Quantum Phase Transitions and Hidden Orders in Low-Dimensional Spin Systems

by Gennady Chitov (Laurentian Univ. Canada)

Wednesday 13 Jun 2012, 11:15 → 12:15 Europe/Zurich

EPFL, Salle 727 du BSP (EPFL)

According to the conventional Landau theory, phases are distinct due to their different symmetries, and a phase transition is always related to a symmetry breaking of the Hamiltonian. The order is described by appropriately chosen local long-range order parameter(s). However, there is a mounting number of examples where the phases and transitions cannot be described by the explicit symmetry breaking and/or local order parameter(s). I will review several known examples of the classical and quantum systems of such type. Then I will discuss our work on the transitions and hidden orders in the dimerized two- and three-leg spin-1/2 ladders. The results of both the mean-field theory and the exact diagonalization technique will be presented. It is shown that the columnar
dimerization pattern is never critical, whereas the staggered configuration possesses a
quantum critical point. Various gapped phases cannot be distinguished by the local Landau
order parameter, but they possess non-local topological string order parameters. We
calculate energies, gaps, string order parameters, and yield estimates of the critical
exponents ν and β. Directions of the future work will be also discussed.