451 research outputs found

    Anisotropic softening of magnetic excitations along the nodal direction in superconducting cuprates

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    The high-T-c cuprate superconductors are close to antiferromagnetic order. Recent measurements of magnetic excitations have reported an intriguing similarity to the spin wavesmagnons- of the antiferromagnetic insulating parent compounds, suggesting that magnons may survive in damped, broadened form throughout the phase diagram. Here we show by resonant inelastic X-ray scattering on Bi2Sr2CaCu2O8+delta (Bi-2212) that the analogy with spin waves is only partial. The magnon-like features collapse along the nodal direction in momentum space and exhibit a photon energy dependence markedly different from the Mott-insulating case. These observations can be naturally described by the continuum of charge and spin excitations of correlated electrons. The persistence of damped magnons could favour scenarios for superconductivity built from quasiparticles coupled to spin fluctuations. However, excitation spectra composed of particle-hole excitations suggest that superconductivity emerges from a coherent treatment of electronic spin and charge in the form of quasiparticles with very strong magnetic correlations

    Mount control system of the ASTRI SST-2M prototype for the Cherenkov Telescope Array

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    The ASTRI SST-2M telescope is an end-to-end prototype proposed for the Small Size class of Telescopes (SST) of the future Cherenkov Telescope Array (CTA). The prototype is installed in Italy at the INAF observing station located at Serra La Nave on Mount Etna (Sicily) and it was inaugurated in September 2014. This paper presents the software and hardware architecture and development of the system dedicated to the control of the mount, health, safety and monitoring systems of the ASTRI SST-2M telescope prototype. The mount control system installed on the ASTRI SST-2M telescope prototype makes use of standard and widely deployed industrial hardware and software. State of the art of the control and automation industries was selected in order to fulfill the mount related functional and safety requirements with assembly compactness, high reliability, and reduced maintenance. The software package was implemented with the Beckhoff TwinCAT version 3 environment for the software Programmable Logical Controller (PLC), while the control electronics have been chosen in order to maximize the homogeneity and the real time performance of the system. The integration with the high level controller (Telescope Control System) has been carried out by choosing the open platform communications Unified Architecture (UA) protocol, supporting rich data model while offering compatibility with the PLC platform. In this contribution we show how the ASTRI approach for the design and implementation of the mount control system has made the ASTRI SST-2M prototype a standalone intelligent machine, able to fulfill requirements and easy to be integrated in an array configuration such as the future ASTRI mini-array proposed to be installed at the southern site of the Cherenkov Telescope Array (CTA)

    An experimental study of novel schemes for low-energy-threshold and low-background particle detection in rare gas crystals

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    There are numerous indications about possible new physics beyond the Standard Model and in particular, one of the major issues refers to the Universe composition. Indeed, the ordinary matter, which is everything we know, seems to account for only a tiny fraction of the Cosmos' mass. The unknown part of the Universe mass is the so-called dark matter which represents one of the greatest mysteries of the modern science. Experimental evidences from cosmological and astrophysical observations in many different contexts, suggest the existence of dark matter particles that interact with ordinary matter mainly under the gravitational force. However, so far, all these particles evaded the detection and a direct proof of their existence is still missing. To overcome this lack, novel complementary detection schemes are necessary to improve the actual limits and the performances of current detectors, opening thus the possibility to novel breakthrough discoveries. This dissertation focuses on a research and development study for a new category of low-energy-threshold particle detector necessary to investigate low-energy-releases within the matter such as the direct investigation of dark matter composition. In this thesis I will present two experimental schemes based on solid matrices of rare gas combined with the in-vacuum single electron detection technology. Small energy releases in unreactive materials, such as solid matrices of rare gas both pure and doped, can be probed exploiting also laser-assisted processes that up-convert the low-energy-release of the incident particle. These mechanisms should lead to a detectable electronic signal triggered by the incoming particle. In such a way, energy threshold ranging from meV to tens of eV could in principle be reached opening-up thus the possibility to probe theoretically well-motivated regions of unexplored electroweak parameter-space and thus test the existence of light dark matter candidates. The work performed during the PhD period refers to the experimental research and development phases about: the rare gas crystals growing techniques and the corresponding set-ups, the electrons' extraction from rare gas crystals to the vacuum environment, and finally the spectroscopic studies on atomic species embedded into rare gas matrices.Oggigiorno, numerose prove sperimentali indicano che il Modello Standard della fisica delle particelle non sia completo, in particolare, una delle questioni più importanti riguarda la composizione dell'Universo. Infatti, la materia ordinaria, che è il costituente di tutto ciò che conosciamo, sembra essere solamente una minima frazione dell'intera massa del Cosmo stesso. La parte sconosciuta della massa dell'Universo viene indicata con il termine materia oscura e rappresenta uno dei più grandi misteri della scienza contemporanea. Le evidenze sperimentali sia su scala astrofisica sia cosmologica in contesti diversi, suggeriscono l'esistenza di particelle di materia oscura che interagiscono con la materia ordinaria mediante la forza gravitazionale. Nonostante queste osservazioni, fino ad ora, queste particelle non sono state mai misurate direttamente e quindi una prova diretta della loro esistenza tutt'ora manca. Per cercare di sopperire a questa lacuna è necessario migliorare le performance dei rivelatori attuali tramite nuovi schemi di rivelazione complementari, aprendo così la possibilità a nuove scoperte fondamentali. La presente dissertazione di dottorato si focalizza su uno studio di ricerca e sviluppo per una nuova categoria di rivelatori di particelle a bassa soglia energetica e a basso fondo. Tali approcci, necessari per investigare piccoli rilasci di energia nella materia, possono ad esempio essere applicabili allo studio della composizione particellare della materia oscura. In questa tesi verranno presentati due schemi sperimentali entrambi basati su matrici cristalline di gas rari o nobili quali neon argon kripton e xenon sia puri sia drogati con metalli alcalini o terre rare. Assieme a questi solidi, si intende sfruttare la tecnologia della rivelazione di singole cariche in vuoto che può essere ottenuta mediante strumenti quali microchannel-plate o channetron. Piccoli rilasci di energia in materiali isolanti quali solidi di gas rari sia drogati sia puri, possono infatti essere sondati da processi che sfruttano fenomeni quali la laser up-conversion per rivelare particelle debolmente interagenti. In questo modo, una soglia energetica che varia dal meV a decine di eV può essere in principio raggiunta, aprendo la possibilità all'indagine di regioni inesplorate dello spazio dei parametri dell'interazione elettrodebole, testando così l'esistenza di candidati leggeri per la materia oscura. Il lavoro che ho svolto durante il PhD riguarda la fase di ricerca e sviluppo relativa a: tecniche di crescita di cristalli di gas rari ed i relativi apparati sperimentali, l'estrazione di elettroni da questi solidi, ed infine studi spettroscopici su specie atomiche intrappolate all'interno di queste matrici cristalline

    Electronic and Magnetic Resonant Inelastic X-ray Study of Cuprates

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    More than twenty five years have passed since the discovery of the first copper oxide based superconductor La2-xBaxCuO4 in 1986, and the intriguing physics of cuprates superconductors continues to fascinate and challenge scientists from all over the world. These relatively new compounds belong to a major class of materials known as strongly correlated materials that show the most disparate physical properties: Mott metal-insulator transitions, High temperature Superconductivity, Kondo and heavy fermion behavior, colossal magnetoresistance, etc. These properties make them attractive for developing new applications, but before this goal is reached a deep investigation of the fundamental electronic properties and the understanding of their tunability is fundamental. In particular high resolution RIXS experiments which represent the main subject of this thesis, became a very powerful tool to investigate the electronic and magnetic properties of strongly correlated materials and in particular high temperature superconductor. The experiment takes advantage of the photon tunability of the synchrotron radiation and the recent technological achievements in the developement of dedicated instrumentation for the construction of the spectrometer (i.e. SAXES). SAXES is installed on the ADRESS beamline at the third generation synchrotron radiation SLS (Swiss Light Source) in Villigen (Switzerland). It operates in the 400-1500 eV energy range and it is designated to have a resolving power higher than 10000 in the whole operational range. In Chapter 1 the RIXS physical process, the distinction between direct and indirect RIXS, the experimental configuration at the ADRESS beamline and a theoretical development for the calculation of the RIXS process will be presented. Chapter 2 is dedicated to an introduction to the vast and still open field of the electronic and magnetic properties of cuprates. The model Hamiltonians for cuprates (Hubbard, Heisenberg, pd and tf Hamiltonians), the Zaanen-Sawatzky-Allen scheme, the rearrangement of the atomic levels due to the crystal field, the core level spectroscopy together with a brief introduction to the problem of the low dimensional quantum magnetism will be presented. Chapter 3 is dedicated to the experiments on the parent compounds of HTSCI have performed at the ADRESS beamline. A systematic study of the crystal field excitations {dd) as a function of the scattering geometry and incident polarizations have been carried out on different samples. An ionic model is presented in order to interpret the Cu L3 edge RIXS in layered cuprates. Then the central topic of the present thesis will be presentend. Measurements on parent compounds of high temperature superconductor along the two high symmetry directions revealed that a correct description of the measured magnon dispersion, needs to take into account an extended Hubbard model with higher order interaction than the nearest neighbour one. I will also present an attempt to enhance the visibility of the spectral feature by a deconvolution method. Chapter 4 presents a wide RIXS study of the superconducting Bi2212 family. I will present our RIXS measurements on Bi2212 superconducting samples along both the antinodal and nodal directions. I will compare our results to other RIXS measurements existing in literature on superconducting samples along the antinodal direction only. Importantly I will show that the observed dispersion along the antinodal direction is very different with respect to the spectra taken along the nodal one. This problem prompted the search for an alternative theoretical model to the one already proposed. The theoretical refinements to this approach are in progress but, already at the actual level they can reproduce the observed differences between the spectra along the two high symmetry crystallographic directions. In Chapter 5 I will show preliminary measurements on cuprite with high resolution. The sharper lineshape with respect to previous measurements, will allow a better comparison with band structure calculations. Moreover these measurements, open the possibility to measure the valence band dispersion of cuprite and related seminconducting materials at the Cu L3 edge. In the appendixes I will present the Lucy-Richardson method for the deconvolution of RIXS data and an explicit calculation for the correction of the self absorption effect in RIXS experiments

    New ideas on prospective low energy threshold detectors for dark matter searches

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    Low energy threshold detectors are necessary in many frontier fields of experimental physics. In particular, these are extremely important for probing possible dark matter (DM) candidates. We present a novel detection approach that exploits the energy levels of atoms maintained at cryogenic temperature. We exploit laser-assisted transitions that are triggered by the absorption of the incident particle in the material and lead to the emission of a fluorescent photon or an electron. In this approach, the incident particle will in fact excite the first low-lying energy level that is then up-converted using an opportune narrow-band laser system. Two different detection schemes are thus possible in our active material: one is based on a photon signal while the other takes advantage of high efficiency in-vacuum charge detection

    AI-based media coding and beyond

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    MPAI-Moving Picture, Audio and Data Coding by Artificial Intelligence is the first body developing data coding standards that have Artificial Intelligence (AI) as its core technology. MPAI believes that universally accessible standards for AI-based data coding can have the same positive effects on AI as standards had on digital media. Elementary components of MPAI standards-AI Modules (AIM)-expose standard interfaces for operation in a standard AI Framework (AIF). As their performance may depend on the technologies used, MPAI expects that competing developers providing AIMs will promote horizontal markets of AI solutions that build on and further promote AI innovation. Finally, the MPAI Framework Licences provide guidelines to IPR holders facilitating the availability of compatible licences to standard users

    CT features of malignant mucinous cystic tumors of the pancreas

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    This study is aimed at evaluating which pre-operative findings at CT are prevailingly associated with histologically malignant appearance of mucinous cystic tumor (MCT) of the pancreas. The CT examinations of 52 female patients affected with pathologically proved MCT were retrospectively evaluated by two radiologists, blinded to the histopathological assessment of the biologic behavior (either benign or malignant). A multivariate logistic regression analysis was performed in order to identify the most important features associated with the malignant nature of MCT. Calcifications in the wall and/or in the septa, thick wall, and septations resulted in the most important features associated with malignancy. The simultaneous presence of all these radiological signs constitutes an almost "certain" marker of malignancy, being the risk equal to 0.95, whereas the simultaneous presence of at least two of them entails a risk of malignancy ranging from 0.56 to 0.74, according to the type of morphological features. On the other hand, the absence of the mentioned radiological features entails a negligible risk of malignancy (0.02). Mucinous cystic tumors characterized by multilocular macrocystic architecture, with thick wall and calcifications in the wall and/or the septa, present the highest risk of malignancy, and advocate prompt surgical intervention. When those signs are absent, the probability of malignancy is lower. In this case a non-operative management is possible
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