Density Response and Electron Pairing

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NONTECHNICAL SUMMARYThis award supports research and education to enhance our understanding of the relationship between how electrons respond to changes in their environment and their tendency to form pairs rather than to remain relatively independent. Pairing of electrons in solids lies at the root of several of the most interesting and useful properties of solids, with superconductivity and inhomogeneous magnetism being two important examples.The 2016 discovery of superconductivity with a critical onset temperature (Tc) above 200 K (-85 F) in trihydrogen sulfide has established a new record for Tc, promoting hydrogen-rich materials as the new frontier in this area of research. This revolution in high-temperature superconductivity comes at the expense of requiring an enclosed pressure cell to provide the two million atmospheres of pressure that induces superconductivity. For progress, theory and simulations must play a much larger role if higher Tc or lower pressure examples are to be discovered. Existing simulations for several candidate hydrides reveal the need to understand at a basic level the interaction of the metallic conduction electrons with a vibrating proton (hydrogen nucleus). This project will use this materials platform to address the fundamental question of why some hydrides are spectacular superconductors, while seemingly similar others are not. Analogous simulations will be applied to other peculiar superconductors (viz. single layers of iron selenide) where a related type of pairing is the best candidate for promoting impressive enhancements of superconducting critical temperatures. The principal investigator will extend and apply relevant theory, and write computer codes for calculations necessary in the study of this problem.By edging its way into technology, superconductivity is achieving broader impacts beyond enhanced fundamental understanding. Examples of applications include strong superconducting magnets in medical magnetic resonance imagers (MRI), the Large Hadron Collider ring used for elementary particle study, ultra-sensitive superconducting quantum interference detectors, superconducting/magnetic technology levitating the world's fastest trains, and demonstration projects for power transmission to residential areas through superconducting cables. Further developments in superconducting materials and properties promise to accelerate the transition of innovations to public use. In addition to research, this award will support the training of young scientists for career paths in science & technology research, development, education, and administration areas.TECHNICAL SUMMARYThe theory of high-temperature superconductivity has been at the forefront of theoretical condensed-matter theory for six decades. The discovery of a new class of high-Tc superconductors moves the field to another, different level. One way it does so is by readjusting the basic scientific paradigm, which has been: experiment leads and stimulates theory. For these hydrides, theory (primarily, Ashcroft's work) appeared well before high-pressure techniques became able to test predictions. The electron-phonon mechanism for hydrogen sulfide and related hydrides might be claimed to be reasonably understood, but the most fundamental quantity -the perturbation caused by a moving proton and how it effects pairing of electrons on the Fermi surface, specifically the matrix element- is hidden inside sophisticated computer simulations without providing any understanding that would enable prediction and design of outstanding superconductors at higher temperature or lower pressure. This project proposes to complete this understanding by calculating and analyzing this scattering atom-by-atom of electrons at the Fermi surface by complementing the conventional theory of electronic linear response with numerical evaluation of the linear response. The added understanding will promote rational material design, versus Edisonian discovery, and a real prospect of even better superconductors by revealing the "genome" of the electron-phonon matrix element. Advances will be applicable to other superconducting materials challenges, especially that of single-layer iron selenide, where it is widely suspected that electron-phonon coupling enhances the underlying, magnetically based, pairing mechanism. Additionally, the very high Tc of hydrogen sulfide together with the electronic fine structure, an important feature that makes it special, presents new challenges in the simulation and basic understanding of behavior (electronic response) that extends beyond the conventional theory of this type of electron pairing. The results of this project are expected to impact the prediction, design, and control of new materials with transport properties required for specific applications, including semiconductors and semimetals, thermoelectric materials, and optimized magnetoelectronic coefficients. In addition to research, this award will support the training of young scientists for career paths in science & technology research, development, education, and administration areas.