Electron Correlations at the Edge of Instability: Complex Materials and Restricted Geometries

这是一个新项目，由一个PI与加州大学圣地亚哥分校的一个成熟的科学家小组合作，研究“高度相关的电子系统”。这些是复杂的系统，在相同的材料系统中发现超导性，磁性，金属导电性和绝缘行为，通常只有成分或结构的微小变化。一个电子带窄，筛选有限，电子-电子相关能高，导致电子系统在不同竞争基态时普遍不稳定。在这种不稳定性的邻域中，降低维数从而增加相关能或增加磁矩的效果可能非常显著。这种复杂性在更传统的材料中是不存在的，在这些材料中，性质通常可以用费米液体理论很成功地描述，而且很少会因为微小的变化而产生戏剧性的效果。在这些复杂的系统中，测量同一样品的所有性质的重要性怎么强调都不过分，因为细微的、通常几乎无法测量的结构或化学变化会极大地影响性质。从低温到任何相变的比热测量揭示了电子系统的重要特性。我们能够测量薄膜和微克大小的单晶的比热，可以在相同的样品上测量磁输运，磁化，红外和隧道光谱。我们计划与Maple、Schuller、Basov和Dynes合作，准备和测量相关能随成分或降维而变化的材料样品，并与理论家Sham、Arovas和Renn合作，为这些材料开发模型。这是一位PI与加州大学圣地亚哥分校的一组知名科学家合作的一个新项目，旨在研究“高度相关的电子系统”。这些材料在凝聚态物理中引起了极大的兴奋，并且具有巨大的应用潜力，例如高温超导体或巨磁电阻。这些材料的相图是复杂的，在相同的材料系统中发现了超导性、磁性、金属导电性或绝缘行为，通常只有成分或结构的微小变化。这种复杂性在传统材料中是不存在的。电子之间的相互作用，导致磁性、超导性或绝缘行为，相互竞争，一个或另一个获胜。因此，这些新材料中的电子似乎处于许多不同类型行为的边缘，根据结构或组成的极其细微的差异，它们有可能走向不同的方向。考虑到这一点，测量同一样品的所有性质的重要性怎么强调都不为过，因为细微的、通常几乎无法测量的结构或化学变化会极大地影响性质。我们在这里有一个独特的能力，可以对其他地方无法测量的样品进行基本测量（比热）。这笔资助将使我们能够利用我们独特的专业知识与他人在样品制备、表征、测量和理论理解方面的专业知识开展合作。* * *

9705300 Hellman This is a new project by a PI who collaborates with a well established group of scientists a UC San Diego to study "highly correlated electron systems". These are complex systems, with superconductivity, magnetism, metallic conduction, and insulating behavior found in the same material system, often with only small changes in composition or structure. One electron bands are narrow, screening is limited, and electron-electron correlation energy is high, leading to a general instability of the electronic system to different competing ground states. In the neighborhood of such instabilities, the effects of reducing dimensionality, thereby increasing the correlation energy, or of adding magnetic moments can be extremely dramatic. This complexity is absent in more traditional materials, where properties can generally be quite successfully described using Fermi liquid theory, and dramatic effects seldom result from small changes. In these complex systems the importance of measuring all properties on the same sample cannot be overstated, since subtle and often nearly unmeasurable structural or chemical changes can drastically affect properties. Measurements of specific heat from low temperature up through any phase transitions reveal important characteristics of the electronic system. Our ability to measure the specific heat of thin films and microgram size single crystals allows measurements of magneto-transport, magnetization, IR and tunneling spectroscopy on the same samples. We plan collaborations with Maple, Schuller, Basov, and Dynes to prepare and measure samples of materials in which correlation energies can be varied, either by composition or reduced dimensionality, and with theorists Sham, Arovas, and Renn to develop models for these materials. %%% This is a new project by a PI who collaborates with a well established group of scientists a UC Sa n Diego to study "highly correlated electron systems". These are materials causing great excitement in condensed matter physics and with great potential for applications, such as high temperature superconductors or colossal magnetoresistance. The phase diagrams of these materials are complex, with superconductivity, magnetism, metallic conduction, or insulating behavior found in the same material system, often with only small changes in composition or structure. This complexity is absent in more traditional materials. The interactions between the electrons, which cause the magnetism, superconductivity, or insulating behavior, compete with each other, with one or another winning. It thus appears that the electrons in these new materials are perched on the edge of many different types of behavior, with the potential to go different ways depending on extremely subtle differences in structure or composition. With this in mind, the importance of measuring all properties on the same sample cannot be overstated, since subtle and often nearly unmeasurable structural or chemical changes can drastically affect properties. We have a unique capability here to make a basic measurement (specific heat) on samples which cannot be measured elsewhere. This grant will allow us to develop collaborations taking advantage of our unique expertise together with others' expertise in sample preparation, characterization, measurement and theoretical understanding. ***