Disorder and the Emergence of Inhomogeneous Phases in Strongly Correlated Electron Systems

University Of Florida, Gainesville FL

Investigators

Abstract

NONTECHNICAL SUMMARY This award supports research and education designed to advance understanding of materials containing itinerant electrons that interact strongly with each other, giving rise to new states of electronic matter with novel properties. The PI will use model calculations for electrons in materials classes known as cuprates and iron-based superconductors, as well as others, to study several challenging problems directed towards a broader understanding of how these materials work. He will also work with a local science museum to prepare exhibits on superconductivity and magnetism for the general public. Superconductivity is one type of quantum state of many electrons in a metal that is characterized by the loss of all electrical resistance and consequently of all dissipation of energy. The PI will explore the interplay between the fundamental physics of superconductors and other phases of matter, including magnetic ones, which tend to form when superconductivity is suppressed. In addition, he will explore the effect of impurities and other defects that are always present when crystals or films are grown, on the interplay between these phases. Thus, the PI will explore models of the effect of impurities and other real-life defects on the properties of these superconducting materials. The PI will focus especially on the theory of how to interpret results from an experimental technique called scanning tunneling spectroscopy. In this method, atomic-resolution images of the quantum states of a material can be collected by applying a voltage to a tiny sharp metal tip as it is scanned over the material's surface. Information obtained from theoretical calculations and the analysis of data from these experiments may provide key insights into the interplay of superconductivity and other states including magnetism and a novel state of electrons that is a quantum mechanical analog of states that occur in liquid crystals and make liquid crystal displays possible, and their interaction with impurities in the material. One consequence of these investigations may be a deeper insight into the nature of high temperature superconductivity and how it can be further optimized to have enormous technological implications. Understanding the properties of these quantum materials and the influence of disorder may also lead to novel properties that can be utilized in new devices and technologies, obtaining unusual sensitivity operating near transitions among competing phases that occur in highly correlated metals. TECHNICAL SUMMARY This award supports theoretical research and education to address long-standing fundamental problems involving the interplay of quenched disorder and various types of competing emergent order in correlated electron systems. The materials to be investigated include cuprate and iron-based superconductors, as well other quantum materials displaying competing orders. The PI will investigate properties of electronic systems through a careful analysis of the behavior of simplified models of interacting electrons on the lattice, informed in some cases by density functional theory-based electronic structure calculations with appropriate inclusion of correlations. There are three main projects: 1. Interplay of nematicity, superconductivity, and disorder. The PI and his group will study the influence of impurities on electronic nematic instabilities in Fe-based superconducting systems and their interplay with superconductivity. Normally it is expected that superconductivity and nematicity compete with one another, as seen clearly in the Barium122 system. This competition may be responsible for the striking enhancement of the transition temperature with electron irradiation in iron selenide, but recent experiments on iron materials that include both selenium and sulfur in proportion have indicated that in some chalcogenide systems nematicity and superconductivity cooperate with each other, and this unusual situation may be responsible for the enhancement of the transition temperature with disorder as observed in iron selenide. A wide range of behavior is possible, and the PI will investigate the phase diagram of simple models that allow for both competition and cooperation, as well as how these phases evolve with disorder. The PI aims to address the basic question: What in a material system influences whether or not nematicity and superconductivity cooperate or compete, and how can disorder affect this balance? 2. Effects of disorder on overdoped cuprates. The PI will perform a series of investigations on the cuprate materials that can be doped well past optimal doping, to show the unexpected effects of scattering from out-of-plane dopants, and test the hypothesis that the overdoped cuprate superconducting state can be well described by the Landau-BCS paradigm. This notion was challenged by recent superfluid density and optical conductivity experiments on lanthanum strontium copper oxide (LSCO) films. The PI and his group will calculate several measurable quantities in the superconducting state of LSCO and related bismuth, thallium and other cuprate compounds using the so-called dirty d-wave theory. One goal will be to examine the effect of disorder as a possible explanation for the disparate transition temperatures in these systems. The PI's work will be complemented by studies of the doping dependence of the intrinsic pairing interaction in Hubbard-type models to see if a consistent picture can be developed where pseudogap physics is apparently absent. Finally, ab initio calculations of out-of-plane dopant impurity potentials will be performed to allow for possible microscopic justification of the established phenomenology. 3. STM of charge and pair density waves. Much of the physical information available on competing order and inhomogeneity arises from scanning tunneling microscopy and spectroscopy on high quality surfaces; yet the theory to interpret such data is available only in very primitive form. The PI will use microscopic Wannier functions from ab initio calculations to construct local Green's functions in both superconducting and metallic states to compare with experiments on charge and density waves, above the sample surface where measurements are actually performed. This technique will be tested on longstanding problems in cuprate and iron-based superconductors, and then extended to study modern issues of competing order in a variety of correlated systems. In particular, the PI and his group will compare microscopic theories of pair density waves in cuprates with STM data. Understanding the properties of correlated electron systems and the influence of disorder may lead to novel materials properties that can be utilized in new devices and technologies, obtaining unusual sensitivity by operating near transitions between competing phases that occur in highly correlated electron systems. This award also supports outreach activities including: designing a new exhibit on electrical conduction and superconductivity for the recently opened Cade Museum for Innovation, organizing U. Florida Activities in support of United Nations Women and Girls in Science Day, and developing and delivering public lectures. Software developed for the calculation of spin fluctuation pairing, and a database for Wannier functions in unconventional superconductors, will be made available through the PI's website and GitHub. This award reflects NSF's statutory mission and has been deemed worthy of support through evaluation using the Foundation's intellectual merit and broader impacts review criteria.