Alladi Ramakrishnan Hall

Classical origins of Landau-incompatible transitions

Abhishodh Prakash

Oxford University

Spontaneous symmetry breaking underpins some of the most important phenomena in condensed matter and statistical physics. A description of direct transitions between symmetry breaking phases in terms of local order parameters is formulated when the symmetry breaking patterns were Landau-compatible i.e. when the unbroken symmetries of one phase is a subset of the other. About twenty years ago, Senthil et al [1] demonstrated that a direct transition between Landau-incompatible symmetry breaking phases was also possible in two-dimensional quantum magnets. Such 'deconfined quantum critical' (DQC) transitions are believed to be exotic and found in interacting quantum systems, often with anomalous symmetries (e.g.: constrained by Lieb-Schultz-Mattis theorems).

In this talk (based on upcoming work with N. Jones), I will demonstrate that such special conditions are unnecessary and Landau-incompatible transitions can be found in a well-known family of classical statistical mechanical models introduced by Jose, Kadanoff, Kirkpatrick and Nelson [2]. All smoking-gun DQC features such as charged defect melting and enhanced symmetries are readily understood. I will also show that a closely related family of models also exhibits another unusual critical phenomenon found in quantum systems- 'unnecessary criticality' where a stable critical surface exists within a single phase of matter analogous to the first-order line separating liquid and gases.

[1] SCIENCE, Vol 303, Issue 5663
[2] Phys. Rev. B 16, 1217 (1977)