2,728 research outputs found
Tunable Polaronic Conduction in Anatase TiO2
Oxygen vacancies created in anatase TiO2 by UV photons (80–130 eV) provide an effective electron-doping mechanism and induce a hitherto unobserved dispersive metallic state. Angle resolved photoemission reveals that the quasiparticles are large polarons. These results indicate that anatase can be tuned from an insulator to a polaron gas to a weakly correlated metal as a function of doping and clarify the nature of conductivity in this material.open1192sciescopu
The free energy in the Derrida--Retaux recursive model
We are interested in a simple max-type recursive model studied by Derrida and
Retaux (2014) in the context of a physics problem, and find a wide range for
the exponent in the free energy in the nearly supercritical regime
Electronic Instability in a Zero-Gap Semiconductor: The Charge-DensityWave in (TaSe4)(2)I
We report a comprehensive study of the paradigmatic quasi-1D compound (TaSe4)(2)I performed by means of angle-resolved photoemission spectroscopy (ARPES) and first-principles electronic structure calculations. We find it to be a zero-gap semiconductor in the nondistorted structure, with non-negligible interchain coupling. Theory and experiment support a Peierls-like scenario for the charge-density wave formation below T-CDW = 263 K, where the incommensurability is a direct consequence of the finite interchain coupling. The formation of small polarons, strongly suggested by the ARPES data, explains the puzzling semiconductor-to-semiconductor transition observed in transport at T-CDW.open114sciescopu
Substrate-induced band gap opening in epitaxial graphene
Graphene has shown great application potentials as the host material for next
generation electronic devices. However, despite its intriguing properties, one
of the biggest hurdles for graphene to be useful as an electronic material is
its lacking of an energy gap in the electronic spectra. This, for example,
prevents the use of graphene in making transistors. Although several proposals
have been made to open a gap in graphene's electronic spectra, they all require
complex engineering of the graphene layer. Here we show that when graphene is
epitaxially grown on the SiC substrate, a gap of ~ 0.26 is produced. This gap
decreases as the sample thickness increases and eventually approaches zero when
the number of layers exceeds four. We propose that the origin of this gap is
the breaking of sublattice symmetry owing to the graphene-substrate
interaction. We believe our results highlight a promising direction for band
gap engineering of graphene.Comment: 10 pages, 4 figures; updated reference
MOSFiT: Modular open source fitter for transients
Much of the progress made in time-domain astronomy is accomplished by
relating observational multi-wavelength time series data to models derived from
our understanding of physical laws. This goal is typically accomplished by
dividing the task in two: collecting data (observing), and constructing models
to represent that data (theorizing). Owing to the natural tendency for
specialization, a disconnect can develop between the best available theories
and the best available data, potentially delaying advances in our understanding
new classes of transients. We introduce MOSFiT: the Modular Open-Source Fitter
for Transients, a Python-based package that downloads transient datasets from
open online catalogs (e.g., the Open Supernova Catalog), generates Monte Carlo
ensembles of semi-analytical light curve fits to those datasets and their
associated Bayesian parameter posteriors, and optionally delivers the fitting
results back to those same catalogs to make them available to the rest of the
community. MOSFiT is designed to help bridge the gap between observations and
theory in time-domain astronomy; in addition to making the application of
existing models and creation of new models as simple as possible, MOSFiT yields
statistically robust predictions for transient characteristics, with a standard
output format that includes all the setup information necessary to reproduce a
given result. As large-scale surveys such as LSST discover entirely new classes
of transients, tools such as MOSFiT will be critical for enabling rapid
comparison of models against data in statistically consistent, reproducible,
and scientifically beneficial ways
Intrinsic and Extrinsic Performance Limits of Graphene Devices on SiO2
The linear dispersion relation in graphene[1,2] gives rise to a surprising
prediction: the resistivity due to isotropic scatterers (e.g. white-noise
disorder[3] or phonons[4-8]) is independent of carrier density n. Here we show
that acoustic phonon scattering[4-6] is indeed independent of n, and places an
intrinsic limit on the resistivity in graphene of only 30 Ohm at room
temperature (RT). At a technologically-relevant carrier density of 10^12 cm^-2,
the mean free path for electron-acoustic phonon scattering is >2 microns, and
the intrinsic mobility limit is 2x10^5 cm^2/Vs, exceeding the highest known
inorganic semiconductor (InSb, ~7.7x10^4 cm^2/Vs[9]) and semiconducting carbon
nanotubes (~1x10^5 cm^2/Vs[10]). We also show that extrinsic scattering by
surface phonons of the SiO2 substrate[11,12] adds a strong temperature
dependent resistivity above ~200 K[8], limiting the RT mobility to ~4x10^4
cm^2/Vs, pointing out the importance of substrate choice for graphene
devices[13].Comment: 16 pages, 3 figure
Atherogenic Lipid Stress Induces Platelet Hyperactivity Through CD36-Mediated Hyposensitivity To Prostacyclin-; The Role Of Phosphodiesterase 3A
Prostacyclin (PGI2) controls platelet activation and thrombosis through a cyclic adenosine monophosphate (cAMP) signalling cascade. However, in patients with cardiovascular diseases this protective mechanism fails for reasons that are unclear. Using both pharmacological and genetic approaches we describe a mechanism by which oxidised low density lipoproteins (oxLDL) associated with dyslipidaemia promote platelet activation through impaired PGI2 sensitivity and diminished cAMP signalling. In functional assays using human platelets, oxLDL modulated the inhibitory effects of PGI2, but not a PDE-insensitive cAMP analogue, on platelet aggregation, granule secretion and in vitro thrombosis. Examination of the mechanism revealed that oxLDL promoted the hydrolysis of cAMP through the phosphorylation and activation of phosphodiesterase 3A (PDE3A), leading to diminished cAMP signalling. PDE3A activation by oxLDL required Src family kinases, Syk and protein kinase C. The effects of oxLDL on platelet function and cAMP signalling were blocked by pharmacological inhibition of CD36, mimicked by CD36-specific oxidised phospholipids and ablated in CD36-/- murine platelets. The injection of oxLDL into wild type mice strongly promoted FeCl3 induced carotid thrombosis in vivo, which was prevented by pharmacological inhibition of PDE3A. Furthermore, blood from dyslipidaemic mice was associated with increased oxidative lipid stress, reduced platelet sensitivity to PGI2 ex vivo and diminished PKA signalling. In contrast, platelet sensitivity to a PDE-resistant cAMP analogue remained normal. Genetic deletion of CD36, protected dyslipidaemic animals from PGI2 hyposensitivity and restored PKA signalling. These data suggest that CD36 can translate atherogenic lipid stress into platelet hyperactivity through modulation of inhibitory cAMP signalling.
Half-Metallic Graphene Nanoribbons
Electrical current can be completely spin polarized in a class of materials
known as half-metals, as a result of the coexistence of metallic nature for
electrons with one spin orientation and insulating for electrons with the
other. Such asymmetric electronic states for the different spins have been
predicted for some ferromagnetic metals - for example, the Heusler compounds-
and were first observed in a manganese perovskite. In view of the potential for
use of this property in realizing spin-based electronics, substantial efforts
have been made to search for half-metallic materials. However, organic
materials have hardly been investigated in this context even though
carbon-based nanostructures hold significant promise for future electronic
device. Here we predict half-metallicity in nanometre-scale graphene ribbons by
using first-principles calculations. We show that this phenomenon is realizable
if in-plane homogeneous electric fields are applied across the zigzag-shaped
edges of the graphene nanoribbons, and that their magnetic property can be
controlled by the external electric fields. The results are not only of
scientific interests in the interplay between electric fields and electronic
spin degree of freedom in solids but may also open a new path to explore
spintronics at nanometre scale, based on graphene
Bipolar supercurrent in graphene
Graphene -a recently discovered one-atom-thick layer of graphite- constitutes
a new model system in condensed matter physics, because it is the first
material in which charge carriers behave as massless chiral relativistic
particles. The anomalous quantization of the Hall conductance, which is now
understood theoretically, is one of the experimental signatures of the peculiar
transport properties of relativistic electrons in graphene. Other unusual
phenomena, like the finite conductivity of order 4e^2/h at the charge
neutrality (or Dirac) point, have come as a surprise and remain to be
explained. Here, we study the Josephson effect in graphene. Our experiments
rely on mesoscopic superconducting junctions consisting of a graphene layer
contacted by two closely spaced superconducting electrodes, where the charge
density can be controlled by means of a gate electrode. We observe a
supercurrent that, depending on the gate voltage, is carried by either
electrons in the conduction band or by holes in the valence band. More
importantly, we find that not only the normal state conductance of graphene is
finite, but also a finite supercurrent can flow at zero charge density. Our
observations shed light on the special role of time reversal symmetry in
graphene and constitute the first demonstration of phase coherent electronic
transport at the Dirac point.Comment: Under review, 12 pages, 4 Figs., suppl. info (v2 identical, resolved
file problems
Valley filter and valley valve in graphene
It is known that the lowest propagating mode in a narrow ballistic ribbon of
graphene may lack the twofold valley degeneracy of higher modes. Depending on
the crystallographic orientation of the ribbon axis, the lowest mode mixes both
valleys or lies predominantly in a single valley (chosen by the direction of
propagation). We show, using a tight-binding model calculation, that a
nonequilibrium valley polarization can be realized in a sheet of graphene, upon
injection of current through a ballistic point contact with zigzag edges. The
polarity can be inverted by local application of a gate voltage to the point
contact region. Two valley filters in series may function as an
electrostatically controlled ``valley valve'', representing a
zero-magnetic-field counterpart to the familiar spin valve.Comment: RevTeX, 4 pages, 5 figure
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