29,572 research outputs found

### Influence of external disturbances and compressibility on free turbulent mixing

It is shown that disturbances in external flow can significantly affect, by as much as an order of magnitude, the turbulent mixing rate in free shear layers and that the length scale of the external flow disturbances is as important as the amplitude. The difference between the effect of wide-band and narrow-band disturbances is stressed. The model for pressure fluctuation term in the kinetic energy equation is included in a two-equation model. The reduced spreading rate in high Mach number, high Reynolds number, adiabatic, free turbulent shear layers is predicted

### Synchronization transition of heterogeneously coupled oscillators on scale-free networks

We investigate the synchronization transition of the modified Kuramoto model
where the oscillators form a scale-free network with degree exponent $\lambda$.
An oscillator of degree $k_i$ is coupled to its neighboring oscillators with
asymmetric and degree-dependent coupling in the form of \couplingcoeff
k_i^{\eta-1}. By invoking the mean-field approach, we determine the
synchronization transition point $J_c$, which is zero (finite) when $\eta >
\lambda-2$ ($\eta < \lambda-2$). We find eight different synchronization
transition behaviors depending on the values of $\eta$ and $\lambda$, and
derive the critical exponents associated with the order parameter and the
finite-size scaling in each case. The synchronization transition is also
studied from the perspective of cluster formation of synchronized vertices. The
cluster-size distribution and the largest cluster size as a function of the
system size are derived for each case using the generating function technique.
Our analytic results are confirmed by numerical simulations.Comment: 11 pages, 3 figures and two table

### Fidelity of Quantum Teleportation through Noisy Channels

We investigate quantum teleportation through noisy quantum channels by
solving analytically and numerically a master equation in the Lindblad form. We
calculate the fidelity as a function of decoherence rates and angles of a state
to be teleported. It is found that the average fidelity and the range of states
to be accurately teleported depend on types of noise acting on quantum
channels. If the quantum channels is subject to isotropic noise, the average
fidelity decays to 1/2, which is smaller than the best possible value 2/3
obtained only by the classical communication. On the other hand, if the noisy
quantum channel is modeled by a single Lindblad operator, the average fidelity
is always greater than 2/3.Comment: 6 pages, 5 figures, accepted for publication in Phys. Rev.

### Ultrafast spectroscopy of propagating coherent acoustic phonons in GaN/InGaN heterostructures

We show that large amplitude, coherent acoustic phonon wavepackets can be
generated and detected in In$_x$Ga$_{1-x}$N/GaN epilayers and heterostructures
in femtosecond pump-probe differential reflectivity experiments. The amplitude
of the coherent phonon increases with increasing Indium fraction $x$ and unlike
other coherent phonon oscillations, both \textit{amplitude} and \textit{period}
are strong functions of the laser probe energy. The amplitude of the
oscillation is substantially and almost instantaneously reduced when the
wavepacket reaches a GaN-sapphire interface below the surface indicating that
the phonon wavepackets are useful for imaging below the surface. A theoretical
model is proposed which fits the experiments well and helps to deduce the
strength of the phonon wavepackets. Our model shows that localized coherent
phonon wavepackets are generated by the femtosecond pump laser in the epilayer
near the surface. The wavepackets then propagate through a GaN layer changing
the local index of refraction, primarily through the Franz-Keldysh effect, and
as a result, modulate the reflectivity of the probe beam. Our model correctly
predicts the experimental dependence on probe-wavelength as well as epilayer
thickness.Comment: 11 pages, 14 figure

### Performance of a prototype active veto system using liquid scintillator for a dark matter search experiment

We report the performance of an active veto system using a liquid
scintillator with NaI(Tl) crystals for use in a dark matter search experiment.
When a NaI(Tl) crystal is immersed in the prototype detector, the detector tags
48% of the internal K-40 background in the 0-10 keV energy region. We also
determined the tagging efficiency for events at 6-20 keV as 26.5 +/- 1.7% of
the total events, which corresponds to 0.76 +/- 0.04 events/keV/kg/day.
According to a simulation, approximately 60% of the background events from U,
Th, and K radioisotopes in photomultiplier tubes are tagged at energies of 0-10
keV. Full shielding with a 40-cm-thick liquid scintillator can increase the
tagging efficiency for both the internal K-40 and external background to
approximately 80%.Comment: Submitted to Nuclear Instruments and Methods in Physics Research
Section

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