Non-Fermi Liquid Behavior, Superconductivity, and Magnetism in Strongly Correlated Electron Systems.

技术总结该奖项支持强关联电子系统的理论研究和教育。研究集中在三个相互关联的领域：1)铜酸盐超导体的物理学，2。)最近发现的铁基磷灰石超导体的物理学，以及3。巡回费米子系统在量子临界点附近的新颖物理。PI将探索这样一种想法，即铜酸盐的物理主要是由接近反铁磁量子临界点驱动的，这是一种自旋涨落情景。在量子临界点，集体自旋激发变得无隙，与它们的相互作用摧毁了相干的费米液体准粒子。同样的相互作用促成了非相干费米子之间的非BCS d波配对。PI计划解决(I)与自旋涨落的相互作用是否引起伪隙物理，以及(Ii)该方法是否可以扩展到真正的强耦合。PI的目标是发展对PNictide超导体的基本理解，包括：尽管有很强的库仑排斥而发展的超导电性的起源，配对带隙的对称性，超导和磁性之间的相互作用，带隙对称性对超导态实验的影响，以及来自配对电子的反馈。PI计划发展一个在量子临界点附近有效的费米液体理论。传统的费米液体描述不起作用，因为自能是频率相关的，而是局域的。这是一个一般性的问题，应该会导致人们对相互作用电子的低能物理有新的理解。本研究的结果可能适用于包括重费米子材料和钴材料在内的更大类别的材料，为研究生提供了良好的训练。PI将让本科生参与这项研究。这项研究还涉及国际合作。非技术总结该奖项支持理论研究和教育，以探索高温超导材料不寻常特性的可能来源。在足够低的温度下，这些材料表现出超导电性--一种物质的电子状态，不会表现出对电流的阻力。这些材料的一个不同寻常和令人兴奋的特征是，第一次出现超导的温度可以比在所有已知材料中观察到的超导发生的最高温度高出大约6倍或更多。如果能够发现在室温附近成为超导的新材料，那么将它们用于电力过渡线路可以节省大量能源，一种全新的超导电子学技术将变得更加实用。但是超导电性是如何在这些材料中产生的呢？PI认为，磁性起着至关重要的作用，特别是当接近一种新的相变时，这种相变发生在绝对零度。波动的磁序可能提供了导致电子配对并转变为超导状态的机制。这项研究项目使用先进的理论方法来确定这一机制的特征，以及它们如何出现在特定超导材料的实验中。这项研究将推进这种超导电性及其可能产生的特殊金属状态的理论。这项研究为研究生提供了很好的培训。PI将让本科生参与这项研究。这项研究还涉及到国际合作。

TECHNICAL SUMMARYThis award supports theoretical research and education on strongly correlated electron systems. The research is focused on three interrelated areas: 1.) the physics of cuprate superconductors, 2.) the physics of recently discovered iron-based pnictide superconductors, and 3.) the novel physics of an itinerant fermionic system near a quantum-critical point.The PI will explore the idea that the physics of the cuprates is primarily driven by proximity to an antiferromagnetic quantum critical point, a spin-fluctuation scenario. At a quantum critical point, collective spin excitations become gapless, and the interaction with them destroys coherent Fermi liquid quasiparticles. The same interaction mediates non-BCS d-wave pairing between incoherent fermions. The PI plans to address (i) whether the interaction with spin fluctuations gives rise to the pseudogap physics and (ii) whether the approach can be extended to a truly strong coupling. The PI aims to develop a fundamental understanding of the pnictide superconductors, including: the origin of superconductivity which develops despite strong Coulomb repulsion, the symmetry of the pairing gap, the interplay between superconductivity and magnetism, the implications of the gap symmetry for the experiments in the superconducting state, and the feedback from the pairing on electrons.The PI plans to develop a Fermi liquid theory valid near a quantum critical point. A conventional Fermi liquid description does not work because the self-energy is frequency dependent but local. This is a general problem which should lead to new understanding of the low-energy physics of interacting electrons. The results of this research may be applicable to a wider class of materials that include the heavy fermion materials and cobaltates.The research provides good training for graduate students. The PI will involve undergraduate students in the research. The research also involves international collaborations.NON-TECHNICAL SUMMARYThis award supports theoretical research and education to pursue a possible origin of the unusual properties of high temperature superconducting materials. At sufficiently low temperatures, these materials exhibit superconductivity - an electronic state of matter that exhibits no resistance to the flow of electricity. An unusual and exciting feature of these materials is that the temperature at which superconductivity first appears can be some 6 times or more higher than the highest temperature at which superconductivity had been observed to occur in all previously known materials. If new materials can be discovered that become superconducting near room temperature, then vast amounts of energy could be saved by using them in power transition lines and a whole new technology of superconducting electronics would become more practical. But how does superconductivity arise in these materials? The PI takes the view that magnetism plays a crucial role, particularly when near a new kind of phase transition that occurs at the absolute zero of temperature. Fluctuating wisps of magnetic order might provide the mechanism that causes electrons to pair up and transform to the superconducting state. This research project uses advanced theoretical methods to determine signatures of this mechanism and how they may appear in experiments on particular superconducting materials. The PI will advance the theory of this kind of superconductivity and the unusual kind of metallic state from which it might spring.The research provides good training for graduate students. The PI will involve undergraduate students in the research. The research also involves international collaborations.