3,296 research outputs found
Symplectic N and time reversal in frustrated magnetism
Identifying the time reversal symmetry of spins as a symplectic symmetry, we
develop a large N approximation for quantum magnetism that embraces both
antiferromagnetism and ferromagnetism. In SU(N), N>2, not all spins invert
under time reversal, so we have introduced a new large N treatment which builds
interactions exclusively out of the symplectic subgroup [SP(N)] of time
reversing spins, a more stringent condition than the symplectic symmetry of
previous SP(N) large N treatments. As a result, we obtain a mean field theory
that incorporates the energy cost of frustrated bonds. When applied to the
frustrated square lattice, the ferromagnetic bonds restore the frustration
dependence of the critical spin in the Neel phase, and recover the correct
frustration dependence of the finite temperature Ising transition.Comment: added reference
Three dimensional generalization of the - Heisenberg model on a square lattice and role of the interlayer coupling
A possibility to describe magnetism in the iron pnictide parent compounds in
terms of the two-dimensional frustrated Heisenberg - model has been
actively discussed recently. However, recent neutron scattering data has shown
that the pnictides have a relatively large spin wave dispersion in the
direction perpendicular to the planes. This indicates that the third dimension
is very important. Motivated by this observation we study the --
model that is the three dimensional generalization of the -
Heisenberg model for and S = 1. Using self-consistent spin wave
theory we present a detailed description of the staggered magnetization and
magnetic excitations in the collinear state. We find that the introduction of
the interlayer coupling suppresses the quantum fluctuations and
strengthens the long range ordering. In the -- model, we find
two qualitatively distinct scenarios for how the collinear phase becomes
unstable upon increasing . Either the magnetization or one of the spin
wave velocities vanishes. For renormalization due to quantum
fluctuations is significantly stronger than for S=1, in particular close to the
quantum phase transition. Our findings for the -- model are of
general theoretical interest, however, the results show that it is unlikely
that the model is relevant to undoped pnictides.Comment: 11 pages, 10 figures. Updated version, several references adde
Bose-Einstein condensation in antiferromagnets close to the saturation field
At zero temperature and strong applied magnetic fields the ground sate of an
anisotropic antiferromagnet is a saturated paramagnet with fully aligned spins.
We study the quantum phase transition as the field is reduced below an upper
critical and the system enters a XY-antiferromagnetic phase. Using a
bond operator representation we consider a model spin-1 Heisenberg
antiferromagnetic with single-ion anisotropy in hyper-cubic lattices under
strong magnetic fields. We show that the transition at can be
interpreted as a Bose-Einstein condensation (BEC) of magnons. The theoretical
results are used to analyze our magnetization versus field data in the organic
compound - (DTN) at very low temperatures. This is the
ideal BEC system to study this transition since is sufficiently low to
be reached with static magnetic fields (as opposed to pulsed fields). The
scaling of the magnetization as a function of field and temperature close to
shows excellent agreement with the theoretical predictions. It allows
to obtain the quantum critical exponents and confirm the BEC nature of the
transition at .Comment: 4 pages, 1 figure. Accepted for publication in PRB
Spin Diffusion in Double-Exchange Manganites
The theoretical study of spin diffusion in double-exchange magnets by means
of dynamical mean-field theory is presented. We demonstrate that the
spin-diffusion coefficient becomes independent of the Hund's coupling JH in the
range of parameters JH*S >> W >> T, W being the bandwidth, relevant to colossal
magnetoresistive manganites in the metallic part of their phase diagram. Our
study reveals a close correspondence as well as some counterintuitive
differences between the results on Bethe and hypercubic lattices. Our results
are in accord with neutron scattering data and with previous theoretical work
for high temperatures.Comment: 4.0 pages, 3 figures, RevTeX 4, replaced with the published versio
Dynamical properties of ultracold bosons in an optical lattice
We study the excitation spectrum of strongly correlated lattice bosons for
the Mott-insulating phase and for the superfluid phase close to localization.
Within a Schwinger-boson mean-field approach we find two gapped modes in the
Mott insulator and the combination of a sound mode (Goldstone) and a gapped
(Higgs) mode in the superfluid. To make our findings comparable with
experimental results, we calculate the dynamic structure factor as well as the
linear response to the optical lattice modulation introduced by Stoeferle et
al. [Phys. Rev. Lett. 92, 130403 (2004)]. We find that the puzzling finite
frequency absorption observed in the superfluid phase could be explained via
the excitation of the gapped (Higgs) mode. We check the consistency of our
results with an adapted f-sum-rule and propose an extension of the experimental
technique by Stoeferle et al. to further verify our findings.Comment: 13 pages, 5 figure
Interacting Boson Theory of the Magnetization Process of the Spin-1/2 Ferromagnetic-Antiferromagnetic Alternating Heisenberg Chain
The low temperature magnetization process of the
ferromagnetic-antiferromagnetic Heisenberg chain is studied using the
interacting boson approximation. In the low field regime and near the
saturation field, the spin wave excitations are approximated by the
function boson gas for which the Bethe ansatz solution is available. The finite
temperature properties are calculated by solving the integral equation
numerically. The comparison is made with Monte Carlo calculation and the limit
of the applicability of the present approximation is discussed.Comment: 4 pages, 7 figure
Dimensional crossover in dipolar magnetic layers
We investigate the static critical behaviour of a uniaxial magnetic layer,
with finite thickness L in one direction, yet infinitely extended in the
remaining d dimensions. The magnetic dipole-dipole interaction is taken into
account. We apply a variant of Wilson's momentum shell renormalisation group
approach to describe the crossover between the critical behaviour of the 3-D
Ising, 2-d Ising, 3-D uniaxial dipolar, and the 2-d uniaxial dipolar
universality classes. The corresponding renormalisation group fixed points are
in addition to different effective dimensionalities characterised by distinct
analytic structures of the propagator, and are consequently associated with
varying upper critical dimensions. While the limiting cases can be discussed by
means of dimensional epsilon expansions with respect to the appropriate upper
critical dimensions, respectively, the crossover features must be addressed in
terms of the renormalisation group flow trajectories at fixed dimensionality d.Comment: 25 pages, Latex, 12 figures (.eps files) and IOP style files include
Collective modes for an array of magnetic dots in the vortex state
The dispersion relations for collective magnon modes for square-planar arrays
of vortex-state magnetic dots, having closure magnetic flux are calculated. The
array dots have no direct contact between each other, and the sole source of
their interaction is the magnetic dipolar interaction. The magnon formalism
using Bose operators along with translational symmetry of the lattice, with the
knowledge of mode structure for the isolated dot, allows the diagonalization of
the system Hamiltonian giving the dispersion relation. Arrays of vortex-state
dots show a large variety of collective mode properties, such as positive or
negative dispersion for different modes. For their description, not only
dipolar interaction of effective magnetic dipoles, but non-dipolar terms common
to higher multipole interaction in classical electrodynamics can be important.
The dispersion relation is shown to be non-analytic as the value of the
wavevector approaches zero for all dipolar active modes of the single dot. For
vortex-state dots the interdot interaction is not weak, because, the dynamical
part (in contrast to the static magnetization of the vortex state) dot does not
contain the small parameter, the ratio of vortex core size to the dot radius.
This interaction can lead to qualitative effects like the formation of modes of
angular standing waves instead of modes with definite azimuthal number known
for the insolated vortex state dot
Spin diffusion and relaxation in three-dimensional isotropic Heisenberg antiferromagnets
A theory is proposed for kinetic effects in isotropic Heisenberg
antiferromagnets at temperatures above the Neel point. A metod based on the
analysis of a set of Feynman diagrams for the kinetic coefficients is developed
for studying the critical dynamics. The scaling behavior of the generalized
coefficient of spin diffusion and relaxation constant in the paramagnetic phase
is studied in terms of the approximation of coupling modes. It is shown that
the kinetic coefficients in an antiferromagnetic system are singular in the
fluctuation region. The corresponding critical indices for diffusion and
relaxation processes are calculated. The scaling dimensionality of the kinetic
coefficients agrees with the predictions of dynamic scaling theory and a
renormalization group analysis. The proposed theory can be used to study the
momentum and frequency dependence of the kinetic parameters, and to determine
the form of the scaling functions. The role of nonlocal correlations and
spin-liquid effects in magnetic systems is briefly discussed.Comment: 10 pages, RevTeX, 3 EPS figures include
Spin and Spin-Wave Dynamics in Josephson Junctions
We extend the Keldysh formulation to quantum spin systems and derive exact
equations of motion. This allows us to explore the dynamics of single spins and
of ferromagnets when these are inserted between superconducting leads. Several
new effects are reported. Chief amongst these are nutations of single S=1/2
spins in Josephson junctions. These nutations are triggered by the
superconducting pairing correlations in the leads. Similarly, we find that on
rather universal grounds, magnets display unconventional spin wave dynamics
when placed in Josephson junctions. These lead to modifications in the
tunneling current.Comment: (14 pages, 5 figures
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