5,146 research outputs found
Superlattice Magnetophonon Resonances in Strongly Coupled InAs/GaSb Superlattices
We report an experimental study of miniband magnetoconduction in
semiconducting InAs/GaSb superlattices. For samples with miniband widths below
the longitudinal optical phonon energy we identify a new superlattice
magnetophonon resonance (SLMPR) caused by resonant scattering of electrons
across the mini-Brillouin zone. This new resonant feature arises directly from
the drift velocity characteristics of the superlattice dispersion and total
magnetic quantisation of the superlattice Landau level minibands.Comment: 9 pages, 8 figures, submitted to Phys. Rev.
Galaxy formation in the Planck cosmology - II. Star-formation histories and post-processing magnitude reconstruction
We adapt the L-Galaxies semi-analytic model to follow the star-formation
histories (SFH) of galaxies -- by which we mean a record of the formation time
and metallicities of the stars that are present in each galaxy at a given time.
We use these to construct stellar spectra in post-processing, which offers
large efficiency savings and allows user-defined spectral bands and dust models
to be applied to data stored in the Millennium data repository.
We contrast model SFHs from the Millennium Simulation with observed ones from
the VESPA algorithm as applied to the SDSS-7 catalogue. The overall agreement
is good, with both simulated and SDSS galaxies showing a steeper SFH with
increased stellar mass. The SFHs of blue and red galaxies, however, show poor
agreement between data and simulations, which may indicate that the termination
of star formation is too abrupt in the models.
The mean star-formation rate (SFR) of model galaxies is well-defined and is
accurately modelled by a double power law at all redshifts: SFR proportional to
, where Gyr, is the age of the
stars and is the loopback time to the onset of galaxy formation; above a
redshift of unity, this is well approximated by a gamma function: SFR
proportional to , where Gyr. Individual
galaxies, however, show a wide dispersion about this mean. When split by mass,
the SFR peaks earlier for high-mass galaxies than for lower-mass ones, and we
interpret this downsizing as a mass-dependence in the evolution of the quenched
fraction: the SFHs of star-forming galaxies show only a weak mass dependence.Comment: Accepted version of the paper, to appear in MNRAS. Compared to the
original version, contains more detail on the post-processing of magnitudes,
including a table of rms magnitude errors. SFHs available on Millennium
database http://gavo.mpa-garching.mpg.de/MyMillennium
Measurement of miniband parameters of a doped superlattice by photoluminescence in high magnetic fields
We have studied a 50/50\AA superlattice of GaAs/AlGaAs
composition, modulation-doped with Si, to produce
cm electrons per superlattice period. The modulation-doping was tailored
to avoid the formation of Tamm states, and photoluminescence due to interband
transitions from extended superlattice states was detected. By studying the
effects of a quantizing magnetic field on the superlattice photoluminescence,
the miniband energy width, the reduced effective mass of the electron-hole
pair, and the band gap renormalization could be deduced.Comment: minor typing errors (minus sign in eq. (5)
Sharp lines in the absorption edge of EuTe and PbEuTe in high magnetic fields
The optical absorption spectra in the region of the \fd transition energies
of epitaxial layers of of EuTe and \PbEuTe, grown by molecular beam epitaxy,
were studied using circularly polarized light, in the Faraday configuration.
Under \sigmam polarization a sharp symmetric absorption line (full width at
half-maximum 0.041 eV) emerges at the low energy side of the band-edge
absorption, for magnetic fields intensities greater than 6 T. The absorption
line shows a huge red shift (35 meV/T) with increasing magnetic fields. The
peak position of the absorption line as a function of magnetic field is
dominated by the {\em d-f} exchange interaction of the excited electron and the
\Euion spins in the lattice. The {\em d-f} exchange interaction energy was
estimated to be eV. In \PbEuTe the same absorption line
is detected, but it is broader, due to alloy disorder, indicating that the
excitation is localized within a finite radius. From a comparison of the
absorption spectra in EuTe and \PbEuTe the characteristic radius of the
excitation is estimated to be \AA.Comment: Journal of Physics: Condensed Matter (2004, at press
Análise do efeito de nanopartículas de prata contra células aderidas e biofilmes de Candida albicans e Candida glabrata
O aumento na resistência dos biofilmes de Candida à terapia antifúngica convencional
tem despertado o interesse no uso da prata como um agente antimicrobiano. Assim, o objetivo
deste trabalho foi avaliar a eficácia antifúngica de nanopartículas de prata (NPs) contra células
aderidas e biofilmes de Candida albicans e Candida glabrata. Métodos: NPs esféricas (5 nm)
foram sintetizadas através da redução do nitrato de prata pelo citrato de sódio. Testes de mínima
concentração inibitória (MCI) foram realizados para as duas espécies de Candida de acordo com o
método da microdiluição. NPs foram aplicadas sobre células aderidas (2 hrs) e biofilmes (48 hrs),
e após 24 horas de contato os biofilmes resultantes foram caracterizados através da contagem do
número de unidades formadoras de colônias (UFCs) e quantificação da biomassa total. Resultados:
Os valores de MCI para C. glabrata foram maiores (0,4 – 3,3 µg/mL) do que para C. albicans
(0,4 – 1,6 µg/mL). NPs foram mais efetivas na redução da biomassa total quando aplicadas sobre
células aderidas do que sobre biofilmes pré-formados. NPs também foram altamente efetivas na
redução das UFCs quando aplicadas sobre as células aderidas de C. glabrata (~70%) e respectivos
biofilmes (~50%). Para as cepas de C. albicans o efeito não foi tão notório, mas também existiu
uma redução no número de UFCs. Conclusão: NPs apresentam potencial como agente antifúngico
alternativo no controle de infecções por espécies de Candida
-Algebras, the BV Formalism, and Classical Fields
We summarise some of our recent works on -algebras and quasi-groups
with regard to higher principal bundles and their applications in twistor
theory and gauge theory. In particular, after a lightning review of
-algebras, we discuss their Maurer-Cartan theory and explain that any
classical field theory admitting an action can be reformulated in this context
with the help of the Batalin-Vilkovisky formalism. As examples, we explore
higher Chern-Simons theory and Yang-Mills theory. We also explain how these
ideas can be combined with those of twistor theory to formulate maximally
superconformal gauge theories in four and six dimensions by means of
-quasi-isomorphisms, and we propose a twistor space action.Comment: 19 pages, Contribution to Proceedings of LMS/EPSRC Durham Symposium
Higher Structures in M-Theory, August 201
An Exact Calculation of the Energy Density of Cosmological Gravitational Waves
In this paper we calculate the Bogoliubov coefficients and the energy density
of the stochastic gravitational wave background for a universe that undergoes
inflation followed by radiation domination and matter domination, using a
formalism that gives the Bogoliubov coefficients as continous functions of
time. By making a reasonable assumption for the equation of state during
reheating, we obtain in a natural way the expected high frequency cutoff in the
spectral energy density.Comment: 12 pages+5 figures, uuencoded file,DF/IST-2.9
Post-Inflationary Reheating
We study a model for reheating that has been much investigated for parametric
resonance, having a quartic interaction of the scalar inflaton with another
scalar field. Attention is particularly on the quantum excitations of the
inflaton field and the metric perturbation with a smooth transition from
quantum to classical stochastic states, followed through from a specific
inflation model to a state including a relativistic fluid. The scalar fields
enter non-perturbatively but the metric enters perturbatively, and the validity
of this latter is assessed. In this model our work seems to point the large
scale curvature parameter changing.Comment: 25 pages, 6 figures. Coding error(misprint) corrected:figures and
some conclusions change
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