521 research outputs found
Application of In Situ Fiberization for fabrication of improved strain isolation pads and graphite epoxy composites
The feasibility of applying the in situ fiberization process to the fabrication of strain isolation pads (SIP) for the Space Shuttle and to the fabrication of graphite-epoxy composites was evaluated. The ISF process involves the formation of interconnected polymer fiber networks by agitation of dilute polymer solutions under controlled conditions. High temperature polymers suitable for SIP use were fiberized and a successful fiberization of polychloro trifluoroethylene, a relatively high melting polymer, was achieved. Attempts to fiberize polymers with greater thermal stability were unsuccessful, apparently due to characteristics caused by the presence of aromaticity in the backbone of such materials. Graphite-epoxy composites were fabricated by interconnecting two dimensional arrays of graphite fiber with polypropylene IS fibers with subsequent epoxy resin impregnation. Mechanical property tests were performed on laminated panels of this material to evaluate intralaminar and interlaminar shear strength, and thus fracture toughness. Test results were generally unpromising
Advanced Strain-Isolation-Pad Material with Bonded Fibrous Construction
The feasibility of utilizing air lay and liquid lay felt deposition techniques to fabricate strain isolation pad (SIP) materials for the Space Shuttle Orbiter was demonstrated. These materials were developed as candidate replacements for the present needled felt SIP used between the ceramic tiles and the aluminum skin on the undersurface of the Orbiter. The SIP materials that were developed consisted of high temperature aramid fibers deposited by controlled fluid (air or liquid) carriers to form low density unbonded felts. The deposited felts were then bonded at the fiber intersections with a small amount of high temperature polyimide resin. This type of bonded felt construction can potentially eliminate two of the problems associated with the present SIP, viz., transmittal of localized stresses into the tiles and load history dependent mechanical response. However, further work is needed to achieve adequate through thickness tensile strength in the bonded felts
Time-dependent Gutzwiller theory of magnetic excitations in the Hubbard model
We use a spin-rotational invariant Gutzwiller energy functional to compute
random-phase-approximation-like (RPA) fluctuations on top of the Gutzwiller
approximation (GA). The method can be viewed as an extension of the previously
developed GA+RPA approach for the charge sector [G. Seibold and J. Lorenzana,
Phys. Rev. Lett. {\bf 86}, 2605 (2001)] with respect to the inclusion of the
magnetic excitations. Unlike the charge case, no assumptions about the time
evolution of the double occupancy are needed in this case. Interestingly, in a
spin-rotational invariant system, we find the correct degeneracy between
triplet excitations, showing the consistency of both computations. Since no
restrictions are imposed on the symmetry of the underlying saddle-point
solution, our approach is suitable for the evaluation of the magnetic
susceptibility and dynamical structure factor in strongly correlated
inhomogeneous systems. We present a detailed study of the quality of our
approach by comparing with exact diagonalization results and show its much
higher accuracy compared to the conventional Hartree-Fock+RPA theory. In
infinite dimensions, where the GA becomes exact for the Gutzwiller variational
energy, we evaluate ferromagnetic and antiferromagnetic instabilities from the
transverse magnetic susceptibility. The resulting phase diagram is in complete
agreement with previous variational computations.Comment: 12 pages, 8 figure
Inhomogeneous Gutzwiller approximation with random phase fluctuations for the Hubbard model
We present a detailed study of the time-dependent Gutzwiller approximation
for the Hubbard model. The formalism, labelled GA+RPA, allows us to compute
random-phase approximation-like (RPA) fluctuations on top of the Gutzwiller
approximation (GA). No restrictions are imposed on the charge and spin
configurations which makes the method suitable for the calculation of linear
excitations around symmetry-broken solutions. Well-behaved sum rules are obeyed
as in the Hartree-Fock (HF) plus RPA approach. Analytical results for a
two-site model and numerical results for charge-charge and current-current
dynamical correlation functions in one and two dimensions are compared with
exact and HF+RPA results, supporting the much better performance of GA+RPA with
respect to conventional HF+RPA theory.Comment: 14 pages, 6 figure
Time-dependent Gutzwiller approximation for the Hubbard model
We develop a time-dependent Gutzwiller approximation (GA) for the Hubbard
model analogous to the time-dependent Hartree-Fock (HF) method. The formalism
incorporates ground state correlations of the random phase approximation (RPA)
type beyond the GA. Static quantities like ground state energy and double
occupancy are in excellent agreement with exact results in one dimension up to
moderate coupling and in two dimensions for all couplings. We find a
substantial improvement over traditional GA and HF+RPA treatments. Dynamical
correlation functions can be easily computed and are also substantially better
than HF+RPA ones and obey well behaved sum rules.Comment: 4 pages, 2 figure
Dynamical charge and spin density wave scattering in cuprate superconductor
We show that a variety of spectral features in high-T_c cuprates can be
understood from the coupling of charge carriers to some kind of dynamical order
which we exemplify in terms of fluctuating charge and spin density waves. Two
theoretical models are investigated which capture different aspects of such
dynamical scattering. The first approach leaves the ground state in the
disordered phase but couples the electrons to bosonic degrees of freedom,
corresponding to the quasi singular scattering associated with the closeness to
an ordered phase. The second, more phenomological approach starts from the
construction of a frequency dependent order parameter which vanishes for small
energies. Both theories capture scanning tunneling microscopy and angle-resoved
photoemission experiments which suggest the protection of quasiparticles close
to the Fermi energy but the manifestation of long-range order at higher
frequencies.Comment: 27 pages, 13 figures, to appear in New J. Phy
Where is the spectral weight in magnetic neutron scattering in the cuprates?
We present estimates in the Hubbard and Heisenberg models for the spectral
weight in magnetic neutron scattering experiments on the cuprates. With the aid
of spin-wave theory and the time dependent Gutzwiller approximation we discuss
how the spectral weight is distributed among the different channels and between
high and low energies. In addition to the well known total moment sum rule we
discuss sum rules for each component of the dynamical structure factor tensor
which are peculiar for spin 1/2 systems. The various factors that reduce the
spectral weight at the relevant energies are singled out and analyzed like:
shielding factors, weight at electronic energies, multimagnon process etc.
Although about 10% ~ 15% of the naively expected weight is detected in
experiments after consideration of these factors the missing weight is within
the experimental uncertainties. A large fraction of the spectral weight is hard
to detect with present experimental conditions.Comment: 16 pages, 13 figures, submitted to PR
Doping dependence of spin excitations in the stripe phase of high-Tc superconductors
Based on the time-dependent Gutzwiller approximation for the extended Hubbard
model we calculate the energy and momentum dependence of spin excitations for
striped ground states. Our starting point correctly reproduces the observed
doping dependence of the incommensurability in La-based cuprates and the
dispersion of magnetic modes in the insulating parent compound. This allows us
to make quantitative predictions for the doping evolution of the dispersion of
magnetic modes in the stripe phase including the energy and intensity of the
resonance peak as well as the velocity of the spin-wave like Goldstone mode. In
the underdoped regime we find a weak linear dependence of
on doping whereas the resonance energy significantly shifts to
higher values when the charge concentration in the stripes starts to deviate
from half-filling for . The velocity is non-monotonous with a
minimum at 1/8 in coincidence with a well known anomaly in . Our
calculations are in good agreement with available experimental data. We also
compare our results with analogous computations based on linear spin-wave
theory.Comment: 18 pages, 14 figures, revised and extended versio
Striped phases in the two-dimensional Hubbard model with long-range Coulomb interaction
We investigate the formation of partially filled domain walls in the
two-dimensional Hubbard model in the presence of long-range interaction. Using
an unrestricted Gutzwiller variational approach we show that: i) the strong
local interaction favors charge segregation in stripe domain walls; ii) The
long-range interaction favors the formation of half-filled vertical stripes
with a period doubling due to the charge and a period quadrupling due to the
spins along the wall. Our results show that, besides the underlying lattice
structure, also the electronic interactions can contribute to determine the
different domain wall textures in Nd doped copper oxides and nickel oxides
Static overscreening and nonlinear response in the Hubbard Model
We investigate the static charge response for the Hubbard model. Using the
Slave-Boson method in the saddle-point approximation we calculate the charge
susceptibility. We find that RPA works quite well close to half-filling,
breaking, of course, down close to the Mott transition. Away from half filling
RPA is much less reliable: Already for very small values of the Hubbard
interaction U, the linear response becomes much more efficient than RPA,
eventually leading to overscreening already beyond quite moderate values of U.
To understand this behavior we give a simple argument, which implies that the
response to an external perturbation at large U should actually be strongly
non-linear. This prediction is confirmed by the results of exact
diagonalization.Comment: 10 pages, 7 figures, RevTe
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