25,670 research outputs found
Evaluation of the micro-carburetor
A prototype sonic, variable-venturi automotive carburetor was evaluated for its effects on vehicle performance, fuel economy, and exhaust emissions. A 350 CID Chevrolet Impala vehicle was tested on a chassis dynamometer over the 1975 Federal Test Procedure, urban driving cycle. The Micro-carburetor was tested and compared with stock and modified-stock engine configurations. Subsequently, the test vehicle's performance characteristics were examined with the stock carburetor and again with the Micro-carburetor in a series of on-road driveability tests. The test engine was then removed from the vehicle and installed on an engine dynamometer. Engine tests were conducted to compare the fuel economy, thermal efficiency, and cylinder-to-cylinder mixture distribution of the Micro-carburetor to that of the stock configuration. Test results show increases in thermal efficiency and improvements in fuel economy at all test conditions. Improve fuel/air mixture preparation is implied from the information presented. Further improvements in fuel economy and exhaust emissions are possible through a detailed recalibration of the Micro-carburetor
Mesoscopic Spin-Boson Models of Trapped Ions
Trapped ions arranged in Coulomb crystals provide us with the elements to
study the physics of a single spin coupled to a boson bath. In this work we
show that optical forces allow us to realize a variety of spin-boson models,
depending on the crystal geometry and the laser configuration. We study in
detail the Ohmic case, which can be implemented by illuminating a single ion
with a travelling wave. The mesoscopic character of the phonon bath in trapped
ions induces new effects like the appearance of quantum revivals in the spin
evolution.Comment: 4.4 pages, 5 figure
Diffusive behavior of a greedy traveling salesman
Using Monte Carlo simulations we examine the diffusive properties of the
greedy algorithm in the d-dimensional traveling salesman problem. Our results
show that for d=3 and 4 the average squared distance from the origin is
proportional to the number of steps t. In the d=2 case such a scaling is
modified with some logarithmic corrections, which might suggest that d=2 is the
critical dimension of the problem. The distribution of lengths also shows
marked differences between d=2 and d>2 versions. A simple strategy adopted by
the salesman might resemble strategies chosen by some foraging and hunting
animals, for which anomalous diffusive behavior has recently been reported and
interpreted in terms of Levy flights. Our results suggest that broad and
Levy-like distributions in such systems might appear due to dimension-dependent
properties of a search space.Comment: accepted in Phys. Rev.
A Kiloparsec-Scale Hyper-Starburst in a Quasar Host Less than 1 Gigayear after the Big Bang
The host galaxy of the quasar SDSS J114816.64+525150.3 (at redshift z=6.42,
when the Universe was <1 billion years old) has an infrared luminosity of
2.2x10^13 L_sun, presumably significantly powered by a massive burst of star
formation. In local examples of extremely luminous galaxies such as Arp220, the
burst of star formation is concentrated in the relatively small central region
of <100pc radius. It is unknown on which scales stars are forming in active
galaxies in the early Universe, which are likely undergoing their initial burst
of star formation. We do know that at some early point structures comparable to
the spheroidal bulge of the Milky Way must have formed. Here we report a
spatially resolved image of [CII] emission of the host galaxy of
J114816.64+525150.3 that demonstrates that its star forming gas is distributed
over a radius of ~750pc around the centre. The surface density of the star
formation rate averaged over this region is ~1000 M_sun/yr/kpc^2. This surface
density is comparable to the peak in Arp220, though ~2 orders of magnitudes
larger in area. This vigorous star forming event will likely give rise to a
massive spheroidal component in this system.Comment: Nature, in press, Feb 5 issue, p. 699-70
Pulsation Period Changes as a Tool to Identify Pre-Zero Age Horizontal Branch Stars
One of the most dramatic events in the life of a low-mass star is the He
flash, which takes place at the tip of the red giant branch (RGB) and is
followed by a series of secondary flashes before the star settles into the
zero-age horizontal branch (ZAHB). Yet, no stars have been positively
identified in this key evolutionary phase, mainly for two reasons: first, this
pre-ZAHB phase is very short compared to other major evolutionary phases in the
life of a star; and second, these pre-ZAHB stars are expected to overlap the
loci occupied by asymptotic giant branch (AGB), HB and RGB stars observed in
the color-magnitude diagram (CMD). We investigate the possibility of detecting
these stars through stellar pulsations, since some of them are expected to
rapidly cross the Cepheid/RR Lyrae instability strip in their route from the
RGB tip to the ZAHB, thus becoming pulsating stars along the way. As a
consequence of their very high evolutionary speed, some of these stars may
present anomalously large period change rates. We constructed an extensive grid
of stellar models and produced pre-ZAHB Monte Carlo simulations appropriate for
the case of the Galactic globular cluster M3 (NGC 5272), where a number of RR
Lyrae stars with high period change rates are found. Our results suggest that
some -- but certainly not all -- of the RR Lyrae stars in M3 with large period
change rates are in fact pre-ZAHB pulsators.Comment: Conference Proceedings HELAS Workshop on 'Synergies between solar and
stellar modelling', Rome, June 2009, Astrophys. Space Sci., in the pres
Relevance of multiple-quasiparticle tunneling between edge states at \nu =p/(2np+1)
We present an explanation for the anomalous behavior in tunneling conductance
and noise through a point contact between edge states in the Jain series
, for extremely weak-backscattering and low temperatures [Y.C.
Chung, M. Heiblum, and V. Umansky, Phys. Rev. Lett. {\bf{91}}, 216804 (2003)].
We consider edge states with neutral modes propagating at finite velocity, and
we show that the activation of their dynamics causes the unexpected change in
the temperature power-law of the conductance. Even more importantly, we
demonstrate that multiple-quasiparticles tunneling at low energies becomes the
most relevant process. This result will be used to explain the experimental
data on current noise where tunneling particles have a charge that can reach
times the single quasiparticle charge. In this paper we analyze the
conductance and the shot noise to substantiate quantitatively the proposed
scenario.Comment: 4 pages, 2 figure
Phase Transitions in Generalised Spin-Boson (Dicke) Models
We consider a class of generalised single mode Dicke Hamiltonians with
arbitrary boson coupling in the pseudo-spin - plane. We find exact
solutions in the thermodynamic, large-spin limit as a function of the coupling
angle, which allows us to continuously move between the simple dephasing and
the original Dicke Hamiltonians. Only in the latter case (orthogonal static and
fluctuating couplings), does the parity-symmetry induced quantum phase
transition occur.Comment: 6 pages, 5 figue
Ratchet effect in dc SQUIDs
We analyzed voltage rectification for dc SQUIDs biased with ac current with
zero mean value. We demonstrate that the reflection symmetry in the
2-dimensional SQUID potential is broken by an applied flux and with appropriate
asymmetries in the dc SQUID. Depending on the type of asymmetry, we obtain a
rocking or a simultaneously rocking and flashing ratchet, the latter showing
multiple sign reversals in the mean voltage with increasing amplitude of the ac
current. Our experimental results are in agreement with numerical solutions of
the Langevin equations for the asymmetric dc SQUID.Comment: 10 pages including 5 Postscript figure
Crossover of phase qubit dynamics in presence of negative-result weak measurement
Coherent dynamics of a superconducting phase qubit is considered in the
presence of both unitary evolution due to microwave driving and continuous
non-unitary collapse due to negative-result measurement. In the case of a
relatively weak driving, the qubit dynamics is dominated by the non-unitary
evolution, and the qubit state tends to an asymptotically stable point on the
Bloch sphere. This dynamics can be clearly distinguished from conventional
decoherence by tracking the state purity and the measurement invariant
(``murity''). When the microwave driving strength exceeds certain critical
value, the dynamics changes to non-decaying oscillations: any initial state
returns exactly to itself periodically in spite of non-unitary dynamics. The
predictions can be verified using a modification of a recent experiment.Comment: 5 pages, 4 eps figure
Dissipative Quantum Ising model in a cold atomic spin-boson mixture
Using cold bosonic atoms with two (hyperfine) ground states, we introduce a
spin-boson mixture which allows to implement the quantum Ising model in a
tunable dissipative environment. The first specie lies in a deep optical
lattice with tightly confining wells and forms a spin array; spin-up/down
corresponds to occupation by one/no atom at each site. The second specie forms
a superfluid reservoir. Different species are coupled coherently via laser
transitions and collisions. Whereas the laser coupling mimics a transverse
field for the spins, the coupling to the reservoir sound modes induces a
ferromagnetic (Ising) coupling as well as dissipation. This gives rise to an
order-disorder quantum phase transition where the effect of dissipation can be
studied in a controllable manner.Comment: 4 pages, 2 figures, 1 table; Title modified and cosmetic change
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