Strongly Correlated Fermi Systems

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This grant supports theoretical research on condensed matter and materials physics. In particular, the research furthers the development and applications of advanced methods to describe electronic materials in which the electrons are strongly interacting (correlated). The methods developed on this grant will find widespread application.Intellectual Merit: Strong correlations in solids drive a wealth of new and unusual physical properties, ranging from complex charge spin and orbital ordering phenomena, unconventional superconductivity, ultrafast nonlinear optical responses, large thermoelectric coefficients, huge volume collapses, and numerous metal-to-insulator transitions. Over a wide range of temperatures and compositions, strongly correlated materials are poorly described by the standard model of solids, which is unable to describe phenomena such as anomalously large metallic resistivities and transfer of spectral weight over large frequency intervals. These materials pose a high intellectual challenge to condensed matter theory, demanding a new theoretical framework to account for their unique physical properties. A nonperturbative methodology is needed to compute their physical properties from first principles, and simple pictures of these complex systems must be developed to accompany analytic and numerical computations. Dynamical Mean Field Theory (DMFT) provides an approach to these interesting issues by reducing the complex many-body problem to self-consistent impurity (or to a cluster of impurity) models, which give a simpler picture of these complex materials and allow the computation of their physical properties in parameter regimes where the standard theory of solids does not apply. Our research links the low energy simplified pictures of the strong correlation phenomena to the computationally intensive calculations that model the microscopic complexity of real materials.Objective and methods: This grant includes the following projects:_ We will develop a first-principles approach for practical computation of the electronic structure of complex correlated materials. This approach will build on renormalization and DMFT ideas. We will explore the unusual properties of the layered cobaltates and the origin of the large thermoelectric response in this class of materials using a combination of model Hamiltonian and first principles techniques. _ We will investigate the description of the unusual electronic excitations present in correlated materials. In particular, we will study the destruction of the Fermi surface as the Mott transition is approached in models describing copper oxides and kappa organic salts with newly developed Cellular DMFT technique. The influence of proximity to different forms of order on this phenomenon will be investigated. We will also explore the evolution of the nature of the electronic excitations in heavy Fermion systems when the Kondo interaction is comparable to the RKKY exchange energy.Broader Impact. Our long-term research objectives are to develop methods and techniques capable of explaining and ultimately predicting the properties of solids that contain correlated electrons. We believe that theory will ultimately play an important role in the design of materials for practical applications. Our advances in understanding materials will provide new avenues for designing them, and strongly correlated materials will play an important role in this area. While developing concepts and tools to gain understanding in this area, we are at the same time training post-doctoral associates graduate and undergraduate students. These individuals are developing the skills needed to analyze and solve complex problems and perform analytic andnumerical computations, skills that can make important contributions to our technological society. Much of this work will be accomplished in collaboration with European groups. This will provide excellent learning experiences for postdoctoral associates and students.