69,880 research outputs found
Radio continuum and polarization study of SNR G57.2+0.8 associated with magnetar SGR1935+2154
We present a radio continuum and linear polarization study of the Galactic
supernova remnant G57.2+0.8, which may host the recently discovered magnetar
SGR1935+2154. The radio SNR shows the typical radio continuum spectrum of a
mature supernova remnant with a spectral index of and
moderate polarized intensity. Magnetic field vectors indicate a tangential
magnetic field, expected for an evolved SNR, in one part of the SNR and a
radial magnetic field in the other. The latter can be explained by an
overlapping arc-like feature, perhaps a pulsar wind nebula, emanating from the
magnetar. The presence of a pulsar wind nebula is supported by the low average
braking index of 1.2, we extrapolated for the magnetar, and the detection of
diffuse X-ray emission around it. We found a distance of 12.5 kpc for the SNR,
which identifies G57.2+0.8 as a resident of the Outer spiral arm of the Milky
Way. The SNR has a radius of about 20 pc and could be as old as 41,000 years.
The SNR has already entered the radiative or pressure-driven snowplow phase of
its evolution. We compared independently determined characteristics like age
and distance for both, the SNR and SGR1935+2154, and conclude that they are
physically related.Comment: accepted by The Astrophysical Journal, 16 pages, 10 figure
Spin relaxation and decoherence of two-level systems
We revisit the concepts of spin relaxation and spin decoherence of two level
(spin-1/2) systems. From two toy-models, we clarify two issues related to the
spin relaxation and decoherence: 1) For an ensemble of two-level particles each
subjected to a different environmental field, there exists an ensemble
relaxation time which is fundamentally different from . When the
off-diagonal coupling of each particle is in a single mode with the same
frequency but a random coupling strength, we show that is finite while
the spin relaxation time of a single spin and the usual ensemble
decoherence time are infinite. 2) For a two-level particle under only a
random diagonal coupling, its relaxation time shall be infinite but its
decoherence time is finite.Comment: 5 pages, 2 figure
Proximity and anomalous field-effect characteristics in double-wall carbon nanotubes
Proximity effect on field-effect characteristic (FEC) in double-wall carbon
nanotubes (DWCNTs) is investigated. In a semiconductor-metal (S-M) DWCNT, the
penetration of electron wavefunctions in the metallic shell to the
semiconducting shell turns the original semiconducting tube into a metal with a
non-zero local density of states at the Fermi level. By using a two-band
tight-binding model on a ladder of two legs, it is demonstrated that anomalous
FEC observed in so-called S-M type DWCNTs can be fully understood by the
proximity effect of metallic phases.Comment: 4 pages, 4 figure
Accurate determination of terahertz optical constants by vector network analyzer of Fabry-Perot response
This paper was published in Optics Letters and is made available as an electronic reprint with the permission of OSA. The paper can be found at the following URL on the OSA website: http://dx.doi.org/10.1364/OL.38.005438. Systematic or multiple reproduction or distribution to multiple locations via electronic or other means is prohibited and is subject to penalties under law.We present a method based on a Fabry-Perot model to efficiently and accurately estimate optical constants of wafer samples in transmission-only measurements performed by a vector network analyzer (VNA). The method is demonstrated on two separate wafer samples: one of silicon and the other of polymethylmethacrylate. Results show that the method can not only acquire optical constants accurately and simply over a broad frequency domain but also overcome the limitations of calculation for dispersive and lossy materials to which existing methods are susceptible, such as those based on VNA-driven quasi-optical transmissometers and terahertz time-domain spectrometry
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