680 research outputs found
Vibration Induced Non-adiabatic Geometric Phase and Energy Uncertainty of Fermions in Graphene
We investigate geometric phase of fermion states under relative vibrations of
two sublattices in graphene by solving time-dependent Sch\"{o}dinger equation
using Floquet scheme. In a period of vibration the fermions acquire different
geometric phases depending on their momenta. There are two regions in the
momentum space: the adiabatic region where the geometric phase can be
approximated by the Berry phase and the chaotic region where the geometric
phase drastically fluctuates in changing parameters. The energy of fermions due
to vibrations shows spikes in the chaotic region. The results suggest a
possible dephasing mechanism which may cause classical-like transport
properties in graphene.Comment: 9 pages, 5 figure
Rigorous derivation of coherent resonant tunneling time and velocity in finite periodic systems
The velocity of resonant tunneling electrons in finite periodic
structures is analytically calculated in two ways. The first method is based on
the fact that a transmission of unity leads to a coincidence of all still
competing tunneling time definitions. Thus, having an indisputable resonant
tunneling time we apply the natural definition
to calculate the velocity. For the second method we
combine Bloch's theorem with the transfer matrix approach to decompose the wave
function into two Bloch waves. Then the expectation value of the velocity is
calculated. Both different approaches lead to the same result, showing their
physical equivalence. The obtained resonant tunneling velocity is
smaller or equal to the group velocity times the magnitude of the complex
transmission amplitude of the unit cell. Only at energies where the unit cell
of the periodic structure has a transmission of unity equals the
group velocity. Numerical calculations for a GaAs/AlGaAs superlattice are
performed. For typical parameters the resonant velocity is below one third of
the group velocity.Comment: 12 pages, 3 figures, LaTe
Localization of quantum wave packets
We study the semiclassical propagation of squeezed Gau{\ss}ian states. We do
so by considering the propagation theorem introduced by Combescure and Robert
\cite{CR97} approximating the evolution generated by the Weyl-quantization of
symbols . We examine the particular case when the Hessian
evaluated at the corresponding solution of
Hamilton's equations of motion is periodic in time. Under this assumption, we
show that the width of the wave packet can remain small up to the Ehrenfest
time. We also determine conditions for ``classical revivals'' in that case.
More generally, we may define recurrences of the initial width. Some of these
results include the case of unbounded classical motion. In the classically
unstable case we recover an exponential spreading of the wave packet as in
\cite{CR97}
Variability of B and Be stars in the LMC/SMC: binaries and pulsations
To study the variability of the 523 B and Be stars observed in the Magellanic
clouds with the VLT-FLAMES, we cross-matched the stars of our sample with the
photometric database MACHO, which provides for each star an 8 years lightcurve.
We searched for long, medium, and short-term periodicity and found the
eclipsing binaries in our sample. For these stars, combining, spectroscopy and
photometry, we were able to provide information on several systems of stars
(systemic velocities, ratios of masses, etc). We also present the ratios of
B-binaries to B-non binaries in the LMC/SMC in comparison with the MW. Note
that this ratio is also an important issue to understand the mechanism of
star-formation at low metallicity. We also found the first multiperiodic B and
Be stars in the SMC, in particular the first SMC Beta Cep and SPB, while,
according to the models, pulsations were not foreseen in low metallicity
environments, i.e. typically in the SMC. Our results show that the instability
strips are shifted towards higher temperatures in comparison with the Milky
Way' strips of pulsating B-type stars. By the fact that we found more pulsating
Be stars than pulsating B stars in the SMC, it seems that the fast rotation
favours the presence of pulsations. However, the ratio of pulsating B-type
stars to "non"-pulsating B-type stars at low metallicity is lower than at high
metallicity.Comment: poster IAUS25
Light propagation through closed-loop atomic media beyond the multiphoton resonance condition
The light propagation of a probe field pulse in a four-level double-lambda
type system driven by laser fields that form a closed interaction loop is
studied. Due to the finite frequency width of the probe pulse, a
time-independent analysis relying on the multiphoton resonance assumption is
insufficient. Thus we apply a Floquet decomposition of the equations of motion
to solve the time-dependent problem beyond the multiphoton resonance condition.
We find that the various Floquet components can be interpreted in terms of
different scattering processes, and that the medium response oscillating in
phase with the probe field in general is not phase-dependent. The phase
dependence arises from a scattering of the coupling fields into the probe field
mode at a frequency which in general differs from the probe field frequency. We
thus conclude that in particular for short pulses with a large frequency width,
inducing a closed loop interaction contour may not be advantageous, since
otherwise the phase-dependent medium response may lead to a distortion of the
pulse shape. Finally, using our time-dependent analysis, we demonstrate that
both the closed-loop and the non-closed loop configuration allow for sub- and
superluminal light propagation with small absorption or even gain. Further, we
identify one of the coupling field Rabi frequencies as a control parameter that
allows to conveniently switch between sub- and superluminal light propagation.Comment: 10 pages, 8 figure
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