401 research outputs found
The electron in three-dimensional momentum space
We study the electron as a system composed by an electron and a photon, using
lowest order perturbation theory. We derive the leading-twist
transverse-momentum-dependent distribution functions for both the electron and
photon in the dressed electron, thereby offering a three-dimensional
description of the dressed electron in momentum space. To obtain the
distribution functions, we apply both the formalism of light-front wave
function overlap representation and the diagrammatic approach. We perform the
calculations both in light-cone gauge and Feynman gauge, and we present a
detailed discussion of the role of the Wilson lines to obtain gauge-independent
results. We provide numerical results and plots for many of the computed
distributions.Comment: 25 pages, 14 figure
Spatial distribution of angular momentum inside the nucleon
We discuss in detail the spatial distribution of angular momentum inside the
nucleon. We show that the discrepancies between different definitions originate
from terms that integrate to zero. Even though these terms can safely be
dropped at the integrated level, they have to be taken into account at the
density level. Using the scalar diquark model, we illustrate our results and,
for the first time, check explicitly that the equivalence between kinetic and
canonical orbital angular momentum persists at the density level, as expected
in a system without gauge degrees of freedom
Advances in Nanoparticles: Synthesis, Characterization, Theoretical Modelling, and Applications
This book focuses on recent advances in the synthesis of nanoparticles, their characterization, and their applications in different fields such as catalysis, photonics, magnetism, and nanomedicine. Nanoparticles receive a large share of the worldwide research activity in contemporary materials science. This is witnessed by the number of scientific papers with ""nanoparticle"" as a keyword, increasing linearly in the last 10 years from about 16,000 in 2009 to about 50,000 in 2019. This impressive widespread interest stems from the basic science of nanoparticles, which constitute a bridge between the molecular and the bulk worlds, as well as from their technological applications. The preparation of nanoparticles is a crossroad of materials science where chemists, physicists, engineers, and even biologists frequently meet, leading to a continuous improvement of existing techniques and to the invention of new methods. The reader interested in nanoparticles synthesis and properties will here find a valuable selection of scientific cases that cannot cover all methods and applications relevant to the field, but still provide an updated overview on the fervent research activity focused on nanoparticles
Time and foreign exchange markets
The definition of time is still an open question when one deals with high
frequency time series. If time is simply the calendar time, prices can be
modeled as continuous random processes and values resulting from transactions
or given quotes are discrete samples of this underlying dynamics. On the
contrary, if one takes the business time point of view, price dynamics is a
discrete random process, and time is simply the ordering according which prices
are quoted in the market. In this paper we suggest that the business time
approach is perhaps a better way of modeling price dynamics than calendar time.
This conclusion comes out from testing probability densities and conditional
variances predicted by the two models against the experimental ones. The data
set we use contains the DEM/USD exchange quotes provided to us by Olsen &
Associates during a period of one year from January to December 1998. In this
period 1,620,843 quotes entries in the EFX system were recorded
A complete census of emitters in NGC 6397
We used a dataset of archival Hubble Space Telescope images obtained through
the F555W, F814W and F656N filters, to perform a complete search for objects
showing emission in the globular cluster NGC 6397. As photometric
diagnostic, we used the color excess in the
- color-color diagram. In the analysed field of view,
we identified 53 emitters. In particular, we confirmed the optical
counterpart to 20 X-ray sources (7 cataclysmic variables, 2 millisecond pulsars
and 11 active binaries) and identified 33 previously unknown sources, thus
significantly enlarging the population of known active binaries in this
cluster. We report the main characteristics for each class of objects.
Photometric estimates of the equivalent width of the emission line,
were derived from the -excess and, for the first time, compared
to the spectroscopic measurements obtained from the analysis of MUSE spectra.
The very good agreement between the spectroscopic and photometric measures
fully confirmed the reliability of the proposed approach to measure the
emission. The search demonstrated the efficiency of this novel
approach to pinpoint and measure -emitters, thus offering a powerful
tool to conduct complete census of objects whose formation and evolution can be
strongly affected by dynamical interactions in star clusters.Comment: Accepted for publication by ApJ; 14 pages, 8 Figures, 1 Tabl
The ESO Spectroscopic facility
We present the concept of a novel facility dedicated to massively-multiplexed
spectroscopy. The telescope has a very wide field Cassegrain focus optimised
for fibre feeding. With a Field of View (FoV) of 2.5 degrees diameter and a
11.4m pupil, it will be the largest etendue telescope. The large focal plane
can easily host up to 16.000 fibres. In addition, a gravity invariant focus for
the central 10 arc-minutes is available to host a giant integral field unit
(IFU). The 3 lenses corrector includes an ADC, and has good performance in the
360-1300 nm wavelength range. The top level science requirements were developed
by a dedicated ESO working group, and one of the primary cases is high
resolution spectroscopy of GAIA stars and, in general, how our Galaxy formed
and evolves. The facility will therefore be equipped with both, high and low
resolution spectrographs. We stress the importance of developing the telescope
and instrument designs simultaneously. The most relevant R\&D aspect is also
briefly discussed.Comment: 6 pages 4 figures , presented at IAU Symposium 334 "rediscovering our
galaxy
Consistent metallicity scale for cool dwarfs and giants. A benchmark test using the Hyades
In several instances chemical abundances of dwarf and giant stars are used
simultaneously under the assumption that they share the same abundance scale.
This assumption might have implications in different astrophysical contexts. We
aim to ascertain a methodology capable of producing a consistent metallicity
scale for giants and dwarfs. To achieve that, we analyzed giants and dwarfs in
the Hyades open cluster. All these stars have archival high-resolution
spectroscopic data obtained with HARPS and UVES. In addition, the giants have
interferometric measurements of the angular diameters. We analyzed the sample
with two methods. The first method constrains the atmospheric parameters
independently from spectroscopy. For that we present a novel calibration of
microturbulence based on 3D model atmospheres. The second method is the
classical spectroscopic based on Fe lines. We also tested two line lists in an
attempt to minimize possible non-LTE effects and to optimize the treatment of
the giants. We show that it is possible to obtain a consistent metallicity
scale between dwarfs and giants. The preferred method should constrain the
three parameters , , and independent of
spectroscopy. In particular, the lines should be chosen to be free of blends in
the spectra of giants. When attention is paid to the line list, the classical
spectroscopic method can also produce consistent results. The metallicities
derived with the well-constrained set of stellar parameters are consistent
independent of the line list used. Therefore, for this cluster we favor the
metallicity of +0.180.03 dex obtained with this method. The classical
spectroscopic analysis, using the line list optimized for the giants, provides
a metallicity of +0.140.03 dex, in agreement with previous works.Comment: 19 pages, 6 figures, 10 tables. Accepted for publication in A&
Operando Raman spectroscopy tracks oxidation-state changes in an amorphous Co oxide material for electrocatalysis of the oxygen evolution reaction
Transition metal oxides are of high interest in both energy storage (batteries) and production of non-fossil fuels by (photo)electrocatalysis. Their functionally crucial charge (oxidation state) changes and electrocatalytic properties are best investigated under electrochemical operation conditions. We established operando Raman spectroscopy for investigation of the atomic structure and oxidation state of a non-crystalline, hydrated, and phosphate-containing Co oxide material (CoCat), which is an electrocatalyst for the oxygen evolution reaction (OER) at neutral pH and is structurally similar to LiCoO2 of batteries. Raman spectra were collected at various sub-catalytic and catalytic electric potentials. 2H labeling suggests Co oxidation coupled to Co—OH deprotonation at catalytic potentials. 18O labeling supports O—O bond formation starting from terminally coordinated oxygen species. Two broad bands around 877 cm−1 and 1077 cm−1 are assigned to CoCat-internal H2PO4-. Raman peaks corresponding to terminal oxide (Co=O) or reactive oxygen species were not detectable; 1000–1200 cm−1 bands were instead assigned to two-phonon Raman scattering. At an increasingly positive potential, the intensity of the Raman bands decreased, which is unexpected and explained by self-absorption relating to CoCat electrochromism. A red-shift of the Co—O Raman bands with increasing potentials was described by four Gaussian bands of potential-dependent amplitudes. By linear combination of Raman band amplitudes, we can follow individually the Co(2+/3+) and Co(3+/4+) redox transitions, whereas previously published x-ray absorption spectroscopy analysis could determine only the averaged Co oxidation state. Our results show how electrochemical operando Raman spectroscopy can be employed as a potent analytical tool in mechanistic investigations on OER catalysis
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