Covalency and Low Dimensionality in Superconducting Pairing

该奖项支持凝聚态物理学的理论研究。 发现或合成具有意想不到和迷人特性的新材料是一个持续的，也许是加速的过程，2001年发现的MgB 2在40 K临界温度下的超导性就是例证。 在没有d电子的简单各向异性化合物中，超导性几乎是其他金属间化合物临界温度的两倍，打破了金属间化合物超导体的所有已知规则。 这些发现引起了材料物理学家的极大兴趣，并对材料物理学的后续发展产生了强烈的影响，无论是知识还是实践。本项目的重点是深入理解和计算这类新的超导体的理论，其中共价性和低维性起着关键作用。 除了MgB 2之外，这个预测的类别还包含：密切相关的Li（1-x）BC系统（预测临界温度远高于40 K）;以Na（x）HfNCl（Tc= 25 K）为代表的电子掺杂氯氮化物;碳哑铃系统Y2 C2Br 2（Tc高达12 K）和新近复苏的C2哑铃材料倍半氧化钇Y2 C3（Tc = 18 K）;最后是高压下的元素Li（Tc = 20 K）。 这些系统显示出很强的共价性、二维性或两者兼而有之。 这一领域的研究是凝聚态物理学家、固体化学家和材料科学家之间的交叉研究，其多方面的研究计划包括：（1）基于第一性原理密度泛函的线性响应理论，以获得成键特性、声子频率、电子-声子矩阵元，最后是Tc本身;（2）第一性原理分子动力学模拟，描述极强耦合情况下离子的动力学特征;（3）图解技术，评价对相互作用电子-声子系统的常规Migdal-Eliashberg理论的微扰修正;（4）强各向异性系统有效热分布函数的构造。 这个项目的目标是获得一个统一的微观理解，基于第一原理（无参数）计算方法，这是足够详细的，不仅提供了一个理解，但也有预测能力，观察到的行为。广泛的材料物理问题，这里涉及的范围从基本的电子结构理论，大规模的动力学模拟，电子和晶格自由度之间的相互作用和能量转移，相互作用的图解微扰理论。 这些问题的结合，再加上与实验学家进行合成和表征的密切互动，使其成为受过广泛教育的计算物理学家的理想培训基地。 在加州大学戴维斯分校的材料研究氛围一般，特别是在物理系，让学生和博士后都成为沉浸在令人兴奋的材料研究，在学术界，研究实验室和工业的职业生涯做准备。 本科生也将参与研究。%该奖项支持凝聚态物理学的理论研究。 发现或合成具有意想不到的和令人着迷的特性的新材料是一个持续的，也许是加速的过程，2001年发现的二硼化镁在40 K临界温度下的超导性就是例证。 在一个简单的各向异性化合物中，超导性几乎是其他金属间化合物临界温度的两倍，打破了金属间超导体的所有已知规则。 这些发现引起了材料物理学家的极大兴趣，并对材料物理学的后续发展产生了强烈的影响，无论是知识还是实践。这里涉及的材料物理问题范围很广，从基本的电子结构理论，到大规模的动力学模拟，到电子和晶格自由度之间的相互作用和能量转移，相互作用的图解微扰理论。 这些问题的结合，再加上与实验学家进行合成和表征的密切互动，使其成为受过广泛教育的计算物理学家的理想培训基地。 在加州大学戴维斯分校的材料研究氛围一般，特别是在物理系，让学生和博士后都成为沉浸在令人兴奋的材料研究，在学术界，研究实验室和工业的职业生涯做准备。 大学生也将参与研究。*

This award supports theoretical research in condensed matter physics. Discovery or synthesis of new materials with unexpected and fascinating properties is a continuing, perhaps accelerating, process exemplified by the discovery in 2001 of superconductivity in MgB2 at a critical temperature of 40K. Superconductivity at almost twice the critical temperature of other intermetallics, in a simple anisotropic compound without d-electrons broke all the known rules for intermetallic superconductors. Such discoveries attract great interest amongst materials physicists, and invariably strongly influence subsequent developments in materials physics, both intellectual and practical.This project focuses on gaining a deep understanding and computational theory of this new class of superconductors, for which covalency and low dimensionality play a critical role. This projected class contains, in addition to MgB2: the closely related Li(1-x)BC system (predicted critical temperature well above 40K); electron-doped chloronitrides typified by Na(x)HfNCl (Tc=25K); the carbon dumbbell system Y2C2Br2 (Tc up to 12K) and a newly revived C2 dumbbell material yttrium sesquicarbide Y2C3 (Tc = 18K); and finally elemental Li under high pressure (Tc = 20K). These systems display strong covalency, two-dimensionality, or both. Research interactions in this area of research are strongly interdisciplinary between condensed matter physicists, solid state chemists, and materials scientists.The multi-pronged research plan includes (1) first principles density-functional based linear response theory, to obtain bonding characteristics, phonon frequencies, electron-phonon matrix elements, and finally Tc itself; (2) first principles molecular dynamics simulations, to characterize the dynamics of ions in situations of extremely strong coupling; (3) diagrammatic techniques to evaluate perturbative corrections to conventional Migdal-Eliashberg theory for interacting electron-phonon systems; and (4) construction of effective thermal distribution functions in systems with strong anisotropy. The objective of this project is to obtain a unified microscopic understanding, based on first principles (parameter-free) computational methods, that is detailed enough to provide not only an understanding of, but also a predictive capability for, the observed behavior.The wide range of materials physics issues that are involved here range from fundamental electronic structure theory, to large-scale dynamical simulations, to interaction and transfer of energy between the electronic and lattice degrees of freedom, to the diagrammatic perturbation theory of the interaction. The combination of these issues, together with close interactions with experimentalists doing synthesis and characterization, make this an ideal training ground for broadly educated computational physicists. The materials research atmosphere at UC-Davis generally, and in the Physics Department in particular, allow students and postdocs alike to become immersed in exciting materials research in preparation for careers in academia, research laboratories and industry. Undergraduates will also participate in the research.%%%This award supports theoretical research in condensed matter physics. Discovery or synthesis of new materials with unexpected and fascinating properties is a continuing, perhaps accelerating, process exemplified by the discovery in 2001 of superconductivity in magnesium di-boride at a critical temperature of 40K. Superconductivity at almost twice the critical temperature of other intermetallic compounds, in a simple anisotropic compound broke all the known rules for intermetallic superconductors. Such discoveries attract great interest amongst materials physicists, and invariably strongly influence subsequent developments in materials physics, both intellectual and practical.The wide range of materials physics issues that are involved here range from fundamental electronic structure theory, to large-scale dynamical simulations, to interaction and transfer of energy between the electronic and lattice degrees of freedom, to the diagrammatic perturbation theory of the interaction. The combination of these issues, together with close interactions with experimentalists doing synthesis and characterization, make this an ideal training ground for broadly educated computational physicists. The materials research atmosphere at UC-Davis generally, and in the Physics Department in particular, allow students and postdocs alike to become immersed in exciting materials research in preparation for careers in academia, research laboratories and industry. Undergraduates will also participate in the research.***