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

University Of Florida, Gainesville FL

Investigators

Abstract

NONTECHNICAL SUMMARY This award supports theoretical and computational research and education designed to advance understanding of materials that conduct electricity but contain electrons that apply strong forces on each other which led to new states of electronic matter. These quantum materials have 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 public, and organize events designed to encourage young women students and researchers to consider a career in physics. Superconductivity is the 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 chemical impurities, where an atom in a crystal is replaced by a different element, as well as effects of other defects always present when crystals or films are grown. In particular, how these impurities determine the phases that are realized will be addressed. For example, “islands” or droplets of a secondary phase may appear in the dominant phase of a superconductor. 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 scanning tunneling spectroscopy experiments. This technique reveals atomic-resolution images of the quantum states of a material by applying a voltage to a tiny sharp metal tip as it is scanned over the material's surface - effectively taking an atomic-scale “photograph” of the quantum state. Information obtained from theoretical calculations and the analysis of data from these experiments may provide key insights into the interplay of superconductivity and other quantum states including magnetism and a novel “electronic nematic” state of electrons that is a quantum mechanical analog of states that make liquid crystal displays possible. One consequence of these investigations may be a deeper insight into the nature of high temperature superconductivity and how it can be further optimized, which could have 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, such as sensors with unusual sensitivity operating near boundaries among competing phases in highly correlated metals. TECHNICAL SUMMARY This award supports theoretical and computational 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 as other quantum materials displaying competing orders. The PI will study 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. 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 illustrate the unexpected effects of scattering from out-of-plane dopants, and test the materials-specific theory of impurity scattering developed using Wannier function-derived scattering potentials from DFT. The PI and his group will develop materials-specific impurity potentials to understand normal- and superconducting-state transport where forward scattering from impurities can be important, in particular the angle-dependent elastic mean free path observed in resistivity measurements, as well as Terahertz conductivity. Some of this work will be done in collaboration with a group at Simon Fraser University. Even in the overdoped cuprates, there is a substantial discrepancy between the resistive Tc and the T where the gap closes. The team will try to calculate how much of the gap filling phenomenon is due to disorder-induced inhomogeneity near the transition, that is, when the system breaks up into small islands of good superconductor that are weakly proximity coupled. These phenomenological studies will be supplemented by microscopic studies of spin fluctuation theory, where the doping dependence of the intrinsic pairing interaction is Hubbard-type and modified by disorder, to study the disappearance of Tc on the overdoped side. 2. 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 pair density waves, calculating the local density of states above the sample surface where measurements are actually performed. In particular, the PI will develop the theory of scanning Josephson spectroscopy to calculate the local critical current using the Wannier function continuum representation, in both the coherent and diffusive regimes to compare with Josephson STM experiments currently being performed. 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. 3. Thermal transport and penetration depth in uranium ditelluride with elastic scattering. The spin-triplet heavy fermion superconductor uranium ditelluride is still quite mysterious and the structure of the superconducting gaps are unknown. Thermal conductivity experiments suggest point nodes, while penetration depth experiments imply that the nodes may be distributed away from high symmetry points. The PI will develop the theory of both experimental probes for triplet states to settle these issues, including nonunitary ones proposed to explain Kerr effect and muon spin resonance indications of time reversal symmetry breaking. He will further investigate the role of disorder in producing these signals. This award also supports outreach activities including: designing a new exhibit on electrical conduction and superconductivity for the recently opened Cade Museum for Innovation 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.