1,515 research outputs found
Checkerboard and stripe inhomogeneities in cuprates
We systematically investigate charge-ordering phases by means of a restricted
and unrestricted Gutzwiller approximation to the single-band Hubbard model with
nearest () and next-nearest neighbor hopping (). When is small,
as appropriate for , stripes are found, whereas in
compounds with larger (such as and
) checkerboard structures are favored. In
contrast to the linear doping dependence found for stripes the charge
periodicity of checkerboard textures is locked to 4 unit cells over a wide
doping range. In addition we find that checkerboard structures are favored at
surfaces.Comment: 5 pages, 3 figure
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
Fermi surface dichotomy on systems with fluctuating order
We investigate the effect of a dynamical collective mode coupled with
quasiparticles at specific wavevectors only. This coupling describes the
incipient tendency to order and produces shadow spectral features at high
energies, while leaving essentially untouched the low energy quasiparticles.
This allows to interpret seemingly contradictory experiments on underdoped
cuprates, where many converging evidences indicate the presence of charge
(stripe or checkerboard) order, which remains instead elusive in the Fermi
surface obtained from angle-resolved photoemission experiments.Comment: 11 pages, 10 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
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
Geographic range size and evolutionary age in birds
Together with patterns of speciation and extinction, post-speciation transformations in the range sizes of individual species determine the form of contemporary species-range-size distributions. However, the methodological problems associated with tracking the dynamics of a species' range size over evolutionary time have precluded direct study of such range-size transformations, although indirect evidence has led to several models being proposed describing the form that they might take. Here, we use independently derived molecular data to estimate ages of species in six monophyletic groups of birds, and examine the relationship between species age and global geographic range size. We present strong evidence that avian range sizes are not static over evolutionary time. In addition, it seems that, with the regular exception of certain taxa (for example island endemics and some threatened species), range-size transformations are non-random in birds. In general, range sizes appear to expand relatively rapidly post speciation; subsequently, and perhaps more gradually, they then decline as species age. We discuss these results with reference to the various models of range-size dynamics that have been proposed
Theory of antibound states in partially filled narrow band systems
We present a theory of the dynamical two-particle response function in the
Hubbard model based on the time-dependent Gutzwiller approximation. The results
are in excellent agreement with exact diagonalization on small clusters and
give reliable results even for high densities, where the usual ladder
approximation breaks down. We apply the theory to the computation of antibound
states relevant for Auger spectroscopy and cold atom physics. A special bonus
of the theory is its computational simplicity.Comment: 4 pages, 3 figure
- …