Massachusetts Institute of Technology
Emergent photons (and beyond) in quantum magnets and optical lattices
Abstract:Quantum spin liquids are a class of states of matter that can arise in insulating magnetic systems. They are paramagnetic down to zero temperature, exhibiting neither magnetic ordering nor any other kind of symmetry-breaking order. However, unlike classical paramagnets, quantum spin liquids are not featureless, but are instead characterized by a variety of subtle structures. In particular, some spin liquid states support a collective excitation that behaves like the photon of electrodynamics, but is entirely distinct from it, and is thus dubbed an emergent photon. Emergent electric and even magnetic charges are also present.
Following a brief review of the basic physics of spin liquids, and some of the experimental motivation to consider such states, I will discuss a simple quantum spin model that has been shown to support emergent photons in its so-called U(1) spin liquid phase [1]. In this case, much of the spin liquid structure can be understood directly, and quite simply, in terms of the spin degrees of freedom. I will also discuss a closely related model that may exhibit some of the same physics, and may potentially be realizable in an appropriately designed system of cold bosonic atoms trapped in an optical lattice [2]. Both of the above systems are three-dimensional, which is crucial for the stability of the simplest U(1) spin liquid. I will conclude by discussing some of the highlights of two-dimensional U(1) spin liquids. These states can only exist as stable phases in the presence of gapless spin-carrying excitations that interact strongly with the emergent photon, and thus exhibit a variety of remarkable properties [3,4].