Computational Studies of Model Hamiltonians for Pnictides and Multiferroic Manganites

技术总结该奖项支持对磷灰石超导体和锰基多铁材料的计算研究。这项研究将基于哈伯德模型哈密顿量，用于需要多轨道形式来正确描述其电子性质的复杂材料家族。模拟将采用Hartree-Fock、精确对角化和密度矩阵重整化群技术的组合，应用于具有多个3D轨道的模型。建立了Hubbard斥力U、Hund耦合JH和电子密度n空间的相图。更具体地说：(I)铁基超导体的发现对基于电子机制解释高临界温度超导电性的想法提出了新的挑战，因为必须同时考虑两到五个Fe轨道才能对这些化合物进行适当的理论描述。PI将使用各种计算和平均场近似来为这些小肽以及竞争中的配对通道建立现实的耦合。我们将继续将理论预测与实验结果进行比较，特别是基于中子散射、角度分辨光电发射和扫描隧道显微镜的结果。我们的结果将对其他也需要多轨道形式的材料具有价值。(Ii)多铁领域具有潜在的技术相关性，并提供了基本概念进步的机会。PI将聚焦于一个新的主题：详细分析基于锰的空穴掺杂多铁化合物的两轨道双交换模型，特别是在小带宽区域；最近的工作揭示了几个新磁态的存在，这些新磁态通过逆Dzyaloshinskii-Moriya相互作用变成铁电。私人投资公司打算对这些新阶段进行详细分析，并调查其奇特性质的起源。此外，PIS将基于金属和绝缘体之间的竞争，继续他们对锰氧化物巨磁电阻效应的研究，重点关注发生巨磁电阻所需的电荷/轨道/自旋有序绝缘体的特殊特性。这项研究的意义超出了所研究的特定材料类别；它将促进对复杂材料的轻视。该项目支持对研究生的培训，他们将在凝聚态和材料物理以及计算物理方面获得广泛的教育。拉丁美洲的研究人员和学生也将参加这一项目，其中包括女科学家，这将有助于扩大对科学的参与。非技术概述该奖项支持有关超导体和多铁材料的凝聚态和材料物理基本方面的理论研究。PI将专注于几种有趣的新材料的理论和计算研究。其中包括最近发现的含有铁的超导化合物，它在比大多数已知超导体更高的温度下表现出超导电性，以及同时具有铁电和磁性的化合物，称为多铁性。多铁性材料在信息技术中具有相当大的应用潜力。超导体是一种对电荷流动没有阻力的材料，与铜等普通金属不同，普通金属对电流流动有阻力，并在电流流动时实际上升温。对超导体的深入了解可能会导致一种方法，提高它们表现出超导电性的最高温度，也就是临界温度，远远高于大气层中的气体氮气是液体的温度，或许最高可达室温。这将导致在电力传输和节能方面的应用。PI将使用计算机模拟、包含基本物理和材料细节的模型以及理论来促进对这些化合物中电子行为的理解，试图揭示它们磁性的深层次根本原因以及它们超导行为背后的原因。PI还将研究多铁化合物；它们的重要性在于潜在地使用电场来‘反转’记录介质中比特的磁取向，提供了一种比目前用于实现这一目标的磁场更有效的方法。在传统的磁性材料中，电场对磁性的影响很小，因为它们的电磁行为几乎是解耦的。然而，在多铁化合物中，它们是强烈相连的，并且可以通过电压来控制磁性行为。该项目还包括对博士生的培训，使他们能够发展对一门令人兴奋的材料类基础科学的研究能力和直觉。他们还将学习如何使用计算来解决凝聚态物质和材料中的问题。将与居住在美国和国外的拉美青年科学家建立密切联系，预计这将有助于增加对物理、材料科学和计算科学感兴趣的拉美裔青年研究人员的数量。

TECHNICAL SUMMARYThis award supports computational studies on the pnictide superconductors and on manganese-based multiferroics. The research will be based on Hubbard model Hamiltonians for families of complex materials that require a multiorbital formalism to properly describe their electronic properties. Simulations will be carried out employing a combination of Hartree-Fock, Exact Diagonalization, and Density Matrix Renormalization Group techniques, applied to models with several 3d orbitals. Phase diagrams in the space of the Hubbard repulsion U, Hund coupling JH, and electronic density n will be constructed. More specifically:(i) The discovery of the Fe-based superconductors has established a new challenge for ideas based on electronic mechanisms to explain high critical temperature superconductivity, since two to five Fe orbitals must be simultaneously considered for a proper theoretical description of these compounds. The PIs will employ a variety of computational and mean-field approximations to establish realistic couplings for these pnictides, as well as the pairing channels that are in competition. Comparison of the theoretical predictions against experimental results, especially those based on neutron scattering, angle-resolved photoemission, and scanning tunneling microscopy, will be pursued. Our results will be of value for other materials that also require a multiorbital formalism.(ii) The field of multiferroics has potential technological relevance and presents the opportunity for fundamental conceptual advance. The PIs will focus on a novel topic: the detailed analysis of two-orbital double-exchange models for hole-doped multiferroic compounds based on manganese, particularly in the regime of small bandwidth; recent work has unveiled the presence of several new magnetic states that become ferroelectric via the inverse Dzyaloshinskii-Moriya interaction. The PIs intend to carry out a detailed analysis of these new phases and investigate the origin of their exotic properties. In addition, the PIs will continue their study of colossal magnetoresistance effects in manganites, based on the competition between metals and insulators, focusing on the special characteristics of the charge/orbital/spin ordered insulator required for colossal magnetoresistance to occur.This research has implications beyond the particular classes of materials studied; it will lead to an advance in understating of complex materials. This project supports the training of graduate students who will obtain a broad education in condensed matter and materials physics, and computational physics. Researchers and students from Latin America will also participate in this project, including female scientists, which will contribute to efforts to broaden participation in science. NONTECHNICAL SUMMARY This award supports theoretical research on fundamental aspects of condensed matter and materials physics involving superconductors and multiferroic materials. The PIs will focus on theoretical and computational studies of several interesting novel materials. These include recently discovered superconducting compounds that contain iron and exhibit superconductivity at higher temperatures than most known superconductors, and compounds that are simultaneously ferroelectric and magnetic, known as multiferroics. Multiferroic materials have considerable potential for applications in information technology. Superconductors are materials that display no resistance to the flow of electrical charge, in contrast to ordinary metals, such as copper, which have resistance to the flow of electricity and actually heat up when current flows. A deeper understanding of superconductors may lead to a way to increase the highest temperature at which they exhibit superconductivity, the critical temperature, well above temperatures where the atomospheric gas nitrogen is a liquid, perhaps up to temperatures as high as room temperature. This would lead to applications in power transmission and energy savings. The PIs will use computer simulations, models that contain essential physics and materials details, and theory to advance understanding of the behavior of electrons in these compounds, attempting to unveil the deep fundamental reasons for their magnetic properties and the reason behind their superconducting behavior. The PIs will also study multiferroic compounds; their importance resides on the potential use of electric fields to 'flip' the magnetic orientation of bits in a recording medium, providing a more efficient method than the use of magnetic fields currently used to achieve this goal. In conventional magnetic materials, an electric field would only slightly affect the magnetic properties because their electric and magnetic behaviors are nearly decoupled. However, in multiferroic compounds, they are strongly linked and magnetic behavior can be controlled with an electric voltage. This project also includes the training of PhD graduate students to enable them to develop research abilities and intuition on the fundamental science of an exciting class of materials. They will also learn how to use computation to solve problems in condensed matter and materials. A close connection with young Latin American scientists, both residing in the USA and abroad, will be developed which is expected to contribute to an increase in the number of Hispanic young researchers that develop an interest in physics, materials science, and computational science.