6,880 research outputs found
On the stability and growth of single myelin figures
Myelin figures are long thin cylindrical structures that typically grow as a
dense tangle when water is added to the concentrated lamellar phase of certain
surfactants. We show that, starting from a well-ordered initial state, single
myelin figures can be produced in isolation thus allowing a detailed study of
their growth and stability. These structures grow with their base at the
exposed edges of bilayer stacks from which material is transported into the
myelin. Myelins only form and grow in the presence of a driving stress; when
the stress is removed, the myelins retract.Comment: 4 pages, 8 figures. Revised version, 1 new figure, additional
reference
Self-organization of traffic jams in cities: effects of stochastic dynamics and signal periods
We propose a cellular automata model for vehicular traffic in cities by
combining (and appropriately modifying) ideas borrowed from the
Biham-Middleton-Levine (BML) model of city traffic and the Nagel-Schreckenberg
(NS) model of highway traffic. We demonstrate a phase transition from the
"free-flowing" dynamical phase to the completely "jammed" phase at a vehicle
density which depends on the time periods of the synchronized signals and the
separation between them. The intrinsic stochasticity of the dynamics, which
triggers the onset of jamming, is similar to that in the NS model, while the
phenomenon of complete jamming through self-organization as well as the final
jammed configurations are similar to those in the BML model. Using our new
model, we have made an investigation of the time-dependence of the average
speeds of the cars in the "free-flowing" phase as well as the dependence of
flux and jamming on the time period of the signals.Comment: 4 pages, REVTEX, 4 eps figures include
Dynamic masses for the close PG1159 binary SDSSJ212531.92-010745.9
SDSSJ212531.92-010745.9 is the first known PG1159 star in a close binary with
a late main sequence companion allowing a dynamical mass determination. The
system shows flux variations with a peak-to-peak amplitude of about 0.7 mag and
a period of about 6.96h. In August 2007, 13 spectra of SDSSJ212531.92-010745.9
covering the full orbital phase range were taken at the TWIN 3.5m telescope at
the Calar Alto Observatory (Alm\'{e}ria, Spain). These confirm the typical
PG1159 features seen in the SDSS discovery spectrum, together with the Balmer
series of hydrogen in emission (plus other emission lines), interpreted as
signature of the companion's irradiated side. A radial velocity curve was
obtained for both components. Using co-added radial-velocity-corrected spectra,
the spectral analysis of the PG1159 star is being refined.
The system's lightcurve, obtained during three seasons of photometry with the
G\"ottingen 50cm and T\"ubingen 80cm telescopes, was fitted with both the
NIGHTFALL and PHOEBE binary simulation programs. An accurate mass determination
of the PG1159 component from the radial velocity measurements requires to first
derive the inclination, which requires light curve modelling and yields further
constraints on radii, effective temperature and separation of the system's
components. From the analysis of all data available so far, we present the
possible mass range for the PG1159 component of SDSSJ212531.92-010745.9.Comment: 8 pages, in "White dwarfs", proceedings of the 16th European White
Dwarf Workshop, eds. E. Garcia-Berro, M. Hernanz, J. Isern, S. Torres, to be
published in J. Phys.: Conf. Se
Optical signature of sub-gap absorption in the superconducting state of Ba(Fe,Co)2As2
The optical conductivity of Ba(FeCo)As shows a
clear signature of the superconducting gap, but a simple -wave description
fails in accounting for the low frequency response. This task is achieved by
introducing an extra Drude peak in the superconducting state representing
sub-gap absorption, other than thermally broken pairs. This extra peak and the
coexisting -wave response respect the total sum rule indicating a common
origin for the carriers. We discuss the possible origins for this absorption as
(i) quasiparticles due to pair-breaking from interband impurity scattering in a
two band gap symmetry model, which includes (ii) the possible
existence of impurity levels within an isotropic gap model; or (iii) an
indication that one of the bands is highly anisotropic.Comment: 5 pages, 4 figure
Generalized Intelligent States for an Arbitrary Quantum System
Generalized Intelligent States (coherent and squeezed states) are derived for
an arbitrary quantum system by using the minimization of the so-called
Robertson-Schr\"odinger uncertainty relation. The Fock-Bargmann representation
is also considered. As a direct illustration of our construction, the
P\"oschl-Teller potentials of trigonometric type will be shosen. We will show
the advantage of the Fock-Bargmann representation in obtaining the generalized
intelligent states in an analytical way. Many properties of these states are
studied
Performance Evaluation of Road Traffic Control Using a Fuzzy Cellular Model
In this paper a method is proposed for performance evaluation of road traffic
control systems. The method is designed to be implemented in an on-line
simulation environment, which enables optimisation of adaptive traffic control
strategies. Performance measures are computed using a fuzzy cellular traffic
model, formulated as a hybrid system combining cellular automata and fuzzy
calculus. Experimental results show that the introduced method allows the
performance to be evaluated using imprecise traffic measurements. Moreover, the
fuzzy definitions of performance measures are convenient for uncertainty
determination in traffic control decisions.Comment: The final publication is available at http://www.springerlink.co
Metrological characterization of the pulsed Rb clock with optical detection
We report on the implementation and the metrological characterization of a
vapor-cell Rb frequency standard working in pulsed regime. The three main parts
that compose the clock, physics package, optics and electronics, are described
in detail in the paper. The prototype is designed and optimized to detect the
clock transition in the optical domain. Specifically, the reference atomic
transition, excited with a Ramsey scheme, is detected by observing the
interference pattern on a laser absorption signal.
\ The metrological analysis includes the observation and characterization of
the clock signal and the measurement of frequency stability and drift. In terms
of Allan deviation, the measured frequency stability results as low as
, being the averaging time, and
reaches the value of few units of for s, an
unprecedent achievement for a vapor cell clock. We discuss in the paper the
physical effects leading to this result with particular care to laser and
microwave noises transferred to the clock signal. The frequency drift, probably
related to the temperature, stays below per day, and no evidence of
flicker floor is observed.
\ We also mention some possible improvements that in principle would lead to
a clock stability below the level at 1 s and to a drift of few units
of per day
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