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

该奖项支持强相关电子系统的理论研究和教育。本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.）最近发现的铁基磷属元素化物超导体的物理学，以及3.）在量子临界点附近的巡回费米子系统的新物理。PI将探索铜氧化物的物理主要是由接近反铁磁量子临界点，自旋波动的情况下驱动的想法。在量子临界点，集体自旋激发变得无隙，与它们的相互作用破坏了相干的费米液体准粒子。同样的相互作用介导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会引起赝能隙物理学，以及（ii）该方法是否可以扩展到真正的强耦合。PI旨在发展对磷属元素化物超导体的基本理解，包括：尽管有很强的库仑排斥，但超导性的起源，配对间隙的对称性，超导性和磁性之间的相互作用，差距对称性对超导态实验的影响，以及电子配对的反馈。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，但局部的。这是一个普遍的问题，应该导致新的理解相互作用的电子的低能物理。本研究的结果可应用于更广泛的材料，包括重费米子材料和钴酸盐，为研究生提供良好的培训。PI将让本科生参与研究。该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，以寻求高温超导材料的不寻常特性的可能起源。在足够低的温度下，这些材料表现出超导性-物质的电子状态，对电流没有阻力。这些材料的一个不寻常的和令人兴奋的特点是，超导性首次出现的温度可以比在所有先前已知的材料中观察到超导性发生的最高温度高出约6倍或更多。如果能够发现在室温附近具有超导性的新材料，那么通过将它们用于电力转换线路，可以节省大量的能量，并且超导电子学的全新技术将变得更加实用。但是超导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，特别是当接近一种在绝对零度下发生的新相变时。磁序的波动可能提供了导致电子配对并转变为超导状态的机制。该研究项目使用先进的理论方法来确定这种机制的特征，以及它们如何在特定超导材料的实验中出现。PI将推进这种超导电性的理论，以及这种超导电性可能产生的不寻常的金属状态。这项研究为研究生提供了很好的培训。PI将让本科生参与研究。这项研究还涉及国际合作。

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.