76 research outputs found
High-Mobility Ambipolar Magnetotransport in Topological Insulator Bi2Se3 Nanoribbons
Nanoribbons of topological insulators (TIs) have been suggested for a variety of applications exploiting the properties of the topologically protected surface Dirac states. In these proposals it is crucial to achieve a high tunability of the Fermi energy, through the Dirac point while preserving a high mobility of the involved carriers. Tunable transport in TI nanoribbons has been achieved by chemical doping of the materials so to reduce the bulk carriers\u27 concentration, however at the expense of the mobility of the surface Dirac electrons, which is substantially reduced. Here we study bare Bi2Se3 nanoribbons transferred on a variety of oxide substrates and demonstrate that the use of a large relative permittivity SrTiO3 substrate enables the Fermi energy to be tuned through the Dirac point and an ambipolar field effect to be obtained. Through magnetotransport and Hall conductance measurements, performed on single Bi2Se3 nanoribbons, we demonstrate that electron and hole carriers are exclusively high-mobility Dirac electrons, without any bulk contribution. The use of SrTiO3 allows therefore an easy field effect gating in TI nanostructures providing an ideal platform to take advantage of the properties of topological surface states
Carbon nanotube film/silicon heterojunction photodetector for new cutting-edge technological devices
Photodetector (PD) devices based on carbon nanotube/n-silicon heterojunction (NSH) have been realized, with a linear response in a large optical power range, proving competitive performances with respect to a recent nanostructure-based detector and those currently available on the market. The core of these devices is a thin semi-transparent and conductive single-walled carbon nanotubes film with a multitask role: junction element, light absorber and transmitter, photocarrier transporting layer, and charge collector. The PD exhibits rise times of some nanoseconds, detecting light from ultraviolet (240 nm) to infrared (1600 nm), and external quantum efficiency reaching 300% in the VIS spectra region
Evolution of faint radio sources in the VIDEO-XMM3 field
© 2013 The Authors Published by Oxford University Press on behalf of the Royal Astronomical SocietyIt has been speculated that low-luminosity radio-loud active galactic nuclei (AGN) have the potential to serve as an important source of AGN feedback, and may be responsible for suppressing star formation activity in massive elliptical galaxies at late times. As such the cosmic evolution of these sources is vitally important to understand the significance of such AGN feedback processes and their influence on the global star formation history of the Universe. In this paper, we present a new investigation of the evolution of faint radio sources out to z ~ 2.5. We combine a 1 square degree Very Large Array radio survey, complete to a depth of 100 μJy, with accurate 10 band photometric redshifts from the following surveys: Visible and Infrared Survey Telescope for Astronomy Deep Extragalactic Observations and Canada-France-Hawaii Telescope Legacy Survey. The results indicate that the radio population experiences mild positive evolution out to z ~ 1.2 increasing their space density by a factor of ~3, consistent with results of several previous studies. Beyond z = 1.2, there is evidence of a slowing down of this evolution. Star-forming galaxies drive the more rapid evolution at low redshifts, z 1.2. The evolution is best fitted by pure luminosity evolution with star-forming galaxies evolving as (1 + z)2.47 ± 0.12 and AGN as (1 + z)1.18 ± 0.21M.Peer reviewe
High Tc superconductivity in superlattices of insulating oxides
We report the occurrence of superconductivity, with maximum Tc = 40 K, in
superlattices (SLs) based on two insulating oxides, namely CaCuO2 and SrTiO3.
In these (CaCuO2)n/(SrTiO3)m SLs, the CuO2 planes belong only to CaCuO2 block,
which is an antiferromagnetic insulator. Superconductivity, confined within few
unit cells at the CaCuO2/SrTiO3 interface, shows up only when the SLs are grown
in a highly oxidizing atmosphere, because of extra oxygen ions entering at the
interfaces. Evidence is reported that the hole doping of the CuO2 planes is
obtained by charge transfer from the interface layers, which act as charge
reservoir.Comment: 18 pages, 8 figure
Cauliflower Mosaic Virus TAV, a Plant Virus Protein That Functions like Ribonuclease H1 and is Cytotoxic to Glioma Cells
Recent comparisons between plant and animal viruses reveal many common principles that underlie how all viruses express their
genetic material, amplify their genomes, and link virion assembly with replication. Cauliflower mosaic virus (CaMV) is not
infectious for human beings. Here, we show that CaMV transactivator/viroplasmin protein (TAV) shares sequence similarity with
and behaves like the human ribonuclease H1 (RNase H1) in reducing DNA/RNA hybrids detected with S9.6 antibody in
HEK293T cells. We showed that TAV is clearly expressed in the cytosol and in the nuclei of transiently transfected human cells,
similar to its distribution in plants. TAV also showed remarkable cytotoxic effects in U251 human glioma cells in vitro. *ese
characteristics pave the way for future analysis on the use of the plant virus protein TAV, as an alternative to human RNAse H1
during gene therapy in human cells
Quasi-stellar objects in the ALHAMBRA survey. I. Photometric redshift accuracy based on a 23 optical-NIR filter photometry
We characterize the ability of the ALHAMBRA survey to assign accurate
photo-z's to BLAGN and QSOs based on their ALHAMBRA very-low-resolution
optical-NIR spectroscopy. A sample of 170 spectroscopically identified BLAGN
and QSOs have been used together with a library of templates (including SEDs
from AGN, normal, starburst galaxies and stars) in order to fit the 23
photometric data points provided by ALHAMBRA in the optical and NIR (20
medium-band optical filters plus the standard JHKs). We find that the ALHAMBRA
photometry is able to provide an accurate photo-z and spectral classification
for ~88% of the spectroscopic sources over 2.5 deg^2 in different areas of the
survey, all of them brighter than m678=23.5 (equivalent to r(SLOAN)~24.0). The
derived photo-z accuracy is better than 1% and comparable to the most recent
results in other cosmological fields. The fraction of outliers (~12%) is mainly
caused by the larger photometric errors for the faintest sources and the
intrinsic variability of the BLAGN/QSO population. A small fraction of outliers
may have an incorrectly assigned spectroscopic redshift. The definition of the
ALHAMBRA survey in terms of the number of filters, filter properties, area
coverage and depth is able to provide photometric redshifts for BLAGN/QSOs with
a precision similar to any previous survey that makes use of medium-band
optical photometry. In agreement with previous literature results, our analysis
also reveals that, in the 0<z<4 redshift interval, very accurate photo-z can be
obtained without the use of near-IR broadband photometry at the expense of a
slight increase of outliers. The NIR importance is expected to increase at
higher redshifts (z>4). These results are relevant for the design of future
optical follow-ups of surveys with a large fraction of BLAGN, as it is the case
for X-rays or radio surveys.Comment: 17 pages, 12 figures. Accepted for publication in A&
Properties of the molecular gas in a starbursting QSO at z=1.83 in the COSMOS field
Using the IRAM 30m telescope, we have detected the CO J=2-1, 4-3, 5-4, and
6-5 emission lines in the millimeter-bright, blank-field selected AGN COSMOS
J100038+020822 at redshift z=1.8275. The sub-local thermodynamic equilibrium
(LTE) excitation of the J=4 level implies that the gas is less excited than
that in typical nearby starburst galaxies such as NGC253, and in the
high-redshift quasars studied to date, such as J1148+5251 or BR1202-0725. Large
velocity gradient (LVG) modeling of the CO line spectral energy distribution
(CO SED; flux density vs. rotational quantum number) yields H2 densities in the
range 10^{3.5}--10^{4.0} cm-3, and kinetic temperatures between 50 K and 200 K.
The H2 mass of (3.6 - 5.4) x 10^{10} M_sun implied by the line intensities
compares well with our estimate of the dynamical mass within the inner 1.5 kpc
of the object. Fitting a two-component gray body spectrum, we find a dust mass
of 1.2 x 10^{9} M_sun, and cold and hot dust temperatures of 42+/-5 K and
160+/-25 K, respectively. The broad MgII line allows us to estimate the mass of
the central black hole as 1.7 x 10^{9} M_sun. Although the optical spectrum and
multi-wavelength SED matches those of an average QSO, the molecular gas content
and dust properties resemble those of known submillimeter galaxies (SMGs). The
optical morphology of this source shows tidal tails that suggest a recent
interaction or merger. Since it shares properties of both starburst and AGN,
this object appears to be in a transition from a strongly starforming
submillimeter galaxy to a QSO.Comment: Accepted for publication in Astronomy & Astrophysics (A&A
On the cosmic evolution of the scaling relations between black holes and their host galaxies: Broad Line AGN in the zCOSMOS survey
(Abriged) We report on the measurement of the rest frame K-band luminosity
and total stellar mass of the hosts of 89 broad line Active Galactic Nuclei
detected in the zCOSMOS survey in the redshift range 1<z<2.2. The unprecedented
multiwavelength coverage of the survey field allows us to disentangle the
emission of the host galaxy from that of the nuclear black hole in their
Spectral Energy Distributions. We derive an estimate of black hole masses
through the analysis of the broad Mg II emission lines observed in the
medium-resolution spectra taken with VIMOS/VLT as part of the zCOSMOS project.
We found that, as compared to the local value, the average black hole to host
galaxy mass ratio appears to evolve positively with redshift, with a best fit
evolution of the form (1+z)^{0.68 \pm0.12 +0.6 -0.3}, where the large
asymmetric systematic errors stem from the uncertainties in the choice of IMF,
in the calibration of the virial relation used to estimate BH masses and in the
mean QSO SED adopted. A thorough analysis of observational biases induced by
intrinsic scatter in the scaling relations reinforces the conclusion that an
evolution of the MBH-M* relation must ensue for actively growing black holes at
early times: either its overall normalization, or its intrinsic scatter (or
both) appear to increase with redshift. This can be interpreted as signature of
either a more rapid growth of supermassive black holes at high redshift, a
change of structural properties of AGN hosts at earlier times, or a significant
mismatch between the typical growth times of nuclear black holes and host
galaxies.Comment: 47 pages, 8 figures. Accepted for publication in Ap
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