155 research outputs found
Donor-acceptor recombination emission in hydrogen-terminated nanodiamond: Novel single-photon source for room-temperature quantum photonics
In fluorescence spectra of nanodiamonds (NDs) synthesized at high pressure
from adamantane and other organic compounds, very narrow (~1 nm) lines of
unknown origin are observed in a wide spectroscopic range from ~500 to 800 nm.
Here, we propose and experimentally substantiate the hypothesis that these
mysterious lines arise from radiative recombination of donor-acceptor pairs
(DAPs). To confirm our hypothesis, we study the fluorescence spectra of undoped
and nitrogen-doped NDs of different sizes, before and after thermal oxidation
of their surface. The results obtained with a high degree of confidence allowed
us to conclude that the DAPs are formed through the interaction of donor-like
substitutional nitrogen present in the diamond lattice, and a 2D layer of
acceptors resulting from the transfer doping effect on the surface of
hydrogen-terminated NDs. A specific behavior of the DAP-induced lines was
discovered in the temperature range of 100-10 K: their energy increases and
most lines are split into 2 or more components with decreasing temperature. It
is shown that the majority of the studied DAP emitters are sources of single
photons, with an emission rate of up to >1 million counts/s at room
temperature, which significantly surpasses that of nitrogen-vacancy and
silicon-vacancy centers under the same detection conditions. Despite an
observed temporal instability in the emission, the DAP emitters of H-terminated
NDs represent a powerful room-temperature single-photon source for quantum
optical technologies
Analysis and use of neural networks as a tool for a rapid non-invasive estimation
Water deficit is one of the most important environmental factors limiting
sustainable crop yields and it requires a reliable tool for fast and precise
quantification. In this work we use simultaneously recorded signals of
photoinduced prompt fluorescence (PF) and delayed fluorescence (DF) as well as
modulated reflection (MR) of light at 820 nm for analysis of the changes in
the photosynthetic activity in detached bean leaves during drying. Depending
on the severity of the water deficit we identify different changes in the
primary photosynthetic processes. When the relative water content (RWC) is
decreased to 60% there is a parallel decrease in the ratio between the rate of
excitation trapping in the Photosystem (PS) II reaction center and the rate of
reoxidation of reduced PSII acceptors. A further decrease of RWC to 20%
suppresses the electron transfer from the reduced plastoquinone pool to the
PSI reaction center. At RWC below values 15%, the reoxidation of the
photoreduced primary quinone acceptor of PSII, QA–, is inhibited and at less
than 5%, the primary photochemical reactions in PSI and II are inactivated.
Using the collected sets of PF, DF and MR signals, we construct and train an
artificial neural network, capable of recognizing the RWC in a series of
“unknown” samples with a correlation between calculated and gravimetrically
determined RWC values of about R2 ≈ 0.98. Our results demonstrate that this is
a reliable method for determination of RWC in detached leaves and after
further development it could be used for quantifying of drought stress of crop
plants in situ. This article is part of a Special Issue entitled:
Photosynthesis Research for Sustainability: from Natural to Artificial
Mg(2)Si(x)Sn(1-x)heterostructures on Si(111) substrate for optoelectronics and thermoelectronics
Thin (50-90 m) non-doped and doped (by Al atoms) Mg2Sn0.6Si0.4 and Mg(2)Sn(0.4)Si(0.6)films with roughness of 1.9-3.7 nm have been grown by multiple deposition and single annealing at 150 degrees C of multilayers formed by repetition deposition of three-layers (Si-Sn-Mg) on Si(111) p-type wafers with 45 cm resistivity. Transmission electron microscopy has shown that the first forming layer is an epitaxial layer of hex-Mg2Sn(300) on Si(111) substrate with thickness not more than 5-7 nm. Epitaxial relationships: hex-Mg2Sn(300)parallel to Si(111), hex-Mg2Sn[001]parallel to Si[-112] and hex-Mg2Sn[030]parallel to Si[110] have been found for the epitaxial layer. But inclusions of cub-Mg2Si were also observed inside hex-Mg2Sn layer. It was found that the remaining part of the film thickness is in amorphous state and has a layered distribution of major elements: Mg, Sn and Mg without exact chemical composition. It was established by optical spectroscopy data that both type films are semiconductor with undispersed region lower 0.18 eV with n(o) = 3.59 +/- 0.01, but only two direct interband transitions with energies 0.75-0.76 eV and 1.2 eV have been determined. The last interband transition has been confirmed by photoreflectance data at room temperature. Fourier transmittance spectroscopy and Raman spectroscopy data have established the formation of stannide, silicide and ternary compositions
On the Properties of Two Pulses Propagating Simultaneously in Different Dispersion Regimes in a Nonlinear Planar Waveguide
Properties of two pulses propagating simultaneously in different dispersion
regimes, anomalous and normal, in a Kerr-type planar waveguide are studied in
the framework of the nonlinear Schroedinger equation. Catastrophic
self-focusing and spatio-temporal splitting of the pulses is investigated. For
the limiting case when the dispersive term of the pulse propagating in the
normal dispersion regime can be neglected an indication of a possibility of a
stable self-trapped propagation of both pulses is obtained.Comment: 18 pages (including 15 eps figures
Sulfides of the Modern Kamchatka Hydrothermal Systems
ABSTRACT Sulfides pyrite, melnikovite-pyrite, marcasite, sphalerite, chalcopyrite, galena, cinnabar, coloradoite, metacinnabar are precipitating at the modern geothermal systems of Kamchatka: Kireunsky, Dvukhyurtochny and Apapel'sky in Central Kamchatka, Vilyuchinsky and Mutnovsky in Southern Kamchatka. Ore deposits are spatially associated with hydrothermal springs. Pyrite is the most common mineral precipitated at the discharge of hydrothermal style. It varies in mode of occurrence, size, inner structure, chemical composition and microstructure. Frequently pyrite occurs as framboids, idiomorphic crystals and their aggregates. By chemical composition, two varieties of pyrite are observed: homogeneous and heterogeneous. Heterogeneity of composition is due to impurities of As, Cu, Sb, Hg and Ag. Au as impurity in pyrite was relieved only in pyrite from Voinovsky hot springs in Southern Kamchatka. Cinnabar is the next most common occurring mineral at the modern hydrothermal systems in Kamchatka. Chalcopyrite, galena, sphalerite and gold are rare minerals. The modern hydrothermal systems in Kamchatka provide the opportunity to study sulfide typomorphism and physico-chemical conditions of the deposition mechanism. We suppose that some of them are the elements of the long-life ore generating hydrothermal systems
Operando tracking of oxidation-state changes by coupling electrochemistry with time-resolved X-ray absorption spectroscopy demonstrated for water oxidation by a cobalt-based catalyst film
Transition metal oxides are promising electrocatalysts for water oxidation, i.e., the oxygen evolution reaction (OER), which is critical in electrochemical production of non-fossil fuels. The involvement of oxidation state changes of the metal in OER electrocatalysis is increasingly recognized in the literature. Tracing these oxidation states under operation conditions could provide relevant information for performance optimization and development of durable catalysts, but further methodical developments are needed. Here, we propose a strategy to use single-energy X-ray absorption spectroscopy for monitoring metal oxidation-state changes during OER operation with millisecond time resolution. The procedure to obtain time-resolved oxidation state values, using two calibration curves, is explained in detail. We demonstrate the significance of this approach as well as possible sources of data misinterpretation. We conclude that the combination of X-ray absorption spectroscopy with electrochemical techniques allows us to investigate the kinetics of redox transitions and to distinguish the catalytic current from the redox current. Tracking of the oxidation state changes of Co ions in electrodeposited oxide films during cyclic voltammetry in neutral pH electrolyte serves as a proof of principle
Electronic structure and optical properties of Ca2Si films grown on silicon different oriented substrates and calculated from first principles
The work considered the growth, optical properties and emerging interband transitions in Ca2Si films grown on silicon substrates with (111), (001), and (110) orientations at two temperatures (250 °C and 300 °C) using the sacrificial-template method. The optimum temperature for MBE single-phase growth of Ca2Si is 250 °C. Calculations of optical functions from the transmission and reflection spectra were carried out within the framework of a two-layer model and by the Kramers–Kronig method. It is shown that the main peaks in the experimental reflection spectra and the optical conductivity calculated according to Kramers–Kronig are in good agreement with each other. Comparison of ab initio calculations of the energy band structure and optical properties of a Ca2Si single crystal and two-dimensional Ca2Si layers with experimental data in the region of high-energy transitions showed good coincidence
Ultrashort filaments of light in weakly-ionized, optically-transparent media
Modern laser sources nowadays deliver ultrashort light pulses reaching few
cycles in duration, high energies beyond the Joule level and peak powers
exceeding several terawatt (TW). When such pulses propagate through
optically-transparent media, they first self-focus in space and grow in
intensity, until they generate a tenuous plasma by photo-ionization. For free
electron densities and beam intensities below their breakdown limits, these
pulses evolve as self-guided objects, resulting from successive equilibria
between the Kerr focusing process, the chromatic dispersion of the medium, and
the defocusing action of the electron plasma. Discovered one decade ago, this
self-channeling mechanism reveals a new physics, widely extending the frontiers
of nonlinear optics. Implications include long-distance propagation of TW beams
in the atmosphere, supercontinuum emission, pulse shortening as well as
high-order harmonic generation. This review presents the landmarks of the
10-odd-year progress in this field. Particular emphasis is laid to the
theoretical modeling of the propagation equations, whose physical ingredients
are discussed from numerical simulations. Differences between femtosecond
pulses propagating in gaseous or condensed materials are underlined. Attention
is also paid to the multifilamentation instability of broad, powerful beams,
breaking up the energy distribution into small-scale cells along the optical
path. The robustness of the resulting filaments in adverse weathers, their
large conical emission exploited for multipollutant remote sensing, nonlinear
spectroscopy, and the possibility to guide electric discharges in air are
finally addressed on the basis of experimental results.Comment: 50 pages, 38 figure
Formation, structure, and optical properties of single-phase CaSi and CaSi2 films on Si substrates
In this paper, we report on optimizing the conditions for subsequently growing single-phase films of calcium monosilicide (CaSi) and calcium disilicide (CaSi2) on single-crystal silicon by reactive deposition epitaxy (RDE) and molecular beam epitaxy (MBE). The temperature range for the growth of CaSi films (400–500 °C) was determined, as well as the temperature range (600–680°C) for the growth of CaSi2 films on silicon with three orientations: (111), (100) and (110). The minimum temperatures for the epitaxial growth of CaSi films by the RDE method and CaSi2 films by the MBE method were determined, amounting to, respectively, T = 475 °C and T = 640 °C. An increase in the ratio of Ca to Si deposition rates to 26 made it possible to grow a large-block CaSi2 epitaxial film with the hR6 structure by the MBE method at T = 680 °C. Raman spectra and reflection spectra from single-phase epitaxial CaSi and CaSi2 films on silicon were recorded and identified for the first time. The correspondence between the experimental reflection spectra and the theoretically calculated reflection spectra in terms of amplitude and peak positions at photon energies of 0.1–6.5 eV has been established. Single-phase CaSi and CaSi2 films retain transparency in the photon energy range 0.4–1.2 eV
Enhanced strange baryon production in Au+Au collisions compared to p+p at sqrts = 200 GeV
We report on the observed differences in production rates of strange and
multi-strange baryons in Au+Au collisions at sqrts = 200 GeV compared to pp
interactions at the same energy. The strange baryon yields in Au+Au collisions,
then scaled down by the number of participating nucleons, are enhanced relative
to those measured in pp reactions. The enhancement observed increases with the
strangeness content of the baryon, and increases for all strange baryons with
collision centrality. The enhancement is qualitatively similar to that observed
at lower collision energy sqrts =17.3 GeV. The previous observations are for
the bulk production, while at intermediate pT, 1 < pT< 4 GeV/c, the strange
baryons even exceed binary scaling from pp yields.Comment: 7 pages, 4 figures. Printed in PR
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