Disorder, Quantum Spin Liquids, and Valence Bond Crystals*

Quantum spin liquids are novel states of matter that exhibit long range entanglement and associated topological properties despite the absence of traditional long range order in the Landau sense.  Whether such states exist in nature or not is an unresolved matter.  Relevant to this discussion is disorder that always exists in real materials.  Here, I will review evidence for quantum spin liquid behavior in several candidate materials, and then comment on the effect of disorder on their properties.  I will then contrast theories that address their novel properties based on either intrinsic QSL physics, or more traditional approaches based on random exchange.  I will close by contrasting QSL ground states with valence bond ones, and how this is influenced by disorder.