1,203 research outputs found
Semi-quantum approach for fast atom diffraction: solving the rainbow divergence
In this work we introduce a distorted wave method, based on the Initial Value
Representation (IVR) approach of the quantum evolution operator, in order to
improve the semiclassical description of rainbow effects in diffraction
patterns produced by grazing scattering of fast atoms from crystal surfaces.
The proposed theory, named Surface Initial Value Representation (SIVR)
approximation, is applied to He atoms colliding with a LiF(001) surface along
low indexed crystallographic channels. For this collision system the SIVR
approach provides a very good representation of the quantum interference
structures of experimental projectile distributions, even in the angular region
around classical rainbow angles where common semiclassical methods diverge.Comment: 8 pages, 5 figure
Influence of the lighting on Fast Atom Diffraction studied via a semi-quantum approach
The influence of the collimating conditions of the incident beam on
diffraction patterns produced by grazing scattering of fast atoms off crystal
surfaces is studied within a semi-quantum approach, named Surface Initial Value
Representation (SIVR) approximation. In this approach we incorporate a
realistic description of the incident particle in terms of the collimating
parameters, which determine the surface area that is coherently illuminated.
The model is applied to He atoms colliding with a LiF(001) surface after
passing through a rectangular aperture. As it was experimentally observed [1],
SIVR spectra as a function of the azimuthal angle are very sensitive to the
width of the collimating slit. We also found that the length of the collimating
aperture affects polar angle distributions, introducing additional interference
structures for the longer collimating slits
Single- and double-slit collimating effects on fast-atom diffraction spectra
Diffraction patterns produced by fast He atoms grazingly impinging on a
LiF(001) surface are investigated focusing on the influence of the beam
collimation. Single- and double- slit collimating devices situated in front of
the beam source are considered. To describe the scattering process we use the
Surface Initial Value Representation (SIVR) approximation, which is a
semi-quantum approach that incorporates a realistic description of the initial
wave packet in terms of the collimating parameters. Our initial wave-packet
model is based on the Van Cittert-Zernike theorem. For a single-slit
collimation the width of the collimating aperture controls the shape of the
azimuthal angle distribution, making different interference mechanisms visible,
while the length of the slit affects the polar angle distribution.
Additionally, we found that by means of a double-slit collimation it might be
possible to obtain a wide polar angle distribution, which is associated with a
large spread of the initial momentum perpendicular to the surface, derived from
the uncertainty principle. It might be used as a simple way to probe the
surface potential for different normal distances
Ocular manifestations in Gorlin-Goltz syndrome
Background: Gorlin-Goltz syndrome, also known as nevoid basal cell carcinoma syndrome, is a rare genetic disorder that is transmitted in an autosomal dominant manner with complete penetrance and variable expressivity. It is caused in 85% of the cases with a known etiology by pathogenic variants in the PTCH1 gene, and is characterized by a wide range of developmental abnormalities and a predisposition to multiple neoplasms. The manifestations are multiple and systemic and consist of basal cell carcinomas in various regions, odontogenic keratocistic tumors and skeletal anomalies, to name the most frequent. Despite the scarce medical literature on the topic, ocular involvement in this syndrome is frequent and at the level of various ocular structures. Our study focuses on the visual apparatus and its annexes in subjects with this syndrome, in order to better understand how this syndrome affects the ocular system, and to evaluate with greater accuracy and precision the nature of these manifestations in this group of patients. Results: Our study confirms the presence of the commonly cited ocular findings in the general literature regarding the syndrome [hypertelorism (45.5%), congenital cataract (18%), nystagmus (9%), colobomas (9%)] and highlights strabismus (63% of the patients), epiretinal membranes (36%) and myelinated optic nerve fiber layers (36%) as the most frequent ophthalmological findings in this group of patients. Conclusions: The presence of characteristic and frequent ocular signs in the Gorlin- Goltz syndrome could help with the diagnostic process in subjects suspected of having the syndrome who do not yet have a diagnosis. The ophthalmologist has a role as part of a multidisciplinary team in managing these patients. The ophthalmological follow-up that these patients require, can allow, if necessary, a timely therapy that could improve the visual prognosis of such patients
Magnetotransport and the upper critical magnetic field in MgB2
Magnetotransport measurements are presented on polycrystalline MgB2 samples.
The resistive upper critical magnetic field reveals a temperature dependence
with a positive curvature from Tc = 39.3 K down to about 20 K, then changes to
a slightly negative curvature reaching 25 T at 1.5 K. The 25- Tesla upper
critical field is much higher than what is known so far on polycrystals of MgB2
but it is in agreement with recent data obtained on epitaxial MgB2 films. The
deviation of Bc2(T) from standard BCS might be due to the proposed two-gap
superconductivity in this compound. The observed quadratic normal-state
magnetoresistance with validity of Kohler's rule can be ascribed to classical
trajectory effects in the low-field limit.Comment: 6 pages, incl. 3 figure
Two Gap State Density in MgB: A True Bulk Property or A Proximity Effect?
We report on the temperature dependence of the quasiparticle density of
states (DOS) in the simple binary compound MgB2 directly measured using
scanning tunneling microscope (STM). To achieve high quality tunneling
conditions, a small crystal of MgB2 is used as a tip in the STM experiment. The
``sample'' is chosen to be a 2H-NbSe2 single crystal presenting an atomically
flat surface. At low temperature the tunneling conductance spectra show a gap
at the Fermi energy followed by two well-pronounced conductance peaks on each
side. They appear at voltages V mV and V mV. With rising temperature both peaks disappear at the Tc of the bulk
MgB2, a behavior consistent with the model of two-gap superconductivity. The
explanation of the double-peak structure in terms of a particular proximity
effect is also discussed.Comment: 4 pages, 3 figure
Self help groups in a city of Tuscany: Reconstruction of the second generation model of work for professionals and services
This study is part of a more extensive project aimed to investigate the effectiveness of self-help group participation in improving quality of life in mental disease. The study is taking place in the Tuscany Region, in Italy. In the first qualitative step of analysis researchers are interested in describing the specific features of the psychiatric self-help movement in Tuscany, comparing different realities, networks, kind of groups. Therefore, our aim is to collect exhaustive information to describe how self-help system work in different provinces at the present moment. The implementation of groups for psychiatric problems is quite young in Italy. Because of a lack of specific regulation in the directives of the Italian health care system, every local service has implemented groups differently, sometimes enhancing, sometimes dismissing them. Prato, near Florence, is one of the more interesting context for the birth of psychiatric self-help movement in the region: public health services improved groups since early 90’s, it was one of the first self-help reality linked to services in the entire region. Now we are in a “second generation” of professionals, and the original meaning of groups seems to be transformed, sometimes misunderstood. Our objectives of study head us toward an in depth analysis of self-help phenomenon in Prato
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