Quantum Phenomena in Solids

NONTECHNICAL SUMMARY All materials are made from elementary constituents -- protons, neutrons, and electrons -- which are governed the laws of quantum mechanics. Often this quantum-ness becomes hidden in their macroscopic aspect, but with care it can be revealed by modern techniques and harnessed to produce novel functionalities. This project focuses on the observation and manipulation of "excitations", which are particle or wave-like disturbances that move within a quantum material and can carry energy, charge, or information with them. The way in which these excitations are created, move, and interact reveal fundamental physics of the quantum world. Such motions also serve as the basis for diverse information and energy technologies. In this project, new theoretical techniques will be developed and specific predictions made to guide the experimental study and use of quantum excitations in materials. The first research thrust is on unconventional magnetic materials known as spin liquids, in which excitations called "spinons" act like fractions of electrons and experience strikingly different forces from those in normal experience. The theory developed in this project will reveal how spinons move and interact with one another, and guide their observation in modern experiments such as neutron scattering. Another type of excitation in quantum materials is the "exciton", which is a sort of pseudo-atom formed from electrons in a semiconductor that can both absorb and emit laser light efficiently. In a second study within this project, theory will be developed to show how dense systems of excitons under intense laser illumination change their quantum states. The final thrust of the project is to develop theoretical techniques to model the motion of quantum excitations on modern quantum computers. The new methods developed therein may enable the next generation of materials modeling on quantum devices. Training of undergraduate and graduate students and postdoctoral researchers is an integral component of this project. These junior scientists will learn forefront areas of condensed matter and quantum theory, as well as develop general scientific, communication, and computational proficiency, through mentorship and collaboration on the research. These skills prepare them to participate productively in the nation's quantum workforce. TECHNICAL SUMMARY The nature of a quantum phase of matter is revealed through its quasiparticle excitations, and these quasparticles control the responses and properties of quantum materials. The goal of this project is advance the theoretical understanding of the dynamics of excitations in exotic quantum states, by developing new theoretical models and techniques, with applications to specific quantum materials of particular interest. The research is divided into three thrusts. The first thrust focuses on quantum spin liquids, in which frustration and quantum fluctuations induce a long-range entangled state which supports fractionalized emergent excitations such as spinons. This project develop the theory of dynamical correlations of spins and holes in spin liquids for detailed comparison with experiments such as inelastic neutron scattering, in order to break the bottleneck limiting progress in spin liquid studies. The second thrust concerns moiré excitons, which are a remarkable new platform for interacting boson physics beyond equilibrium, and the proposed work will elucidate the non-equilibrium aspects which are unique and novel, in particular the many-body physics of moiré excitons under strong laser illumination. The final thrust of the project is to develop theoretical techniques to model the dynamics of quasiparticles on modern quantum computers. The new methods developed therein may enable the next generation of materials modeling on quantum devices. Training of undergraduate and graduate students and postdoctoral researchers is an integral component of this project. These junior scientists will learn forefront areas of condensed matter and quantum theory, as well as develop general scientific, communication, and computational proficiency, through mentorship and collaboration on the research. These skills prepare them to participate productively in the nation's quantum workforce. 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.