2,717 research outputs found
Some properties of frustrated spin systems: extensions and applications of Lieb-Schupp approach
Lieb and Schupp have obtained, using certain version of ``spin-reflection
positivity'' method, a number of ground-state properties for frustrated
Heisenberg models. One group of these results is related to singlet nature of
ground state and it needs an assumption of reflection symmetry present in the
system. In this paper, it is shown that the result holds also for other
symmetries (inversion etc.). The second Lieb-Schupp result is relation between
ground-state energies of certain systems. In the paper, this relation is
applied to multidimensional models on various lattices.Comment: 15 pages, 8 eps figures, revtex
Spiral order by disorder and lattice nematic order in a frustrated Heisenberg antiferromagnet on the honeycomb lattice
Motivated by recent experiments on BiMnO(NO), we study a
frustrated - Heisenberg model on the two dimensional (2D) honeycomb
lattice. The classical - Heisenberg model on the two dimensional (2D)
honeycomb lattice has N\'eel order for , it
exhibits a one-parameter family of degenerate incommensurate spin spiral ground
states where the spiral wave vector can point in any direction. Spin wave
fluctuations at leading order lift this accidental degeneracy in favor of
specific wave vectors, leading to spiral order by disorder. For spin ,
quantum fluctuations are, however, likely to be strong enough to melt the
spiral order parameter over a wide range of . Over a part of this
range, we argue that the resulting state is a valence bond solid (VBS) with
staggered dimer order - this VBS is a nematic which breaks lattice rotational
symmetry. Our arguments are supported by comparing the spin wave energy with
the energy of the dimer solid obtained using a bond operator formalism. Turning
to the effect of thermal fluctuations on the spiral ordered state, any nonzero
temperature destroys the magnetic order, but the discrete rotational symmetry
of the lattice remains broken resulting in a thermal analogue of the nematic
VBS. We present arguments, supported by classical Monte Carlo simulations, that
this nematic transforms into the high temperature symmetric paramagnet via a
thermal phase transition which is in the universality class of the classical
3-state Potts (clock) model in 2D. We discuss the possible relevance of our
results for honeycomb magnets, such as BiMO(NO) (with
M=Mn,V,Cr), and bilayer triangular lattice magnets.Comment: Slightly revise
Dirac method and symplectic submanifolds in the cotangent bundle of a factorizable Lie group
In this work we study some symplectic submanifolds in the cotangent bundle of
a factorizable Lie group defined by second class constraints. By applying the
Dirac method, we study many issues of these spaces as fundamental Dirac
brackets, symmetries, and collective dynamics. This last item allows to study
integrability as inherited from a system on the whole cotangent bundle, leading
in a natural way to the AKS theory for integrable systems
Inhomogeneity Induces Resonance Coherence Peaks in Superconducting BSCCO
In this paper we analyze, using scanning tunneling spectroscopy, the density
of electronic states in nearly optimally doped BSCCO in zero field. Focusing on
the superconducting gap, we find patches of what appear to be two different
phases in a background of some average gap, one with a relatively small gap and
sharp large coherence peaks and one characterized by a large gap with broad
weak coherence peaks. We compare these spectra with calculations of the local
density of states for a simple phenomenological model in which a 2 xi_0 * 2
xi_0 patch with an enhanced or supressed d-wave gap amplitude is embedded in a
region with a uniform average d-wave gap.Comment: 4 pages, 3 figure
Suppression of Dimer Correlations in the Two-Dimensional - Heisenberg Model: an Exact Diagonalization Study
We present an exact diagonalization study of the ground state of the
spin-half model. Dimer correlation functions and the susceptibility
associated to the breaking of the translational invariance are calculated for
the and the clusters. These results -- especially when
compared to the one dimensional case, where the occurrence of a dimerized phase
for large enough frustration is well established -- suggest either a
homogeneous spin liquid or, possibly, a dimerized state with a rather small
order parameter
Thermodynamics of the quantum easy-plane antiferromagnet on the triangular lattice
The classical XXZ triangular-lattice antiferromagnet (TAF) shows both an
Ising and a BKT transition, related to the chirality and the in-plane spin
components, respectively. In this paper the quantum effects on the
thermodynamic quantities are evaluated by means of the pure-quantum
self-consistent harmonic approximation (PQSCHA), that allows one to deal with
any spin value through classical MC simulations. We report the internal energy,
the specific heat, and the in-plane correlation length of the quantum XX0 TAF,
for S=1/2, 1, 5/2. The quantum transition temperatures turn out to be smaller
the smaller the spin, and agree with the few available theoretical and
numerical estimates.Comment: 4 pages,3 postscript figure
Violation of the Luttinger sum rule within the Hubbard model on a triangular lattice
The frequency-moment expansion method is developed to analyze the validity of
the Luttinger sum rule within the Mott-Hubbard insulator, as represented by the
generalized Hubbard model at half filling and large . For the particular
case of the Hubbard model with nearest-neighbor hopping on a triangular lattice
lacking the particle-hole symmetry results reveal substantial violation of the
sum rule.Comment: 4 pages, 2 figure
A Recursive Method of the Stochastic State Selection for Quantum Spin Systems
In this paper we propose the recursive stochastic state selection method, an
extension of the recently developed stochastic state selection method in Monte
Carlo calculations for quantum spin systems. In this recursive method we use
intermediate states to define probability functions for stochastic state
selections. Then we can diminish variances of samplings when we calculate
expectation values of the powers of the Hamiltonian. In order to show the
improvement we perform numerical calculations of the spin-1/2
anti-ferromagnetic Heisenberg model on the triangular lattice. Examining
results on the ground state of the 21-site system we confide this method in its
effectiveness. We also calculate the lowest and the excited energy eigenvalues
as well as the static structure factor for the 36-site system. The maximum
number of basis states kept in a computer memory for this system is about 3.6 x
10**7. Employing a translationally invariant initial trial state, we evaluate
the lowest energy eigenvalue within 0.5 % of the statistical errors.Comment: 14 pages, 1 figur
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