oalib

Publish in OALib Journal

ISSN: 2333-9721

APC: Only $99

Submit

Any time

2017 ( 2 )

2015 ( 6 )

2014 ( 8 )

2013 ( 11 )

Custom range...

Search Results: 1 - 10 of 115 matches for " Arzhang Ardavan "
All listed articles are free for downloading (OA Articles)
Page 1 /115
Display every page Item
Inadequacies in the conventional treatment of the radiation field of moving sources
Houshang Ardavan,Arzhang Ardavan,John Singleton,Joseph Fasel,Andrea Schmidt
Physics , 2009, DOI: 10.1063/1.3215978
Abstract: There is a fundamental difference between the classical expression for the retarded electromagnetic potential and the corresponding retarded solution of the wave equation that governs the electromagnetic field. While the boundary contribution to the retarded solution for the {\em potential} can always be rendered equal to zero by means of a gauge transformation that preserves the Lorenz condition, the boundary contribution to the retarded solution of the wave equation governing the {\em field} may be neglected only if it diminishes with distance faster than the contribution of the source density in the far zone. In the case of a source whose distribution pattern both rotates and travels faster than light {\em in vacuo}, as realized in recent experiments, the boundary term in the retarded solution governing the field is by a factor of the order of $R^{1/2}$ {\em larger} than the source term of this solution in the limit that the distance $R$ of the boundary from the source tends to infinity. This result is consistent with the prediction of the retarded potential that part of the radiation field generated by a rotating superluminal source decays as $R^{-1/2}$, instead of $R^{-1}$, a prediction that is confirmed experimentally. More importantly, it pinpoints the reason why an argument based on a solution of the wave equation governing the field in which the boundary term is neglected (such as appears in the published literature) misses the nonspherical decay of the field.
Morphology of the nonspherically decaying radiation generated by a rotating superluminal source: reply to comment
Houshang Ardavan,Arzhang Ardavan,John Singleton,Joseph Fasel,Andrea Schmidt
Physics , 2008, DOI: 10.1364/JOSAA.25.002167
Abstract: The fact that the formula used by Hannay in his Comment is "from a standard text on electrodynamics" neither warrants that it is universally applicable, nor that it is unequivocally correct. We have explicitly shown [J. Opt. Soc. Am. A 25, 543 (2008)] that,since it does not include the boundary contribution toward the value of the field, the formula in question is not applicable when the source is extended and has a distribution pattern that rotates faster than light in vacuo. The neglected boundary term in the retarded solution to the wave equation governing the electromagnetic field forms the basis of diffraction theory. If this term were identically zero, for the reasons given by Hannay, the iffraction of electromagnetic waves through apertures on a surface enclosing a source would have been impossible. If this term were identically zero, for the reasons given by Hannay, the diffraction of electromagnetic waves through apertures on a surface enclosing a source would have been impossible.
Response to "Comment on `Method of handling the divergences in the radiation theory of sources that move faster than their own waves'" [J. Math. Phys. 40, 4331 (1999)]
Houshang Ardavan,Arzhang Ardavan,John Singleton,Joseph Fasel,Andrea Schmidt
Physics , 2008,
Abstract: There is a fundamental difference between the classical expression for the retarded electromagnetic potential and the corresponding retarded solution of the wave equation that governs the electromagnetic field. While the boundary contribution to the retarded solution for the potential can always be rendered equal to zero by means of a gauge transformation that preserves the Lorenz condition, the boundary contribution to the retarded solution of the wave equation governing the field may be neglected only if it diminishes with distance faster than the contribution of the source density in the far zone. In the case of a source whose distribution pattern rotates superluminally (i.e., faster than the speed of light in vacuo), the boundary term in the retarded solution governing the field is by a factor of the order of R^(1/2) larger than the source term of this solution in the limit where the distance R of the boundary from the source tends to infinity. This result is consistent with the prediction of the retarded potential that the radiation field generated by a rotating superluminal source decays as 1/R^(1/2), instead of 1/R. It also explains why an argument based on the solution of the wave equation governing the field in which the boundary term is neglected, such as Hannay presents in his Comment, misses the nonspherical decay of the field.
Morphology of the nonspherically decaying radiation beam generated by a rotating superluminal source
Houshang Ardavan,Arzhang Ardavan,John Singleton,Joseph Fasel,Andrea Schmidt
Physics , 2006, DOI: 10.1364/JOSAA.24.002443
Abstract: We consider the nonspherically decaying radiation field that is generated by a polarization current with a superluminally rotating distribution pattern in vacuum, a field that decays with the distance $\subP{R}$ from its source as $\subP{R}^{-1/2}$, instead of $\subP{R}^{-1}$. It is shown (i) that the nonspherical decay of this emission remains in force at all distances from its source independently of the frequency of the radiation, (ii) that the part of the source that makes the main contribution toward the value of the nonspherically decaying field has a filamentary structure whose radial and azimuthal widths become narrower (as $\subP{R}^{-2}$ and $\subP{R}^{-3}$, respectively), the farther the observer is from the source, (iii) that the loci on which the waves emanating from this filament interfere constructively delineate a radiation `subbeam' that is nondiffracting in the polar direction, (iv) that the cross-sectional area of each nondiffracting subbeam increases as $\subP{R}$, instead of $\subP{R}^2$, so that the requirements of conservation of energy are met by the nonspherically decaying radiation automatically, and (v) that the overall radiation beam within which the field decays nonspherically consists, in general, of the incoherent superposition of such coherent nondiffracting subbeams. These findings are related to the recent construction and use of superluminal sources in the laboratory and numerical models of the emission from them. We also briefly discuss the relevance of these results to the giant pulses received from pulsars.
Electron spin ensemble strongly coupled to a three-dimensional microwave cavity
Eisuke Abe,Hua Wu,Arzhang Ardavan,John J. L. Morton
Physics , 2011, DOI: 10.1063/1.3601930
Abstract: We demonstrate the strong coupling between an electron spin ensemble and a three-dimensional cavity in a reflection geometry. We also find that an anticrossing in the cavity/spin spectrum can be observed under conditions that the collective coupling strength $g_c$ is smaller than the spin linewidth $\gamma_s$ or the cavity linewidth. We identify a ratio of $g_c$ to $\gamma_s$ ($g_c/\gamma_s >$ 0.64) as a condition to observe a splitting in the cavity frequency. Finally, we confirm that $g_c$ scales with $\sqrt{N}$, where $N$ is the number of polarized spins.
Coherent spin control by electrical manipulation of the magnetic anisotropy
Richard E George,James P Edwards,Arzhang Ardavan
Physics , 2012, DOI: 10.1103/PhysRevLett.110.027601
Abstract: High-spin paramagnetic manganese defects in polar piezoelectric zinc oxide exhibit a simple almost axial anisotropy and phase coherence times of the order of a millisecond at low temperatures. The anisotropy energy is tunable using an externally applied electric field. This can be used to control electrically the phase of spin superpositions and to drive spin transitions with resonant microwave electric fields.
A Maximum-Likelihood Analysis of Observational Data on Fluxes and Distances of Radio Pulsars: Evidence for Violation of the Inverse-Square Law
John Singleton,Pinaki Sengupta,John Middleditch,Todd L. Graves,Mario R. Perez,Houshang Ardavan,Arzhang Ardavan
Physics , 2009,
Abstract: We analyze pulsar fluxes at 1400 MHz ($S_{1400}$) and distances ($d$) extracted from the Parkes Multibeam Survey. Under the assumption that distribution of pulsar luminosities is distance-independent, we find that either (a) pulsar fluxes diminish with distance according to a non-standard power law, due, we suggest, to the presence of a component with $S_{1400} \propto 1/d$, or (b) that there are very significant (i.e. order of magnitude) errors in the dispersion-measure method for estimating pulsar distances. The former conclusion (a) supports a model for pulsar emission that has also successfully explained the frequency spectrum of the Crab and 8 other pulsars over 16 orders of magnitude of frequency, whilst alternative (b) would necessitate a radical re-evaluation of both the dispersion-measure method and current ideas about the distribution of free electrons within our Galaxy.
A new mechanism for generating broadband pulsar-like polarization
Houshang Ardavan,Arzhang Ardavan,Joseph Fasel,John Middleditch,Mario Perez,Andrea Schmidt,John Singleton
Physics , 2009,
Abstract: Observational data imply the presence of superluminal electric currents in pulsar magnetospheres. Such sources are not inconsistent with special relativity; they have already been created in the laboratory. Here we describe the distinctive features of the radiation beam that is generated by a rotating superluminal source and show that (i) it consists of subbeams that are narrower the farther the observer is from the source: subbeams whose intensities decay as 1/R instead of 1/R^2 with distance (R), (ii) the fields of its subbeams are characterized by three concurrent polarization modes: two modes that are 'orthogonal' and a third mode whose position angle swings across the subbeam bridging those of the other two, (iii) its overall beam consists of an incoherent superposition of such coherent subbeams and has an intensity profile that reflects the azimuthal distribution of the contributing part of the source (the part of the source that approaches the observer with the speed of light and zero acceleration), (iv) its spectrum (the superluminal counterpart of synchrotron spectrum) is broader than that of any other known emission and entails oscillations whose spacings and amplitudes respectively increase and decrease algebraically with increasing frequency, and (v) the degree of its mean polarization and the fraction of its linear polarization both increase with frequency beyond the frequency for which the observer falls within the Fresnel zone. We also compare these features with those of the radiation received from the Crab pulsar.
Magnetic field sensors using 13-spin cat states
Stephanie Simmons,Jonathan A. Jones,Steven D. Karlen,Arzhang Ardavan,John J. L. Morton
Physics , 2009, DOI: 10.1103/PhysRevA.82.022330
Abstract: Measurement devices could benefit from entangled correlations to yield a measurement sensitivity approaching the physical Heisenberg limit. Building upon previous magnetometric work using pseudo-entangled spin states in solution-state NMR, we present two conceptual advancements to better prepare and interpret the pseudo-entanglement resource as well as the use of a 13-spin cat state to measure the local magnetic field with a sensitivity beyond the standard quantum limit.
Efficient Dynamic Nuclear Polarization at High Magnetic Fields
Gavin W. Morley,Johan van Tol,Arzhang Ardavan,Kyriakos Porfyrakis,Jinying Zhang,G. Andrew D. Briggs
Physics , 2006, DOI: 10.1103/PhysRevLett.98.220501
Abstract: By applying a new technique for dynamic nuclear polarization involving simultaneous excitation of electronic and nuclear transitions, we have enhanced the nuclear polarization of the nitrogen nuclei in 15N@C60 by a factor of 1000 at a fixed temperature of 3 K and a magnetic field of 8.6 T, more than twice the maximum enhancement reported to date. This methodology will allow the initialization of the nuclear qubit in schemes exploiting N@C60 molecules as components of a quantum information processing device.
Page 1 /115
Display every page Item


Home
Copyright © 2008-2017 Open Access Library. All rights reserved.