3,212 research outputs found
Thermal broadening of the J-band in disordered linear molecular aggregates: A theoretical study
We theoretically study the temperature dependence of the J-band width in
disordered linear molecular aggregates, caused by dephasing of the exciton
states due to scattering on vibrations of the host matrix. In particular, we
consider inelastic one- and two-phonon scattering between different exciton
states (energy-relaxation-induced dephasing), as well as elastic two-phonon
scattering of the excitons (pure dephasing). The exciton states follow from
numerical diagonalization of a Frenkel Hamiltonian with diagonal disorder; the
scattering rates between them are obtained using the Fermi Golden Rule. A
Debye-like model for the one- and two-phonon spectral densities is used in the
calculations. We find that, owing to the disorder, the dephasing rates of the
individual exciton states are distributed over a wide range of values. We also
demonstrate that the dominant channel of two-phonon scattering is not the
elastic one, as is often tacitly assumed, but rather comes from a similar
two-phonon inelastic scattering process. In order to study the temperature
dependence of the J-band width, we simulate the absorption spectrum, accounting
for the dephasing induced broadening of the exciton states. We find a power-law
(T^p) temperature scaling of the effective homogeneous width, with an exponent
p that depends on the shape of the spectral density of host vibrations. In
particular, for a Debye model of vibrations, we find p ~ 4, which is in good
agreement with experimental data on J-aggregates of pseudoisocyanine [J. Phys.
Chem. A 101, 7977 (1997)].Comment: 14 pages, 7 figure
Explicit asymptotic velocity of the boundary between particles and antiparticles
On the real line initially there are infinite number of particles on the
positive half-line., each having one of negative velocities
. Similarly, there are infinite number of
antiparticles on the negative half-line, each having one of positive
velocities . Each particle moves with constant
speed, initially prescribed to it. When particle and antiparticle collide, they
both disappear. It is the only interaction in the system. We find explicitly
the large time asymptotics of - the coordinate of the last collision
before between particle and antiparticle.Comment: 25 page
Photon recoil momentum in a Bose-Einstein condensate of a dilute gas
We develop a "minimal" microscopic model to describe a
two-pulse-Ramsay-interferometer-based scheme of measurement of the photon
recoil momentum in a Bose-Einstein condensate of a dilute gas [Campbell et al.,
Phys. Rev. Lett. 94, 170403 (2005)]. We exploit the truncated coupled
Maxwell-Schroedinger equations to elaborate the problem. Our approach provides
a theoretical tool to reproduce essential features of the experimental results.
Additionally, we enable to calculate the quantum-mechanical mean value of the
recoil momentum and its statistical distribution that provides a detailed
information about the recoil event.Comment: 6 pages, 4 figure
Leptonic origin of the 100 MeV gamma-ray emission from the Galactic Centre
The Galactic centre is a bright gamma-ray source with the GeV-TeV band
spectrum composed of two distinct components in the 1-10 GeV and 1-10 TeV
energy ranges. The nature of these two components is not clearly understood. We
investigate the gamma-ray properties of the Galactic centre to clarify the
origin of the observed emission. We report imaging, spectral, and timing
analysis of data from 74 months of observations of the Galactic centre by
FERMI/LAT gamma-ray telescope complemented by sub-MeV data from approximately
ten years of INTEGRAL/PICsIT observations. We find that the Galactic centre is
spatially consistent with the point source in the GeV band. The tightest 3
sigma upper limit on its radius is 0.13 degree in the 10-300 GeV energy band.
The spectrum of the source in the 100 MeV energy range does not have a
characteristic turnover that would point to the pion decay origin of the
signal. Instead, the source spectrum is consistent with a model of inverse
Compton scattering by high-energy electrons. In this a model, the GeV bump in
the spectrum originates from an episode of injection of high-energy particles,
which happened ~300 years ago. This injection episode coincides with the known
activity episode of the Galactic centre region, previously identified using
X-ray observations. The hadronic model of source activity could be still
compatible with the data if bremsstrahlung emission from high-energy electrons
was present in addition to pion decay emission.Comment: To match the accepted versio
DNA double helices for single molecule electronics
The combination of self-assembly and electronic properties as well as its
true nanoscale dimensions make DNA a promising candidate for a building block
of single molecule electronics. We argue that the intrinsic double helix
conformation of the DNA strands provides a possibility to drive the electric
current through the DNA by the perpendicular electric (gating) field. The
transistor effect in the poly(G)-poly(C) synthetic DNA is demonstrated within a
simple model approach. We put forward experimental setups to observe the
predicted effect and discuss possible device applications of DNA. In
particular, we propose a design of the single molecule analog of the Esaki
diode.Comment: 4 pages, 4 figur
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