614 research outputs found
Magnetic fluctuations from stripes in cuprates
Within the time-dependent Gutzwiller approximation for the Hubbard model we
compute the magnetic fluctuations of vertical metallic stripes with parameters
appropriate for LaBaCuO (LBCO). For bond- and
site-centered stripes the excitation spectra are similar, consisting of a
low-energy incommensurate acoustic branch which merges into a ``resonance
peak'' at the antiferromagnetic wave vector and several high-energy optical
branches. The acoustic branch is similar to the result of theories assuming
localized spins whereas the optical branches are significantly different.
Results are in good agreement with a recent inelastic neutron study of LBCO.Comment: 4 pages, 2 eps figure
Does the Heisenberg model describe the multimagnon spin dynamics in antiferromagnetic CuO layers ?
We compute the absorption spectrum for multimagnon excitations assisted by
phonons in insulating layered cuprates using exact diagonalization in clusters
of up to 32 sites. The resulting line shape is very sensitive to the underlying
magnetic Hamiltonian describing the spin dynamics. For the usual Heisenberg
description of undoped Cu-O planes we find, in accordance with experiment, a
two-magnon peak followed by high energy side bands. However the relative weight
of the side bands is too small to reproduce the experiment. An extended
Heisenberg model including a sizable four-site cyclic exchange term is shown to
be consistent with the experimental data.Comment: To appear in Physical Review Letter
Theory of Phonon-Assisted Multimagnon Optical Absorption and Bimagnon States in Quantum Antiferromagnets
We calculate the effective charge for multimagnon infrared (IR) absorption
assisted by phonons in a perovskite like antiferromagnet and we compute the
spectra for two magnon absorption using interacting spin-wave theory. The full
set of equations for the interacting two magnon problem is presented in the
random phase approximation for arbitrary total momentum of the magnon pair. The
spin wave theory results fit very well the primary peak of recent measured
bands in the parent insulating compounds of cuprate superconductors. The line
shape is explained as being due to the absorption of one phonon plus a new
quasiparticle excitation of the Heisenberg Hamiltonian that consists off a long
lived virtual bound state of two magnons (bimagnon). The bimagnon states have
well defined energy and momentum in a substantial portion of the Brillouin
zone. The higher energy bands are explained as one phonon plus higher
multimagnon absorption processes. Other possible experiments for observing
bimagnons are proposed. In addition we predict the line shape for the spin one
system LaNiO.Comment: Modified version of the paper to be published in PR
Density functional theory with adaptive pair density
We propose a density functional to find the ground state energy and density
of interacting particles, where both the density and the pair density can
adjust in the presence of an inhomogeneous potential. As a proof of principle
we formulate an a priori exact functional for the inhomogeneous Hubbard model.
The functional has the same form as the Gutzwiller approximation but with an
unknown kinetic energy reduction factor. An approximation to the functional
based on the exact solution of the uniform problem leads to a substantial
improvement over the local density approximation
Odd parity charge density-wave scattering in cuprates
We investigate a model where superconducting electrons are coupled to a
frequency dependent charge-density wave (CDW) order parameter Delta(w). Our
approach can reconcile the simultaneous existence of low energy Bogoljubov
quasiparticles and high energy electronic order as observed in scanning
tunneling microscopy (STM) experiments. The theory accounts for the contrast
reversal in the STM spectra between positive and negative bias observed above
the pairing gap. An intrinsic relation between scattering rate and
inhomogeneities follows naturally.Comment: 5 pages, 3 figure
Incommensurability and spin excitations of diagonal stripes in cuprates
Based on the time-dependent Gutzwiller approximation we study the possibility
that the diagonal incommensurate spin scattering in strongly underdoped
lanthanum cuprates originates from antiferromagnetic domain walls (stripes).
Calculation of the dynamic spin response for stripes in the diagonal phase
yields the characteristic hour glass dispersion with the crossing of low energy
Goldstone and high-energy branches at a characteristic energy Ecross at the
antiferromagnetic wave-vector Q_{AF}. The high energy part is close to the
parent antiferromagnet. Our results suggest that inelastic neutron scattering
experiments on strongly underdoped lanthanum cuprates can be understood as due
to a mixture of bond centered and site centered stripe configurations with
substantial disorder.Comment: 4 pages, 5 figure
Coarse grained models in Coulomb-frustrated phase separation
Competition between interactions on different length scales leads to
self-organized textures in classical as well as quantum systems. This pattern
formation phenomenon has been invoked to explain some intriguing properties of
a large variety of strongly correlated electronic systems that includes for
example high temperature superconductors and colossal magnetoresistance
manganites. We classify the more common situations in which Coulomb frustrated
phase separation can occur and review their properties.Comment: 13 pages, 4 figures. Presented at "Phase Separation in Electronic
Systems", Crete 200
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