Covalency and Low Dimensionality in Superconducting Pairing

该奖项支持凝聚态物理的理论研究。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，例如2001年发现的MgB2在40K临界温度下的超导性。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，并且不可避免地强烈影响了材料物理学在知识和实践方面的后续发展。本项目的重点是获得对这类新型超导体的深刻理解和计算理论，其中共价和低维起着关键作用。除MgB2外，该预测类还包含密切相关的Li(1-x)BC体系（预测临界温度远高于40K）；以Na(x)HfNCl （Tc=25K）为表征的电子掺杂氯氮化合物；碳哑铃体系Y2C2Br2 （Tc = 12K）和新复活的C2哑铃材料倍碳化钇Y2C3 (Tc = 18K)；最后是高压（Tc = 20K）下的单质Li。这些体系表现出很强的共价性、二维性或两者兼而有之。在这个研究领域的研究互动是凝聚态物理学家，固态化学家和材料科学家之间强烈的跨学科研究。多管齐下的研究计划包括：(1)基于第一性原理的密度泛函线性响应理论，得到键合特性、声子频率、电子-声子矩阵元素，最后得到Tc本身；(2)第一性原理分子动力学模拟，表征离子在极强耦合情况下的动力学特性；(3)评价电子-声子相互作用系统的传统Migdal-Eliashberg理论的微扰修正的图解技术；(4)构造强各向异性系统的有效热分布函数。该项目的目标是基于第一性原理（无参数）计算方法获得统一的微观理解，该方法足够详细，不仅可以提供对观察到的行为的理解，还可以提供预测能力。这里涉及的材料物理问题范围广泛，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，到相互作用的图解摄动理论。这些问题的结合，加上与做合成和表征的实验家的密切互动，使这里成为受过广泛教育的计算物理学家的理想训练场。加州大学戴维斯分校的材料研究氛围，尤其是物理系，让学生和博士后都能沉浸在令人兴奋的材料研究中，为在学术界、研究实验室和工业界的职业生涯做准备。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年发现的二硼化镁在40K临界温度下的超导性就是一个例子。一种简单的各向异性化合物的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，并且不可避免地强烈影响了材料物理学在知识和实践方面的后续发展。这里涉及的材料物理问题范围广泛，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，到相互作用的图解摄动理论。这些问题的结合，加上与做合成和表征的实验家的密切互动，使这里成为受过广泛教育的计算物理学家的理想训练场。加州大学戴维斯分校的材料研究氛围，尤其是物理系，让学生和博士后都能沉浸在令人兴奋的材料研究中，为在学术界、研究实验室和工业界的职业生涯做准备。本科生也将参与研究

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.***