3,913 research outputs found
Longitudinal excitations in quantum antiferromagnets
By extending our recently proposed magnon-density-waves to low dimensions, we
investigate, using a microscopic many-body approach, the longitudinal
excitations of the quasi-one-dimensional (quasi-1d) and quasi-2d Heisenberg
antiferromagnetic systems on a bipartite lattice with a general spin quantum
number. We obtain the full energy spectrum of the longitudinal mode as a
function of the coupling constants in the original lattice Hamiltonian and find
that it always has a non-zero energy gap if the ground state has a long-range
order and becomes gapless for the pure isotropic 1d model. The numerical value
of the minimum gap in our approximation agrees with that of a longitudinal mode
observed in the quasi-1d antiferromagnetic compound KCuF at low
temperature. It will be interesting to compare values of the energy spectrum at
other momenta if their experimental results are available.Comment: 19 pages, 4 figure
Diagrammatic approach in the variational coupled-cluster method
Recently, as demonstrated by an antiferromagnetic spin-lattice application,
we have successfully extended the coupled-cluster method (CCM) to a variational
formalism in which two sets of distribution functions are introduced to
evaluate Hamiltonian expectation. We calculated these distribution functions by
employing an algebraic scheme. Here we present an alternative calculation based
on a diagrammatic technique. Similar to the method of correlated-basis
functionals (CBF), a generating functional is introduced and calculated by a
linked-cluster expansion in terms of diagrams which are categorized and
constructed according to a few simple rules and using correlation coefficients
and Pauli exclusion principle (or Pauli line) as basic elements. Infinite
resummations of diagrams can then be done in a straightforward manner. One such
resummation, which includes all so-called ring diagrams and ignores Pauli
exclusion principle, reproduces spin-wave theory (SWT). Approximations beyond
SWT are also given. Interestingly, one such approximation including all
so-called super-ring diagrams by a resummation of infinite Pauli lines in
additional to resummations of ring diagrams produces a convergent, precise
number for the order-parameter of the one-dimensional isotropic model, contrast
to the well-known divergence of SWT. We also discuss the direct relation
between our variational CCM and CBF and discuss a possible unification of the
two theories.Comment: 18 pages, 9 figure
Excited states of the quasi-one-dimensional hexagonal quantum antiferromagnets
We investigate the excited states of the quasi-one-dimensional quantum
antiferromagnets on hexagonal lattices, including the longitudinal modes based
on the magnon-density waves. A model Hamiltonian with a uniaxial single-ion
anisotropy is first studied by a spin-wave theory based on the one-boson
method; the ground state thus obtained is employed for the study of the
longitudinal modes. The full energy spectra of both the transverse modes (i.e.,
magnons) and the longitudinal modes are obtained as functions of the
nearest-neighbor coupling and the anisotropy constants. We have found two
longitudinal modes due to the non-collinear nature of the triangular
antiferromagnetic order, similar to that of the phenomenological field theory
approach by Affleck. The excitation energy gaps due to the anisotropy and the
energy gaps of the longitudinal modes without anisotropy are then investigated.
We then compare our results for the longitudinal energy gaps at the magnetic
wavevectors with the experimental results for several antiferromagnetic
compounds with both integer and non-integer spin quantum numbers, and we find
good agreement after the higher-order contributions are included in our
calculations.Comment: 7 pages, 5 figure
Excited states of quantum many-body interacting systems: A variational coupled-cluster description
We extend recently proposed variational coupled-cluster method to describe
excitation states of quantum many-body interacting systems. We discuss, in
general terms, both quasiparticle excitations and quasiparticle-density-wave
excitations (collective modes). In application to quantum antiferromagnets, we
reproduce the well-known spin-wave excitations, i.e. quasiparticle magnons of
spin . In addition, we obtain new, spin-zero magnon-density-wave
excitations which has been missing in Anserson's spin-wave theory. Implications
of these new collective modes are discussed.Comment: 17 pages, 4 figure
Coupled-cluster calculations for the ground- and excited-states of the spin-half XXZ model
The coupled-cluster method is applied to the spin-1/2 antiferromagnetic XXZ
model on a square lattice by employing an approximation which contains two-body
long-range correlations and high-order four-body local correlations.
Improvement is found for the ground-state energy, sublattice magnetization, and
the critical anisotropy when comparing with the approximation including the
two-body correlations alone. We also obtain the full excitation spectrum which
is in good agreement with the quantum Monte Carlo results and the high-order
spin-wave theory.Comment: 20 pages, 6 figure
Exact Results of Dimerization Order Parameter in Antiferromagnetic Chains
After a proper definition of the dimerization order parameter for a spin-
system, I show that this order parameter in the ()
antiferromagnetic chains (or equivalently the spin- chains with
Hamiltonians which project out singlet states) is, in the thermodynamic limit,
directly proportional to the staggered-magnetization in the corresponding
spin- chains which had already been mapped onto the
chains.Comment: 8 pages, plain Te
On the scalar nonet in the extended Nambu Jona-Lasinio model
We discuss the lightest scalar resonances, , ,
and in the extended Nambu Jona-Lasinio model. We find
that the model parameters can be tuned, but unnaturally, to accommodate for
those scalars except the . We also discuss problems encountered in
the K Matrix unitarization approximation by using counting technique.Comment: 23 pages 3 eps figures, To appear in Nucl. Phys.
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