CAREER: Correlated Topological Phases in Rare-earth-based Compounds

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Non-Technical Abstract: Materials can be classified into two categories based on the movement of the carriers: metals and insulators. Metals conduct electricity because of the free movement of the electrons whereas insulators cannot conduct electricity. The topological insulator (TI) is a new state of matter in which a material is non-conducting throughout its bulk but is guaranteed to have very stable conducting states on its surface. This surface conduction is protected against disturbance by the basic constraints of the material's underlying symmetry. Topological properties of materials are a fascinating subject of study in contemporary condensed matter physics. The discovery of the first TI tremendously accelerated research into phases of matter characterized by non-trivial topological states such as Dirac, Weyl and nodal-line semimetals, the topological crystalline insulator, and the topological superconductor. Dirac and Weyl semimetals provide a platform to study both bulk (linearly dispersive) and surface (Fermi arc) states. It is important to note that, to date, most of the observed topological materials realized experimentally are found in weakly correlated electronic band systems where the correlation between the electrons are largely ignored. This research proposes to discover and understand the electron correlation in topological materials. Topological quantum materials promise to bring upon a new age in areas such as spintronics and quantum computation among other next generation technologies. Progress in these areas helps to increase the ability to store and process larger amounts of data at higher speeds. There are other applications of strategic interest such as cryptography and safe communications. This project aims to help recruit, mentor, and prepare underrepresented minority students into Physics PhD programs through the American Physical Society (APS)-funded Bridge Program as well as to increase the production of highly qualified high school Physics teachers through "PhysTEC" program. Education materials based on the progress of the quantum materials are prepared to help students, teachers and the general public.Technical Abstract: The field of topological materials has grown exponentially since the first three-dimensional topological insulator material was discovered in 2007. It is currently impacting large bodies of condensed matter physics, chemistry, materials sciences, and engineering communities worldwide. Moreover, most work thus far has focused on simple materials where electronic correlation effects can be largely ignored, and the topological properties are well described in terms of a single-particle band structure. The introduction of strong electron correlation opens up a novel field of research in a topological insulating state. This project aims to discover and understand the unusual Dirac fermion states in strongly correlated electron systems, the nature of the correlations in these novel states are distinctly different from those found in graphene, topological insulators, and Weyl semimetal. Specifically, this project utilizes high-resolution angle-, spin- and time-resolved photoemission spectroscopy techniques to study the electronic and spin properties as well as the momentum resolved dynamical properties of the bulk and symmetry-protected surface properties of rare-earth (4f)-based compounds to discover and understand the possible signature of the strongly correlated topological phase. The planned research provides the electronic signatures for the correlated topological phase, heavy Weyl fermion phase and magnetic topological insulator phases, as well as their light-matter interactions, thus opening up a new direction of research into the topological and Weyl phases facilitated by strong correlations. The educational component of the project is to motivate students from high schools to universities to pursue quantum materials research so that they become potential leaders in the field of condensed matter physics, spectroscopy, vacuum technology, laser technology and nanoscience. The educational plans create a system that inspires, educates, and provides the opportunities necessary for the younger generations to take steps and become the scientists of tomorrow.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.