6 research outputs found
The magnetization process in the 2-dimensional J_1-J_2 model
We study the alpha = J_2/J_1-dependence of the magnetization process in the
J_1-J_2 model on a square lattice with frustrating couplings J_2 along the
diagonals. Perturbation expansions around alpha=J_2/J_1=0 and 1/alpha=0$ yield
an adequate description of the magnetization curve in the antiferromagnetic and
collinear antiferromagnetic phase, respectively. The transition from one phase
to the other (0.5 < alpha < 0.7) leaves pronounced structures in the
longitudinal and transverse structure factors at p=(pi,pi) and p=(0,pi).Comment: 10 pages, 10 figures, RevTe
Magnetic order in the Shastry-Sutherland model
The ground state properties of the Shastry-Sutherland model in the presence
of an external field are investigated by means of variational states built up
from unpaired spins (monomers) and singlet pairs of spins (dimers). The minimum
of the energy is characterized by specific monomer-dimer configurations, which
visualize the magnetic order in the sectors with fixed magnetization M=S/N. A
change in the magnetic order is observed if the frustrating coupling alpha
exceeds a critical value alpha_c(M), which depends on M. Special attention is
paid to the ground state configurations at M=1/4, 1/6, 1/8.Comment: 9 pages, 9 figures, RevTe
Charge density plateaux and insulating phases in the model with ladder geometry
We discuss the occurrence and the stability of charge density plateaux in
ladder-like systems (at zero magnetization M=0) for the cases of 2- and
3-leg ladders. Starting from isolated rungs at zero leg coupling, we study the
behaviour of plateaux-related phase transitions by means of first order
perturbation theory and compare our results with Lanczos diagonalizations for
ladders () with increasing leg couplings. Furthermore we
discuss the regimes of rung and leg couplings that should be favoured for the
appearance of the charge density plateaux.Comment: 10 pages, 7 figures, RevTex
A numerical study of the formation of magnetisation plateaus in quasi one-dimensional spin-1/2 Heisenberg models
We study the magnetisation process of the one dimensional spin-1/2
antiferromagnetic Heisenberg model with modulated couplings over j=1,2,3 sites.
It turns out that the evolution of magnetisation plateaus depends on j and on
the wave number q of the modulation according to the rule of Oshikawa, et al. A
mapping of two- and three-leg zig-zag ladders on one dimensional systems with
modulated couplings yields predictions for the occurence of magnetization
plateaus. The latter are tested by numerical computations with the DMRG
algorithm.Comment: 7 pages, 10 figures, accepted for publication in Euro. Phys. J.
Static and dynamic structure factors in the Haldane phase of the bilinear-biquadratic spin-1
The excitation spectra of the T=0 dynamic structure factors for the spin,
dimer, and trimer fluctuation operators as well as for the newly defined center
fluctuation operator in the one-dimensional S=1 Heisenberg model wi th
isotropic bilinear and biquadratic exchange are
investigated via the recursion method for systems with up to N=18 site s over
the predicted range, , of the topologically ordered
Haldane phase. The four static and dynamic structure factors probe t he
ordering tendencies in the various coupling regimes and the elementary and
composite excitations which dominate the T=0 dynamics. At (VBS point), the dynamically relevant spectra in the invariant
subspaces with total spin are dominated by a branch of magnon
states , by continua of two-magnon scattering states , and by discrete branches of two-magnon bound states with positive
interaction energy . The dimer and trimer spectra at ar e
found to consist of single modes with -independent excitation energies
and , where is
the ground-state energy per site. The basic structure of the dynamically
relevant excitation spectrum remains the same over a substantial parameter
range within the Haldane phase. At the transition to the dimerized phase
(), the two-magnon excitations turn into two-spinon excitations.Comment: 12 pages, 4 Postscript figure