8,410 research outputs found
Computation of inflationary cosmological perturbations in chaotic inflationary scenarios using the phase-integral method
The phase-integral approximation devised by Fr\"oman and Fr\"oman, is used
for computing cosmological perturbations in the quadratic chaotic inflationary
model. The phase-integral formulas for the scalar and tensor power spectra are
explicitly obtained up to fifth order of the phase-integral approximation. We
show that, the phase integral gives a very good approximation for the shape of
the power spectra associated with scalar and tensor perturbations as well as
the spectral indices. We find that the accuracy of the phase-integral
approximation compares favorably with the numerical results and those obtained
using the slow-roll and uniform approximation methods.Comment: 21 pages, RevTex, to appear in Phys. Rev
Media Access Schemes for Indirect Diffused Free-Space Optical Networks
In this paper, we propose a set of three media access control (MAC) schemes for an indirect diffused light free-space optical communications (ID-FSOCs). ID-FSOC has been recently proposed to establish wireless high-speed (i.e., ≥1 Gbps) network access using FSO from stations that have no line-of-sight (LOS) with the access point. ID-FSOC employs a diffuse reflector (DR) to uniformly reflect diffused light from an incident laser to all directions, except towards the DR. To establish a link, ID-FSOC requires LOS between the transmitter and the DR and between DR and the receiver. In this way, ID-FSOC relaxes the location of stations as long as they keep LOS to the DR. We analyze the performance and scalability of proposed schemes. We also consider the impact of the zoom-in time of a receiver in our evaluations. Our results show that our proposed MAC schemes achieve high channel utilization and higher throughput than carrier-sense multiple access schemes
Utilizing Fast Spin Echo MRI to Reduce Image Artifacts and Improve Implant/Tissue Interface Detection in Refractory Parkinson's Patients with Deep Brain Stimulators
Introduction:. In medically refractory Parkinson's disease (PD) deep-brain stimulation (DBS) is an effective therapeutic tool. Postimplantation MRI is important in assessing tissue damage and DBS lead placement accuracy. We wanted to identify which MRI sequence can detect DBS leads with smallest artifactual signal void, allowing better tissue/electrode edge conspicuity. Methods:. Using an IRB approved protocol 8 advanced PD patients were imaged within MR conditional safety guidelines at low RF power (SAR ≤ 0.1 W/kg) in coronal plane at 1.5T by various sequences. The image slices were subjectively evaluated for diagnostic quality and the lead contact diameters were compared to identify a sequence least affected by metallic leads. Results and Discussion. Spin echo and fast spin echo based low SAR sequences provided acceptable image quality with comparable image blooming (enlargement) of stimulator leads. The mean lead diameters were 2.2 ± 0.1 mm for 2D, 2.1 ± 0.1 mm for 3D, and 4.0 ± 0.2 mm for 3D MPRAGE sequence. Conclusion:. Low RF power spin echo and fast spin echo based 2D and 3D FSE sequences provide acceptable image quality adjacent to DBS leads. The smallest artifactual blooming of stimulator leads is present on 3D FSE while the largest signal void appears in the 3D MPRAGE sequence
Supertubes versus superconducting tubes
In this paper we show the relationship between cylindrical D2-branes and
cylindrical superconducting membranes described by a generic effective action
at the bosonic level. In the first case the extended objects considered, arose
as blown up type IIA superstrings to D2-branes, named supertubes. In the second
one, the cosmological objects arose from some sort of field theories. The
Dirac-Born-Infeld action describing supertubes is shown to be equivalent to the
generic effective action describing superconducting membranes via a special
transformation.Comment: Version with minor text changes with respect to the already publishe
Computation of inflationary cosmological perturbations in the power-law inflationary model using the phase-integral method
The phase-integral approximation devised by Fr\"oman and Fr\"oman, is used
for computing cosmological perturbations in the power-law inflationary model.
The phase-integral formulas for the scalar and tensor power spectra are
explicitly obtained up to ninth-order of the phase-integral approximation. We
show that, the phase-integral approximation exactly reproduces the shape of the
power spectra for scalar and tensor perturbations as well as the spectral
indices. We compare the accuracy of the phase-integral approximation with the
results for the power spectrum obtained with the slow-roll and uniform
approximation methods.Comment: 16 pages, Revtex, to appear in Physical Review
Influence of the photon - neutrino processes on magnetar cooling
The photon-neutrino processes ,
and are investigated
in the presence of a strongly magnetized and dense electron-positron plasma.
The amplitudes of the reactions and
are obtained. In the case of a cold degenerate
plasma contributions of the considering processes to neutrino emissivity are
calculated. It is shown that contribution of the process to neutrino emissivity is supressed in comparision with the
contributions of the processes and
. The constraint on the magnetic field strength in the
magnetar outer crust is obtained.Comment: 8 pages, LaTeX, 2 PS figures, based on the talk presented by D.A.
Rumyantsev at the XV International Seminar Quarks'2008, Sergiev Posad, Moscow
Region, May 23-29, 2008, to appear in the Proceeding
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