Condensed Matter Seminar Series
Landau Forbidden Phase Transitions
and Emergent Photons in Quantum Magnets
Monday October 18, 11:30 am, Room 234, Physics Building
Host: Shailesh Chandrasekharan
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.