6,412 research outputs found
Black Extended Objects, Naked Singularities and P-Branes
We treat the horizons of charged, dilaton black extended objects as quantum
mechanical objects. We show that the S matrix for such an object can be written
in terms of a p-brane-like action. The requirements of unitarity of the S
matrix and positivity of the p-brane tension equivalent severely restrict the
number of space-time dimensions and the allowed values of the dilaton parameter
a. Generally, black objects transform at the extremal limit into p-branes.Comment: 9 pages, REVTE
Signatures of Fermi surface reconstruction in Raman spectra of underdoped cuprates
We have calculated the Raman B and B spectra as a function of
temperature, as well as doping, for the underdoped cuprates, using a model
based on the resonating valence-bond spin-liquid. We discuss changes in
intensity and peak position brought about by the presence of a pseudogap and
the implied Fermi surface reconstruction, which are elements of this model.
Signatures of Fermi surface reconstruction are evident as a sharp rise in the
doping dependence of the antinodal to nodal peak ratio which occurs below the
quantum critical point. The temperature dependence of the B polarization
can be used to determine if the superconducting gap is limited to the Fermi
pocket, as seen in angle resolved photoemission spectroscopy, or extends
beyond. We find that the slope of the linear low energy B spectrum
maintains its usual d-wave form, but with an effective gap which reflects the
gap amplitude projected on the Fermi pocket. Our calculations capture the main
qualitative features revealed in the extensive data set available on the
HgBaCuO (Hg-1201) cuprate.Comment: 13 pages, 14 figure
Analysis of body calcium (regional changes in body calcium by in vivo neutron activation analysis)
The effect of space flight on urine and fecal calcium loss was documented during the three long-term Skylab flights. Neutron activation analysis was used to determine regional calcium loss. Various designs for regional analysis were investigated
Dilatonic Black Holes, Naked Singularities and Strings
We extend a previous calculation which treated Schwarschild black hole
horizons as quantum mechanical objects to the case of a charged, dilaton black
hole. We show that for a unique value of the dilaton parameter `a', which is
determined by the condition of unitarity of the S matrix, black holes transform
at the extremal limit into strings.Comment: 8 pages, REVTE
Fast high-efficiency integrated waveguide photodetectors using novel hybrid vertical/butt coupling geometry
We report a novel coupling geometry for integrated waveguide photodetectorsâa hybrid vertical coupling/butt coupling scheme that allows the integration of fast, efficient, photodetectors with conventional double heterostructure waveguides. It can be employed to yield a planar, or pseudo-planar, surface that supports further levels of integration. The approach is demonstrated with a 25-”m-long p-i-n detector integrated with an InP/InGaAsP/InP waveguide, which displays a high (~90%) efficiency and large (~15 GHz) bandwidth. This is the fastest high-efficiency integrated waveguide photodetector reported to date
In vivo nuclear magnetic resonance imaging
A number of physiological changes have been demonstrated in bone, muscle and blood after exposure of humans and animals to microgravity. Determining mechanisms and the development of effective countermeasures for long duration space missions is an important NASA goal. The advent of tomographic nuclear magnetic resonance imaging (NMR or MRI) gives NASA a way to greatly extend early studies of this phenomena in ways not previously possible; NMR is also noninvasive and safe. NMR provides both superb anatomical images for volume assessments of individual organs and quantification of chemical/physical changes induced in the examined tissues. The feasibility of NMR as a tool for human physiological research as it is affected by microgravity is demonstrated. The animal studies employed the rear limb suspended rat as a model of mucle atrophy that results from microgravity. And bedrest of normal male subjects was used to simulate the effects of microgravity on bone and muscle
Monolithic InP-Based Grating Spectrometer for Wavelength-Division Multiplexed Systems at 1.5 ÎŒm
A monolithic InP-based grating spectrometer for use in wavelength-division multiplexed systems at 1.5 ÎŒm is reported.
The spectrometer uses a single etched reflective focusing diffraction grating and resolves >50 channels at 1 nm spacing with a ~0.3nm channel width and at least 19dB channel isolation. Operation is essentially of the state of the input polarisation
CPSA: A Cyber-Physical Security Assessment Tool for Situational Awareness in Smart Grid
It has now become critical and important to understanding the nature of cyber-attacks and their impact on the physical operation of emerging smart electricity grids. Modeling and simulation provide a cost-effective means to develop frameworks and algorithms that address cyber-physical security challenges facing the smart grid. Existing simulation tools support either the communication network or the power system, but not both together. Thus, it is difficult to explore the effects of cyber-physical attacks on power system dynamics and operations. In order to bridge this gap, a cyber-physical co-simulator is required. In this paper, we present a novel integrated cyber-physical security co-simulator tool capable of cyber-physical security assessment (CPSA), which simulates the communication network and the power system together. The tool identifies future vulnerable states and bad measurements and guides the operator at the control center on taking appropriate action to minimize disruption of the physical power system operation due to cyber-attack. The developed tool can be used in understanding of power system monitoring, analyzing the nature of cyber-attacks, detecting bad measurement data, bad command, disabled devices and understand their impact on the operation of the power system
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