7,718 research outputs found
The Nonlinear Optical Effects of Opening a Gap in Graphene
Graphene possesses remarkable electronic, optical and mechanical properties
that have taken the research of two-dimensional relativistic condensed matter
systems to prolific levels. However, the understanding of how its nonlinear
optical properties are affected by relativistic-like effects has been broadly
uncharted. It has been recently shown that highly-nontrivial currents can be
generated in free-standing samples, notably leading to the generation of even
harmonics. Since graphene monolayers are centrosymmetric media, for which such
harmonic generation is deemed inaccessible, this light-driven phenomenon is
both startling and promising. More realistically, graphene samples are often
deposited on a dielectric substrate, leading to additional intricate
interactions. Here, we present a treatment to study this instance by gapping
the spectrum and we show this leads to the appearance of a Berry phase in the
carrier dynamics. We analyse the role of such a phase in the generated
nonlinear current and conclude that it suppresses odd-harmonic generation. The
pump energy can be tuned to the energy gap to yield interference among odd
harmonics mediated by interband transitions, allowing even harmonics to be
generated. Our results and general methodology pave the way for understanding
the role of gap-opening physical factors in the nonlinear optics of hexagonal
two-dimensional lattices.Comment: 5 figure
Relative entropy minimizing noisy non-linear neural network to approximate stochastic processes
A method is provided for designing and training noise-driven recurrent neural
networks as models of stochastic processes. The method unifies and generalizes
two known separate modeling approaches, Echo State Networks (ESN) and Linear
Inverse Modeling (LIM), under the common principle of relative entropy
minimization. The power of the new method is demonstrated on a stochastic
approximation of the El Nino phenomenon studied in climate research
First-Principle Description of Correlation Effects in Layered Materials
We present a first-principles description of anisotropic materials
characterized by having both weak (dispersion-like) and strong covalent bonds,
based on the Adiabatic--Connection Fluctuation--Dissipation Theorem within
Density Functional Theory. For hexagonal boron nitride the in-plane and out of
plane bonding as well as vibrational dynamics are well described both at
equilibrium and when the layers are pulled apart. Also bonding in covalent and
ionic solids is described. The formalism allows to ping-down the deficiencies
of common exchange-correlation functionals and provides insight towards the
inclusion of dispersion interactions into the correlation functional.Comment: Accepted for publication in Physical Review Letter
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
Heat Capacity Mapping Mission (HCMM) program: Study of geological structure of Sicily and other Italian areas
The usefulness of thermal inertia mapping in discriminating geolithological units was investigated using Sardinia and the Gulf of Orosei as test sites. Software designed for LANDSAT data were modified and improved for HCMM tapes. A first attempt was made to compare the geological cross section, the topography, the IR radiance, and the thermal inertia along selected profiles of the test site. Thermal inertia profiles appear smoothed in comparison with the thermal radiance. The lowest apparent thermal inertia (ATI) was found on granitic and basaltic outcrops where their image is of sufficient extent, while ATI is higher on carbonatic and dolomitic or moist deposits. Almost every fault is marked by a jump of ATI, the interval being sometimes of the order of one pixel. This seems to demonstrate the ability of ATI to detect contacts or tectonically disturbed zones with a good resolution. It seems more difficult to measure the differences in ATI between homogeneous materials having different lithology. Ground surveys conducted and a simulation model of diurnal temperatures of rocks having different thermal inertia are discussed
Toward the Standardization of the BVL_RU: An Instrument for Speech and Language Assessment of Russian-speaking Children
The Battery for the assessment of speech and language development in children from 4 to 12 years (BVL_4-12; [1]) was originally developed for Italian-speaking children and currently is under adaptation into several European languages including Russian. The BVL_4-12 consists of three parts and includes tasks assessing oral production, comprehension and repetition skills in children. This article describes the process of adaptation of the BVL_4-12 into Russia and focuses on the instructions’ translation and standardization. It presents the results of the tasks instructions’ clarity evaluation by an expert panel including Russian-speaking specialists constantly working with children of a target age in Russia and Italy (N = 7) and a cohort of children from 4.06 to10.10 including monolinguals with typical language development, children previously diagnosed with primary language impairment (PLI) and heritage Russian speakers (N = 84). Overall, 10 task instructions were judged as absolutely clear and 5 task instructions were somewhat unclear to some of the participants. Further analysis ofthe age of the participants who rated the instructions as ‘unclear’ was performed. Some of the youngest participants, whose age did not exceed 6.10, found that the instructions for the following tasks were not clear: phonological fluency; sentence completion; grammatical judgments; idiom comprehension, and comprehension of linguistic prosody. However, the minimum inter-rater agreement among the sample was reached. The potential explanation of the results of the study is proposed in the Discussion section.
Keywords: language assessment, Russian, children, SLI, task instruction
- …