Abstract

One of the appealing aspects of condensed matter physics is the emergence of new collective behavior and quasiparticle excitations as a consequence of interactions between particles. This talk begins with one-dimensional materials, which have long been known to support unusual metallic states in which the electron spin and charge separate and travel with different velocities. However, the collective physics of even simple systems, including the paradigmatic Heisenberg spin chain, only became clear in the last few years: even at room temperature, the spins form an unusual quantum fluid described by the Kardar-Parisi-Zhang universality class familiar from soft condensed matter physics. This prediction leads to consequences for neutron scattering experiments under reasonable conditions, which were recently observed by a study of KCuF3 and also in an ultracold atomic system. A different kind of breakup or fractionalization of the electron can happen in higher dimensions, where new insight is coming from concepts and methods originating in quantum information theory. The last part of the talk discusses the renaissance of interest in interacting spins on a triangular lattice, following on the original spin liquid idea introduced by Anderson in 1973. An increasing body of evidence from theory and experiment suggests that many competing kinds of spin liquid compete on the triangular lattice, and we discuss some recent experimental and theoretical progress.