CAREER: Nonlinear THz electrodynamics of spin quasiparticles

University Of Illinois At Urbana-Champaign, Urbana IL

Investigators

Abstract

This award is funded in whole or in part under the American Rescue Plan Act of 2021 (Public Law 117-2). Nontechnical abstract: Nearly all existing computing devices are based on the manipulation of the charge of electrons. Unfortunately, fundamental limits at which charge information can be transmitted and preserved is slowing down and preventing breakthrough technological advances in next-generation computing. Recent research has indicated that this impasse can be overcome by using the collective motion of the spin of electrons. The spin of an electron is like a microscopic bar magnet. Most magnetic materials can be thought of as consisting of these tiny electron magnets aligned and fixed in a geometric pattern. In certain materials, however, the quantum interactions between these electron magnets causes them to move synchronously rather than remaining aligned. These collective behaviors are called emergent spin quasiparticles, and they are predicted to have the unique ability to store and transmit information in the form of angular momentum while not carrying any electron charge. In this way, spin quasiparticles can potentially enable faster, more energy-efficient and robust computation than is currently possible. However, these particles are hard to detect, and the underlying rules governing their behavior are poorly understood. This research project fills this knowledge gap by developing experimental methods based on powerful light sources to directly observe and manipulate these spin quasiparticles. Simultaneously, the educational component of this project increases exposure of middle school students to modern physics concepts (such as quantum mechanics) through a traveling and interactive physics demonstration show called the “Physics Van.” Undergraduate students at the University of Illinois are also coached as part of this project to lead these demos, visit local schools, and mentor middle school students on viable future pathways into science and engineering fields. This project is jointly funded by the Condensed Matter Physics (CMP) and the Electronic and Photonic Materials (EPM) programs of the Division of Materials Research (DMR). Technical abstract: In a variety of magnetic insulators, strong interactions, geometric frustration, and low dimensionality intertwine to give emergent spin quasiparticles that can be viable platforms for quantum computation and efficient spintronics. Examples include fractionalized spin excitations (spinons) and antiferromagnetic magnon currents. This experimental project uses nonlinear optical response in the terahertz (THz) range to measure, characterize and ultimately manipulate these spin quasiparticles. The specific objectives are (1) identifying distinctive experimental signatures of spinons in the nonlinear THz susceptibility of quantum magnets as indicated by recent theoretical work; (2) driving coherent antiferromagnet currents in insulators using strong THz light-spin coupling; and (3) understanding decoherence and relaxation mechanisms of THz-induced spin excitations. The research effort is implemented by using newly developed multidimensional nonlinear THz spectroscopies on magnetic heterostructures having resonators specifically designed to enhance THz light-matter coupling. This approach allows highly controllable and adaptable settings for inducing the strongest possible light-matter coupling. Materials under study include two-dimensional quantum spin liquid candidates, one-dimensional spin chains that are known to host spinons, and trivial antiferromagnetic insulators. The nonlinear magnetic susceptibility and shift magnon currents in these materials is measured to systematically determine relaxation and decoherence properties of spin quasiparticles. This work establishes a framework for discovering new spin quasiparticles and implements nonlinear THz spectroscopy as a general method to probe material properties inaccessible in conventional linear spectroscopies. 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.