Condensed Matter Seminar Series

Landau Forbidden Phase Transitions and Emergent Photons in Quantum Magnets

Ashvin Vishwanath

UC Berkeley

Monday October 18, 11:30 am, Room 234, Physics Building

Abstract: Our modern understanding of phase transitions is built on Landau's theory, which, among other things, dictates when it may be possible to have a continuous transition between two phases. For example, a continuous transition between a superfluid state and a density wave is forbidden without special fine tuning in this theory, since different orders exist on either side of the transition . In this talk I describe how in certain two dimensional quantum systems (e.g. the spin-half antiferromagnet on the square lattice) interference effects from Berry phases can lead to precisely such `Landau forbidden' quantum phase transitions. This can allow for a direct continuous transition between a Neel phase and a Valence Bond Solid phase, without special fine tuning. Moreover, the critical point, at which certain topological defects - hedgehog configurations of the `spins' - are absent, is most naturally described in terms of fractionalized excitations (e.g. spinons with spin one half) coupled to emergent photons. In contrast, the phases on either side of the transition are conventional.