19,079 research outputs found
First clear evidence of quantum chaos in the bound states of an atomic nucleus
We study the spectral fluctuations of the Pb nucleus using the
complete experimental spectrum of 151 states up to excitation energies of
MeV recently identified at the Maier-Leibnitz-Laboratorium at Garching,
Germany. For natural parity states the results are very close to the
predictions of Random Matrix Theory (RMT) for the nearest-neighbor spacing
distribution. A quantitative estimate of the agreement is given by the Brody
parameter , which takes the value for regular systems and
for chaotic systems. We obtain which
is, to our knowledge, the closest value to chaos ever observed in experimental
bound states of nuclei. By contrast, the results for unnatural parity states
are far from RMT behavior. We interpret these results as a consequence of the
strength of the residual interaction in Pb, which, according to
experimental data, is much stronger for natural than for unnatural parity
states. In addition our results show that chaotic and non-chaotic nuclear
states coexist in the same energy region of the spectrum.Comment: 9 pages, 1 figur
Ratchet behavior in nonlinear Klein-Gordon systems with point-like inhomogeneities
We investigate the ratchet dynamics of nonlinear Klein-Gordon kinks in a
periodic, asymmetric lattice of point-like inhomogeneities. We explain the
underlying rectification mechanism within a collective coordinate framework,
which shows that such system behaves as a rocking ratchet for point particles.
Careful attention is given to the kink width dynamics and its role in the
transport. We also analyze the robustness of our kink rocking ratchet in the
presence of noise. We show that the noise activates unidirectional motion in a
parameter range where such motion is not observed in the noiseless case. This
is subsequently corroborated by the collective variable theory. An explanation
for this new phenomenom is given
Stellar populations in the Carina region: The Galactic plane at l = 291
Previous studies of the Carina region have revealed its complexity and
richness as well as a significant number of early-type stars. In many cases,
these studies only concentrated on the central region or were not homogeneous.
This latter aspect, in particular, is crucial because very different ages and
distances for key clusters have been claimed in recent years. The aim of this
work is to study in detail an area of the Galactic plane in Carina. We analyze
the properties of different stellar populations and focus on a sample of open
clusters and their population of YSOs and highly reddened early stars. We also
studied the stellar mass distribution in these clusters and the possible
scenario of their formation. Finally, we outline the Galactic spiral structure
in this direction. We obtained photometric data for six young open clusters
located in Carina at l = 291, and their adjacent stellar fields, which we
complemented with spectroscopic observations of a few selected targets. We also
culled additional information from the literature. Our results provide more
reliable estimates of distances, color excesses, masses, and ages of the
stellar populations in this direction. We estimate the basic parameters of the
studied clusters and find that they identify two overdensities of young stellar
populations. We find evidence of PMS populations inside them, with an apparent
coeval stellar formation in the most conspicuous clusters. We also discuss
apparent age and distance gradients in the direction NW-SE. We study the mass
distributions of several clusters in the region. They consistently show a
canonical IMF slope. We discover and characterise an abnormally reddened
massive stellar population. Spectroscopic observations of ten stars of this
latter population show that all selected targets were massive OB stars. Their
location is consistent with the position of the Car-Sag spiral arm.Comment: 15 pages, 13 figure
Late time tails of the massive vector field in a black hole background
We investigate the late-time behavior of the massive vector field in the
background of the Schwarzschild and Schwarzschild-de Sitter black holes. For
Schwarzschild black hole, at intermediately late times the massive vector field
is represented by three functions with different decay law , ,
, while at asymptotically late times
the decay law is universal, and does not
depend on the multipole number . Together with previous study of massive
scalar and Dirac fields where the same asymptotically late-time decay law was
found, it means, that the asymptotically late-time decay law \emph{does not depend} also \emph{on the spin} of the field under
consideration. For Schwarzschild-de Sitter black holes it is observed two
different regimes in the late-time decay of perturbations: non-oscillatory
exponential damping for small values of and oscillatory quasinormal mode
decay for high enough . Numerical and analytical results are found for these
quasinormal frequencies.Comment: one author and new material are adde
Coulomb blockade without potential barriers
We study transport through a strongly correlated quantum dot and show that
Coulomb blockade can appear even in the presence of perfect contacts. This
conclusion arises from numerical calculations of the conductance for a
microscopic model of spinless fermions in an interacting chain connected to
each lead via a completely open channel. The dependence of the conductance on
the gate voltage shows well defined Coulomb blockade peaks which are sharpened
as the interaction strength is increased. Our numerics is based on the
embedding method and the DMRG algorithm. We explain the emergence of Coulomb
blockade with perfect contacts by a reduction of the effective coupling matrix
elements between many-body states corresponding to successive particle numbers
in the interacting region. A perturbative approach, valid in the strong
interaction limit, yields an analytic expression for the interaction-induced
suppression of the conductance in the Coulomb blockade regime.Comment: Fixed problems with eps figure
100 MHz Amplitude and Polarization Modulated Optical Source for Free-Space Quantum Key Distribution at 850 nm
We report on an integrated photonic transmitter of up to 100 MHz repetition
rate, which emits pulses centered at 850 nm with arbitrary amplitude and
polarization. The source is suitable for free space quantum key distribution
applications. The whole transmitter, with the optical and electronic components
integrated, has reduced size and power consumption. In addition, the
optoelectronic components forming the transmitter can be space-qualified,
making it suitable for satellite and future space missions.Comment: 6 figures, 2 table
Optimization of soliton ratchets in inhomogeneous sine-Gordon systems
Unidirectional motion of solitons can take place, although the applied force
has zero average in time, when the spatial symmetry is broken by introducing a
potential , which consists of periodically repeated cells with each cell
containing an asymmetric array of strongly localized inhomogeneities at
positions . A collective coordinate approach shows that the positions,
heights and widths of the inhomogeneities (in that order) are the crucial
parameters so as to obtain an optimal effective potential that yields
a maximal average soliton velocity. essentially exhibits two
features: double peaks consisting of a positive and a negative peak, and long
flat regions between the double peaks. Such a potential can be obtained by
choosing inhomogeneities with opposite signs (e.g., microresistors and
microshorts in the case of long Josephson junctions) that are positioned close
to each other, while the distance between each peak pair is rather large. These
results of the collective variables theory are confirmed by full simulations
for the inhomogeneous sine-Gordon system
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