576,775 research outputs found
Meson-Nucleon Vertex Form Factors at Finite Temperature Using a Soft Pion Form Factor
The temperature and density dependence of the meson-nucleon vertex form
factors is studied in the framework of thermofield dynamics. Results are
obtained for two rather different nucleon-nucleon potentials: the usual Bonn
potential and the variation with a softer form factor, due to Holinde
and Thomas. In general, the results show only a modest degree of sensitivity to
the choice of interaction.Comment: 5 pages, uses revtex
Deterministic creation of stationary entangled states by dissipation
We propose a practical physical system for creation of a stationary
entanglement by dissipation without employing the environment engineering
techniques. The system proposed is composed of two perfectly distinguishable
atoms, through their significantly different transition frequencies, with only
one atom addressed by an external laser field. We show that the arrangement
would easily be realized in practice by trapping the atoms at the distance
equal to the quarter-wavelength of a standing-wave laser field and locating one
of the atoms at a node and the other at the successive antinode of the wave.
The undesirable dipole-dipole interaction between the atoms, that could be
large at this small distance, is adjusted to zero by a specific initial
preparation of the atoms or by a specific polarization of the atomic dipole
moments. Following this arrangement, we show that the dissipative relaxation
can create a stationary entanglement on demand by tuning the Rabi frequency of
the laser field to the difference between the atomic transition frequencies.
The laser field dresses the atom and we identify that the entangled state
occurs when the frequency of one of the Rabi sidebands of the driven atom tunes
to frequency of the undriven atom. It is also found that this system behaves as
a cascade open system where the fluorescence from the dressed atom drives the
other atom with no feedback.Comment: Published versio
Fitting isochrones to open cluster photometric data III. Estimating metallicities from UBV photometry
The metallicity is a critical parameter that affects the correct
determination fundamental characteristics stellar cluster and has important
implications in Galactic and Stellar evolution research. Fewer than 10 % of the
2174 currently catalog open clusters have their metallicity determined in the
literature. In this work we present a method for estimating the metallicity of
open clusters via non-subjective isochrone fitting using the cross-entropy
global optimization algorithm applied to UBV photometric data. The free
parameters distance, reddening, age, and metallicity simultaneously determined
by the fitting method. The fitting procedure uses weights for the observational
data based on the estimation of membership likelihood for each star, which
considers the observational magnitude limit, the density profile of stars as a
function of radius from the center of the cluster, and the density of stars in
multi-dimensional magnitude space. We present results of [Fe/H] for nine
well-studied open clusters based on 15 distinct UBV data sets. The [Fe/H]
values obtained in the ten cases for which spectroscopic determinations were
available in the literature agree, indicating that our method provides a good
alternative to determining [Fe/H] by using an objective isochrone fitting. Our
results show that the typical precision is about 0.1 dex
Machining and grinding of ultrahigh-strength steels and stainless steel alloys
Machining and grinding of ultrahigh-strength steels and stainless steel alloy
Current-driven vortex dynamics in untwinned superconducting single crystals
Current-driven vortex dynamics of type-II superconductors in the weak-pinning limit is investigated by quantitatively studying the current-dependent vortex dissipation of an untwinned YBa2Cu3O7 single crystal. For applied current densities (J) substantially larger than the critical current density (Jc), non-linear resistive peaks appear below the thermodynamic first-order vortex-lattice melting transition temperature (Tm), in contrast to the resistive hysteresis in the low-current limit (J < Jc). These resistive peaks are quantitatively analysed in terms of the current-driven coherent and plastic motion of vortex bundles in the vortex-solid phase, and the non-linear current - voltage characteristics are found to be consistent with the collective flux-creep model. The effects of high-density random point defects on the vortex dynamics are also investigated via proton irradiation of the same single crystal. Neither resistive hysteresis at low currents nor peak effects at high currents are found after the irradiation. Furthermore, the current-voltage characteristics within the instrumental resolution become completely ohmic over a wide range of currents and temperatures, despite theoretical predictions of much larger Jc-values for the given experimental variables. This finding suggests that the vortex-glass phase, a theoretically proposed low-temperature vortex state which is stabilized by point disorder and has a vanishing resistivity, may become unstable under applied currents significantly smaller than the theoretically predicted Jc. More investigation appears necessary in order to resolve this puzzling issue
Thermally activated escape rates of uniaxial spin systems with transverse field
Classical escape rates of uniaxial spin systems are characterized by a
prefactor differing from and much smaller than that of the particle problem,
since the maximum of the spin energy is attained everywhere on the line of
constant latitude: theta=const, 0 =< phi =< 2*pi. If a transverse field is
applied, a saddle point of the energy is formed, and high, moderate, and low
damping regimes (similar to those for particles) appear. Here we present the
first analytical and numerical study of crossovers between the uniaxial and
other regimes for spin systems. It is shown that there is one HD-Uniaxial
crossover, whereas at low damping the uniaxial and LD regimes are separated by
two crossovers.Comment: 4 PR pages, 3 figures, final published versio
Testing the theory of grain growth and fragmentation by millimeter observations of protoplanetary disks
Context. Observations at sub-millimeter and mm wavelengths will in the near
future be able to resolve the radial dependence of the mm spectral slope in
circumstellar disks with a resolution of around a few AU at the distance of the
closest star-forming regions.
Aims. We aim to constrain physical models of grain growth and fragmentation
by a large sample of (sub-)mm observations of disks around pre-main sequence
stars in the Taurus-Auriga and Ophiuchus star-forming regions.
Methods. State-of-the-art coagulation/fragmentation and disk-structure codes
are coupled to produce steady-state grain size distributions and to predict the
spectral slopes at (sub-)mm wavelengths.
Results. This work presents the first calculations predicting the mm spectral
slope based on a physical model of grain growth. Our models can quite naturally
reproduce the observed mm-slopes, but a simultaneous match to the observed
range of flux levels can only be reached by a reduction of the dust mass by a
factor of a few up to about 30 while keeping the gas mass of the disk the same.
This dust reduction can either be due to radial drift at a reduced rate or
during an earlier evolutionary time (otherwise the predicted fluxes would
become too low) or due to efficient conversion of dust into larger, unseen
bodies.Comment: Accepted for publication in A&A Letters. 5 pages, 3 figure
A Vehicular Traffic Flow Model Based on a Stochastic Acceleration Process
A new vehicular traffic flow model based on a stochastic jump process in
vehicle acceleration and braking is introduced. It is based on a master
equation for the single car probability density in space, velocity and
acceleration with an additional vehicular chaos assumption and is derived via a
Markovian ansatz for car pairs. This equation is analyzed using simple driver
interaction models in the spatial homogeneous case. Velocity distributions in
stochastic equilibrium, together with the car density dependence of their
moments, i.e. mean velocity and scattering and the fundamental diagram are
presented.Comment: 27 pages, 6 figure
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