31,221 research outputs found
The host galaxies of long-duration GRBs in a cosmological hierarchical scenario
We developed a Monte Carlo code to generate long-duration gamma ray burst
(LGRB) events within cosmological hydrodynamical simulations consistent with
the concordance model. As structure is assembled, LGRBs are generated in the
substructure that formed galaxies today. We adopted the collapsar model so that
LGRBs are produced by single, massive stars at the final stage of their
evolution. We found that the observed properties of the LGRB host galaxies
(HGs) are reproduced if LGRBs are also required to be generated by low
metallicity stars. The low metallicity condition imposed on the progenitor
stars of LGRBs selects a sample of HGs with mean gas abundances of 12 + log O/H
\~ 8.6. For z<1 the simulated HGs of low metallicity LGRB progenitors tend to
be faint, slow rotators with high star formation efficiency, compared with the
general galaxy population, in agreement with observations. At higher redshift,
our results suggest that larger systems with high star formation activity could
also contribute to the generation of LGRBs from low metallicity progenitors
since the fraction of low metallicity gas available for star formation
increases for all systems with look-back time. Under the hypothesis of our LGRB
model, our results support the claim that LGRBs could be unbiased tracers of
star formation at high redshifts.Comment: Final revised version with minor changes. 9 pages, 9 figures,
mn2e.cls. To appear in MNRA
Coherent phenomena in semiconductors
A review of coherent phenomena in photoexcited semiconductors is presented.
In particular, two classes of phenomena are considered: On the one hand the
role played by optically-induced phase coherence in the ultrafast spectroscopy
of semiconductors; On the other hand the Coulomb-induced effects on the
coherent optical response of low-dimensional structures.
All the phenomena discussed in the paper are analyzed in terms of a
theoretical framework based on the density-matrix formalism. Due to its
generality, this quantum-kinetic approach allows a realistic description of
coherent as well as incoherent, i.e. phase-breaking, processes, thus providing
quantitative information on the coupled ---coherent vs. incoherent--- carrier
dynamics in photoexcited semiconductors.
The primary goal of the paper is to discuss the concept of quantum-mechanical
phase coherence as well as its relevance and implications on semiconductor
physics and technology. In particular, we will discuss the dominant role played
by optically induced phase coherence on the process of carrier photogeneration
and relaxation in bulk systems. We will then review typical field-induced
coherent phenomena in semiconductor superlattices such as Bloch oscillations
and Wannier-Stark localization. Finally, we will discuss the dominant role
played by Coulomb correlation on the linear and non-linear optical spectra of
realistic quantum-wire structures.Comment: Topical review in Semiconductor Science and Technology (in press)
(Some of the figures are not available in electronic form
Metal-free α-trifluoromethylthiolation and α-trifluoromethylselenolation of carbonyl derivatives
The incorporation of a SCF3 or a SeCF3 group into organic molecules is a topic of great interest, especially for the pharmaceutical and agrochemical industries. Due to their high lipophilicity and high electron-withdrawing character (Hansch lipophilicity parameter piR =1.44 (SCF3) vs piR =1.29 (SeCF3)), these moieties represent a powerful opportunity to influence the pharmacokinetics properties of a drug molecule.
In the last years, new structural units, rising from the association between chalcogens and fluorinated moieties, have been introduced into carbonyl compounds, as emerging class with potential applications on several fields. New reagents have been developed as sources of electrophilic SCF3 and SeCF3 groups; however, a widespread use of such fluorinated compounds is hampered by the very limited number of strategies available for their preparation. In this contest, we have developed two methodologies for the preparation of alpha-SCF3 and alpha-SeCF3 substituted carbonyl derivatives starting from non activated ketones or their derivatives
Dissipation and Decoherence in Nanodevices: a Generalized Fermi's Golden Rule
We shall revisit the conventional adiabatic or Markov approximation, which
--contrary to the semiclassical case-- does not preserve the positive-definite
character of the corresponding density matrix, thus leading to highly
non-physical results. To overcome this serious limitation, originally pointed
out and partially solved by Davies and co-workers almost three decades ago, we
shall propose an alternative more general adiabatic procedure, which (i) is
physically justified under the same validity restrictions of the conventional
Markov approach, (ii) in the semiclassical limit reduces to the standard
Fermi's golden rule, and (iii) describes a genuine Lindblad evolution, thus
providing a reliable/robust treatment of energy-dissipation and dephasing
processes in electronic quantum devices. Unlike standard master-equation
formulations, the dependence of our approximation on the specific choice of the
subsystem (that include the common partial trace reduction) does not threaten
positivity, and quantum scattering rates are well defined even in case the
subsystem is infinitely extended/has continuous spectrum.Comment: 6 pages, 0 figure
Dissipation and Decoherence in Nanodevices: a Generalized Fermi's Golden Rule
We shall revisit the conventional adiabatic or Markov approximation, which
--contrary to the semiclassical case-- does not preserve the positive-definite
character of the corresponding density matrix, thus leading to highly
non-physical results. To overcome this serious limitation, originally pointed
out and partially solved by Davies and co-workers almost three decades ago, we
shall propose an alternative more general adiabatic procedure, which (i) is
physically justified under the same validity restrictions of the conventional
Markov approach, (ii) in the semiclassical limit reduces to the standard
Fermi's golden rule, and (iii) describes a genuine Lindblad evolution, thus
providing a reliable/robust treatment of energy-dissipation and dephasing
processes in electronic quantum devices. Unlike standard master-equation
formulations, the dependence of our approximation on the specific choice of the
subsystem (that include the common partial trace reduction) does not threaten
positivity, and quantum scattering rates are well defined even in case the
subsystem is infinitely extended/has continuous spectrum.Comment: 6 pages, 0 figure
On the Geometry of Surface Stress
We present a fully general derivation of the Laplace--Young formula and
discuss the interplay between the intrinsic surface geometry and the extrinsic
one ensuing from the immersion of the surface in the ordinary euclidean
three-dimensional space. We prove that the (reversible) work done in a general
surface deformation can be expressed in terms of the surface stress tensor and
the variation of the intrinsic surface metric
Dissipation and Decoherence in Nanodevices: a Generalized Fermi's Golden Rule
We shall revisit the conventional adiabatic or Markov approximation, which
--contrary to the semiclassical case-- does not preserve the positive-definite
character of the corresponding density matrix, thus leading to highly
non-physical results. To overcome this serious limitation, originally pointed
out and partially solved by Davies and co-workers almost three decades ago, we
shall propose an alternative more general adiabatic procedure, which (i) is
physically justified under the same validity restrictions of the conventional
Markov approach, (ii) in the semiclassical limit reduces to the standard
Fermi's golden rule, and (iii) describes a genuine Lindblad evolution, thus
providing a reliable/robust treatment of energy-dissipation and dephasing
processes in electronic quantum devices. Unlike standard master-equation
formulations, the dependence of our approximation on the specific choice of the
subsystem (that include the common partial trace reduction) does not threaten
positivity, and quantum scattering rates are well defined even in case the
subsystem is infinitely extended/has continuous spectrum.Comment: 6 pages, 0 figure
The Long Term Optical Variability of the BL Lac object S5 0716+714: Evidence for a Precessing Jet
We present the historic light curve of the BL Lac object S5 0716+714,
spanning the time interval from 1953 to 2003, built using Asiago archive plates
and our recent CCD observations, together with literature data. The source
shows an evident long term variability, over which well known short term
variations are superposed. In particular, in the period from 1961 to 1983 the
mean brightness of S5 0716+714 remained significantly fainter than that
observed after 1994. Assuming a constant variation rate of the mean magnitude
we can estimate a value of about 0.11 magnitude/year. The simultaneous
occurrence of decreasing ejection velocities of superluminal moving components
in the jet reported by Bach et al. (2005) suggests that both phenomena are
related to the change of the direction of the jet to the line of sight from
about 5 to 0.7 degrees for an approximately constant bulk Lorentz factor of
about 12. A simple explanation is that of a precessing relativistic jet, which
should presently be close to the smallest orientation angle. One can therefore
expect in the next ten years a decrease of the mean brightness of about 1
magnitude.Comment: to appear on The Astronomical Journal, 17 pages, 7 figures. Fig.2 is
given as a separated jpg fil
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