Quantum spin liquids

Abstract

Years ago, Lev Landau taught us how to think about distinct phases of matter through an order parameter that characterizes the symmetry-broken state relative to the symmetry-preserving state from which it emerges. More recently, however, it has been realized that not all phases of matter are captured by this paradigm. This was spectacularly demonstrated by the discovery of fractional quantum Hall states in the 1980s. Over the years, it has been elucidated that these states, along with their exotic excitations—quasiparticles carrying a rational fraction of the elementary charge of an electron—are the consequence of topological properties of ground state wave functions with a special type of long-range quantum entanglement. One might wonder whether analogous phenomena occur for spins. Whether these “quantum spin liquids” actually exist in nature has been the subject of much investigation.