559 research outputs found
Exactly solvable approximating models for Rabi Hamiltonian dynamics
The interaction between an atom and a one mode external driving field is an
ubiquitous problem in many branches of physics and is often modeled using the
Rabi Hamiltonian. In this paper we present a series of analytically solvable
Hamiltonians that approximate the Rabi Hamiltonian and compare our results to
the Jaynes-Cummings model which neglects the so-called counter-rotating term in
the Rabi Hamiltonian. Through a unitary transformation that diagonlizes the
Jaynes-Cummings model, we transform the counter-rotating term into separate
terms representing several different physical processes. By keeping only
certain terms, we can achieve an excellent approximation to the exact dynamics
within specified parameter ranges
Quantum versus Semiclassical Description of Selftrapping: Anharmonic Effects
Selftrapping has been traditionally studied on the assumption that
quasiparticles interact with harmonic phonons and that this interaction is
linear in the displacement of the phonon. To complement recent semiclassical
studies of anharmonicity and nonlinearity in this context, we present below a
fully quantum mechanical analysis of a two-site system, where the oscillator is
described by a tunably anharmonic potential, with a square well with infinite
walls and the harmonic potential as its extreme limits, and wherein the
interaction is nonlinear in the oscillator displacement. We find that even
highly anharmonic polarons behave similar to their harmonic counterparts in
that selftrapping is preserved for long times in the limit of strong coupling,
and that the polaronic tunneling time scale depends exponentially on the
polaron binding energy. Further, in agreement, with earlier results related to
harmonic polarons, the semiclassical approximation agrees with the full quantum
result in the massive oscillator limit of small oscillator frequency and strong
quasiparticle-oscillator coupling.Comment: 10 pages, 6 figures, to appear in Phys. Rev.
Spin and Conductance-Peak-Spacing Distributions in Large Quantum Dots: A Density Functional Theory Study
We use spin-density-functional theory to study the spacing between
conductance peaks and the ground-state spin of 2D model quantum dots with up to
200 electrons. Distributions for different ranges of electron number are
obtained in both symmetric and asymmetric potentials. The even/odd effect is
pronounced for small symmetric dots but vanishes for large asymmetric ones,
suggesting substantially stronger interaction effects than expected. The
fraction of high-spin ground states is remarkably large.Comment: 4 pages, 3 figure
Enhanced allergic sensitisation related to parental smoking
The objective of this study was to assess the role of parental smoking in changes, after a four year interval (1983-7), in the prevalence and severity of the atopic state in 166 pre-adolescent children. Allergy skin prick tests were related to parental smoking habits and their changes during this same interval. The total number of cigarettes smoked by parents decreased in 56 families while it increased in only 16. Boys had significantly more persistently positive skin tests and changed more frequently from negative to positive. The skin test index did not show significant changes in girls. This index did not change in children of persistent non-smokers or those starting to smoke during this period, while it increased among sons of those that quit smoking and of persistent smokers. This was not only due to those boys who became skin test positive during follow up. When analysis was restricted to 14 boys who had been skin test positive in 1983 and whose parents were persistent smokers, the index increased in eight, remained unchanged in four, and decreased in only two. This report supports the hypothesis that parental smoking is a factor that, together with specific allergenic exposure, may enhance allergic sensitisation in children
Active crustal extension and strain accumulation from GPS data in the Molise region (central-southern Apennines, Italy)
In this paper, we report new GPS measurements which indicate active NE-SW
extension and strain accumulation in the Molise region (Apennines, Italy). The GPS
observations were collected during campaigns on benchmarks of the dense IGM95
network (average distance 20 km), spanning a maximum observation interval of 13
years, and have been integrated with measurements from the available permanent GPS
sites. Considering the differential motion of the GPS sites, located on the Tyrrhenian
and Adriatic coasts, we can evaluate a 4-5 mm/yr extension accommodated across this
part of the Apennines. The velocity field exhibits clusters of sites with homogeneous
velocity vectors, outlining two main divergence areas, both characterized by the
largest velocity gradients: one near Venafro and the other near Isernia where two
primary active faults and several historical earthquakes have been documented. These
results suggest that an active extension in this part of the Apennines can be currently
distributed between the two faults systems associated with the largest earthquakes of
this region
Coseismic and post-seismic slip of the 2009 L'Aquila (central Italy) MW 6.3 earthquake and implications for seismic potential along the Campotosto fault from joint inversion of high-precision levelling, InSAR and GPS data
After the April 6th 2009 MW 6.3 (ML 5.9) L'Aquila earthquake (central Italy), we re-measured more than 100 km
of high-precision levelling lines in the epicentral area. The joint inversion of the levelling measurements with
InSAR and GPS measurements, allowed us to derive new coseismic and post-seismic slip distributions and to de-
scribe, with high resolution details on surface displacements, the activation and the slip distribution of a second-
ary fault during the aftershock sequence that struck the Campotosto area (major event MW 5.2). Coseismic slip on
the Paganica fault occurred on one main asperity, while the afterslip distribution shows a more complex pattern,
occurring on three main patches, including both slips on the shallow portions and on the deeper parts of the rup-
ture plane. The comparison between coseismic and post-seismic slip distributions strongly suggests that afterslip
was triggered at the edges of the coseismic asperity. The activation of a segment of the Campotosto fault during
the aftershock sequence, with a good correlation between the estimated slipping area, moment release and
distribution of aftershocks, raises the opportunity to discuss the local seismic hazard following the occurrence
of the 2009 L'Aquila mainshock. The Campotosto fault appears capable of generating earthquakes as large as his-
torical events in the region (M N 6.5) or as small as the ones associated with the 2009 sequence. In the case that
the Campotosto fault is accumulating a significant portion of the current interseismic deformation, the 2009 MW
N 5 events will have released only a small amount of the accumulated elastic strain, and then a significant hazard
still remains in the area. Continuing geodetic monitoring and a densification of the GPS networks in the region are
therefore needed to estimate the tectonic loading across the different recognized active fault systems in this part
of the Apennines.Published168-1852T. Tettonica attivaJCR Journalrestricte
L\'{e}vy scaling: the Diffusion Entropy Analysis applied to DNA sequences
We address the problem of the statistical analysis of a time series generated
by complex dynamics with a new method: the Diffusion Entropy Analysis (DEA)
(Fractals, {\bf 9}, 193 (2001)). This method is based on the evaluation of the
Shannon entropy of the diffusion process generated by the time series imagined
as a physical source of fluctuations, rather than on the measurement of the
variance of this diffusion process, as done with the traditional methods. We
compare the DEA to the traditional methods of scaling detection and we prove
that the DEA is the only method that always yields the correct scaling value,
if the scaling condition applies. Furthermore, DEA detects the real scaling of
a time series without requiring any form of de-trending. We show that the joint
use of DEA and variance method allows to assess whether a time series is
characterized by L\'{e}vy or Gauss statistics. We apply the DEA to the study of
DNA sequences, and we prove that their large-time scales are characterized by
L\'{e}vy statistics, regardless of whether they are coding or non-coding
sequences. We show that the DEA is a reliable technique and, at the same time,
we use it to confirm the validity of the dynamic approach to the DNA sequences,
proposed in earlier work.Comment: 24 pages, 9 figure
Dynamics of the Hubbard model: a general approach by time dependent variational principle
We describe the quantum dynamics of the Hubbard model at semi-classical
level, by implementing the Time-Dependent Variational Principle (TDVP)
procedure on appropriate macroscopic wavefunctions constructed in terms of
su(2)-coherent states. Within the TDVP procedure, such states turn out to
include a time-dependent quantum phase, part of which can be recognized as
Berry's phase. We derive two new semi-classical model Hamiltonians for
describing the dynamics in the paramagnetic, superconducting, antiferromagnetic
and charge density wave phases and solve the corresponding canonical equations
of motion in various cases. Noticeably, a vortex-like ground state phase
dynamics is found to take place for U>0 away from half filling. Moreover, it
appears that an oscillatory-like ground state dynamics survives at the Fermi
surface at half-filling for any U. The low-energy dynamics is also exactly
solved by separating fast and slow variables. The role of the time-dependent
phase is shown to be particularly interesting in the ordered phases.Comment: ReVTeX file, 38 pages, to appear on Phys. Rev.
The inhibition of the highly expressed miR-221 and miR-222 impairs the growth of prostate carcinoma xenografts in mice
MiR-221 and miR-222 are two highly homologous microRNAs whose upregulation has been recently described in several types of human tumors, for some of which their oncogenic role was explained by the discovery of their target p27, a key cell cycle regulator. We previously showed this regulatory relationship in prostate carcinoma cell lines in vitro, underlying the role of miR-221/222 as inducers of proliferation and tumorigenicity
Resonance-assisted tunneling in near-integrable systems
Dynamical tunneling between symmetry related invariant tori is studied in the
near-integrable regime. Using the kicked Harper model as an illustration, we
show that the exponential decay of the wave functions in the classically
forbidden region is modified due to coupling processes that are mediated by
classical resonances. This mechanism leads to a substantial deviation of the
splitting between quasi-degenerate eigenvalues from the purely exponential
decrease with 1 / hbar obtained for the integrable system. A simple
semiclassical framework, which takes into account the effect of the resonance
substructure on the KAM tori, allows to quantitatively reproduce the behavior
of the eigenvalue splittings.Comment: 4 pages, 2 figures, gzipped tar file, to appear in Phys. Rev. Lett,
text slightly condensed compared to first versio
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