A gauge approach to the "pseudogap" phenomenology of the spectral weight in high Tc cuprates

We assume the t-t'-J model to describe the CuO_2 planes of hole-doped cuprates and we adapt the spin-charge gauge approach, previously developed for the t-J model, to describe the holes in terms of a spinless fermion carrying the charge (holon) and a neutral boson carrying spin 1/2 (spinon), coupled by a slave-particle gauge field. In this framework we consider the effects of a finite density of incoherent holon pairs in the normal state. Below a crossover temperature, identified as the experimental "upper pseudogap", the scattering of the "quanta" of the phase of the holon-pair field against holons reproduces the phenomenology of Fermi arcs coexisting with gap in the antinodal region. We thus obtain a microscopic derivation of the main features of the hole spectra due to pseudogap. This result is obtained through a holon Green function which follows naturally from the formalism and analytically interpolates between a Fermi liquid-like and a d-wave superconductor behavior as the coherence length of the holon pair order parameter increases. By inserting the gauge coupling with the spinon we construct explicitly the hole Green function and calculate its spectral weight and the corresponding density of states. So we prove that the formation of holon pairs induces a depletion of states on the hole Fermi surface. We compare our results with ARPES and tunneling experimental data. In our approach the hole preserves a finite Fermi surface until the superconducting transition, where it reduces to four nodes. Therefore we propose that the gap seen in the normal phase of cuprates is due to the thermal broadening of the SC-like peaks masking the Fermi-liquid peak. The Fermi arcs then correspond to the region of the Fermi surface where the Fermi-liquid peak is unmasked.Comment: 10 figures, comments and references added, 2 figures change

A collimation system for ELI-NP Gamma Beam System - design and simulation of performance

The purpose of this study was to evaluate the performance and refine the design of the collimation system for the gamma radiation source (GBS) currently being realised at ELI-NP facility. The gamma beam, produced by inverse Compton scattering, will provide a tunable average energy in the range between 0.2 and 20 MeV, an energy bandwidth 0.5% and a flux of about 108 photons/s. As a result of the inverse Compton interaction, the energy of the emitted radiation is related to the emission angle, it is maximum in the backscattering direction and decreases as the angle increase [1,2]. Therefore, the required energy bandwidth can be obtained only by developing a specific collimation system of the gamma beam, i.e. filtering out the radiation emitted at larger angles. The angular acceptance of the collimation for ELI-NP-GBS must be continuously adjustable in a range from about 700 to 60 μrad, to obtain the required parameters in the entire energy range. The solution identified is a stack of adjustable slits, arranged with a relative rotation around the beam axis to obtain an hole with an approximately circular shape. In this contribution, the final collimation design and its performance evaluated by carrying out a series of detailed Geant4 simulations both of the high-energy and the low-energy beamline are presented

Validation of a hemodynamic model for the study of the cerebral venous outflow system using MR imaging and echo- Color doppler data

BACKGROUND AND PURPOSE: A comprehensive parameter model was developed to investigate correlations between cerebral hemodynamics and alterations in the extracranial venous circulation due to posture changes and/or extracranial venous obstruction (stenosis). The purpose of this work was to validate the simulation results by using MR imaging and echo-color Doppler experimental blood flow data in humans. MATERIALS AND METHODS: To validate the model outcomes, we used supine average arterial and venous extracerebral blood flow, obtained by using phase-contrast MR imaging from 49 individuals with stenosis in the acquisition plane at the level of the disc between the second and third vertebrae of the left internal jugular vein, 20 with stenosis in the acquisition plane at the level of the disc between the fifth and sixth vertebrae of the right internal jugular vein, and 38 healthy controls without stenosis. Average data from a second group of 10 healthy volunteers screened with an echo-color Doppler technique were used to evaluate flow variations due to posture change. RESULTS: There was excellent agreement between experimental and simulated supine flows. Every simulated CBF fell inside the standard error from the corresponding average experimental value, as well as most of the simulated extracerebral arterial flow (extracranial blood flow from the head and face, measured at the level of the disc between second and third vertebrae) and venous flows. Simulations of average jugular and vertebral blood flow variations due to a change of posture from supine to upright also matched the experimental data. CONCLUSIONS: The good agreement between simulated and experimental results means that the model can correctly reproduce the main factors affecting the extracranial circulation and could be used to study other types of stenotic conditions not represented by the experimental data

EuroGammaS gamma characterisation system for ELI-NP-GBS: The nuclear resonance scattering technique

A Gamma Beam Characterisation System has been designed by the EuroGammaS association for thecommissioning and development of the Extreme Light Infrastructure-Nuclear Physics Gamma Beam System(ELI-NP-GBS) to be installed in Magurele, Romania. The characterisation system consists of four elements: aCompton spectrometer, a sampling calorimeter, a nuclear resonant scattering spectrometer (NRSS) and a beamprofile imager. In this paper, the nuclear resonant scattering spectrometer system, designed to perform anabsolute energy calibration for the gamma beam, will be describe

Dual energy imaging in mammography: Cross-talk study in a Si array detector

Abstract One of the main limitation to the extensive use of breast-cancer screening as a prevention method is the relatively high X-ray dose released to the patient. A new approach is under study in which two quasi-monochromatic beamswith mean energies of 18.0 and 36.0 keV -are produced simultaneously, starting from an X-ray tube, by means of a monochromator based on a pyrolytic graphite crystal. The two beams are superimposed in space. The removal of the energy components with low content of diagnostic information from the spectrum, leads to a reduction of the dose released to patients maintaining (or improving) the image quality. The two quasi-monochromatic beams impinge on the patient and then are detected with a solid-state array detector; the image results as the difference between the transmitted intensities of the two detected beams. In this work, the performances of two different electronic readouts and three pixel widths of a silicon position sensitive array detector are simulated and described in order to minimize cross-talk effects between adjacent pixels. The use of a detector with spectrometric capabilities is necessary to separate, by means of thresholds, the high energy photons from the low energy ones

X-ray topographic determination of the granular structure in a graphite mosaic crystal: a three-dimensional reconstruction

Section topographs recorded at different spatial locations and at different rocking angles of a highly oriented pyrolytic graphite (HOPG) crystal allow three-dimensional maps of the local angular-dependent scattering power to be obtained. This is performed with a direct reconstruction from the intensity distribution on such topographs. The maps allow the extraction of information on local structural parameters such as size, form and internal mosaic spread of crystalline domains. This data analysis leads to a new method for the characterization of mosaic crystals. Perspectives and limits of applicability of this method are discussed

Influence of the serotonin transporter 5HTTLPR polymorphism on symptom severity in irritable bowel syndrome

5HTTLPR polymorphism of serotonin transporter yields short (S) and long (L) alleles. SS and LS genotypes are associated with reduced expression of serotonin transporter. This cross-sectional study investigated the association of 5HTTLPR with symptom severity of irritable bowel syndrome (IBS). Patients with IBS (Rome III) and healthy controls were included. Genomic DNA was extracted from saliva, and 5HTTLPR alleles were assessed by polymerase chain reaction. IBS symptom severity was evaluated by means of IBS-SSS questionnaire. Two hundreds and four IBS patients (159 females; mean age: 39.6±12.3 years; 106 with constipation: C-IBS; 98 with diarrhea: D-IBS) and 200 healthy controls (154 females; mean age: 40.4±15.8 years) were enrolled. The overall IBS-SSS value was higher in LS/SS than LL patients (319.0±71.5 versus 283.8±62.3; P = 0.0006). LS/SS patients had also higher values of abdominal pain (59.7±21.0 versus 51.0±18.8; P = 0.020) and bowel dissatisfaction (80.1±23.9 versus 70.5±22.8; P = 0.035). The overall IBS-SSS values in C-IBS and D-IBS patients were 317.2±68.3 and 296.1±71.4, respectively (P = 0.192), with significantly higher values for abdominal distension (65.0±24.4 versus 51.4±24.8; P = 0.0006), but not for bowel dissatisfaction (80.5±21.7 versus 72.9±25.7; P = 0.138). Frequencies of 5HTTLPR genotypes did not differ significantly when comparing IBS patients (overall or upon stratification in C-IBS and D-IBS) with healthy controls. In conclusion, the LS and SS genotypes are significantly correlated with IBS symptom severity, although their possible direct causal role remains to be proven. In addition, the present findings do not support an association of 5HTTLPR with IBS or its clinical presentation in terms of bowel habit predominance

X-ray topographic determination of the granular structure in a graphite mosaic crystal : a three-dimensional reconstruction

Section topographs recorded at different spatial locations and at different rocking angles of a highly oriented pyrolytic graphite (HOPG) crystal allow three-dimensional maps of the local angular-dependent scattering power to be obtained. This is performed with a direct reconstruction from the intensity distribution on such topographs. The maps allow the extraction of information on local structural parameters such as size, form and internal mosaic spread of crystalline domains. This data analysis leads to a new method for the characterization of mosaic crystals. Perspectives and limits of applicability of this method are discussed

Silicon strip detectors for two-dimensional soft X-ray imaging at normal incidence

A simple prototype system for static two-dimensional soft X-ray imaging using silicon microstrip detectors irradiated at normal incidence is presented. Radiation sensors consist of single-sided silicon detectors made from 300 mum thick wafers, read by RX64 ASICs. Data acquisition and control is performed by a Windows PC workstation running dedicated LabVIEW routines, connected to the sensors through a PCI-DIO-96 interface. Two-dimensional images are obtained by scanning a lead collimator with a thin slit perpendicular to the strip axis, along the whole detector size; the several strip profiles (slices) taken at each position are then put together to form a planar image. Preliminary results are presented, illustrating the high-resolution imaging capabilities of the system with soft X-rays. (C) 2003 Elsevier B.V. All rights reserved.info:eu-repo/semantics/publishedVersio