1,816 research outputs found
Covariant Lattice Theory and t'Hooft's Formulation
We show that 't Hooft's representation of (2+1)-dimensional gravity in terms
of flat polygonal tiles is closely related to a gauge-fixed version of the
covariant Hamiltonian lattice theory. 't Hooft's gauge is remarkable in that it
leads to a Hamiltonian which is a linear sum of vertex Hamiltonians, each of
which is defined modulo . A cyclic Hamiltonian implies that ``time'' is
quantized. However, it turns out that this Hamiltonian is {\it constrained}. If
one chooses an internal time and solves this constraint for the ``physical
Hamiltonian'', the result is not a cyclic function. Even if one quantizes {\it
a la Dirac}, the ``internal time'' observable does not acquire a discrete
spectrum. We also show that in Euclidean 3-d lattice gravity, ``space'' can be
either discrete or continuous depending on the choice of quantization. Finally,
we propose a generalization of 't Hooft's gauge for Hamiltonian lattice
formulations of topological gravity dimension 4.Comment: 10 pages of text. One figure available from J.A. Zapata upon reques
Oscillatory decay of a two-component Bose-Einstein condensate
We study the decay of a two-component Bose-Einstein condensate with negative
effective interaction energy. With a decreasing atom number due to losses, the
atom-atom interaction becomes less important and the system undergoes a
transition from a bistable Josephson regime to the monostable Rabi regime,
displaying oscillations in phase and number. We study the equations of motion
and derive an analytical expression for the oscillation amplitude. A quantum
trajectory simulation reveals that the classical description fails for low
emission rates, as expected from analytical considerations. Observation of the
proposed effect will provide evidence for negative effective interaction.Comment: 4 pages, 3 figue
Voltage rectification by a SQUID ratchet
We argue that the phase across an asymmetric dc SQUID threaded by a magnetic
flux can experience an effective ratchet (periodic and asymmetric) potential.
Under an external ac current, a rocking ratchet mechanism operates whereby one
sign of the time derivative of the phase is favored. We show that there exists
a range of parameters in which a fixed sign (and, in a narrower range, even a
fixed value) of the average voltage across the ring occurs, regardless of the
sign of the external current dc component.Comment: 4 pages, 4 EPS figures, uses psfig.sty. Revised version, to appear in
Physical Review Letters (26 August 1996
QuizMap: Open social student modeling and adaptive navigation support with TreeMaps
In this paper, we present a novel approach to integrate social adaptive navigation support for self-assessment questions with an open student model using QuizMap, a TreeMap-based interface. By exposing student model in contrast to student peers and the whole class, QuizMap attempts to provide social guidance and increase student performance. The paper explains the nature of the QuizMap approach and its implementation in the context of self-assessment questions for Java programming. It also presents the design of a semester-long classroom study that we ran to evaluate QuizMap and reports the evaluation results. © 2011 Springer-Verlag Berlin Heidelberg
Compact x-ray source based on burst-mode inverse Compton scattering at 100 kHz
A design for a compact x-ray light source (CXLS) with flux and brilliance
orders of magnitude beyond existing laboratory scale sources is presented. The
source is based on inverse Compton scattering of a high brightness electron
bunch on a picosecond laser pulse. The accelerator is a novel high-efficiency
standing-wave linac and RF photoinjector powered by a single ultrastable RF
transmitter at x-band RF frequency. The high efficiency permits operation at
repetition rates up to 1 kHz, which is further boosted to 100 kHz by operating
with trains of 100 bunches of 100 pC charge, each separated by 5 ns. The entire
accelerator is approximately 1 meter long and produces hard x-rays tunable over
a wide range of photon energies. The colliding laser is a Yb:YAG solid-state
amplifier producing 1030 nm, 100 mJ pulses at the same 1 kHz repetition rate as
the accelerator. The laser pulse is frequency-doubled and stored for many
passes in a ringdown cavity to match the linac pulse structure. At a photon
energy of 12.4 keV, the predicted x-ray flux is
photons/second in a 5% bandwidth and the brilliance is in pulses with RMS pulse
length of 490 fs. The nominal electron beam parameters are 18 MeV kinetic
energy, 10 microamp average current, 0.5 microsecond macropulse length,
resulting in average electron beam power of 180 W. Optimization of the x-ray
output is presented along with design of the accelerator, laser, and x-ray
optic components that are specific to the particular characteristics of the
Compton scattered x-ray pulses.Comment: 25 pages, 24 figures, 54 reference
Josephson effect between trapped Bose-Einstein condensates
We study the Josephson effect between atomic Bose-Einstein condensates. By
drawing on an electrostatic analogy, we derive a semiclassical functional
expression for the three-dimensional Josephson coupling energy in terms of the
condensate density. Estimates of the capacitive energy and of the Josephson
plasma frequency are also given. The effect of dissipation due to the
incoherent exchange of normal atoms is analysed. We conclude that coherent
Josephson dynamics may already be observable in current experimental systems.Comment: 4 pages, RevTe
Creep-resistant composites of alumina and single-wall carbon nanotubes
Composites of alumina Al2O3 ceramic and single-wall carbon nanotubes (SWNTs) have been tested in uniaxial compression at 1300 and 1350 °C (Ar atmosphere), and they have been found to be about two orders of magnitude more creep-resistant compared to a pure alumina of about the same grain size (0.5 micras). This is attributed to partial blocking of grain-boundary sliding by SWNTs in the composites. Since the grain boundaries in the ceramic/SWNTs composites are amenable to being engineered, this constitutes an attractive approach to the design of creep-resistant ceramic composites
Three intermediate-mass YSOs with different properties emerging from the same natal cloud in IRAS 00117+6412
We observed with the VLA, PdBI, and SMA the centimeter and millimeter
continuum, N2H+(1-0), and CO(2-1) emission associated with a dusty cloud
harboring a nascent cluster with intermediate-mass protostars. At centimeter
wavelengths we found a strong source, tracing a UCHII region, at the eastern
edge of the dusty cloud, with a shell-like structure, and with the
near-infrared counterpart falling in the center of the shell. This is
presumably the most massive source of the forming cluster. About 15'' to the
west of the UCHII region and well embedded in the dusty cloud, we detected a
strong millimeter source, MM1, associated with centimeter and near-infrared
emission. MM1 seems to be driving a prominent high-velocity CO bipolar outflow,
and is embedded in a ridge of dense gas traced by N2H+. We estimated that MM1
is an intermediate-mass source in the Class 0/I phase. About 15'' to the south
of MM1, and still more deeply embedded in the dusty cloud, we detected a
compact millimeter source, MM2, with neither centimeter nor near-infrared
emission, but with water maser emission. MM2 is associated with a clump of
N2H+, whose kinematics reveal a clear velocity gradient and additionally we
found signposts of infall motions. MM2, being deeply embedded within the dusty
cloud, with an associated water maser but no hints of CO outflow emission, is
an intriguing object, presumably of intermediate mass. In conclusion, the UCHII
region is found at the border of a dusty cloud which is currently undergoing
active star formation. Two intermediate-mass protostars in the dusty cloud seem
to have formed after the UCHII region and have different properties related to
the outflow phenomenon.Comment: accepted to Astronomy and Astrophysic
Chemical-potential standard for atomic Bose-Einstein condensates
When subject to an external time periodic perturbation of frequency , a
Josephson-coupled two-state Bose-Einstein condensate responds with a constant
chemical potential difference , where is Planck's constant
and is an integer. We propose an experimental procedure to produce
ac-driven atomic Josephson devices that may be used to define a standard of
chemical potential. We investigate how to circumvent some of the specific
problems derived from the present lack of advanced atom circuit technology. We
include the effect of dissipation due to quasiparticles, which is essential to
help the system relax towards the exact Shapiro resonance, and set limits to
the range of values which the various physical quantities must have in order to
achieve a stable and accurate chemical potential difference between the
macroscopic condensates.Comment: 13 pages, 4 figure
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