Kets of broken symmetry in the derivation of model Hamiltonians

Daniel Klein, David C. Foyt

A new variational derivation scheme for the Heisenberg exchange Hamiltonian is presented. A many-body variational Ansatz for the multistate ket appearing in the derivation is described and is applied to a simple model problem containing the qualitatively important different types of exchange contributions. Advantages of the present derivation over others are briefly indicated.

Manuel Rispler, Kasper Duivenvoorden, Norbert Schuch

Abstract We consider projected entangled pair state (PEPS) models with a global Z N symmetry, which are constructed from Z N -symmetric tensors and are thus Z N -invariant wavefunctions, and study the occurence of long-range order and symmetry breaking in these systems. First, we show that long-range order in those models is accompanied by a degeneracy in the so-called transfer operator of the system. We subsequently use this degeneracy to determine the nature of the symmetry broken states, i.e. those stable under arbitrary perturbations, and provide a succinct characterization in terms of the fixed points of the transfer operator (i.e. the different boundary conditions) in the individual symmetry sectors. We verify our findings numerically through the study of a Z 3 -symmetric model, and show that the entanglement Hamiltonian derived from the symmetry broken states is quasi-local (unlike the one derived from the symmetric state), reinforcing the locality of the entanglement Hamiltonian for gapped phases.

B. H. Brandow

Manuel Rispler, Kasper Duivenvoorden, Norbert Schuch

We consider Projected Entangled Pair State (PEPS) models with a global $\mathbb Z_N$ symmetry, which are constructed from $\mathbb Z_N$-symmetric tensors and are thus $\mathbb Z_N$-invariant wavefunctions, and study the occurence of long-range order and symmetry breaking in these systems. First, we show that long-range order in those models is accompanied by a degeneracy in the so-called transfer operator of the system. We subsequently use this degeneracy to determine the nature of the symmetry broken states, i.e., those stable under arbitrary perturbations, and provide a succinct characterization in terms of the fixed points of the transfer operator (i.e.\ the different boundary conditions) in the individual symmetry sectors. We verify our findings numerically through the study of a $\mathbb Z_3$-symmetric model, and show that the entanglement Hamiltonian derived from the symmetry broken states is quasi-local (unlike the one derived from the symmetric state), reinforcing the locality of the entanglement Hamiltonian for gapped phases.