157 research outputs found
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Nonlinear Torsional Wave Beams
An evolution equation with cubic nonlinearity is presented for a torsional wave beam in an isotropic elastic solid. Analytical solutions are presented for the fundamental and third harmonic in the far field of a uniform circular source. Numerical results are presented for harmonic beam patterns at an intermediate distance between the near and far fields, and for a torsional waveform with shocks.Applied Research Laboratorie
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Abating low-frequency noise using encapsulated gas bubbles
Air bubbles may be used to reduce radiated underwater noise. Two modalities of sound attenuation by air bubbles were shown to provide a reduction in radiated sound: bubble acoustic resonance damping and acoustic impedance mismatching. The bubbles used for acoustic resonance damping were manifested using gas-filled containers coupled to a support, and the acoustic impedance mismatching bubbles were created using a cloud of freely-rising bubbles, which were both used to surround an underwater sound source.Board of Regents, University of Texas Syste
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Underwater noise abatement panel and resonator structure
A system for reducing noise emissions in underwater environments is presented. The system can be extended to applications in any two-fluid environments where one fluid (gas) is contained in an enclosed resonator volume connected to the outside environment at an open end of the resonator body. The resonators act as gas-containing (e.g., air) Helmholtz resonators constructed into solid panels that are submerged in the fluid medium (e.g., sea water) in the vicinity of a noise generating source. The oscillations of the trapped air volume in the resonators causes reduction of certain noise energy and a general reduction in the transmitted noise in the environment of the system.Board of Regents, University of Texas Syste
Quantitative determination of bond order and lattice distortions in nickel oxide heterostructures by resonant x-ray scattering
We present a combined study of Ni -edge resonant x-ray scattering and
density functional calculations to probe and distinguish electronically driven
ordering and lattice distortions in nickelate heterostructures. We demonstrate
that due to the low crystal symmetry, contributions from structural distortions
can contribute significantly to the energy-dependent Bragg peak intensities of
a bond-ordered NdNiO reference film. For a LaNiO-LaAlO superlattice
that exhibits magnetic order, we establish a rigorous upper bound on the
bond-order parameter. We thus conclusively confirm predictions of a dominant
spin density wave order parameter in metallic nickelates with a
quasi-two-dimensional electronic structure
Strain and composition dependence of the orbital polarization in nickelate superlattices
A combined analysis of x-ray absorption and resonant reflectivity data was
used to obtain the orbital polarization profiles of superlattices composed of
four-unit-cell-thick layers of metallic LaNiO3 and layers of insulating RXO3
(R=La, Gd, Dy and X=Al, Ga, Sc), grown on substrates that impose either
compressive or tensile strain. This superlattice geometry allowed us to partly
separate the influence of epitaxial strain from interfacial effects controlled
by the chemical composition of the insulating blocking layers. Our quantitative
analysis reveal orbital polarizations up to 25%. We further show that strain is
the most effective control parameter, whereas the influence of the chemical
composition of the blocking layers is comparatively small.Comment: 9 pages, 8 figure
Probing Exfoliated Graphene Layers and Their Lithiation with Microfocused X-rays
X-ray diffraction is measured on individual bilayer and multilayer graphene single-crystals and combined with electrochemically induced lithium intercalation. In-plane Bragg peaks are observed by grazing incidence diffraction. Focusing the incident beam down to an area of about 10 μm × 10 μm, individual flakes are probed by specular X-ray reflectivity. By deploying a recursive Parratt algorithm to model the experimental data, we gain access to characteristic crystallographic parameters of the samples. Notably, it is possible to directly extract the bi/multilayer graphene c-axis lattice parameter. The latter is found to increase upon lithiation, which we control using an on-chip peripheral electrochemical cell layout. These experiments demonstrate the feasibility of in situ X-ray diffraction on individual, micron-sized single crystallites of few- and bilayer two-dimensional materials
A 4-unit-cell superstructure in optimally doped YBa2Cu3O6.92 superconductor
Using high-energy diffraction we show that a 4-unit-cell superstructure,
q0=(1/4,0,0), along the shorter Cu-Cu bonds coexists with superconductivity in
optimally doped YBCO. A complex set of anisotropic atomic displacements on
neighboring CuO chain planes, BaO planes, and CuO2 planes, respectively,
correlated over ~3-6 unit cells gives rise to diffuse superlattice peaks. Our
observations are consistent with the presence of Ortho-IV nanodomains
containing these displacements.Comment: Corrected typo in abstrac
Stripe orders in the extended Hubbard model
We study stripe orders of charge and spin density waves in the extended
Hubbard model with the nearest-neighbor Coulomb repulsion V within the mean
field approximation. We obtain V vs. T(temperature) phase diagram for the
on-site Coulomb interaction U/t=8.0 and the filling n=0.8, here t is a
nearest-neighbor transfer energy. Our result shows that the diagonal stripe
spin density wave state (SDW) is stable for small V, but for large V the most
stable state changes to a charge density wave-antiferromagnetic (CDW-AF) state.
Especially we find at low temperature and for a certain range of value of V, a
vertical stripe CDW-AF state becomes stable.Comment: LaTeX 9 pages, 17 figures, uses jpsj.st
The Charge Ordered State from Weak to Strong Coupling
We apply the Dynamical Mean Field Theory to the problem of charge ordering.
In the normal state as well as in the Charge Ordered (CO) state the existence
of polarons, i.e. electrons strongly coupled to local lattice deformation, is
associated to the qualitative properties of the Lattice Polarization
Distribution Function (LPDF). At intermediate and strong coupling a CO state
characterized by a certain amount of thermally activated defects arise from the
spatial ordering of preexisting randomly distributed polarons. Properties of
this particular CO state gives a qualitative understanding of the low frequency
behavior of optical conductivity of perovskites.Comment: 4 pages, 3 figures, to be published in J. of Superconductivity
(proceedings Stripes 98
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