This 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.
This 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.
