Computational Studies of Multiorbital Model Hamiltonians for Iron-Based Superconductors in Quasi One-Dimensional Geometries.

该奖项支持理论和计算研究和教育，重点研究一系列铁基材料，其中大多数在低温下成为超导体。超导体在低于某一临界温度时不损耗电能。然而，20世纪初发现的传统超导体的临界温度非常低（接近绝对零度）。如果超导的临界温度可以提高到接近室温，那么大量的新技术进步将立即成为可能。20世纪80年代末，人们发现了一类铜基类陶瓷磁性化合物（所谓的铜酸盐），其临界温度介于绝对零度和室温之间，这是提高超导体临界温度的一个里程碑。虽然在传统超导体中产生超导性的机制已经被很好地理解，但对铜酸盐中产生超导性的机制的基本理解一直是一个重大的挑战。2008年，铁基超导材料的发现为高温超导之谜增加了一块新的拼图。铁基材料家族非常丰富，它们的性质介于传统超导体和铜之间。了解它们的晶格、电子的空间排列和磁性自由度相互作用的方式，揭示出铁基材料的有趣特性，将揭示导致高温超导的机制。为了实现这一目标，必须建立新的理论框架，以便研究具有不同空间排列的电子彼此强烈相互作用的系统。本项目的目标是开发捕捉铁基材料本质的模型，并用强大的计算方法和分析技术对其进行研究，以便(i)解释新出现的实验数据，（ii）为寻求合成具有潜在技术相关特性的材料的实验学家提供指导。这项活动的另一个重要方面是培养博士生，他们将学习如何使用计算来解决凝聚态物理和材料科学中的问题。将与居住在美国和国外的拉丁美洲青年科学家建立密切联系。预计这将有助于增加对物理、材料和计算科学感兴趣的年轻西班牙裔研究人员的数量。该奖项支持理论和计算研究和教育，重点是复杂氧化物材料的模型哈密顿研究，这些材料需要多轨道哈伯德形式来准确描述其电子特性。最近铁基高温超导体的发现已经建立了一个新的研究领域，基于超导的电子机制的想法可以被测试。与超导氧化铜材料相比，在铁衍生物和硫族化合物中，必须同时考虑几个铁三维轨道才能对这些化合物进行适当的理论描述。基于这一动机，将对需要多轨道哈伯德形式来描述其电子特性的复杂材料族的模型哈密顿量进行计算研究。这项工作的结果不仅将阐明铁基超导体的性质，而且还将指导其他需要多轨道形式的材料的研究。该项目的重点将放在准一维材料上，因为有具有这些特性的真实材料，如BaFe2Se3（两条腿的梯子）和TlFeSe2（链），而且因为一维几何形状的计算比二维几何形状的计算要精确得多。许多体计算将通过密度矩阵重整化群和精确对角化技术的组合来执行，这两种技术都处理静态和动态量，但它们也将由二维的Hartree-Fock近似来补充。将讨论的物理方面包括(i)构建具有不同的哈伯德排斥、亨德耦合和电子密度的相图，特别关注轨道选择性莫特跃迁；（ii）动力学响应的计算，以便将理论与角分辨光电发射和中子散射实验进行比较；（iii）对成对形成及其主要通道的调查，特别是在两条腿的梯子中；（四）研究电子行业可能出现的自组织趋势。调查将涉及凝聚态物理中当前感兴趣的各种主题。对铁基超导体的多轨道模型哈密顿量相图的研究将使pi能够追求概念上新颖的研究领域，涉及具有磁性，轨道，电荷和配对排序倾向的奇异状态，这将导致对需要多轨道形式主义建模的复杂材料的改进视图。该项目的另一个重要方面是培养博士生，他们将学习如何使用计算来解决凝聚态物理和材料科学中的问题。将与居住在美国和国外的拉丁美洲青年科学家建立密切联系。预计这将有助于增加对物理、材料和计算科学感兴趣的年轻西班牙裔研究人员的数量。

NON-TECHNICAL SUMMARYThis award supports theoretical and computational research and education focused on studies of a family of iron-based materials most of which become superconductors at low temperatures. Superconductors conduct electricity without any power loss below a certain critical temperature. However, this critical temperature for traditional superconductors discovered in the early 20th century is very low (near absolute zero). If the critical temperature for superconductivity could be raised to near room temperature, a plethora of novel technological advances would instantly be possible. A milestone in the quest to raise the critical temperature of superconductors was achieved in the late 1980s with the discovery of a family of copper-based ceramic-like magnetic compounds (the so-called cuprates) with critical temperatures that are halfway between absolute zero and room temperature. While the mechanism that produces superconductivity in traditional superconductors is fairly well understood, achieving a fundamental understanding of the mechanism that leads to superconductivity in the cuprates has been a significant challenge. A new piece of the puzzle of high temperature superconductivity was added by the discovery of iron-based superconducting materials in 2008. The iron-based family of materials is very rich and their properties fall somewhere between those of traditional superconductors and the cuprates. Understanding the way in which their crystal lattice, spatial arrangement of electrons, and magnetic degrees of freedom interact to bring out the interesting properties of the iron-based materials will shed light on the mechanism that leads to high temperature superconductivity.To achieve this goal new theoretical frameworks have to be established in order to study systems where electrons with different spatial arrangements strongly interact with each other. The goal of the present project is to develop models that capture the essence of the iron-based materials and study them with powerful computational approaches and analytical techniques in order to (i) explain the emerging experimental data, and (ii) offer guidance to experimentalists in their quest to synthetize materials with potential technologically relevant properties. Another important aspect of this activity is the training of PhD students who will learn how to use computation to solve problems in condensed matter physics and materials science. A close connection with young Latin American scientists, both residing in the USA and abroad, will be developed. This is expected to contribute to an increase in the number of young Hispanic researchers with an interest in physics, materials, and computational science. TECHNICAL SUMMARYThis award supports theoretical and computational research and education focused on model Hamiltonian studies of complex oxide materials that require a multi-orbital Hubbard formalism for accurate description of their electronic properties. The recent discovery of the iron-based high critical temperature superconductors has established a new area of research where ideas based on electronic mechanisms for superconductivity can be tested. Compared with the superconducting copper oxide materials, in the iron pnictides and chalcogenides several Fe 3d orbitals must be considered simultaneously for a proper theoretical description of these compounds. Based on this motivation, computational studies of model Hamiltonians for families of complex materials that require a multi-orbital Hubbard formalism to describe their electronic properties will be performed. The results of this effort will not only elucidate the properties of iron-based superconductors, but they will also guide the study of other materials that require a multi-orbital formalism. The focus of the project will be on quasi-one-dimensional materials because there are real materials with these characteristics, such as BaFe2Se3 (two-leg ladders) and TlFeSe2 (chains), and because the computational calculations are considerably more accurate in one dimensional geometries than in two. The many body calculations will be performed by a combination of Density Matrix Renormalization Group and Exact Diagonalization techniques, both addressing static and dynamical quantities, but they will also be supplemented by Hartree-Fock approximations in two dimensions. The physical aspects that will be addressed include (i) construction of phase diagrams with varying the Hubbard repulsion, Hund coupling, and electronic density with special focus on the orbital-selective Mott transition; (ii) calculation of dynamical responses in order to compare theory against angle-resolved photoemission and neutron scattering experiments; (iii) investigation of pair formation and their dominant channels, particularly in two-leg ladders; and (iv) studies of possible self-organization tendencies in the electronic sector.The investigations will address a variety of topics of current interest in Condensed Matter Physics. The study of the phase diagrams of multi-orbital model Hamiltonians for the iron-based superconductors will allow the PIs to pursue conceptually novel areas of research involving exotic states with magnetic, orbital, charge, and pairing ordering tendencies that will lead to an improved view of complex materials that require a multi-orbital formalism for their modeling. Another important aspect of this project is the training of PhD students who will learn how to use computation to solve problems in condensed matter physics and materials science. A close connection with young Latin American scientists, both residing in the USA and abroad, will be developed. This is expected to contribute to an increase in the number of young Hispanic researchers with an interest in physics, materials, and computational science.