56 research outputs found
Mixed ab initio quantum mechanical and Monte Carlo calculations of secondary emission from SiO2 nanoclusters
A mixed quantum mechanical and Monte Carlo method for calculating Auger
spectra from nanoclusters is presented. The approach, based on a cluster
method, consists of two steps. Ab initio quantum mechanical calculations are
first performed to obtain accurate energy and probability distributions of the
generated Auger electrons. In a second step, using the calculated line shape as
electron source, the Monte Carlo method is used to simulate the effect of
inelastic losses on the original Auger line shape. The resulting spectrum can
be directly compared to 'as-acquired' experimental spectra, thus avoiding
background subtraction or deconvolution procedures. As a case study, the O K-LL
spectrum from solid SiO2 is considered. Spectra computed before or after the
electron has traveled through the solid, i.e., unaffected or affected by
extrinsic energy losses, are compared to the pertinent experimental spectra
measured within our group. Both transition energies and relative intensities
are well reproduced.Comment: 9 pageg, 5 figure
Lorentz Symmetry in QFT on Quantum Bianchi I Space-Time
We develop the quantum theory of a scalar field on LQC Bianchi I geometry. In
particular, we focus on single modes of the field: the evolution equation is
derived from the quantum scalar constraint, and it is shown that the same
equation can be obtained from QFT on an "classical" effective geometry. We
investigate the dependence of this effective space-time on the wavevector of
the mode (which could in principle generate a deformation in local
Lorentz-symmetry), focusing our attention on the dispersion relation. We prove
that when we disregard backreaction no Lorentz-violation is present, despite
the effective metric being different than the classical Bianchi I one. A
preliminary analysis of the correction due to inclusion of backreaction is
briefly discussed in the context of Born-Oppenheimer approximation.Comment: 14 pages, v3. Corrected a reference in the bibliograph
Electronic excitation spectra of cerium oxides: from ab initio dielectric response functions to Monte Carlo electron transport simulations
Nanomaterials made of the cerium oxides CeO and CeO have a broad
range of applications, from catalysts in automotive, industrial or energy
operations to promising materials to enhance hadrontherapy effectiveness in
oncological treatments. To elucidate the physico-chemical mechanisms involved
in these processes, it is of paramount importance to know the electronic
excitation spectra of these oxides, which are obtained here through
high-accuracy linear-response time-dependent density functional theory
calculations. In particular, the macroscopic dielectric response functions
of both bulk CeO and CeO are derived, which compare
remarkably well with the available experimental data. These results stress the
importance of appropriately accounting for local field effects to model the
dielectric function of metal oxides. Furthermore, we reckon the materials
energy loss functions \mbox{Im} (-1/\bar{\epsilon}), including the accurate
evaluation of the momentum transfer dispersion from first-principles. In this
respect, by using a Mermin-type parametrization we are able to model the
contribution of different electronic excitations to the dielectric loss
function. Finally, from the knowledge of the electron inelastic mean free path,
together with the elastic mean free path provided by the relativistic Mott
theory, we carry out statistical Monte Carlo (MC) charge transport simulations
to reproduce the major features of the reported experimental reflection
electron energy loss (REEL) spectra of cerium oxides. The good agreement with
REEL experimental data strongly supports our approach based on MC modelling
informed by ab initio calculated electronic excitation spectra in a broad range
of momentum and energy transfers.Comment: 21 pages, 19 figure
Inflation from non-minimally coupled scalar field in loop quantum cosmology
The FRW model with non-minimally coupled massive scalar field has been
investigated in LQC framework. Considered form of the potential and coupling
allows applications to Higgs driven inflation. Out of two frames used in the
literature to describe such systems: Jordan and Einstein frame, the latter one
is applied. Specifically, we explore the idea of the Einstein frame being the
natural 'environment' for quantization and the Jordan picture having an
emergent nature. The resulting dynamics qualitatively modifies the standard
bounce paradigm in LQC in two ways: (i) the bounce point is no longer marked by
critical matter energy density, (ii) the Planck scale physics features the
'mexican hat' trajectory with two consecutive bounces and rapid expansion and
recollapse between them. Furthermore, for physically viable coupling strength
and initial data the subsequent inflation exceeds 60 e-foldings.Comment: Clarity improved. Replaced with revised version accepted in JCA
CSF parvalbumin levels reflect interneuron loss linked with cortical pathology in multiple sclerosis
INTRODUCTION AND METHODS: In order to verify whether parvalbumin (PVALB), a protein specifically expressed by GABAergic interneurons, could be a MS-specific marker of grey matter neurodegeneration, we performed neuropathology/molecular analysis of PVALB expression in motor cortex of 40 post-mortem progressive MS cases, with/without meningeal inflammation, and 10 control cases, in combination with cerebrospinal fluid (CSF) assessment. Analysis of CSF PVALB and neurofilaments (Nf-L) levels combined with physical/cognitive/3TMRI assessment was performed in 110 naïve MS patients and in 32 controls at time of diagnosis. RESULTS: PVALB gene expression was downregulated in MS (fold change = 3.7 ± 1.2, P < 0.001 compared to controls) reflecting the significant reduction of PVALB+ cell density in cortical lesions, to a greater extent in MS patients with high meningeal inflammation (51.8, P < 0.001). Likewise, post-mortem CSF-PVALB levels were higher in MS compared to controls (fold change = 196 ± 36, P < 0.001) and correlated with decreased PVALB+ cell density (r = -0.64, P < 0.001) and increased MHC-II+ microglia density (r = 0.74, P < 0.01), as well as with early age of onset (r = -0.69, P < 0.05), shorter time to wheelchair (r = -0.49, P < 0.05) and early age of death (r = -0.65, P < 0.01). Increased CSF-PVALB levels were detected in MS patients at diagnosis compared to controls (P = 0.002). Significant correlation was found between CSF-PVALB levels and cortical lesion number on MRI (R = 0.28, P = 0.006) and global cortical thickness (R = -0.46, P < 0.001), better than Nf-L levels. CSF-PVALB levels increased in MS patients with severe cognitive impairment (mean ± SEM:25.2 ± 7.5 ng/mL) compared to both cognitively normal (10.9 ± 2.4, P = 0.049) and mild cognitive impaired (10.1 ± 2.9, P = 0.024) patients. CONCLUSIONS: CSF-PVALB levels reflect loss of cortical interneurons in MS patients with more severe disease course and might represent an early, new MS-specific biomarker of cortical neurodegeneration, atrophy, and cognitive decline
Radiation-induced melting in coherent X-ray diffractive imaging at the nanoscale
Coherent X-ray diffraction techniques play an increasingly significant role in imaging nanoscale structures which range from metallic and semiconductor samples to biological objects. The conventional knowledge about radiation damage effects caused by ever higher brilliance X-ray sources has to be critically revised while studying nanostructured materials
Backscattering of electrons from selected oxides: MgO, SiO
The inelastic and elastic cross-sections of an interacting
electron beam with MgO, SiO2, and Al2O3 have been calculated
and tabulations are provided for primary electron energies lower than 10 keV.
By using the tabulated cross-sections, a Monte-Carlo simulation is
utilized to obtain the backscattering coefficient in the energy
range 1–10 ke
Reducing Hydrogen Permeation through Metals
Metal–hydrogen systems are of great basic and technological interest in connection to the
role of hydrogen as a clean energy carrier. Frequently, metal systems are involved in hydrogen
purification, storage, and engines making use of this fuel. The presence of hydrogen in a metallic
matrix gives rise to modifications of electrical, optical and mechanical properties. Hydrogen
accumulation in metals may cause damage to the material by also producing fracture, thus limiting
operating lifetime. Reducing the hydrogen permeation is an important task also for the fusion
reactors: it is well known, indeed, that tritium is radioactive so that it is very important to be able to
confine tritium during the nuclear fusion process. The theoretical study of permeation is thus of
fundamental importance to obtain efficient barriers to permeation. Hydrogen trapping sites have a
great influence on the hydrogen permeation through a slab sample. The diffusion of the hydrogen in
a crystal is generally described by a parabolic partial differential equation with appropriate boundary
conditions. The numerical simulation code PHM (Permeation of Hydrogen through Metals),
realized for the study of the permeation of hydrogen in presence of trapping sites, is here described
and utilized for the analysis of the influence of reversible and irreversible traps on the diffusion of
hydrogen in a metal
Hydrogen permeation through a slab sample in the case of high hydrogen concentration
Hydrogen trapping sites have a great influence on the hydrogen permeation through a slab sample. The diffusion of the hydrogen in a crystal is generally governed by a parabolic partial differential equation: a numerical simulation code, realized for the study of the permeation of hydrogen in presence of trapping sites, has been utilized for the analysis of the influence of reversible and irreversible traps on the diffusion of hydrogen in a metal for the case of high (not negligible) hydrogen concentration with boundary conditions which cannot be treated analytically
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