1,618 research outputs found
Electric field and exciton structure in CdSe nanocrystals
Quantum Stark effect in semiconductor nanocrystals is theoretically
investigated, using the effective mass formalism within a
Baldereschi-Lipari Hamiltonian model for the hole states. General expressions
are reported for the hole eigenfunctions at zero electric field. Electron and
hole single particle energies as functions of the electric field
() are reported. Stark shift and binding energy of the
excitonic levels are obtained by full diagonalization of the correlated
electron-hole Hamiltonian in presence of the external field. Particularly, the
structure of the lower excitonic states and their symmetry properties in CdSe
nanocrystals are studied. It is found that the dependence of the exciton
binding energy upon the applied field is strongly reduced for small quantum dot
radius. Optical selection rules for absorption and luminescence are obtained.
The electric-field induced quenching of the optical spectra as a function of
is studied in terms of the exciton dipole matrix element. It
is predicted that photoluminescence spectra present anomalous field dependence
of the emission lines. These results agree in magnitude with experimental
observation and with the main features of photoluminescence experiments in
nanostructures.Comment: 9 pages, 7 figures, 1 tabl
Obtención de gas de síntesis por tratamiento térmico en microondas de biomasa y biogás
En este trabajo se estudia la pirólisis, tanto en microondas como en horno eléctrico, de un residuo procedente de la industria del café, con objeto de comparar la influencia del calentamiento en las características de las fracciones resultantes. La alta proporción de gas de síntesis (H2 + CO) obtenida con el microondas (hasta del 73%) es debida al hecho que la auto-gasificación del carbonizado con el CO2 liberado y que la descomposición catalítica de CH4 se ven favorecidas por este tipo de calentamiento.Así, los experimentos de estas dos últimas reacciones llevadas a cabo de manera individual, con ambos sistemas de calentamiento, demuestran que se dan mayores conversiones en el microondas. En el caso de la descomposición de CH4, la conversión, que inicialmente alcanza valores próximos al 100%, experimentaun descenso importante al cabo de pocos minutos como consecuencia de la formación de depósitos de carbono que bloquean los centros activos de la superficie del catalizador. Sin embargo, una combinación de ambas reacciones, i.e. reformado de CH4 con CO2, mejora el proceso de conversión de CH4 a H2, debido a que el CO2 gasifica los depósitos de carbono generados, permitiendo una regeneración “in situ” del catalizador. Estos resultados han servido como punto de partida para el desarrollo de un nuevo procedimiento que permitiría convertir biogás en gas de síntesis
Cuantificación mediante estudio estereológico del sistema microfisural del granito de El Berrocal (Sistema Central, Toledo, España)
This paper describes an exarnple of the use of stereological techniques for the quantification of some characteristics of the microcrack network of rocks, in this case the El Berrocal granite (Sistema Central, Toledo, Spain). From data measured in sections, therefore 2-D, 3-D characteristics of the microcrack network have been obtained. The quantified characteristics are the microcrack density (in terms of the frequency distribution of the specific surface of microcracks in relation to their apertures) and the relative frequency of the microcrack aperture. These procedures have been applied to images under scanning electron microscopy, so microcracks with apparent apertures over 0,l pm. have been measured. The so called "vertical sections" have been prepared to assure a correct sampling. From the measurement of the apparent apertures of microcracks and applying stereological procedures, the real apertures and the specific surface of each class of microcracks are obtained. Besides of the characteristics of the microcrack network of the rock as a whole, data of the microcracks associated to each of the main minerals (quartz, feldspars and micas) are presented separately, allowing the interpretation of the contribution of each mineral to the microfractography of the rock. In El Berrocal granite, the microcracks with a real aperture less than 1 m are the most abundant. The microcracks are more important in quartz and, above all, in feldspars (with more than 60% of the total network of the rock), than in micas. When considering the microcrack density of each mineral, and taking into account the mineral composition of the rock too, the feldspar is also the more cracked mineral in the rock. Nevertheless, the frequency distribution of the specific surface in relation to the microcrack aperture is quite similar in any of the three mirierals. It is intended with this paper to show the possibilities of the stereology that, using simple, fast and economic procedures, provides quantitative information about the characteristics of the microcrack network of rocks that can be very useful in many geological studies
Strain-free Ge/GeSiSn quantum cascade lasers based on L-valley intersubband transitions
The authors propose a Ge/Ge0.76Si0.19Sn0.05 quantum cascade laser using intersubband transitions at L valleys of the conduction band which has a “clean” offset of150 meV situated below other energy valleys (Γ,X). The entire structure is strain-free because the lattice-matched Ge and Ge0.76Si0.19Sn0.05 layers are to be grown on a relaxed Ge buffer layer on a Si substrate. Longer lifetimes due to the weaker scattering of nonpolar optical phonons reduce the threshold current and potentially lead to room temperature operation
Star-forming dwarf galaxies in the Virgo cluster: the link between molecular gas, atomic gas, and dust
We present CO(1-0) and CO(2-1) observations of a sample of 20
star-forming dwarfs selected from the Herschel Virgo Cluster Survey, with
oxygen abundances ranging from 12 + log(O/H) ~ 8.1 to 8.8. CO emission is
observed in ten galaxies and marginally detected in another one. CO fluxes
correlate with the FIR 250 m emission, and the dwarfs follow the same
linear relation that holds for more massive spiral galaxies extended to a wider
dynamical range. We compare different methods to estimate H2 molecular masses,
namely a metallicity-dependent CO-to-H2 conversion factor and one dependent on
H-band luminosity. The molecular-to-stellar mass ratio remains nearly constant
at stellar masses <~ 10 M, contrary to the atomic hydrogen
fraction, M/M, which increases inversely with M. The flattening
of the M/M ratio at low stellar masses does not seem to be related
to the effects of the cluster environment because it occurs for both
HI-deficient and HI-normal dwarfs. The molecular-to-atomic ratio is more
tightly correlated with stellar surface density than metallicity, confirming
that the interstellar gas pressure plays a key role in determining the balance
between the two gaseous components of the interstellar medium. Virgo dwarfs
follow the same linear trend between molecular gas mass and star formation rate
as more massive spirals, but gas depletion timescales, , are not
constant and range between 100 Myr and 6 Gyr. The interaction with the Virgo
cluster environment is removing the atomic gas and dust components of the
dwarfs, but the molecular gas appears to be less affected at the current stage
of evolution within the cluster. However, the correlation between HI deficiency
and the molecular gas depletion time suggests that the lack of gas
replenishment from the outer regions of the disc is lowering the star formation
activity.Comment: 19 pages, 11 figures, accepted for publication in Astronomy &
Astrophysic
The internationalisation of the Spanish SME sector
As part of a wider research program, we analysed the theoretical framework and the recent developments of the process of internationalisation (transnationalisation) of the small- and medium-sized enterprises in Spain. The paper highlights the main trends and barriers of this internationalisation process. Methodology included document analyses, interviews, and the analyses of statistical databases
Far-infrared vibrational properties of high-pressure-high-temperature C60 polymers and the C60 dimer
We report high-resolution far-infrared transmission measurements of the 2 + 2 cycloaddition C-60 dimer and two-dimensional rhombohedral and one-dimensional orthorhombic high-pressure high-temperature C60 polymers. In the spectral region investigated(20-650 cm(-1)), we see no low-energy interball modes, but symmetry breaking of the linked C-60 balls is evident in the complex spectrum of intramolecular modes. Experimental features suggest large splittings or frequency shifts of some IhC60-derived modes that are activated by symmetry reduction, implying that the balls are strongly distorted in these structures. We have calculated the vibrations of all three systems by first-principles quantum molecular dynamics and use them to assign the predominant IhC60 symmetries of observed modes. Pur calculations show unprecedentedly large downshifts of T-1u(2)-derived modes and extremely large splittings of other modes, both of which are consistent with the experimental spectra. For the rhombohedral and orthorhombic polymers, the T-1u(2)-derived mode that is polarized along the bonding direction is calculated to downshift below any T-1u(1)-derived modes. We also identify a previously unassigned feature near 610 cm(-1) in all three systems as a widely split or shifted mode derived from various silent IhC60 vibrations, confirming a strong perturbation model for these linked fullerene structures
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