3,236 research outputs found
Effects of disorder on the optical gap of (Zn,Mg)(S,Se)
The electronic properties and optical gap of (Zn,Mg)(S,Se) wide-gap solid solutions are studied using ab initio techniques and starting from the previously determined atomistic structure of the alloy. Compositional disorder is shown to close the gap substantially with respect to the predictions of the virtual-crystal approximation. The bowing of the fundamental gap versus composition predicted by our calculations is in very good agreement with experiments available for the Zn(S,Se) pseudobinary alloy. At temperatures typical of molecular-beam epitaxy growth, the quaternary alloy displays a rather large amount of short-range order whose effect is to slightly but unmistakably open the gap. Our results agree well with recent experimental data for the quaternary alloy. (C) 1999 American Institute of Physics. [S0003-6951(99)02044-6]
Structure and stability of graphene nanoribbons in oxygen, carbon dioxide, water, and ammonia
We determine, by means of density functional theory, the stability and the
structure of graphene nanoribbon (GNR) edges in presence of molecules such as
oxygen, water, ammonia, and carbon dioxide. As in the case of
hydrogen-terminated nanoribbons, we find that the most stable armchair and
zigzag configurations are characterized by a non-metallic/non-magnetic nature,
and are compatible with Clar's sextet rules, well known in organic chemistry.
In particular, we predict that, at thermodynamic equilibrium, neutral GNRs in
oxygen-rich atmosphere should preferentially be along the armchair direction,
while water-saturated GNRs should present zigzag edges. Our results promise to
be particularly useful to GNRs synthesis, since the most recent and advanced
experimental routes are most effective in water and/or ammonia-containing
solutions.Comment: accepted for publication in PR
Structure, Stability, Edge States and Aromaticity of Graphene Ribbons
We determine the stability, the geometry, the electronic and magnetic
structure of hydrogen-terminated graphene-nanoribbons edges as a function of
the hydrogen content of the environment by means of density functional theory.
Antiferromagnetic zigzag ribbons are stable only at extremely-low ultra-vacuum
pressures. Under more standard conditions, the most stable structures are the
mono- and di-hydrogenated armchair edges and a zigzag edge reconstruction with
one di- and two mono-hydrogenated sites. At high hydrogen-concentration
``bulk'' graphene is not stable and spontaneously breaks to form ribbons, in
analogy to the spontaneous breaking of graphene into small-width nanoribbons
observed experimentally in solution. The stability and the existence of exotic
edge electronic-states and/or magnetism is rationalized in terms of simple
concepts from organic chemistry (Clar's rule)Comment: 4 pages, 3 figures, accepted for publication by Physical Review
Letter
A new and efficient approach to time-dependent density-functional perturbation theory for optical spectroscopy
Using a super-operator formulation of linearized time-dependent
density-functional theory, the dynamical polarizability of a system of
interacting electrons is given a matrix continued-fraction representation whose
coefficients can be obtained from the non-symmetric block-Lanczos method. The
resulting algorithm allows for the calculation of the {\em full spectrum} of a
system with a computational workload which is only a few times larger than that
needed for {\em static} polarizabilities within time-independent
density-functional perturbation theory. The method is demonstrated with the
calculation of the spectrum of benzene, and prospects for its application to
the large-scale calculation of optical spectra are discussed.Comment: 4 pages, 2 figure
Implicit neural representations for unsupervised super-resolution and denoising of 4D flow MRI
4D flow MRI is a non-invasive imaging method that can measure blood flow velocities over time. However, the velocity fields detected by this technique have limitations due to low resolution and measurement noise. Coordinate-based neural networks have been researched to improve accuracy, with SIRENs being suitable for super-resolution tasks. Our study investigates SIRENs for time-varying 3-directional velocity fields measured in the aorta by 4D flow MRI, achieving denoising and super-resolution. We trained our method on voxel coordinates and benchmarked our approach using synthetic measurements and a real 4D flow MRI scan. Our optimized SIREN architecture outperformed state-of-the-art techniques, producing denoised and super-resolved velocity fields from clinical data. Our approach is quick to execute and straightforward to implement for novel cases, achieving 4D super-resolution
Formation of p-n junction in polymer electrolyte-top gated bilayer graphene transistor
We show simultaneous p and n type carrier injection in bilayer graphene
channel by varying the longitudinal bias across the channel and the top gate
voltage. The top gate is applied electrochemically using solid polymer
electrolyte and the gate capacitance is measured to be 1.5 , a
value about 125 times higher than the conventional SiO back gate
capacitance. Unlike the single layer graphene, the drain-source current does
not saturate on varying the drain-source bias voltage. The energy gap opened
between the valence and conduction bands using top and back gate geometry is
estimated.Comment: 16 pages, 6 figure
Ab-initio Molecular Dynamics study of electronic and optical properties of silicon quantum wires: Orientational Effects
We analyze the influence of spatial orientation on the optical response of
hydrogenated silicon quantum wires. The results are relevant for the
interpretation of the optical properties of light emitting porous silicon. We
study (111)-oriented wires and compare the present results with those
previously obtained within the same theoretical framework for (001)-oriented
wires [F. Buda {\it et al.}, {\it Phys. Rev. Lett.} {\bf 69}, 1272, (1992)]. In
analogy with the (001)-oriented wires and at variance with crystalline bulk
silicon, we find that the (111)-oriented wires exhibit a direct gap at whose value is largely enhanced with respect to that found in bulk
silicon because of quantum confinement effects. The imaginary part of the
dielectric function, for the external field polarized in the direction of the
axis of the wires, shows features that, while being qualitatively similar to
those observed for the (001) wires, are not present in the bulk. The main
conclusion which emerges from the present study is that, if wires a few
nanometers large are present in the porous material, they are
optically active independently of their specific orientation.Comment: 14 pages (plus 6 figures), Revte
TRANSCONJUNCTIVAL NONVITRECTOMIZING VITREOUS SURGERY VERSUS 25-GAUGE VITRECTOMY IN PATIENTS WITH EPIRETINAL MEMBRANE: A Prospective Randomized Study
Midterm Outcomes of the Nellix Endovascular Aneurysm Sealing System: A Dual-Center Experience
Purpose: To report midterm outcomes of the Nellix Endovascular Aneurysm Sealing (EVAS) System in the treatment of abdominal aortic aneurysm (AAA). Methods: Between September 2013 and July 2014, 64 AAA patients (mean age 76.6±6.8 years; 61 men) were treated with the EVAS system at 2 centers (only procedures performed at least 12 months prior to the analysis were included). Most patients were treated for a stable AAA, while 1 patient was treated for a ruptured aneurysm. Mean aneurysm diameter was 57.3±9.3 mm. The proximal neck measured a mean 21.5±3.3 mm in diameter and 27.0±12.1 mm long; the neck angle was 16.9°±19.3°. Eleven (17.2%) patients were treated outside the instructions for use (IFU). Results: Technical success was achieved in 63 (98.4%) of 64 patients; 1 type Ia endoleak was treated intraoperatively. One (1.6%) aneurysm-related death occurred at 4 months due to a secondary aortoenteric fistula. Overall, endoleaks occurred in 3 (4.7%) patients (2 type Ia, 1 type II). The estimated rates for 18-month overall survival, freedom from aneurysm-related death, and freedom from secondary interventions were 92.7%, 98.4%, and 95.0%, respectively. Patients treated outside the IFU had a significantly higher incidence of device-related complications (p=0.03). Conclusion: The use of the Nellix device in everyday clinical practice is safe and offers promising midterm results. The risk of secondary aortoenteric fistula requires further analysis. Longer follow-up is needed to assess the actual efficacy of the device, although the risk of migration with late endoleak seems low
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