We discuss possible quantum disordered states of the O(3) nonlinear \ensuremath{\sigma} model in which time-reversal invariance (T) and parity (P) are spontaneously broken. We show that there is spin-charge separation in these T- and P-broken disordered states. In one of the disordered states the quasiparticles are half-fermions, or semions, as in the Kalmeyer-Laughlin approach to the resonating-valence-bond state. We also write down the effective Lagrangian for the quasiparticles in these disordered phases.We discuss possible quantum disordered states of the O(3) nonlinear \ensuremath{\sigma} model in which time-reversal invariance (T) and parity (P) are spontaneously broken. We show that there is spin-charge separation in these T- and P-broken disordered states. In one of the disordered states the quasiparticles are half-fermions, or semions, as in the Kalmeyer-Laughlin approach to the resonating-valence-bond state. We also write down the effective Lagrangian for the quasiparticles in these disordered phases.

Physical Review LettersJul 24, 198937 citations

Stochastic resonance phenomena in spin chains

́Angel Rivas, Oxtoby, NP, Huelga, SF

May 1, 200914 citations

Spin-Peierls, valence-bond solid, and Néel ground states of low-dimensional quantum antiferromagnets

N. Read, Subir Sachdev A

We examine large-N limits of the nearest-neighbor SU(N) quantum antiferromagnets on bipartite lattices in d=1,2. In d=2 the model displays a transition from a N\'eel to a disordered phase. The properties of the disordered phase close to the phase boundary are crucially dependent upon the nature of ``hedgehog''-like instanton tunneling events. We calculate the Berry phases of the instantons and show that, at scales larger than the spin-correlation length, the system can be described by a Coulomb plasma of instantons with complex fugacities. The properties of the Coulomb plasma vary periodically with the ``spin'' ${\mathit{n}}_{\mathit{c}}$ of the states at each site, with periodicity given by the coordination number Z of the lattice [ ${\mathit{n}}_{\mathit{c}}$ =2S for SU(2)]. For ${\mathit{n}}_{\mathit{c}}$ \ensuremath{\ne}0 (mod Z) the disordered phase has a broken lattice symmetry with spin-Peierls order, while for ${\mathit{n}}_{\mathit{c}}$ =0 (mod Z), the ground state is a valence-bond solid state with no broken symmetry. Related topological effects for the d=1 chain lead to spin-Peierls order for odd ${\mathit{n}}_{\mathit{c}}$ . These results are for a class of models which have, at sites of the A sublattice, representations of SU(N) described by a Young tableau with a single row, and the conjugate on the B sublattice. Similar results are also obtained for representations with m rows, using U(m) gauge theory.We examine large-N limits of the nearest-neighbor SU(N) quantum antiferromagnets on bipartite lattices in d=1,2. In d=2 the model displays a transition from a N\'eel to a disordered phase. The properties of the disordered phase close to the phase boundary are crucially dependent upon the nature of ``hedgehog''-like instanton tunneling events. We calculate the Berry phases of the instantons and show that, at scales larger than the spin-correlation length, the system can be described by a Coulomb plasma of instantons with complex fugacities. The properties of the Coulomb plasma vary periodically with the ``spin'' ${\mathit{n}}_{\mathit{c}}$ of the states at each site, with periodicity given by the coordination number Z of the lattice [${\mathit{n}}_{\mathit{c}}$=2S for SU(2)]. For ${\mathit{n}}_{\mathit{c}}$\ensuremath{\ne}0 (mod Z) the disordered phase has a broken lattice symmetry with spin-Peierls order, while for ${\mathit{n}}_{\mathit{c}}$=0 (mod Z), the ground state is a valence-bond solid state with no broken symmetry. Related topological effects for the d=1 chain lead to spin-Peierls order for odd ${\mathit{n}}_{\mathit{c}}$. These results are for a class of models which have, at sites of the A sublattice, representations of SU(N) described by a Young tableau with a single row, and the conjugate on the B sublattice. Similar results are also obtained for representations with m rows, using U(m) gauge theory.

Physical review. B, Condensed matterSep 1, 1990492 citations

Stochastic resonance phenomena in spin chains

A. Rivas, Neil P. Oxtoby, S. F. Huelga

arXiv:0901.1791The European Physical Journal BMar 21, 200922 citations

{\mathit{n}}_{\mathit{c}}

of the states at each site, with periodicity given by the coordination number Z of the lattice [

{\mathit{n}}_{\mathit{c}}

=2S for SU(2)]. For

{\mathit{n}}_{\mathit{c}}

\ensuremath{\ne}0 (mod Z) the disordered phase has a broken lattice symmetry with spin-Peierls order, while for

{\mathit{n}}_{\mathit{c}}

=0 (mod Z), the ground state is a valence-bond solid state with no broken symmetry. Related topological effects for the d=1 chain lead to spin-Peierls order for odd

{\mathit{n}}_{\mathit{c}}

. These results are for a class of models which have, at sites of the A sublattice, representations of SU(N) described by a Young tableau with a single row, and the conjugate on the B sublattice. Similar results are also obtained for representations with m rows, using U(m) gauge theory.