CAREER: Electrical and Thermoelectric Transport Beyond the Metal/Insulator Paradigm

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Nontechnical AbstractElectronic materials are traditionally dichotomized into two broad classes: metals and insulators. In this dichotomy, metals are materials that can conduct electricity via a "Fermi surface" of electrons, and insulators are materials that do not conduct electricity and have no such Fermi surface. But the last decade has uncovered whole new classes of materials, "topological materials," that defy the established dichotomy. Topological materials are able to conduct electricity but have no Fermi surface of electrons. The possibilities for new technology and new physical phenomena inherent in this combination of properties remain largely to be explored.This award supports an integrated plan of research, outreach, and education. The research activities aim to explore, theoretically, the possibilities for new bulk phenomena in topological materials. The research focuses in particular on thermoelectric properties (the conversion of heat to electric power) and on the nature of the transition from conducting to non-conducting states. Key questions include these: 1) which conditions and which materials are conducive to achieving an efficient conversion of heat to electric power?, and 2) what new forms of the metal-to-insulator transition are possible, and what are their experimental signatures?New topological thermoelectric materials may have applications in both waste-heat recovery and more efficient cooling. Understanding new forms of the metal-insulator transition could lead to new generations of electronic devices.The outreach and education activities make concrete steps toward improving the visibility and accessibility of condensed matter physics as a field, and toward broadening participation within it. Activities include (i) developing lesson plans that introduce electricity and electric materials to elementary school students in underserved communities, (ii) developing problem sets and lectures for the community of high school students participating in olympiad-level physics competitions, (iii) building an inclusive research group and developing new courses at the graduate and undergraduate level, (iv) mentoring students from underrepresented demographic groups through the Bridge Program at Ohio State University, and (v) developing social media content that introduces condensed matter physics to the public.Technical AbstractThis award supports an integrated plan of research, outreach, and education that is based on the transport properties of electronic materials. The research focuses primarily on topological materials, which harbor the possibility for fundamentally new kinds of transport properties and electronic phases compared to those possible in traditional metals and insulators. Exploring these possibilities requires theories that go beyond the non-interacting, free-particle descriptions that led to the discovery and classification of topological materials. The aim of this research is to produce such theories, focusing in particular on bulk thermoelectric transport and on the nature of the metal-insulator transition.One thrust of the research is concerned with the thermoelectric response of topological semimetals and its dependence on magnetic field. The PI and his research team will consider magnetic Weyl semimetals, compensated topological semimetals, and other topological materials beyond Dirac/Weyl. The goal of this work is to identify conditions and material properties that are conducive to achieving large thermopower and large thermoelectric efficiency. A second thrust considers how topological systems admit novel bulk metal-insulator transitions, beyond the canonical Anderson and Mott perspectives. The research team will consider different scenarios for such transitions, including in strongly disordered systems, hydrodynamic systems, and in electron systems with nonperturbative electron-phonon coupling. The goal of these works is to uncover possible new electronic quantum phenomena, which have strong manifestations in the electrical conductivity, and to illustrate how they may appear in real materials.Integrated with these research efforts is a comprehensive plan for outreach and education that will take concrete steps toward improving the visibility and accessibility of condensed-matter physics as a field, and toward broadening participation within it. Activities include (i) developing lesson plans that introduce electricity and electric materials to elementary school students in underserved communities, (ii) developing problem sets and lectures for the community of high-school students participating in olympiad-level physics competitions, (iii) building an inclusive group and developing new courses at the graduate and undergraduate level, (iv) mentoring students from underrepresented demographic groups through the Bridge Program at Ohio State University, and (v) developing social media content that introduces condensed-matter physics to the public.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.