3,343 research outputs found
Approximations for many-body Green's functions: insights from the fundamental equations
Several widely used methods for the calculation of band structures and photo
emission spectra, such as the GW approximation, rely on Many-Body Perturbation
Theory. They can be obtained by iterating a set of functional differential
equations relating the one-particle Green's function to its functional
derivative with respect to an external perturbing potential. In the present
work we apply a linear response expansion in order to obtain insights in
various approximations for Green's functions calculations. The expansion leads
to an effective screening, while keeping the effects of the interaction to all
orders. In order to study various aspects of the resulting equations we
discretize them, and retain only one point in space, spin, and time for all
variables. Within this one-point model we obtain an explicit solution for the
Green's function, which allows us to explore the structure of the general
family of solutions, and to determine the specific solution that corresponds to
the physical one. Moreover we analyze the performances of established
approaches like over the whole range of interaction strength, and we
explore alternative approximations. Finally we link certain approximations for
the exact solution to the corresponding manipulations for the differential
equation which produce them. This link is crucial in view of a generalization
of our findings to the real (multidimensional functional) case where only the
differential equation is known.Comment: 17 pages, 7 figure
Current applications of 3d printing in neurosurgery
Medical implications of 3-dimensional (3D) printing technology have progressed with increasingly used especially in surgical fields. 3D printing techniques are practical and anatomically accurate methods of producing patient specific models for medical education, surgical planning, training and simulation, and implants production for the assessment and treatment of neurosurgical diseases. This article presents the main directions of 3D printing models application in neurosurgery
Neuroendoscopic surgery in hydrocephalus
Hydrocephalus represents one of the most frequent pathologic entities requiring neurosurgical intervention. Even in present times the treatment of hydrocephalus is a highly debated subject. Neuroendoscopy is one of the technique posibility. The present study includes patients with different types of hydrocephalus classified according to imagistic criteria (MRI). Neuroendoscopic therapeutic options are presented for different types of hydrocephalus and the postoperative results of these procedures are revealed by imaging. In conclusion, we consider neuroendoscopy useful in each type of hydrocephalus
Special features of the Be2He fragmentation in emulsion at an energy of 1.2~A~GeV
The results of investigations of the relativistic Be nucleus
fragmentation in emulsion which entails the production of two He fragments of
an energy of 1.2~A~GeV are presented. The results of the angular measurements
of the Be2He events are analyzed.
The BeBe+n fragmentation channel involving the Be decay from
the ground (0) and the first excited (2) states to two
particles is observed to be predominant.Comment: 10 pages, 6 figures, conference: Conference on Physics of Fundamental
Interactions, Moscow, Russia, 5-9 Dec 2005 (Author's translation
Preliminary study of thrombogenicity induced by the nanoparticle surface coating of intracranial stents
Endovascular treatment of intracranial aneurysms with intracranial stents was proven to be clinically safe and effective, but is still associated with a risk of thromboembolic complications. Stent thrombosis could be a sever complication associated with specific stent surface coatings and designs. Standardized in vitro tests for investigation of thrombogenicity induced by different nanomaterials were used as the basic method in carrying out the present study. Therefore, the aim of this study was to evaluate the thrombogenicity of three different nanomaterials (ZnO, TiO2 si Fe3O4) possible used as surface coating for intracranial stents. This study is based on a procedure for in vitro analyses of plasma coagulation time. To measure the plasma coagulation time, platelet-poor plasma from human whole blood was in vitro exposed to nanoparticles and analysed in prothrombin (PT) and activated partial thromboplastin (APTT)
Some segmental morphological and morphometrical features of the intima and media of the aortic wall in Chinchilla lanigera
Background: The aim of this study is to describe the morphology, morphometry and ultrastructure of segments of the thoracic and abdominal aorta portions in Chinchilla lanigera. Thickness measurements of the tunica intima and media complex of the aorta were taken. Materials and methods: In all observed specimens, the thickness values for the tunica intima and media complex of the cranial thoracic aorta were significantly higher (mean: 702.19 ÎŒm) when compared to the values of other analysed aortic segments (means: 354.18 ÎŒm; 243.55 ÎŒm). Complex statistical methods were used to assess the differences between various aortic segments. Results and Conclusions: The components of the vessel walls show variations in structure and thickness, presumably due to an adaptation to functional demand
Low-Temperature Mobility of Surface Electrons and Ripplon-Phonon Interaction in Liquid Helium
The low-temperature dc mobility of the two-dimensional electron system
localized above the surface of superfluid helium is determined by the slowest
stage of the longitudinal momentum transfer to the bulk liquid, namely, by the
interaction of surface and volume excitations of liquid helium, which rapidly
decreases with temperature. Thus, the temperature dependence of the
low-frequency mobility is \mu_{dc} = 8.4x10^{-11}n_e T^{-20/3} cm^4 K^{20/3}/(V
s), where n_e is the surface electron density. The relation
T^{20/3}E_\perp^{-3} << 2x10^{-7} between the pressing electric field (in
kV/cm) and temperature (in K) and the value \omega < 10^8 T^5 K^{-5}s^{-1} of
the driving-field frequency have been obtained, at which the above effect can
be observed. In particular, E_\perp = 1 kV/cm corresponds to T < 70 mK and
\omega/2\pi < 30 Hz.Comment: 4 pages, 1 figur
- âŠ