8,938 research outputs found
Conditional Spectrum Computation Incorporating Multiple Causal Earthquakes and Ground‐Motion Prediction Models
The Conditional Spectrum (CS) is a target spectrum (with conditional mean and conditional standard deviation) that links seismic hazard information with ground motion selection for nonlinear dynamic analysis. Probabilistic seismic hazard analysis (PSHA) estimates the ground motion hazard by incorporating the aleatory uncertainties in all earthquake scenarios and resulting ground motions as well as the epistemic uncertainties in ground motion prediction models (GMPMs) and seismic source models. Typical CS calculations to date are produced for a single earthquake scenario using a single GMPM, but more precise use requires consideration of at least multiple causal earthquakes and multiple GMPMs that are often considered in a PSHA computation. This paper presents the mathematics underlying these more precise CS calculations. Despite requiring more effort to compute than approximate calculations using a single causal earthquake and GMPM, the proposed approach produces an exact output that has a theoretical basis. To demonstrate the results of this approach and compare the exact and approximate calculations, several example calculations are performed for real sites in the western U.S. (WUS). The results also provide some insights regarding the circumstances under which approximate results are likely to closely match more exact results. To facilitate these more precise calculations for real applications, the exact CS calculations can now be performed for real sites in the U.S. using new deaggregation features in the U.S. Geological Survey hazard mapping tools. Details regarding this implementation are discussed in this paper
Calculated NMR T_2 relaxation due to vortex vibrations in cuprate superconductors
We calculate the rate of transverse relaxation arising from vortex motion in
the mixed state of YBa_2Cu_3O_7 with the static field applied along the c axis.
The vortex dynamics are described by an overdamped Langevin equation with a
harmonic elastic free energy. We find that the variation of the relaxation with
temperature, average magnetic field, and local field is consistent with
experiments; however, the calculated time dependence is different from what has
been measured and the value of the rates calculated is roughly two orders of
magnitude slower than what is observed. Combined with the strong experimental
evidence pointing to vortex motion as the dominant mechanism for T_2
relaxation, these results call into question a prior conclusion that vortex
motion is not significant in T_1 measurements in the vortex state.Comment: 6 pages, 5 figures, to be published in Phys. Rev.
Perturbed Three Vortex Dynamics
It is well known that the dynamics of three point vortices moving in an ideal
fluid in the plane can be expressed in Hamiltonian form, where the resulting
equations of motion are completely integrable in the sense of Liouville and
Arnold. The focus of this investigation is on the persistence of regular
behavior (especially periodic motion) associated to completely integrable
systems for certain (admissible) kinds of Hamiltonian perturbations of the
three vortex system in a plane. After a brief survey of the dynamics of the
integrable planar three vortex system, it is shown that the admissible class of
perturbed systems is broad enough to include three vortices in a half-plane,
three coaxial slender vortex rings in three-space, and `restricted' four vortex
dynamics in a plane. Included are two basic categories of results for
admissible perturbations: (i) general theorems for the persistence of invariant
tori and periodic orbits using Kolmogorov-Arnold-Moser and Poincare-Birkhoff
type arguments; and (ii) more specific and quantitative conclusions of a
classical perturbation theory nature guaranteeing the existence of periodic
orbits of the perturbed system close to cycles of the unperturbed system, which
occur in abundance near centers. In addition, several numerical simulations are
provided to illustrate the validity of the theorems as well as indicating their
limitations as manifested by transitions to chaotic dynamics.Comment: 26 pages, 9 figures, submitted to the Journal of Mathematical Physic
Exact Master Equation and Quantum Decoherence of Two Coupled Harmonic Oscillators in a General Environment
In this paper we derive an exact master equation for two coupled quantum
harmonic oscillators interacting via bilinear coupling with a common
environment at arbitrary temperature made up of many harmonic oscillators with
a general spectral density function. We first show a simple derivation based on
the observation that the two-harmonic oscillator model can be effectively
mapped into that of a single harmonic oscillator in a general environment plus
a free harmonic oscillator. Since the exact one harmonic oscillator master
equation is available [Hu, Paz and Zhang, Phys. Rev. D \textbf{45}, 2843
(1992)], the exact master equation with all its coefficients for this two
harmonic oscillator model can be easily deduced from the known results of the
single harmonic oscillator case. In the second part we give an influence
functional treatment of this model and provide explicit expressions for the
evolutionary operator of the reduced density matrix which are useful for the
study of decoherence and disentanglement issues. We show three applications of
this master equation: on the decoherence and disentanglement of two harmonic
oscillators due to their interaction with a common environment under Markovian
approximation, and a derivation of the uncertainty principle at finite
temperature for a composite object, modeled by two interacting harmonic
oscillators. The exact master equation for two, and its generalization to ,
harmonic oscillators interacting with a general environment are expected to be
useful for the analysis of quantum coherence, entanglement, fluctuations and
dissipation of mesoscopic objects towards the construction of a theoretical
framework for macroscopic quantum phenomena.Comment: 35 pages, revtex, no figures, 2nd version, references added, to
appear in PR
Scanning tunneling microscopy of lnAs/GaSb superlattices: Subbands, interface roughness, and interface asymmetry
Scanning tunneling microscopy and spectroscopy is used to characterize InAs/GaSb superlattices, grown by molecular-beam epitaxy. Roughness at the interfaces between InAs and GaSb layers is directly observed in the images, and a quantitative spectrum of this roughness is obtained. Electron subbands in the InAs layers are resolved in spectroscopy. Asymmetry between the interfaces of InAs grown on GaSb compared with GaSb grown on In As is seen in voltage-dependent imaging. Detailed
spectroscopic study of the interfaces reveals some subtle differences between the two in terms of their valence-band onsets and conduction-band state density. These differences are interpreted in a model in which the GaSb on InAs interface has an abrupt InSb-like structure, but at the InAs on GaSb interface some Sb grading occurs into the InAs overlayer
BGF-YOLO: Enhanced YOLOv8 with Multiscale Attentional Feature Fusion for Brain Tumor Detection
You Only Look Once (YOLO)-based object detectors have shown remarkable
accuracy for automated brain tumor detection. In this paper, we develop a novel
BGF-YOLO architecture by incorporating Bi-level Routing Attention (BRA),
Generalized feature pyramid networks (GFPN), and Fourth detecting head into
YOLOv8. BGF-YOLO contains an attention mechanism to focus more on important
features, and feature pyramid networks to enrich feature representation by
merging high-level semantic features with spatial details. Furthermore, we
investigate the effect of different attention mechanisms and feature fusions,
detection head architectures on brain tumor detection accuracy. Experimental
results show that BGF-YOLO gives a 4.7% absolute increase of mAP
compared to YOLOv8x, and achieves state-of-the-art on the brain tumor detection
dataset Br35H. The code is available at https://github.com/mkang315/BGF-YOLO
The Democratic Peace Theory and Its Problems
This essay discusses the democratic peace theory from the prespective of both its proponents and opponents. The puzzle of the democratic peace theory has long been debated methodologically and empirically. Both have a strong argument to support their views, however. This essay highlights the debate by focusing on three problems of the democratic peace theory. First, the differences of the definitions of democracy, war, and peace that demonstrates the lack of robustness in the democratic peace theory. Second, democracy by force has often failed to establish peace whether International or domestic peace and therefore the promotion of democarcy around the world have been seen as a justification of democratic intervention to other sovereign states. Third, the democratic peace theory does not always apply in new emerging democratic countries. As a result, it raises a question whether the democratic peace theory or an ideology
Elastodynamics of radially inhomogeneous spherically anisotropic elastic materials in the Stroh formalism
A method is presented for solving elastodynamic problems in radially
inhomogeneous elastic materials with spherical anisotropy, i.e.\ materials such
that in a spherical coordinate system
. The time harmonic displacement field is expanded in a separation of variables form with dependence on
described by vector spherical harmonics with -dependent
amplitudes. It is proved that such separation of variables solution is
generally possible only if the spherical anisotropy is restricted to transverse
isotropy with the principal axis in the radial direction, in which case the
amplitudes are determined by a first-order ordinary differential system.
Restricted forms of the displacement field, such as ,
admit this type of separation of variables solutions for certain lower material
symmetries. These results extend the Stroh formalism of elastodynamics in
rectangular and cylindrical systems to spherical coordinates.Comment: 15 page
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Discussion On Device Structures And Hermetic Encapsulation For SiOx Random Access Memory Operation In Air
An edge-free structure and hermetic encapsulation technique are presented that enable SiOx-based resistive random-access memory (RRAM) operation in air. A controlled etch study indicates that the switching filament is close to the SiOx surface in devices with an exposed SiOx edge. Electrical test of encapsulated, edge-free devices in 1 atmosphere air indicates stable switching characteristics, unlike devices with an edge. This work demonstrates that SiOx RRAM is able to operate in air with proper encapsulation and an edge-free structure. The resistive switching failure mechanism when operating in air is explained by the oxidation of hydrogen-complexed defects in the switching filament. (C) 2014 AIP Publishing LLC.National Science Foundation IIP-1127537Microelectronics Research Cente
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