4,013 research outputs found
PIN31 COST-EFFECTIVENESS ANALYSIS OF ONCEDAILY MODIFIED RELEASE CLARITHROMYCIN VERSUS CONVENTIONAL TWICE-DAILY GENERIC CLARITHROMYCIN FOR THE TREATMENT OF RESPIRATORY TRACT INFECTIONS
Excitons in boron nitride nanotubes: dimensionality effects
We show that the optical absorption spectra of boron nitride (BN) nanotubes
are dominated by strongly bound excitons. Our first-principles calculations
indicate that the binding energy for the first and dominant excitonic peak
depends sensitively on the dimensionality of the system, varying from 0.7 eV in
bulk hexagonal BN via 2.1 eV in the single sheet of BN to more than 3 eV in the
hypothetical (2,2) tube. The strongly localized nature of this exciton dictates
the fast convergence of its binding energy with increasing tube diameter
towards the sheet value. The absolute position of the first excitonic peak is
almost independent of the tube radius and system dimensionality. This provides
an explanation for the observed "optical gap" constancy for different tubes and
bulk hBN [R. Arenal et al., to appear in Phys. Rev. Lett. (2005)].Comment: 5 pages, 2 figure
Efficient formalism for large scale ab initio molecular dynamics based on time-dependent density functional theory
A new "on the fly" method to perform Born-Oppenheimer ab initio molecular
dynamics (AIMD) is presented. Inspired by Ehrenfest dynamics in time-dependent
density functional theory, the electronic orbitals are evolved by a
Schroedinger-like equation, where the orbital time derivative is multiplied by
a parameter. This parameter controls the time scale of the fictitious
electronic motion and speeds up the calculations with respect to standard
Ehrenfest dynamics. In contrast to other methods, wave function orthogonality
needs not be imposed as it is automatically preserved, which is of paramount
relevance for large scale AIMD simulations.Comment: 5 pages, 3 color figures, revtex4 packag
Impact of the Electronic Band Structure in High-Harmonic Generation Spectra of Solids
An accurate analytic model describing the microscopic mechanism of high-harmonic generation (HHG) in solids is derived. Extensive first-principles simulations within a time-dependent density-functional framework corroborate the conclusions of the model. Our results reveal that (i)Â the emitted HHG spectra are highly anisotropic and laser-polarization dependent even for cubic crystals; (ii)Â the harmonic emission is enhanced by the inhomogeneity of the electron-nuclei potential; the yield is increased for heavier atoms; and (iii)Â the cutoff photon energy is driver-wavelength independent. Moreover, we show that it is possible to predict the laser polarization for optimal HHG in bulk crystals solely from the knowledge of their electronic band structure. Our results pave the way to better control and optimize HHG in solids by engineering their band structure.European Research Council (Grant ERC-2015-AdG-694097)European Cooperation in the Field of Scientific and Technical Research (Organization) (Action Grant MP1306)German Science Foundation. Hamburg Centre for Ultrafast Imaging-Structure, Dynamics and Control of Matter at the Atromic ScaleGerman Science Foundation (Grant SPP1840 SOLSTICE
Theoretical Study of One-dimensional Chains of Metal Atoms in Nanotubes
Using first-principles total-energy pseudopotential calculations, we have
studied the properties of chains of potassium and aluminum in nanotubes. For BN
tubes, there is little interaction between the metal chains and the tubes, and
the conductivity of these tubes is through carriers located at the inner part
of the tube. In contrast, for small radius carbon nanotubes, there are two
types of interactions: charge-transfer (dominant for alkali atoms) leading to
strong ionic cohesion, and hybridization (for multivalent metal atoms)
resulting in a smaller cohesion. For Al-atomic chains in carbon tubes, we show
that both effects contribute. New electronic properties related to these
confined atomic chains of metal are analyzed.Comment: 12 pages + 3 figure
Optical excitations in hexagonal nanonetwork materials
Optical excitations in hexagonal nanonetwork materials, for example,
Boron-Nitride (BN) sheets and nanotubes, are investigated theoretically. The
bonding of BN systems is positively polarized at the B site, and is negatively
polarized at the N site. There is a permanent electric dipole moment along the
BN bond, whose direction is from the B site to the N site. When the exciton
hopping integral is restricted to the nearest neighbors, the flat band of the
exciton appears at the lowest energy. The higher optical excitations have
excitation bands similar to the electronic bands of graphene planes and carbon
nanotubes. The symmetry of the flat exciton band is optically forbidden,
indicating that the excitons related to this band will show quite long lifetime
which will cause strong luminescence properties.Comment: 4 pages; 3 figures; proceedings of "XVIth International Winterschool
on Electronic Properties of Novel Materials (IWEPNM2002)
Polariton panorama
In this brief review, we summarize and elaborate on some of the nomenclature of polaritonic phenomena and systems as they appear in the literature on quantum materials and quantum optics. Our summary includes at least 70 different types of polaritonic light–matter dressing effects. This summary also unravels a broad panorama of the physics and applications of polaritons. A constantly updated version of this review is available at https://infrared.cni.columbia.edu
Ultra high temperature ceramic composite materials
Ultra-high temperature ceramics (UHTCs) are materials that have been demonstrated to withstand temperatures up to around 3000°C, thermal fluxes of ~17 MWm-2 and gas velocities of around Mach 0.6. Thus, they offer potential for use in applications such as leading edges and engine parts for hypervelocity vehicles. Under the Domain 8 of the MCM-ITP (Materials and Components for Missiles – Innovation and Technology Partnership) programme, research has been carried out investigating UHTC composites consisting of carbon fibre (Cf) preforms impregnated with HfB2 powders. Whilst the initial impregnation route resulted in preforms with high and uniform powder loadings, this was not true for large samples. As a result, the mechanical properties showed a high degree of scatter. Nevertheless, samples with higher final densities showed higher strengths. Thus a new impregnation route has been developed that results in both higher and more homogeneous powder loading. This has led to higher strengths and even greater temperature and ablation resistance with the only penalty being an increase in component mass. A prototype jet vane has been successfully produced
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