在几种混合价材料上进行晶体生长，测量和器件制造，以揭示混合价物理学的本质，并通过扩展探索其不寻常的电子和热特性，用于超高频振荡器和量子计量学等技术应用。该项目纳入了几个教育组成部分，包括课程开发、K-12推广和学生科学培训。该项目为来自圣安娜贫困地区的高中志愿者以及加州大学欧文分校加州少数民族参与联盟计划的本科研究员提供实习机会，以激发他们的兴趣，并为科学和技术职业做好准备。技术摘要 稀土化合物中通常具有良好局域性和磁性的4f电子在某些材料中可以成为巡回电子，其有效电子质量比自由电子大几个数量级，从而导致不寻常的“强关联”超导，磁性和半导体状态，这些最后被称为近藤绝缘体。理论表明，近藤绝缘体SmB 6属于所谓的拓扑绝缘体的异国情调类，具有有趣的技术潜力的表面导电状态。SmB 6是目前唯一一种强相关的f波段半导体，实验证明了这一点。SmB 6的半导体能隙尺度约为40 K，表面导电态仅在4K以下发展。主要研究人员发现，单轴应变强烈地增加了差距能量，并将表面导电状态的发展移动到200 K，接近于对应用有用的范围。 该项目的目标是发现是什么设定了能隙尺度和伴随的表面态的发展，如何控制这种尺度以及在哪里可以找到其他强相关的拓扑绝缘体。这将在SmB 6和其他近藤绝缘体的单晶合金中进行，通过在环境压力和应变下的传输，磁性和热力学测量。没有理由假设SmB 6是一种独特的材料，了解能隙尺度的设定可以为发现具有应用潜力的其他强相关半导体提供理论基础。

Non-technical abstract: Wonder what happens to the electronic properties of a material when you stretch it? In an electronic material, the valence of an atom tells us the strength of its combining power with other atoms, and does not usually change with time. In some very special materials, called 'mixed valence compounds', the valence rapidly fluctuates between more than one value. It is recently discovered by the principle investigator that stretching some of these mixed valence compounds could slow down or speed up the valence fluctuation. As a result, the electronic properties are effectively tuned to meet the requirements of device applications. In this project, the research team investigates the effect of stretching on mixed valence in several so-called 'strongly correlated materials'. Cousins of these materials have already found technological importance: high temperature superconductors for MRI diagnosis, giant magneto-resistance materials inside iPads, just to name two. This project pursues crystal growth, measurements and device fabrication on several mixed valence materials to unveil the nature of mixed valence physics and by extension explore their unusual electronic and thermal properties for technological applications such as ultra-high frequency oscillator and quantum metrology. Several educational components are integrated into this project, consisting of course development, K-12 outreach and scientific training for students. Internships for high school volunteers from disadvantaged regions in Santa Ana, as well as undergraduate researcher from UC Irvine's California Alliance for Minority Participation program are enabled by this project to generate interest and prepare for careers in science and technology.Technical Abstract The normally well localized and magnetic 4f-electrons in rare earth compounds can in some materials become itinerant electrons with effective electron mass orders of magnitude larger than that of free electrons, leading variously to unusual 'strongly correlated' superconducting, magnetic and semiconducting states, these last known as Kondo insulators. Theory suggests that the Kondo insulator SmB6 belongs to the exotic class of so-called topological insulators possessing a surface conducting state with interesting technological potential. SmB6 is at present the only such strongly correlated f-band semiconductor for which experiment finds good evidence for this. The semiconducting energy gap scale in SmB6 is approximately 40K with the surface conducting state developing only below 4K. The principal investigators find that uniaxial strain strongly increases the gap energy and moves the development of the surface conducting state to 200K, approaching a range useful for application. The goals of this project are to discover what sets the energy gap scale and accompanying development of the surface state, how this scale can be controlled and where other strongly correlated topological insulators can be found. This will be pursued in single crystal alloys of SmB6 and other Kondo insulators via transport, magnetic and thermodynamic measurements at ambient pressure and under strain. There is no reason to suppose that SmB6 is a unique material and understanding what sets the energy gap scale can present the rationale for discovery of other strongly correlated semiconductors in its exotic class with potential for application.