25,078 research outputs found
Structural relaxation and rheological response of a driven amorphous system
The interplay between the structural relaxation and the rheological response
of a binary LJ glass former is studied via MD simulations. In the quiescent
state, the model is well known for its sluggish dynamics and a two step
relaxation of correlation functions at low temperatures. An ideal glass
transition temperature of has been identified in the previous
studies via the analysis of the system's dynamics in the frame work of the mode
coupling theory of the glass transition [W. Kob and H.C. Andersen, PRE 51, 4626
(1995)]. Here, we test wether a signature of this ideal glass transition can
also be found under shear. Indeed, the following distinction in the structural
relaxation is found: In the supercooled state, the structural relaxation is
dominated by the shear at relatively high shear rates, , whereas
at sufficiently low the (shear-independent) equilibrium
relaxation is recovered. In contrast to this, the structural relaxation of a
\emph{glass} is always driven by shear. This distinct behavior of the
correlation functions is also reflected in the rheological response. In the
supercooled state, the shear viscosity, , decreases with increasing shear
rate (shear thinning) at high shear rates, but then converges toward a constant
as the is decreased below a (temperature-dependent) threshold
value. Below , on the other hand, the shear viscosity grows as suggesting a divergence at . Thus,
within the accessible observation time window, a transition toward a
non-ergodic state seems to occur in the driven glass as the driving force
approaches zero.Comment: 12 pages, 9 figure
Ultra-compact optical auto-correlator based on slow-light enhanced third harmonic generation in a silicon photonic crystal waveguide
The ability to use coherent light for material science and applications is
directly linked to our ability to measure short optical pulses. While
free-space optical methods are well-established, achieving this on a chip would
offer the greatest benefit in footprint, performance, flexibility and cost, and
allow the integration with complementary signal processing devices. A key goal
is to achieve operation at sub-Watt peak power levels and on sub-picosecond
timescales. Previous integrated demonstrations require either a temporally
synchronized reference pulse, an off-chip spectrometer, or long tunable delay
lines. We report the first device capable of achieving single-shot time-domain
measurements of near-infrared picosecond pulses based on an ultra-compact
integrated CMOS compatible device, with the potential to be fully integrated
without any external instrumentation. It relies on optical third-harmonic
generation in a slow-light silicon waveguide. Our method can also serve as a
powerful in-situ diagnostic tool to directly map, at visible wavelengths, the
propagation dynamics of near-infrared pulses in photonic crystals.Comment: 20 pages, 6 figures, 38 reference
A simulation study of energy transport in the Hamiltonian XY-model
The transport properties of the planar rotator model on a square lattice are
analyzed by means of microcanonical and non--equilibrium simulations. Well
below the Kosterlitz--Thouless--Berezinskii transition temperature, both
approaches consistently indicate that the energy current autocorrelation
displays a long--time tail decaying as t^{-1}. This yields a thermal
conductivity coefficient which diverges logarithmically with the lattice size.
Conversely, conductivity is found to be finite in the high--temperature
disordered phase. Simulations close to the transition temperature are insted
limited by slow convergence that is presumably due to the slow kinetics of
vortex pairs.Comment: Submitted to Journal of Statistical Mechanics: theory and experimen
The Hunt for Exomoons with Kepler (HEK): II. Analysis of Seven Viable Satellite-Hosting Planet Candidates
From the list of 2321 transiting planet candidates announced by the Kepler
Mission, we select seven targets with favorable properties for the capacity to
dynamically maintain an exomoon and present a detectable signal. These seven
candidates were identified through our automatic target selection (TSA)
algorithm and target selection prioritization (TSP) filtering, whereby we
excluded systems exhibiting significant time-correlated noise and focussed on
those with a single transiting planet candidate of radius less than 6 Earth
radii. We find no compelling evidence for an exomoon around any of the seven
KOIs but constrain the satellite-to-planet mass ratios for each. For four of
the seven KOIs, we estimate a 95% upper quantile of M_S/M_P<0.04, which given
the radii of the candidates, likely probes down to sub-Earth masses. We also
derive precise transit times and durations for each candidate and find no
evidence for dynamical variations in any of the KOIs. With just a few systems
analyzed thus far in the in-going HEK project, projections on eta-moon would be
premature, but a high frequency of large moons around
Super-Earths/Mini-Neptunes would appear to be incommensurable with our results
so far.Comment: 32 pages, 11 figures, 23 tables, Accepted to Ap
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