Computational Studies of Model Hamiltonians for Pnictides and Multiferroic Manganites

技术概述：该奖项支持镍超导体和锰基多铁性材料的计算研究。这项研究将以哈伯德模型哈密顿量为基础，用于需要多轨道形式来正确描述其电子特性的复杂材料族。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，应用于具有多个三维轨道的模型。将构造Hubbard斥力U、Hund耦合JH和电子密度n空间中的相图。更具体地说：(i)铁基超导体的发现对基于电子机制来解释高温超导性的想法提出了新的挑战，因为必须同时考虑2到5个铁轨道才能对这些化合物进行适当的理论描述。pi将采用各种计算和平均场近似来建立这些粒子的实际耦合，以及竞争中的配对通道。将进行理论预测与实验结果的比较，特别是基于中子散射、角分辨光电发射和扫描隧道显微镜的预测。我们的结果将对其他同样需要多轨道形式的材料有价值。多铁性领域具有潜在的技术相关性，并为基本概念的进步提供了机会。pi将专注于一个新的主题：详细分析基于锰的空穴掺杂多铁化合物的双轨道双交换模型，特别是在小带宽下；最近的工作揭示了几种通过逆Dzyaloshinskii-Moriya相互作用变成铁电态的新磁态的存在。pi打算对这些新相进行详细分析，并调查其奇异性质的来源。此外，pi将继续他们在金属和绝缘体之间竞争的基础上对锰的巨磁电阻效应进行研究，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。这项研究的影响超出了所研究的特定材料类别；它将导致对复杂材料的理解的进步。该项目支持研究生的培养，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。来自拉丁美洲的研究人员和学生也将参与这个项目，其中包括女性科学家，这将有助于扩大科学参与的努力。该奖项支持凝聚态物质和材料物理学基础方面的理论研究，包括超导体和多铁材料。pi将侧重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，在比大多数已知超导体更高的温度下表现出超导性，以及同时具有铁电性和磁性的化合物，即多铁性化合物。多铁性材料在信息技术领域具有巨大的应用潜力。超导体是一种对电荷流动没有抵抗力的材料，与铜等普通金属不同，普通金属对电流流动有抵抗力，并且在电流流动时实际上会发热。对超导体的更深入的了解可能会导致一种方法来提高它们表现出超导性的最高温度，即临界温度，远高于大气中气体氮是液态的温度，也许会达到室温。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，以及理论来推进对这些化合物中电子行为的理解，试图揭示其磁性和超导行为背后的深层基本原因。pi还将研究多铁化合物；它们的重要性在于利用电场“翻转”记录介质中比特的磁性方向，提供了一种比目前使用磁场实现这一目标更有效的方法。在传统的磁性材料中，电场只会轻微地影响磁性，因为它们的电和磁行为几乎是解耦的。然而，在多铁化合物中，它们是紧密相连的，磁性行为可以用电压来控制。该项目还包括对博士研究生的培训，使他们能够在令人兴奋的材料类别的基础科学上发展研究能力和直觉。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年轻拉丁美洲科学家建立密切联系，预计这将有助于增加对物理学、材料科学和计算科学感兴趣的西班牙裔年轻研究人员的数量。

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.