Quantum Coherence: Quantum Spin Ice and Lattice Gauge Theory

Abstract

In this chapter, we address the effects of symmetry-allowed terms which induce quantum dynamics in a range of models close to the classical spin ice point. Specifically, we focus on Coulombic quantum spin liquid states, in which a highly entangled massive superposition of spin ice states is formed, allowing for dramatic quantum effects. In the perturbative limit near classical spin ice, a compact U(1) lattice gauge theory applies, and affords a direct description of the simplest such state. Supplementing the gauge theory with matter fields provides the key to a physically-motivated non-perturbative parton approach, which allows a description of the phase diagram more broadly. Throughout the presentation we use and discuss how results from lattice gauge theory translate to the context of quantum spin ice. We include a somewhat pedagogical presentation of duality and of the excitations of Coulombic spin liquids, and a new discussion of the wavefunctions of the various phases of quantum spin ice, not previously published in the literature. The latter provides some intuitive insight and may be a useful reference point for future variational approaches. Finally, we draw a thorough comparison between classical and quantum spin ice, before addressing some frontier topics such as the more frustrated version of quantum spin ice, quantum phase transitions, numerics and disorder.