601 research outputs found
Enhanced thermionic-dominated photoresponse in graphene Schottky junctions
Vertical heterostructures of van der Waals materials enable new pathways to
tune charge and energy transport characteristics in nanoscale systems. We
propose that graphene Schottky junctions can host a special kind of
photoresponse which is characterized by strongly coupled heat and charge flows
that run vertically out of the graphene plane. This regime can be accessed when
vertical energy transport mediated by thermionic emission of hot carriers
overwhelms electron-lattice cooling as well as lateral diffusive energy
transport. As such, the power pumped into the system is efficiently extracted
across the entire graphene active area via thermionic emission of hot carriers
into a semiconductor material. Experimental signatures of this regime include a
large and tunable internal responsivity with a non-monotonic
temperature dependence. In particular, peaks at electronic
temperatures on the order of the Schottky potential and has a large
upper limit ( when ). Our proposal opens up new approaches for engineering the
photoresponse in optically-active graphene heterostructures.Comment: 6 pages, 2 figure
Normal state magnetotransport properties of -FeSe superconductors
We present -FeSe magnetotransport data, and describe them
theoretically. Using a simplified microscopic model with two correlated
effective orbitals, we determined the normal state electrical conductivity and
Hall coefficient, using Kubo formalism. With model parameters relevant for
Fe-chalcogenides, we describe the observed effect of the structural transition
on the ab-plane electrical resistivity, as well as on the magnetoresistance.
Temperature-dependent Hall coefficient data were measured at 16 Tesla, and
their theoretical description improves upon inclusion of moderate electron
correlations. We confirm the effect of the structural transition on the
electronic structure, finding deformation-induced band splittings comparable to
those reported in angle-resolved photoemission.Comment: 6 pages, 5 figure
Dynamical mass of the O-type supergiant in Zeta Orionis A
A close companion of Zeta Orionis A was found in 2000 with the Navy Precision
Optical Interferometer (NPOI), and shown to be a physical companion. Because
the primary is a supergiant of type O, for which dynamical mass measurements
are very rare, the companion was observed with NPOI over the full 7-year orbit.
Our aim was to determine the dynamical mass of a supergiant that, due to the
physical separation of more than 10 AU between the components, cannot have
undergone mass exchange with the companion. The interferometric observations
allow measuring the relative positions of the binary components and their
relative brightness. The data collected over the full orbital period allows all
seven orbital elements to be determined. In addition to the interferometric
observations, we analyzed archival spectra obtained at the Calar Alto, Haute
Provence, Cerro Armazones, and La Silla observatories, as well as new spectra
obtained at the VLT on Cerro Paranal. In the high-resolution spectra we
identified a few lines that can be associated exclusively to one or the other
component for the measurement of the radial velocities of both. The combination
of astrometry and spectroscopy then yields the stellar masses and the distance
to the binary star. The resulting masses for components Aa of 14.0 solar masses
and Ab of 7.4 solar masses are low compared to theoretical expectations, with a
distance of 294 pc which is smaller than a photometric distance estimate of 387
pc based on the spectral type B0III of the B component. If the latter (because
it is also consistent with the distance to the Orion OB1 association) is
adopted, the mass of the secondary component Ab of 14 solar masses would agree
with classifying a star of type B0.5IV. It is fainter than the primary by about
2.2 magnitudes in the visual. The primary mass is then determined to be 33
solar masses
C II abundances in early-type stars: solution to a notorious non-LTE problem
We address a long-standing discrepancy between non-LTE analyses of the
prominent C II 4267 and 6578/82 A multiplets in early-type stars. A
comprehensive non-LTE model atom of C II is constructed based on critically
selected atomic data. This model atom is used for an abundance study of six
apparently slow-rotating main-sequence and giant early B-type stars.
High-resolution and high-S/N spectra allow us to derive highly consistent
abundances not only from the classical features but also from up to 18 further
C II lines in the visual - including two so far unreported emission features
equally well reproduced in non-LTE. These results require the stellar
atmospheric parameters to be determined with care. A homogeneous (slightly)
sub-solar present-day carbon abundance from young stars in the solar vicinity
(in associations and in the field) of log C/H +12= 8.29+/-0.03 is indicated.Comment: 8 pages, 5 figure
An asteroseismic study of the O9V star HD 46202 from CoRoT space-based photometry
The O9V star HD 46202, which is a member of the young open cluster NGC 2244,
was observed by the CoRoT satellite in October/November 2008 during a short run
of 34 days. From the very high-precision light curve, we clearly detect beta
Cep-like pulsation frequencies with amplitudes of ~0.1 mmag and below. A
comparison with stellar models was performed using a chi^2 as a measure for the
goodness-of-fit between the observed and theoretically computed frequencies.
The physical parameters of our best-fitting models are compatible with the ones
deduced spectroscopically. A core overshooting parameter alpha_ov = 0.10 +-
0.05 pressure scale height is required. None of the observed frequencies are
theoretically excited with the input physics used in our study. More
theoretical work is thus needed to overcome this shortcoming in how we
understand the excitation mechanism of pulsation modes in such a massive star.
A similar excitation problem has also been encountered for certain pulsation
modes in beta Cep stars recently modelled asteroseismically.Comment: Accepted for publication in Astronomy and Astrophysics on 17/12/2010,
9 pages, 7 figures, 4 table
Anomalous Proximity Effect in Underdoped YBaCuO Josephson Junctions
Josephson junctions were photogenerated in underdoped thin films of the
YBaCuO family using a near-field scanning optical microscope.
The observation of the Josephson effect for separations as large as 100 nm
between two wires indicates the existence of an anomalously large proximity
effect and show that the underdoped insulating material in the gap of the
junction is readily perturbed into the superconducting state. The critical
current of the junctions was found to be consistent with the conventional
Josephson relationship. This result constrains the applicability of SO(5)
theory to explain the phase diagram of high critical temperature
superconductors.Comment: 11 pages, 4 figure
Modelo anual de la dinámica sedimentaria en una marisma mareal mediterránea.
Se ha detectado un patrón estacional en la dinámica sedimentaria en zonas de marisma baja, relacionado
con la distribución anual de las precipitaciones caracterÃstica del clima mediterráneo, que induce cambios en el
caudal de los rÃos y en la cantidad y distribución de los aportes de partÃculas finas que entran en el estuario. En
este escenario se pone de manifiesto cómo la presencia de vegetación (Spartina maritima) favorece la acreción y
estabilización de sedimentos. En planicies desprovistas de vegetación, los mayores niveles de acreción se registran
durante primavera y principios de verano. Las lluvias otoñales determinan un aumento del caudal de los rÃos,
con un incremento en el aporte de sedimentos y removilización de éstos en las zonas desnudas. Parte de estos sedimentos
quedarán entonces retenidos en las zonas provistas de vegetación. Durante este periodo de otoño es cuando
esta vegetación alcanza sus mayores valores de biomasa, maximizando asà su papel como estabilizador de sedimentos.We have found a seasonal pattern operating on the lower areas of a tidal marsh, in relation to the characteristic
annual distribution of the rainfall in a Mediterranean climate which induces changes in river flows and in
quantity and distribution of loam sediments entering the estuary. Also, the presence of vegetation (S. maritima)
favours the accretion rate and the sediment stabilization. Bare muds record the maximum values of accretion during
Spring and early Summer. Autumn rainfalls increase the river flow what means a new input of sediments and the
movilization of the old ones from the bare muds; the vegetation raises the maximum biomass and it traps and stabilizes
these sediments.Dirección General de Investigación CientÃfica y Técnica. Gobierno de España-PB94-145
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