We consider classical normal modes and noninteracting bosonic excitations in disordered systems. We emphasize generic aspects of such problems and parallels with disordered, noninteracting systems of fermions, and discuss in particular the relevance for bosonic excitations of symmetry classes known in the fermionic context. We also stress important differences between bosonic and fermionic problems. One of these follows from the fact that ground-state stability of a system requires all bosonic excitation energy levels to be positive, while stability in systems of noninteracting fermions is ensured by the exclusion principle, whatever the single-particle energies. As a consequence, simple models of uncorrelated disorder are less useful for bosonic systems than for fermionic ones, and it is generally important to study the excitation spectrum in conjunction with the problem of constructing a disorder-dependent ground state: we show how a mapping to an operator with chiral symmetry provides a useful tool for doing this. A second difference involves the distinction for bosonic systems between excitations which are Goldstone modes and those which are not. In the case of Goldstone modes we review established results illustrating the fact that disorder decouples from excitations in the low-frequency limit, above a critical dimension ${d}_{c},$ which in different circumstances takes the values ${d}_{c}=2$ and ${d}_{c}=0.$ For bosonic excitations which are not Goldstone modes, we argue that an excitation density varying with frequency as $\ensuremath{\rho}(\ensuremath{\omega})\ensuremath{\propto}{\ensuremath{\omega}}^{4}$ is a universal feature in systems with ground states that depend on the disorder realization. We illustrate our conclusions with extensive analytical and some numerical calculations for a variety of models in one dimension.We consider classical normal modes and noninteracting bosonic excitations in disordered systems. We emphasize generic aspects of such problems and parallels with disordered, noninteracting systems of fermions, and discuss in particular the relevance for bosonic excitations of symmetry classes known in the fermionic context. We also stress important differences between bosonic and fermionic problems. One of these follows from the fact that ground-state stability of a system requires all bosonic excitation energy levels to be positive, while stability in systems of noninteracting fermions is ensured by the exclusion principle, whatever the single-particle energies. As a consequence, simple models of uncorrelated disorder are less useful for bosonic systems than for fermionic ones, and it is generally important to study the excitation spectrum in conjunction with the problem of constructing a disorder-dependent ground state: we show how a mapping to an operator with chiral symmetry provides a useful tool for doing this. A second difference involves the distinction for bosonic systems between excitations which are Goldstone modes and those which are not. In the case of Goldstone modes we review established results illustrating the fact that disorder decouples from excitations in the low-frequency limit, above a critical dimension ${d}_{c},$ which in different circumstances takes the values ${d}_{c}=2$ and ${d}_{c}=0.$ For bosonic excitations which are not Goldstone modes, we argue that an excitation density varying with frequency as $\ensuremath{\rho}(\ensuremath{\omega})\ensuremath{\propto}{\ensuremath{\omega}}^{4}$ is a universal feature in systems with ground states that depend on the disorder realization. We illustrate our conclusions with extensive analytical and some numerical calculations for a variety of models in one dimension.

arXiv:cond-mat/0305445Physical review. B, Condensed matterOct 15, 2003144 citations

Local magnetic field distributions. II. Further results

T. C. Choy, David Sherrington, M. F. Thomsen +1 more

The concept and character of the local magnetic field distribution P(h), developed in an earlier paper, is extended to higher space and spin dimensions and to several special but spatially periodic systems, disorder being the subject of a future paper. For Ising spins on standard lattices, P(h) is shown to have a shape which is dimension dependent but relatively coordination independent. While in two dimensions (2D) P(h) has a dip at h=0 at ${T}_{c}$ , in 3D it is extremely flat near the origin, in 4D it is pseudo-Gaussian, while in all dimensions it is qualitatively similar to a discrete form of the corresponding universal block-spin probability function. Bethe lattices, studied for all spin dimensions, are shown to be quite different. Results are also presented for P(h) and the ground-state degeneracies of some one-dimensional frustrated systems exhibiting high-degeneracy disorder points.The concept and character of the local magnetic field distribution P(h), developed in an earlier paper, is extended to higher space and spin dimensions and to several special but spatially periodic systems, disorder being the subject of a future paper. For Ising spins on standard lattices, P(h) is shown to have a shape which is dimension dependent but relatively coordination independent. While in two dimensions (2D) P(h) has a dip at h=0 at ${T}_{c}$, in 3D it is extremely flat near the origin, in 4D it is pseudo-Gaussian, while in all dimensions it is qualitatively similar to a discrete form of the corresponding universal block-spin probability function. Bethe lattices, studied for all spin dimensions, are shown to be quite different. Results are also presented for P(h) and the ground-state degeneracies of some one-dimensional frustrated systems exhibiting high-degeneracy disorder points.

Physical review. B, Condensed matterJun 1, 19857 citations

Exact ground states of disordered systems

Jan Hermann Mienke

Jan 1, 20020 citations

Phase shifts and bound states in one-dimensional problems

Pedro Miguel Echenique

American Journal of PhysicsMar 1, 19871 citation

Order and disorder in SU( N ) simplex solid antiferromagnets

Yury Kiselev, S. A. Parameswaran, Daniel P. Arovas

Abstract. We study the structure of quantum ground states of simplex solid models, which are generalizations of the valence bond construction for quantum antiferromagnets originally proposed by Affleck, Kennedy, Lieb, and Tasaki (AKLT) (1987 Phys. Rev. Lett . 59 799). Whereas the AKLT states are created by application of bond singlet operators for spins, the simplex solid construction is based on N -simplex singlet operators for spins. In both cases, a discrete one-parameter family of translationally-invariant models with exactly solvable ground states is defined on any regular lattice, and the equal time ground state correlations are given by the finite temperature correlations of an associated classical model on the same lattice, owing to the product form of the wave functions when expressed in a coherent state representation. We study these classical companion models via a mix of Monte Carlo simulations, mean-field arguments, and low-temperature effective field theories. Our analysis reveals that the ground states of edge- and face-sharing cubic lattice simplex solid models are long range ordered for sufficiently large values of the discrete parameter, whereas the ground states of the models on the kagome (2D) and hyperkagome (3D) lattices are always quantum disordered. The kagome simplex solid exhibits strong local order absent in its three-dimensional hyperkagome counterpart, a contrast that we rationalize with arguments similar to those leading to ‘order by disorder’.Abstract. We study the structure of quantum ground states of simplex solid models, which are generalizations of the valence bond construction for quantum antiferromagnets originally proposed by Affleck, Kennedy, Lieb, and Tasaki (AKLT) (1987 Phys. Rev. Lett . 59 799). Whereas the AKLT states are created by application of bond singlet operators for spins, the simplex solid construction is based on N -simplex singlet operators for spins. In both cases, a discrete one-parameter family of translationally-invariant models with exactly solvable ground states is defined on any regular lattice, and the equal time ground state correlations are given by the finite temperature correlations of an associated classical model on the same lattice, owing to the product form of the wave functions when expressed in a coherent state representation. We study these classical companion models via a mix of Monte Carlo simulations, mean-field arguments, and low-temperature effective field theories. Our analysis reveals that the ground states of edge- and face-sharing cubic lattice simplex solid models are long range ordered for sufficiently large values of the discrete parameter, whereas the ground states of the models on the kagome (2D) and hyperkagome (3D) lattices are always quantum disordered. The kagome simplex solid exhibits strong local order absent in its three-dimensional hyperkagome counterpart, a contrast that we rationalize with arguments similar to those leading to ‘order by disorder’.

Journal of Statistical Mechanics Theory and ExperimentJan 1, 20161 citation

{d}_{c},

which in different circumstances takes the values

{d}_{c}=2

and

{d}_{c}=0.

For bosonic excitations which are not Goldstone modes, we argue that an excitation density varying with frequency as $\ensuremath{\rho}(\ensuremath{\omega})\ensuremath{\propto}{\ensuremath{\omega}}^{4}$ is a universal feature in systems with ground states that depend on the disorder realization. We illustrate our conclusions with extensive analytical and some numerical calculations for a variety of models in one dimension.

{T}_{c}

, in 3D it is extremely flat near the origin, in 4D it is pseudo-Gaussian, while in all dimensions it is qualitatively similar to a discrete form of the corresponding universal block-spin probability function. Bethe lattices, studied for all spin dimensions, are shown to be quite different. Results are also presented for P(h) and the ground-state degeneracies of some one-dimensional frustrated systems exhibiting high-degeneracy disorder points.