14 research outputs found
The J_1-J_2 antiferromagnet with Dzyaloshinskii-Moriya interaction on the square lattice: An exact diagonalization study
We examine the influence of an anisotropic interaction term of
Dzyaloshinskii-Moriya (DM) type on the groundstate ordering of the J_1-J_2
spin-1/2-Heisenberg antiferromagnet on the square lattice. For the DM term we
consider several symmetries corresponding to different crystal structures. For
the pure J_1-J_2 model there are strong indications for a quantum spin liquid
in the region of 0.4 < J_2/J_1 < 0.65. We find that a DM interaction influences
the breakdown of the conventional antiferromagnetic order by i) shifting the
spin liquid region, ii) changing the isotropic character of the groundstate
towards anisotropic correlations and iii) creating for certain symmetries a net
ferromagnetic moment.Comment: 7 pages, RevTeX, 6 ps-figures, to appear in J. Phys.: Cond. Ma
Generalized calculation of magnetic coupling constants for Mott-Hubbard insulators: Application to ferromagnetic Cr compounds
Using a Rayleigh-Schr\"odinger perturbation expansion of multi-band Hubbard
models, we present analytic expressions for the super-exchange coupling
constants between magnetic transition metal ions of arbitrary separation in
Mott-Hubbard insulators. The only restrictions are i) all ligand ions are
closed shell anions and ii) all contributing interaction paths are of equal
length. For short paths, our results essentially confirm the
Goodenough-Kanamori-Anderson rules, yet in general there does not exist any
simple rule to predict the sign of the magnetic coupling constants. The most
favorable situation for ferromagnetic coupling is found for ions with less than
half filled d shells, the (relative) tendency to ferromagnetic coupling
increases with increasing path length. As an application, the magnetic
interactions of the Cr compounds RbCrCl, CrCl, CrBr and CrI
are investigated, all of which except CrCl are ferromagnets.Comment: 13 pages, 6 eps figures, submitted to Phys Rev
Thermodynamic properties of the two-dimensional S=1/2 Heisenberg antiferromagnet coupled to bond phonons
By applying a quantum Monte Carlo procedure based on the loop algorithm we
investigate thermodynamic properties of the two-dimensional antiferromagnetic
S=1/2 Heisenberg model coupled to Einstein phonons on the bonds. The
temperature dependence of the magnetic susceptibility, mean phonon occupation
numbers and the specific heat are discussed in detail. We study the spin
correlation function both in the regime of weak and strong spin phonon coupling
(coupling constants g=0.1, w=8J and g=2, w=2J, respectively). A finite size
scaling analysis of the correlation length indicates that in both cases long
range Neel order is established in the ground state.Comment: 10 pages, 13 figure
Triplet Dispersion in CuGeO_3: Perturbative Analysis
We reconsider the 2d model for CuGeO_3 introduced previously (Phys. Rev.
Lett. 79, 163 (1997)). Using a computer aided perturbation method based on flow
equations we expand the 1-triplet dispersion up to 10th order. The expansion is
provided as a polynom in the model parameters. The latter are fixed by fitting
the theoretical result to experimental data obtained by INS. For a dimerization
delta = 0.08(1) we find an excellent agreement with experiment. This value is
at least 2 to 3 times higher than values deduced previously from 1d chain
approaches. For the intrachain frustration alpha_0 we find a smaller value of
0.25(3). The existence of interchain frustration conjectured previously is
confirmed by the analysis of temperature dependent susceptibility.Comment: 8 pages, 10 figures, submitted to Phys. Rev.
Exchange coupling in Eu monochalcogenides from first principles
Using a density functional method with explicit account for strong Coulomb
repulsion within the 4f shell, we calculate effective exchange parameters and
the corresponding ordering temperatures of the (ferro)magnetic insulating Eu
monochalcogenides (EuX; X=O,S,Se,Te) at ambient and elevated pressure
conditions. Our results provide quantitative account of the many-fold increase
of the Curie temperatures with applied pressure and reproduce well the
enhancement of the tendency toward ferromagnetic ordering across the series
from telluride to oxide, including the crossover from antiferromagnetic to
ferromagnetic ordering under pressure in EuTe and EuSe. The first and second
neighbor effective exchange are shown to follow different functional
dependencies. Finally, model calculations indicate a significant contribution
of virtual processes involving the unoccupied f states to the effective
exchange.Comment: 4 pages, 6 figure
Generalized hole-particle transformations and spin reflection positivity in multi-orbital systems
We propose a scheme combining spin reflection positivity and generalized
hole-particle and orbital transformations to characterize the symmetry
properties of the ground state for some correlated electron models on bipartite
lattices. In particular, we rigorously determine at half-filling and for
different regions of the parameter space the spin, orbital and pairing
pseudospin of the ground state of generalized two-orbital Hubbard models which
include the Hund's rule coupling.Comment: 6 pages, 2 figure
Efimov effect in quantum magnets
Physics is said to be universal when it emerges regardless of the underlying
microscopic details. A prominent example is the Efimov effect, which predicts
the emergence of an infinite tower of three-body bound states obeying discrete
scale invariance when the particles interact resonantly. Because of its
universality and peculiarity, the Efimov effect has been the subject of
extensive research in chemical, atomic, nuclear and particle physics for
decades. Here we employ an anisotropic Heisenberg model to show that collective
excitations in quantum magnets (magnons) also exhibit the Efimov effect. We
locate anisotropy-induced two-magnon resonances, compute binding energies of
three magnons and find that they fit into the universal scaling law. We propose
several approaches to experimentally realize the Efimov effect in quantum
magnets, where the emergent Efimov states of magnons can be observed with
commonly used spectroscopic measurements. Our study thus opens up new avenues
for universal few-body physics in condensed matter systems.Comment: 7 pages, 5 figures; published versio