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.
The main outcome of this award has been the development of computational techniques and the study of models for iron-based superconducting materials and multiferroic manganites. The iron-based superconductors are compounds with a complex structure characterized by planes formed by iron and other elements such as arsenic, phosphorus (pnictides) or selenium (selenides), that become superconducting, a state in which current flows without resistance, at a relatively "high" temperature of -360F degrees (traditionally superconductivity occurs at about -455F). A technological dream is to obtain superconductivity at room temperature and some copper-based materials exhibit superconductivity at about -190F. However, the mechanism of superconductivity in the copper and iron-based materials is unknown and its understanding is crucial to advance the creation of promising materials. The manganites are another family of complex layered compounds based on planes of manganese and oxygen that have a property, colossal magnetoresistance, of technological interest. These materials can change from insulators to metals under the effects of relatively small magnetic fields. The common threads relating manganites to pnictides and selenides are the complex atomic structures and the strong interaction of their electrons. When electrons interact strongly the traditional techniques that succesfully allowed the study of metals and band insulators in which 20th century technology was based can no longer be used. This complicated problems that cannot be solved exactly require the development of a variety of approaches that together will reveal their complex properties. The contribution provided under this award has three main components: 1) Development of minimal models that capture some of the properties of the materials to be studied. A minimal model may be defined in two or even one spatial dimension rather than 3, or with a resticted number of degrees of freedom which means that while the actual material may have electrons in multiple atomic orbitals, localized spins, and the ions may be vibrating, a model that only focus on the spins, for example, may allow to unveil the magnetic properties observed in experiments; 2) Numerical studies of the minimal models in small systems, with very few atoms, is necessary because it is the only regime in which exact and unbiased solutions to the problem can be obtained; 3) The final goal is the development of approximate methods under the guidance obtained from the numerical results. One example of how this approach was implemented under this effort was the development of a model in which three orbitals (the number of orbitals expected to be relevant in the pnictides) were populated by electrons interacting with an arbitrary Coulomb interaction and Hund coupling (magnetic interaction between the electrons) in only one spatial dimension. The advantage of the lower dimension is that a numerically exact technique known as Density Matrix Renormalization Group, can be used to obtain the properties of the model for the whole range of Coulomb and Hund interactions characterized by the parameters U and J. The figure attached shows the "phase diagram" that allows to visualize that the properties of the material strongly depend on the values of U and J. There are regions where the system is metallic as indicated by the letter M on the left part of the figure, while for larger values of U and J a region labeled OSMP in the figure and characterized by insulating properties in some orbitals and metallic in others is observed. A finer structure was found in this region: i ) a block (B) regime characterized by ferromagnetic blocks (the spins of the electrons are parallel to each other) coupled antiferromagnetically (with the spins in each block pointing in antiparallel directions) and a pure ferromagnetic state indicated by FM in the figure. The block region has already been observed experimentally in selenides and experimentalists now know about the possibility of other intersting phases to be found in materials that can be described by different values of J and U. In addition, materials characterized by such rich phase diagrams have the potential to develop technologically useful properties since the addition of doping or the application of external fields may allow to transition from one of the phases to another. In addition, neighboring phases may coexist at the nanoscale, once again, providing fertile ground for technological applications. The results of the phase diagram allow for the development of mean-field states that allow a guided study of the properties of larger systems not only in 1 but also in 2 or even 3 dimensions. Under this award 3 students were trained and obtained PH.D.'s. The skills obtained allowed them to find jobs in national security, software development, and academia.
