486 research outputs found
Three-Body and One-Body Channels of the Auger Core-Valence-Valence decay: Simplified Approach
We propose a computationally simple model of Auger and APECS line shapes from
open-band solids. Part of the intensity comes from the decay of unscreened
core-holes and is obtained by the two-body Green's function ,
as in the case of filled bands. The rest of the intensity arises from screened
core-holes and is derived using a variational description of the relaxed ground
state; this involves the two-holes-one-electron propagator , which
also contains one-hole contributions. For many transition metals, the two-hole
Green's function can be well described by the Ladder
Approximation, but the three-body Green's function poses serious further
problems. To calculate , treating electrons and holes on equal
footing, we propose a practical approach to sum the series to all orders. We
achieve that by formally rewriting the problem in terms of a fictitious
three-body interaction. Our method grants non-negative densities of states,
explains the apparent negative-U behavior of the spectra of early transition
metals and interpolates well between weak and strong coupling, as we
demonstrate by test model calculations.Comment: AMS-LaTeX file, 23 pages, 8 eps and 3 ps figures embedded in the text
with epsfig.sty and float.sty, submitted to Phys. Rev.
W=0 pairing in Hubbard and related models of low-dimensional superconductors
Lattice Hamiltonians with on-site interaction have W=0 solutions, that
is, many-body {\em singlet} eigenstates without double occupation. In
particular, W=0 pairs give a clue to understand the pairing force in repulsive
Hubbard models. These eigenstates are found in systems with high enough
symmetry, like the square, hexagonal or triangular lattices. By a general
theorem, we propose a systematic way to construct all the W=0 pairs of a given
Hamiltonian. We also introduce a canonical transformation to calculate the
effective interaction between the particles of such pairs. In geometries
appropriate for the CuO planes of cuprate superconductors, armchair
Carbon nanotubes or Cobalt Oxides planes, the dressed pair becomes a bound
state in a physically relevant range of parameters. We also show that W=0 pairs
quantize the magnetic flux like superconducting pairs do. The pairing mechanism
breaks down in the presence of strong distortions. The W=0 pairs are also the
building blocks for the antiferromagnetic ground state of the half-filled
Hubbard model at weak coupling. Our analytical results for the
Hubbard square lattice, compared to available numerical data, demonstrate that
the method, besides providing intuitive grasp on pairing, also has quantitative
predictive power. We also consider including phonon effects in this scenario.
Preliminary calculations with small clusters indicate that vector phonons
hinder pairing while half-breathing modes are synergic with the W=0 pairing
mechanism both at weak coupling and in the polaronic regime.Comment: 42 pages, Topical Review to appear in Journal of Physics C: Condensed
Matte
Auger transition from orbitally degenerate systems: Effects of screening and multielectron excitations
We calculate Auger spectra given by the two-hole Green's function from
orbitally degenerate Hubbard-like models as a function of correlation strength
and band filling. The resulting spectra are qualitatively different from those
obtained from fully-filled singly degenerate models due to the presence of
screening dynamics and multielectron excitations. Application to a real system
shows remarkable agreement with experimental results leading to
reinterpretation of spectral features.Comment: To appear in Phy. Rev. Let
Are violations to temporal Bell inequalities there when somebody looks?
The possibility of observing violations of temporal Bell inequalities,
originally proposed by Leggett as a mean of testing the quantum mechanical
delocalization of suitably chosen macroscopic bodies, is discussed by taking
into account the effect of the measurement process. A general criterion
quantifying this possibility is defined and shown not to be fulfilled by the
various experimental configurations proposed so far to test inequalities of
different forms.Comment: 7 pages, 1 eps figure, needs europhys.sty and euromacr.tex, enclosed
in the .tar.gz file; accepted for publication in Europhysics Letter
Curie-Weiss model of the quantum measurement process
A hamiltonian model is solved, which satisfies all requirements for a
realistic ideal quantum measurement. The system S is a spin-\half, whose
-component is measured through coupling with an apparatus A=M+B, consisting
of a magnet \RM formed by a set of spins with quartic infinite-range
Ising interactions, and a phonon bath \RB at temperature . Initially A is
in a metastable paramagnetic phase. The process involves several time-scales.
Without being much affected, A first acts on S, whose state collapses in a very
brief time. The mechanism differs from the usual decoherence. Soon after its
irreversibility is achieved. Finally the field induced by S on M, which may
take two opposite values with probabilities given by Born's rule, drives A into
its up or down ferromagnetic phase. The overall final state involves the
expected correlations between the result registered in M and the state of S.
The measurement is thus accounted for by standard quantum statistical mechanics
and its specific features arise from the macroscopic size of the apparatus.Comment: 5 pages Revte
Gut microbial composition in different castes and developmental stages of the invasive hornet Vespa velutina nigrithorax
Social insects are successful animal invaders. Their survival and success, and in some cases also their impact on invaded ecosystem functioning, is often mediated by symbiosis with microorganisms. Here, we report a comprehensive comparative characterization of the gut microbial communities of different castes and developmental stages of the invasive hornet Vespa velutina nigrithorax. The species recently colonized Europe, becoming a high ecological and economic concern, as it threatens pollinator survival and competes with native hornet species. We used targeted meta-genomics to describe the yeasts and bacteria gut communities of individuals of different reproductive phenotypes (workers and future queens), life stages (larvae, newly emerged individuals and adults) and colony non-living samples (nest paper and larval faeces). Bacilli, Gammaproteobacteria, Actinobacteria, Alphaproteobacteria were the most abundant classes of bacteria, and Saccharomycetes, Dothideomycetes, Tremellomycetes and Eurotiomycetes were the most represented yeast classes. We found that the microbial compositions significantly differ across developmental stages and castes, with yeast and bacterial communities switching in frequency and abundance during ontogeny and according to reproductive phenotype. Moreover, the gut microbial communities poorly mirror those found in the nest, suggesting that hornets possess a specific microbial signature. Our results provide the first metagenomic resource of the microbiome of V. velutina in Europe and suggest the importance of considering life stages, reproductive phenotypes and nest influence in order to obtain a comprehensive picture of social insect microbial communities
Theory of Spin-Resolved Auger-Electron Spectroscopy from Ferromagnetic 3d-Transition Metals
CVV Auger electron spectra are calculated for a multi-band Hubbard model
including correlations among the valence electrons as well as correlations
between core and valence electrons. The interest is focused on the
ferromagnetic 3d-transition metals. The Auger line shape is calculated from a
three-particle Green function. A realistic one-particle input is taken from
tight-binding band-structure calculations. Within a diagrammatic approach we
can distinguish between the \textit{direct} correlations among those electrons
participating in the Auger process and the \textit{indirect} correlations in
the rest system. The indirect correlations are treated within second-order
perturbation theory for the self-energy. The direct correlations are treated
using the valence-valence ladder approximation and the first-order perturbation
theory with respect to valence-valence and core-valence interactions. The
theory is evaluated numerically for ferromagnetic Ni. We discuss the
spin-resolved quasi-particle band structure and the Auger spectra and
investigate the influence of the core hole.Comment: LaTeX, 12 pages, 8 eps figures included, Phys. Rev. B (in press
Is preoperative ultrasound tumor size a prognostic factor in endometrial carcinoma patients?
Objective: We aimed to assess the prognostic value of preoperative ultrasound tumor size in EC through a single center, observational, retrospective, cohort study. Methods: Medical records and electronic clinical databases were searched for all consecutive patients with EC, preoperative ultrasound scans available to ad hoc estimate tumor size, and a follow-up of at least 2-year, at our Institution from January 2010 to June 2018. Patients were divided into two groups based on different dimensional cut-offs for the maximum tumor diameter: 2, 3 and 4 cm. Differences in overall survival (OS), disease specific survival (DSS) and progression-free survival (PFS) were assessed among the groups by using the Kaplan–Meier estimator and the log-rank test. Results: 108 patients were included in the study. OS, DSS and PFS did not significantly differ between the groups based on the different tumor diameter cut-offs. No significant differences were found among the groups sub-stratified by age, BMI, FIGO stage, FIGO grade, lymphovascular space invasion status, myometrial invasion, lymph nodal involvement, histotype, and adjuvant treatment. Conclusions: Preoperative ultrasound tumor size does not appear as a prognostic factor in EC women
Institutional change in the European Parliament:balancing legislative ethics and parliamentary independence
- …