8,680 research outputs found
Instanton-noninstanton transition in nonintegrable tunneling processes: A renormalized perturbation approach
The instanton-noninstanton (I-NI) transition in the tunneling process, which
has been numerically observed in classically nonintegrable quantum maps, can be
described by a perturbation theory based on an integrable Hamiltonian
renormalized so as to incorporate the integrable part of the map. The
renormalized perturbation theory is successfully applied to the two quantum
maps, the H\'enon and standard maps. In spite of different nature of tunneling
in the two systems, the I-NI transition exhibits very common characteristics.
In particular, the manifestation of I-NI transition is obviously explained by a
remarkable quenching of the renormalized transition matrix element. The
enhancement of tunneling probability after the transition can be understood as
a sudden change of the tunneling mechanism from the instanton to quite a
different mechanism supported by classical flows just outside of the
stable-unstable manifolds of the saddle on the top of the potential barrier.Comment: 6 pages, 4 figure
The kinetic mechanism of bacterial ribosome recycling.
Bacterial ribosome recycling requires breakdown of the post-termination complex (PoTC), comprising a messenger RNA (mRNA) and an uncharged transfer RNA (tRNA) cognate to the terminal mRNA codon bound to the 70S ribosome. The translation factors, elongation factor G and ribosome recycling factor, are known to be required for recycling, but there is controversy concerning whether these factors act primarily to effect the release of mRNA and tRNA from the ribosome, with the splitting of the ribosome into subunits being somewhat dispensable, or whether their main function is to catalyze the splitting reaction, which necessarily precedes mRNA and tRNA release. Here, we utilize three assays directly measuring the rates of mRNA and tRNA release and of ribosome splitting in several model PoTCs. Our results largely reconcile these previously held views. We demonstrate that, in the absence of an upstream Shine-Dalgarno (SD) sequence, PoTC breakdown proceeds in the order: mRNA release followed by tRNA release and then by 70S splitting. By contrast, in the presence of an SD sequence all three processes proceed with identical apparent rates, with the splitting step likely being rate-determining. Our results are consistent with ribosome profiling results demonstrating the influence of upstream SD-like sequences on ribosome occupancy at or just before the mRNA stop codon
Surface tension of electrolytes: Hydrophilic and hydrophobic ions near an interface
We calculate the ion distributions around an interface in fluid mixtures of
highly polar and less polar fluids (water and oil) for two and three ion
species. We take into account the solvation and image interactions between ions
and solvent. We show that hydrophilic and hydrophobic ions tend to undergo a
microphase separation at an interface, giving rise to an enlarged electric
double layer. We also derive a general expression for the surface tension of
electrolyte systems, which contains a negative electrostatic contribution
proportional to the square root of the bulk salt density. The amplitude of this
square-root term is small for hydrophilic ion pairs, but is much increased for
hydrophilic and hydrophobic ion pairs. For three ion species including
hydrophilic and hydrophobic ions, we calculate the ion distributions to explain
those obtained by x-ray reflectivity measurements.Comment: 8 figure
Experimental demonstration of entanglement assisted coding using a two-mode squeezed vacuum state
We have experimentally realized the scheme initially proposed as quantum
dense coding with continuous variables [Ban, J. Opt. B \textbf{1}, L9 (1999),
and Braunstein and Kimble, \pra\textbf{61}, 042302 (2000)]. In our experiment,
a pair of EPR (Einstein-Podolski-Rosen) beams is generated from two independent
squeezed vacua. After adding two-quadrature signal to one of the EPR beams, two
squeezed beams that contain the signal were recovered. Although our squeezing
level is not sufficient to demonstrate the channel capacity gain over the
Holevo limit of a single-mode channel without entanglement, our channel is
superior to conventional channels such as coherent and squeezing channels. In
addition, optical addition and subtraction processes demonstrated are
elementary operations of universal quantum information processing on continuous
variables.Comment: 4 pages, 4 figures, submitted to Phys. Rev.
Probing the effect of point defects on the leakage blocking capability of Al0.1Ga0.9N/Si structures using a monoenergetic positron beam
Vacancy-type defects in Al0.1Ga0.9N were probed using a monoenergetic positron beam. Al0.1Ga0.9N layers with different carbon doping concentrations ([C] = 5 x 10(17) -8 x 10(19) cm(-3)) were grown on Si substrates by metalorganic vapor phase epitaxy. The major defect species in Al0.1Ga0.9N was determined to be a cation vacancy (or cation vacancies) coupled with nitrogen vacancies and/or with carbon atoms at nitrogen sites (C(N)s). The charge state of the vacancies was positive because of the electron transfer from the defects to C-N-related acceptors. The defect charge state was changed from positive to neutral when the sample was illuminated with photon energy above 1.8 eV, and this energy range agreed with the yellow and blue luminescence. For the sample with high [C], the charge transition of the vacancies under illumination was found to be suppressed, which was attributed to the trapping of emitted electrons by C-N-related acceptors. With increasing [C], the breakdown voltage under the reverse bias condition increased. This was explained by the trapping of the injected electrons by the positively charged vacancies and C-N-related acceptors
On the phase of quark determinant in lattice QCD with finite chemical potential
We investigate the phase of the quark determinant with finite chemical
potential in lattice QCD using both analytic and numerical methods. Applying
the winding number expansion and the hopping parameter expansion to the
logarithm of the determinant, we show that the absolute value of the phase has
an upper bound that grows with the spatial volume but decreases exponentially
with an increase in the temporal extent of the lattice. This analytic but
approximate result is confirmed with a numerical study in four-flavor QCD in
which the phase is calculated exactly. Since the phase is well controlled on
lattices with larger time extents, we try the phase reweighting method in a
region beyond where the Taylor expansion method cannot be applied.
Working in four-flavor QCD, we find a first-order like behavior on a lattice at which was previously observed by Kentucky
group with the canonical method. We also show that the winding number expansion
has a nice convergence property beyond . We expect that this expansion
is useful to study the high density region of the QCD phase diagram at low
temperatures.Comment: 21 page
Optical and electrical Barkhausen noise induced by recording ferroelectric domain holograms
Ferroelectric domain gratings with periods of the order of an optical wavelength are induced in strontium barium niobate by photorefractive space-charge fields. We measure the Barkhausen noise in current and diffraction efficiency while optically recording domain gratings and show that the two are strongly correlated in time. Significant random depolarization occurs under high-intensity illumination. We deduce the kinetics of the domain inversion process from the shape of the current transients
Electromagnetic radiation due to naked singularity formation in self-similar gravitational collapse
Dynamical evolution of test fields in background geometry with a naked
singularity is an important problem relevant to the Cauchy horizon instability
and the observational signatures different from black hole formation. In this
paper we study electromagnetic perturbations generated by a given current
distribution in collapsing matter under a spherically symmetric self-similar
background. Using the Green's function method, we construct the formula to
evaluate the outgoing energy flux observed at the future null infinity. The
contributions from "quasi-normal" modes of the self-similar system as well as
"high-frequency" waves are clarified. We find a characteristic power-law time
evolution of the outgoing energy flux which appears just before naked
singularity formation, and give the criteria as to whether or not the outgoing
energy flux diverges at the future Cauchy horizon.Comment: 20 pages, 7 figures, references added to match the published versio
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