164 research outputs found
Progettazione di prove sperimentali per la calibrazione del modello di rottura di Johnson-Cook
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
The momentum and photon energy dependence of the circular dichroic photoemission in the bulk Rashba semiconductors BiTeX (X = I, Br, Cl)
Bulk Rashba systems BiTeX (X = I, Br, Cl) are emerging as important
candidates for developing spintronics devices, because of the coexistence of
spin-split bulk and surface states, along with the ambipolar character of the
surface charge carriers. The need of studying the spin texture of strongly
spin-orbit coupled materials has recently promoted circular dichroic Angular
Resolved Photoelectron Spectroscopy (cd-ARPES) as an indirect tool to measure
the spin and the angular degrees of freedom. Here we report a detailed photon
energy dependent study of the cd-ARPES spectra in BiTeX (X = I, Br and Cl). Our
work reveals a large variation of the magnitude and sign of the dichroism.
Interestingly, we find that the dichroic signal modulates differently for the
three compounds and for the different spin-split states. These findings show a
momentum and photon energy dependence for the cd-ARPES signals in the bulk
Rashba semiconductor BiTeX (X = I, Br, Cl). Finally, the outcome of our
experiment indicates the important relation between the modulation of the
dichroism and the phase differences between the wave-functions involved in the
photoemission process. This phase difference can be due to initial or final
state effects. In the former case the phase difference results in possible
interference effects among the photo-electrons emitted from different atomic
layers and characterized by entangled spin-orbital polarized bands. In the
latter case the phase difference results from the relative phases of the
expansion of the final state in different outgoing partial waves.Comment: 6 pages, 4 figure
Giant ambipolar Rashba effect in a semiconductor: BiTeI
We observe a giant spin-orbit splitting in bulk and surface states of the
non-centrosymmetric semiconductor BiTeI. We show that the Fermi level can be
placed in the valence or in the conduction band by controlling the surface
termination. In both cases it intersects spin-polarized bands, in the
corresponding surface depletion and accumulation layers. The momentum splitting
of these bands is not affected by adsorbate-induced changes in the surface
potential. These findings demonstrate that two properties crucial for enabling
semiconductor-based spin electronics -- a large, robust spin splitting and
ambipolar conduction -- are present in this material.Comment: 4 pages, 3 figure
Electronic Instability in a Zero-Gap Semiconductor: The Charge-DensityWave in (TaSe4)(2)I
International audienceWe 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
Enhanced insulin sensitivity in skeletal muscle and liver by physiological overexpression of SIRT6
Objective: Available treatment for obesity and type 2 diabetes mellitus (T2DM) is suboptimal. Thus, identifying novel molecular target(s) exerting protective effects against these metabolic imbalances is of enormous medical significance. Sirt6 loss- and gain-of-function studies have generated confounding data regarding the role of this sirtuin on energy and glucose homeostasis, leaving unclear whether activation or inhibition of SIRT6 may be beneficial for the treatment of obesity and/or T2DM. Methods: To address these issues, we developed and studied a novel mouse model designed to produce eutopic and physiological overexpression of SIRT6 (Sirt6BAC mice). These mutants and their controls underwent several metabolic analyses. These include whole-blood reverse phase high-performance liquid chromatography assay, glucose and pyruvate tolerance tests, hyperinsulinemic-euglycemic clamp assays, and assessment of basal and insulin-induced level of phosphorylated AKT (p-AKT)/AKT in gastrocnemius muscle. Results: Sirt6BAC mice physiologically overexpress functionally competent SIRT6 protein. While Sirt6BAC mice have normal body weight and adiposity, they are protected from developing high-caloric-diet (HCD)-induced hyperglycemia and glucose intolerance. Also, Sirt6BAC mice display increased circulating level of the polyamine spermidine. The ability of insulin to suppress endogenous glucose production was significantly enhanced in Sirt6BAC mice compared to wild-type controls. Insulin-stimulated glucose uptake was increased in Sirt6BAC mice in both gastrocnemius and soleus muscle, but not in brain, interscapular brown adipose, or epididymal adipose tissue. Insulin-induced p-AKT/AKT ratio was increased in gastrocnemius muscle of Sirt6BAC mice compared to wild-type controls. Conclusions: Our data indicate that moderate, physiological overexpression of SIRT6 enhances insulin sensitivity in skeletal muscle and liver, engendering protective actions against diet-induced T2DM. Hence, the present study provides support for the anti-T2DM effect of SIRT6 and suggests SIRT6 as a putative molecular target for anti-T2DM treatment
New Mechanism for Spin-Orbit Splitting of Conduction States in Surface Alloys
We present Angle-Resolved Photoelectron Spectroscopy (ARPES) data on spin-orbit split states in two XAg2 surface alloys grown on an Ag(111) substrate, and of the Si(111)-AgBiAg2 trilayer system. We briefly discuss the origin of the unusually large energy and momentum splitting, and the possibility of tuning by surface engineering the spin polarization at the Fermi level. [DOI: 10.1380/ejssnt.2009.264
Atomically smooth films of CsSb: a chemically robust visible light photocathode
Alkali antimonide semiconductor photocathodes provide a promising platform
for the generation of high brightness electron beams, which are necessary for
the development of cutting-edge probes including x-ray free electron lasers and
ultrafast electron diffraction. However, to harness the intrinsic brightness
limits in these compounds, extrinsic degrading factors, including surface
roughness and contamination, must be overcome. By exploring the growth of CsxSb
thin films monitored by in situ electron diffraction, the conditions to
reproducibly synthesize atomically smooth films of CsSb on 3C-SiC (100) and
graphene coated TiO2 (110) substrates are identified, and detailed structural,
morphological, and electronic characterization is presented. These films
combine high quantum efficiency in the visible (up to 1.2% at 400 nm), an
easily accessible photoemission threshold of 550 nm, low surface roughness
(down to 600 pm on a 1 um scale), and a robustness against oxidation up to 15
times greater then Cs3Sb. These properties suggest that CsSb has the potential
to operate as an alternative to CsSb in electron source applications where
the demands of the vacuum environment might otherwise preclude the use of
traditional alkali antimonides.Comment: 11 pages, 6 figures, 1 tabl
MOKA3D: An innovative approach to 3D gas kinematic modelling. I. Application to AGN ionized outflows
Studying the feedback process of Active Galactic Nuclei (AGN) requires
characterising multiple kinematical components, such as rotating gas and
stellar disks, outflows, inflows, and jets. To compare the observed properties
with theoretical predictions of galaxy evolution and feedback models and to
assess the mutual interaction and energy injection rate into the interstellar
medium (ISM), one usually relies on simplified kinematic models. These models
have several limitations, as they often do not take into account projection
effects, beam smearing and the surface brightness distribution of the emitting
medium. Here, we present MOKA3D, an innovative approach to model the 3D gas
kinematics from integral field spectroscopy observations. In this first paper,
we discuss its application to the case of AGN ionised outflows, whose observed
clumpy emission and apparently irregular kinematics are only marginally
accounted for by existing kinematical models. Unlike previous works, our model
does not assume the surface brightness distribution of the gas, but exploits a
novel procedure to derive it from the observations by reconstructing the 3D
distribution of emitting clouds and providing accurate estimates of the
spatially resolved outflow physical properties (e.g. mass rate, kinetic
energy). As an example, we demonstrate the capabilities of our method by
applying it to three nearby Seyfert-II galaxies observed with MUSE at the VLT
and selected from the MAGNUM survey, showing that the complex kinematic
features observed can be described by a conical outflow with a constant radial
velocity field and a clumpy distribution of clouds.Comment: 17 pages, 14 figure
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