9,902 research outputs found
Quantum Fluctuations in the Chirped Pendulum
An anharmonic oscillator when driven with a fast, frequency chirped voltage
pulse can oscillate with either small or large amplitude depending on whether
the drive voltage is below or above a critical value-a well studied classical
phenomenon known as autoresonance. Using a 6 GHz superconducting resonator
embedded with a Josephson tunnel junction, we have studied for the first time
the role of noise in this non-equilibrium system and find that the width of the
threshold for capture into autoresonance decreases as the square root of T, and
saturates below 150 mK due to zero point motion of the oscillator. This unique
scaling results from the non-equilibrium excitation where fluctuations, both
quantum and classical, only determine the initial oscillator motion and not its
subsequent dynamics. We have investigated this paradigm in an electrical
circuit but our findings are applicable to all out of equilibrium nonlinear
oscillators.Comment: 5 pages, 4 figure
MnAs dots grown on GaN(0001)-(1x1) surface
MnAs has been grown by means of MBE on the GaN(0001)-(1x1) surface. Two
options of initiating the crystal growth were applied: (a) a regular MBE
procedure (manganese and arsenic were delivered simultaneously) and (b)
subsequent deposition of manganese and arsenic layers. It was shown that
spontaneous formation of MnAs dots with the surface density of 1
cm and cm, respectively (as observed by AFM),
occurred for the layer thickness higher than 5 ML. Electronic structure of the
MnAs/GaN systems was studied by resonant photoemission spectroscopy. That led
to determination of the Mn 3d - related contribution to the total density of
states (DOS) distribution of MnAs. It has been proven that the electronic
structures of the MnAs dots grown by the two procedures differ markedly. One
corresponds to metallic, ferromagnetic NiAs-type MnAs, the other is similar to
that reported for half-metallic zinc-blende MnAs. Both system behave
superparamagnetically (as revealed by magnetization measurements), but with
both the blocking temperatures and the intra-dot Curie temperatures
substantially different. The intra-dot Curie temperature is about 260 K for the
former system while markedly higher than room temperature for the latter one.
Relations between growth process, electronic structure and other properties of
the studied systems are discussed. Possible mechanisms of half-metallic MnAs
formation on GaN are considered.Comment: 20+ pages, 8 figure
A Normal Stellar Disk in the Galaxy Malin 1
Since its discovery, Malin 1 has been considered the prototype and most
extreme example of the class of giant low surface brightness disk galaxies.
Examination of an archival Hubble Space Telescope I-band image reveals that
Malin 1 contains a normal stellar disk that was not previously recognized,
having a central I-band surface brightness of mu_0 = 20.1 mag arcsec^-2 and a
scale length of 4.8 kpc. Out to a radius of ~10 kpc, the structure of Malin 1
is that of a typical SB0/a galaxy. The remarkably extended, faint outer
structure detected out to r~100 kpc appears to be a photometrically distinct
component and not a simple extension of the inner disk. In terms of its disk
scale length and central surface brightness, Malin 1 was originally found to be
a very remote outlier relative to all other known disk galaxies. The presence
of a disk of normal size and surface brightness in Malin 1 suggests that such
extreme outliers in disk properties probably do not exist, but underscores the
importance of the extended outer disk regions for a full understanding of the
structure and formation of spiral galaxies.Comment: 13 pages, 7 figures. To appear in AJ. Typographical error correcte
Shimura curve computations via K3 surfaces of Neron-Severi rank at least 19
It is known that K3 surfaces S whose Picard number rho (= rank of the
Neron-Severi group of S) is at least 19 are parametrized by modular curves X,
and these modular curves X include various Shimura modular curves associated
with congruence subgroups of quaternion algebras over Q. In a family of such K3
surfaces, a surface has rho=20 if and only if it corresponds to a CM point on
X. We use this to compute equations for Shimura curves, natural maps between
them, and CM coordinates well beyond what could be done by working with the
curves directly as we did in ``Shimura Curve Computations'' (1998) =
Comment: 16 pages (1 figure drawn with the LaTeX picture environment); To
appear in the proceedings of ANTS-VIII, Banff, May 200
Transient and Highly Polarized Double-Peaked H-alpha Emission in the Seyfert 2 Nucleus of NGC 2110
We have discovered an extremely broad, double-peaked H-alpha emission line in
the polarized flux spectrum of NGC 2110, establishing that this well-studied
Seyfert 2 galaxy contains a disk-like hidden broad-line region (BLR). Several
properties of NGC 2110 suggest that it is an obscured twin of Arp 102B, the
prototypical double-peaked emission-line active galactic nucleus (AGN). A
comparison between our data and previous spectra of NGC 2110 indicates that the
double-peaked H-alpha feature is transient. The presence of a disk-like BLR in
NGC 2110 has important implications for AGNs: it expands the range of
properties exhibited by Seyfert 2 galaxies, and the fact that the BLR is
obscured by a torus-like structure provides the first evidence that
double-peaked emitters and classical Seyfert nuclei may have the same basic
parsec-scale geometry.Comment: 5 pages, including 3 postscript figures. Uses emulateapj. Accepted
for publication in ApJ Letter
Measurement of the reaction \gamma p \TO K^ + \Lambda(1520) at photon energies up to 2.65 GeV
The reaction \gamma p \TO K^+\Lambda(1520) was measured in the energy range
from threshold to 2.65 GeV with the SAPHIR detector at the electron stretcher
facility ELSA in Bonn. The production cross section was
analyzed in the decay modes , , , and
as a function of the photon energy and the squared
four-momentum transfer . While the cross sections for the inclusive
reactions rise steadily with energy, the cross section of the process \gamma p
\TO K^+\Lambda(1520) peaks at a photon energy of about 2.0 GeV, falls off
exponentially with , and shows a slope flattening with increasing photon
energy. The angular distributions in the -channel helicity system indicate
neither a nor a exchange dominance. The interpretation of the
as a molecule is not supported.Comment: 11 pages, 16 figures, 4 table
Measurement of gamma p --> K+ Lambda and gamma p --> K+ Sigma0 at photon energies up to 2.6 GeV
The reactions gamma p --> K+ Lambda and gamma p --> K+ Sigma0 were measured
in the energy range from threshold up to a photon energy of 2.6 GeV. The data
were taken with the SAPHIR detector at the electron stretcher facility, ELSA.
Results on cross sections and hyperon polarizations are presented as a function
of kaon production angle and photon energy. The total cross section for Lambda
production rises steeply with energy close to threshold, whereas the Sigma0
cross section rises slowly to a maximum at about E_gamma = 1.45 GeV. Cross
sections together with their angular decompositions into Legendre polynomials
suggest contributions from resonance production for both reactions. In general,
the induced polarization of Lambda has negative values in the kaon forward
direction and positive values in the backward direction. The magnitude varies
with energy. The polarization of Sigma0 follows a similar angular and energy
dependence as that of Lambda, but with opposite sign.Comment: 21 pages, 25 figures, submitted to Eur. Phys. J.
Is the Broad-Line Region Clumped or Smooth? Constraints from the H alpha Profile in NGC 4395, the Least Luminous Seyfert 1 Galaxy
The origin and configuration of the gas which emits broad lines in Type I
active galactic nuclei is not established yet. The lack of small-scale
structure in the broad emission-line profiles is consistent with a smooth gas
flow, or a clumped flow with many small clouds. An attractive possibility for
the origin of many small clouds is the atmospheres of bloated stars, an origin
which also provides a natural mechanism for the cloud confinement. Earlier
studies of the broad-line profiles have already put strong lower limits on the
minimum number of such stars, but these limits are sensitive to the assumed
width of the lines produced by each cloud. Here we revisit this problem using
high-resolution Keck spectra of the H alpha line in NGC 4395, which has the
smallest known broad-line region (~10^14 cm). Only a handful of the required
bloated stars (each having r~10^14 cm) could fit into the broad-line region of
NGC 4395, yet the observed smoothness of the H alpha line implies a lower limit
of ~10^4-10^5 on the number of discrete clouds. This rules out conclusively the
bloated-stars scenario, regardless of any plausible line-broadening mechanisms.
The upper limit on the size of the clouds is ~10^12 cm, which is comparable to
the size implied by photoionization models. This strongly suggests that gas in
the broad-line region is structured as a smooth rather than a clumped flow,
most likely in a rotationally dominated thick disk-like configuration. However,
it remains to be clarified why such a smooth, gravity-dominated flow generates
double-peaked emission lines only in a small fraction of active galactic
nuclei.Comment: 12 pages, including 3 figures, accepted for publication in The
Astrophysical Journa
Diffraction and boundary conditions in semi-classical open billiards
The conductance through open quantum dots or quantum billiards shows
fluctuations, that can be explained as interference between waves following
different paths between the leads of the billiard. We examine such systems by
the use of a semi-classical Green's functions. In this paper we examine how the
choice of boundary conditions at the lead mouths affect the diffraction. We
derive a new formula for the S-matrix element. Finally we compare
semi-classical simulations to quantum mechanical ones, and show that this new
formula yield superior results.Comment: 7 pages, 4 figure
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