3,610 research outputs found
Investigations of afterpulsing and detection efficiency recovery in superconducting nanowire single-photon detectors
We report on the observation of a non-uniform dark count rate in
Superconducting Nanowire Single Photon Detectors (SNSPDs), specifically
focusing on an afterpulsing effect present when the SNSPD is operated at a high
bias current regime. The afterpulsing exists for real detection events
(triggered by input photons) as well as for dark counts (no laser input). In
our standard set-up, the afterpulsing is most likely to occur at around 180 ns
following a detection event, for both real counts and dark counts. We
characterize the afterpulsing behavior and speculate that it is not due to the
SNSPD itself but rather the amplifiers used to boost the electrical output
signal from the SNSPD. We show that the afterpulsing indeed disappears when we
use a different amplifier with a better low frequency response. We also examine
the short-lived enhancement of detection efficiency during the recovery of the
SNSPD due to temporary perturbation of the bias and grounding conditions
Black hole mass and accretion rate of active galactic nuclei with double-peaked broad emission lines
(Abridged) Using an empirical relation between the broad line region size and
optical continuum luminosity, we estimated the black hole mass and accretion
rate for 135 AGNs with double-peaked broad emission lines in two samples. With
black hole masses from to , these
AGNs have the dimensionless accretion rates (Eddington ratios) between 0.001
and 0.1, and the bolometric luminosity between and
, both being significantly larger than those of several
previously known low-luminosity double-peaked AGNs. The optical-X-ray spectra
indices, , of these high-luminosity double-peaked AGNs is between
1 and 1.9. Modest correlations of the value with the Eddington
ratio and bolometric luminosity indicate that double-peaked AGNs with higher
Eddington ratio or higher luminosity tend to have larger value.
Therefore we suggested that the accretion process in some high-luminosity
double-peaked AGNs is probably different from that of low-luminosity objects
where an ADAF-like accretion flow was thought to exist. This is also supported
by the presence of possible big blue bumps in the spectra of some double-peaked
AGNs with higher Eddington ratios. We noticed that the prototype double-peaked
emission line AGN, Arp 102B, may be an ``intermediate'' object between the high
and low luminosity double-peaked AGNs. In addition, we found an apparent strong
anti-correlation between the peak separation of double-peaked profile and
Eddington ratio. If it is real, it may provide us a clue to understand why
double-peaked broad emission lines were hardly found in luminous AGNs with
Eddington ratio larger than 0.1.Comment: 24 pages, 6 figures, accepted by Ap
Earth Matter Effects in Detection of Supernova Neutrinos
We calculated the matter effect, including both the Earth and supernova, on
the detection of neutrinos from type II supernovae at the proposed Daya Bay
reactor neutrino experiment. It is found that apart from the dependence on the
flip probability P_H inside the supernova and the mass hierarchy of neutrinos,
the amount of the Earth matter effect depends on the direction of the incoming
supernova neutrinos, and reaches the biggest value when the incident angle of
neutrinos is around 93^\circ. In the reaction channel \bar{\nu}_e + p --> e^+ +
n the Earth matter effect can be as big as about 12%. For other detection
processes the amount of the Earth matter effect is a few per cent.Comment: 13 pages, 5 figure
Inelastic cotunneling induced decoherence and relaxation, charge and spin currents in an interacting quantum dot under a magnetic field
We present a theoretical analysis of several aspects of nonequilibirum
cotunneling through a strong Coulomb-blockaded quantum dot (QD) subject to a
finite magnetic field in the weak coupling limit. We carry this out by
developing a generic quantum Heisenberg-Langevin equation approach leading to a
set of Bloch dynamical equations which describe the nonequilibrium cotunneling
in a convenient and compact way. These equations describe the time evolution of
the spin variables of the QD explicitly in terms of the response and
correlation functions of the free reservoir variables. This scheme not only
provides analytical expressions for the relaxation and decoherence of the
localized spin induced by cotunneling, but it also facilitates evaluations of
the nonequilibrium magnetization, the charge current, and the spin current at
arbitrary bias-voltage, magnetic field, and temperature. We find that all
cotunneling events produce decoherence, but relaxation stems only from {\em
inelastic} spin-flip cotunneling processes. Moreover, our specific calculations
show that cotunneling processes involving electron transfer (both spin-flip and
non-spin-flip) contribute to charge current, while spin-flip cotunneling
processes are required to produce a net spin current in the asymmetric coupling
case. We also point out that under the influence of a nonzero magnetic field,
spin-flip cotunneling is an energy-consuming process requiring a sufficiently
strong external bias-voltage for activation, explaining the behavior of
differential conductance at low temperature: in particular, the splitting of
the zero-bias anomaly in the charge current and a broad zero-magnitude "window"
of differential conductance for the spin current near zero-bias-voltage.Comment: 15 pages, 5 figures, published version, to appear in Phys. Rev.
Zero Modes in Electromagnetic Form Factors of the Nucleon in a Light-Cone Diquark Model
We use a diquark model of the nucleon to calculate the electromagnetic form
factors of the nucleon described as a scalar and axialvector diquark bound
state. We provide an analysis of the zero-mode contribution in the diquark
model. We find there are zero-mode contributions to the form factors arising
from the instantaneous part of the quark propagator, which cannot be neglected
compared with the valence contribution but can be removed by the choice of wave
function. We also find that the charge and magnetic radii and magnetic moment
of the proton can be reproduced, while the magnetic moment of the neutron is
too small. The dipole shape of the form factors, and
can be reproduced. The ratio decreases
with but too fast.Comment: 22 pages, 6 pages, accepted by J.Phys.
Chandra View of DA 530: A Sub-Energetic Supernova Remnant with a Pulsar Wind Nebula?
Based on a Chandra ACIS observation, we report the detection of an extended
X-ray feature close to the center of the remnant DA 530 with 5.3 sigma above
the background within a circle of 20'' radius. This feature, characterized by a
power-law with the photon index gamma=1.6+-0.8 and spatially coinciding with a
nonthermal radiosource, most likely represents a pulsar wind nebula. We have
further examined the spectrum of the diffuse X-ray emission from the remnant
interior with a background-subtracted count rate of ~0.06 counts s^-1 in
0.3-3.5 keV. The spectrum of the emission can be described by a thermal plasma
with a temperature of ~0.3-0.6 keV and a Si over-abundance of >~7 solar. These
spectral characteristics, together with the extremely low X-ray luminosity,
suggest that the remnant arises from a supernova with an anomalously low
mechanical energy (<10^50 ergs). The centrally-filled thermal X-ray emission of
the remnant may indicate an early thermalization of the SN ejecta by the
circum-stellar medium. Our results suggest that the remnant is likely the
product of a core-collapsed SN with a progenitor mass of 8-12 Msun. Similar
remnants are probably common in the Galaxy, but have rarely been studied.Comment: 23 pages, 7 figures, accepted for publication in ApJ; complete the
abstract on astro-ph and correct some typo
Symbolic Manipulators Affect Mathematical Mindsets
Symbolic calculators like Mathematica are becoming more commonplace among
upper level physics students. The presence of such a powerful calculator can
couple strongly to the type of mathematical reasoning students employ. It does
not merely offer a convenient way to perform the computations students would
have otherwise wanted to do by hand. This paper presents examples from the work
of upper level physics majors where Mathematica plays an active role in
focusing and sustaining their thought around calculation. These students still
engage in powerful mathematical reasoning while they calculate but struggle
because of the narrowed breadth of their thinking. Their reasoning is drawn
into local attractors where they look to calculation schemes to resolve
questions instead of, for example, mapping the mathematics to the physical
system at hand. We model the influence of Mathematica as an integral part of
the constant feedback that occurs in how students frame, and hence focus, their
work
Superconductivity in Ti-doped Iron-Arsenide Compound Sr4Cr0.8Ti1.2O6Fe2As2
Superconductivity was achieved in Ti-doped iron-arsenide compound
Sr4Cr0.8Ti1.2O6Fe2As2 (abbreviated as Cr-FeAs-42622). The x-ray diffraction
measurement shows that this material has a layered structure with the space
group of \emph{P4/nmm}, and with the lattice constants a = b = 3.9003 A and c =
15.8376 A. Clear diamagnetic signals in ac susceptibility data and
zero-resistance in resistivity data were detected at about 6 K, confirming the
occurrence of bulk superconductivity. Meanwhile we observed a superconducting
transition in the resistive data with the onset transition temperature at 29.2
K, which may be induced by the nonuniform distribution of the Cr/Ti content in
the FeAs-42622 phase, or due to some other minority phase.Comment: 3 pages, 3 figure
Experimental demonstration of phase-remapping attack in a practical quantum key distribution system
Unconditional security proofs of various quantum key distribution (QKD)
protocols are built on idealized assumptions. One key assumption is: the sender
(Alice) can prepare the required quantum states without errors. However, such
an assumption may be violated in a practical QKD system. In this paper, we
experimentally demonstrate a technically feasible "intercept-and-resend" attack
that exploits such a security loophole in a commercial "plug & play" QKD
system. The resulting quantum bit error rate is 19.7%, which is below the
proven secure bound of 20.0% for the BB84 protocol. The attack we utilize is
the phase-remapping attack (C.-H. F. Fung, et al., Phys. Rev. A, 75, 32314,
2007) proposed by our group.Comment: 16 pages, 6 figure
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