Computational Studies of Model Hamiltonians for Pnictides and Multiferroic Manganites

University Of Tennessee Knoxville, Knoxville TN

Investigators

Abstract

TECHNICAL SUMMARY This 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.