351 research outputs found
Plane waves from double extended spacetimes
We study exact string backgrounds (WZW models) generated by nonsemisimple
algebras which are obtained as double extensions of generic D--dimensional
semisimple algebras. We prove that a suitable change of coordinates always
exists which reduces these backgrounds to be the product of the nontrivial
background associated to the original algebra and two dimensional Minkowski.
However, under suitable contraction, the algebra reduces to a Nappi--Witten
algebra and the corresponding spacetime geometry, no more factorized, can be
interpreted as the Penrose limit of the original background. For both
configurations we construct D--brane solutions and prove that {\em all} the
branes survive the Penrose limit. Therefore, the limit procedure can be used to
extract informations about Nappi--Witten plane wave backgrounds in arbitrary
dimensions.Comment: 27 pages, no figures, references adde
Accounting for spin fluctuations beyond LSDA in the density functional theory
We present a method to correct the magnetic properties of itinerant systems
in local spin density approximation (LSDA) and we apply it to the
ferromagnetic-paramagnetic transition under pressure in a typical itinerant
system, NiAl. We obtain a scaling of the critical fluctuations as a
function of pressure equivalent to the one obtained within Moryia's theory.
Moreover we show that in this material the role of the bandstructure is crucial
in driving the transition. Finally we calculate the magnetic moment as a
function of pressure, and find that it gives a scaling of the Curie temperature
that is in good agreement with the experiment. The method can be easily
extended to the antiferromagnetic case and applied, for instance, to the
Fe-pnictides in order to correct the LSDA magnetic moment.Comment: 7 pages, 4 figure
Quantum radiation from superluminal refractive index perturbations
We analyze in detail photon production induced by a superluminal refractive
index perturbation in realistic experimental operating conditions. The
interaction between the refractive index perturbation and the quantum vacuum
fluctuations of the electromagnetic field leads to the production of photon
pairs.Comment: 4 page
Zero point motion and direct/indirect bandgap crossover in layered transition-metal dichalcogenides
Two-dimensional transition-metal dichalcogendes (es. MoS, WS,
MoSe, \ldots) are among the most promising materials for bandgap
engineering. Widely studied in these compounds, by means of ab-initio
techniques, is the possibility of tuning the direct-indirect gap character by
means of in-plane strain. In such kind of calculations however the lattice
degrees of freedom are assumed to be classical and frozen. In this paper we
investigate in details the dependence of the bandgap character (direct vs.
indirect) on the out-of-plane distance between the two chalcogen planes in
each unit. Using DFT calculations, we show that the bandgap character is
indeed highly sensitive on the parameter , in monolayer as well as in
bilayer and bulk compounds, permitting for instance the switching from indirect
to direct gap and from indirect to direct gap in monolayer systems. This
scenario is furthermore analyzed in the presence of quantum lattice fluctuation
induced by the zero-point motion. On the basis of a quantum analysis, we argue
that the direct-indirect bandgap transitions induced by the out-of-plane strain
as well by the in-plane strain can be regarded more as continuous crossovers
rather than as real sharp transitions. The consequences on the physical
observables are discussed.Comment: 12 pages, 9 eps figures include
Spacetime geometries and light trapping in travelling refractive index perturbations
In the framework of transformation optics, we show that the propagation of a
locally superluminal refractive index perturbation (RIP) in a Kerr medium can
be described, in the eikonal approximation, by means of a stationary metric,
which we prove to be of Gordon type. Under suitable hypotheses on the RIP, we
obtain a stationary but not static metric, which is characterized by an
ergosphere and by a peculiar behaviour of the geodesics, which are studied
numerically, also accounting for material dispersion. Finally, the equation to
be satisfied by an event horizon is also displayed and briefly discussed.Comment: 14 pages, 7 figure
Fermi surface shrinking and interband coupling in iron-based pnictides
Recent measurements of Fermi surface with de Haas-van Alphen oscillations in
LaFePO showed a shrinking of the Fermi pockets with respect to first-principle
LDA calculations, suggesting an energy shift of the hole and electrons bands
with respect to LDA. We show that these shifts are a natural consequence of the
strong particle-hole asymmetry of electronic bands in pnictides, and that they
provide an indirect experimental evidence of a dominant interband scattering in
these systems.Comment: 4 latex pages, 4 eps figures included, last version as in the
accepted for
Euler angles for G2
We provide a simple parametrization for the group G2, which is analogous to
the Euler parametrization for SU(2). We show how to obtain the general element
of the group in a form emphasizing the structure of the fibration of G2 with
fiber SO(4) and base H, the variety of quaternionic subalgebras of octonions.
In particular this allows us to obtain a simple expression for the Haar measure
on G2. Moreover, as a by-product it yields a concrete realization and an
Einstein metric for H.Comment: 21 pages, 2 figures, some misprints correcte
Reply to Comment on: Hawking radiation from ultrashort laser pulse filaments
A comment by R. Schutzhold et al. raises possible concerns and questions
regarding recent measurements of analogue Hawking radiation. We briefly reply
to the opinions expressed in the comment and sustain that the origin of the
radiation may be understood in terms of Hawking emission
Fast Production of Cellulose Nanocrystals by Hydrolytic-Oxidative Microwave-Assisted Treatment
In contrast to conventional approaches, which are considered to be energy- and
time-intensive, expensive, and not green, herein, we report an alternative microwave-assisted
ammonium persulfate (APS) method for cellulose nanocrystals (CNCs) production, under pressurized
conditions in a closed reaction system. The aim was to optimize the hydrolytic-oxidative patented
procedure (US 8,900,706), replacing the conventional heating with a faster process that would allow
the industrial scale production of the nanomaterial and make it more appealing to a green economy.
A microwave-assisted process was performed according to dierent time\u2013temperature programs,
varying the ramp (from 5 to 40 min) and the hold heating time (from 60 to 90 min), at a fixed reagent
concentration and weight ratio of the raw material/APS solution. Dierences in composition, structure,
and morphology of the nanocrystals, arising fromtraditional and microwave methods, were studied by
several techniques (TEM, Fourier transform infrared spectroscopy (FTIR)-attenuated total reflectance
(ATR), dynamic light scattering (DLS), electrophoretic light scattering (ELS), thermogravimetric
analysis (TGA), X-ray diraction (XRD)), and the extraction yields were calculated. Fine tuning
the microwave treatment variables, it was possible to realize a simple, cost-eective way for faster
materials\u2019 preparation, which allowed achieving high-quality CNCs, with a defined hydrodynamic
diameter (150 nm) and zeta potential (0.040 V), comparable to those obtained using conventional
heating, in only 90 min instead of 16 h
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