Bose glass and Mott glass of quasiparticles in a doped quantum magnet

The low-temperature states of bosonic fluids exhibit fundamental quantum effects at the macroscopic scale: the best-known examples are Bose-Einstein condensation and superfluidity, which have been tested experimentally in a variety of different systems. When bosons interact, disorder can destroy condensation, leading to a 'Bose glass'. This phase has been very elusive in experiments owing to the absence of any broken symmetry and to the simultaneous absence of a finite energy gap in the spectrum. Here we report the observation of a Bose glass of field-induced magnetic quasiparticles in a doped quantum magnet (bromine-doped dichloro-tetrakis-thiourea-nickel, DTN). The physics of DTN in a magnetic field is equivalent to that of a lattice gas of bosons in the grand canonical ensemble; bromine doping introduces disorder into the hopping and interaction strength of the bosons, leading to their localization into a Bose glass down to zero field, where it becomes an incompressible Mott glass. The transition from the Bose glass (corresponding to a gapless spin liquid) to the Bose-Einstein condensate (corresponding to a magnetically ordered phase) is marked by a universal exponent that governs the scaling of the critical temperature with the applied field, in excellent agreement with theoretical predictions. Our study represents a quantitative experimental account of the universal features of disordered bosons in the grand canonical ensemble.

Performance of Down syndrome subjects during a coincident timing task

Abstract Background The time synchronization is a very important ability for the acquisition and performance of motor skills that generate the need to adapt the actions of body segments to external events of the environment that are changing their position in space. Down Syndrome (DS) individuals may present some deficits to perform tasks with synchronization demand. We aimed to investigate the performance of individuals with DS in a simple Coincident Timing task. Method 32 individuals were divided into 2 groups: the Down syndrome group (DSG) comprised of 16 individuals with average age of 20 (+/− 5 years old), and a control group (CG) comprised of 16 individuals of the same age. All individuals performed the Simple Timing (ST) task and their performance was measured in milliseconds. The study was conducted in a single phase with the execution of 20 consecutive trials for each participant. Results There was a significant difference in the intergroup analysis for the accuracy adjustment - Absolute Error (Z = 3.656, p = 0.001); and for the performance consistence - Variable Error (Z = 2.939, p = 0.003). Conclusion DS individuals have more difficulty in integrating the motor action to an external stimulus and they also present more inconsistence in performance. Both groups presented the same tendency to delay their motor responses.

Time-of-flight two-photon photoemission spectromicroscopy with femtosecond laser radiation

Time-of-flight photoemission spectromicroscopy was used to measure and compare the two-photon photoemission (2PPE) spectra of Cu and Ag nanoparticles with linear dimensions ranging between 40 nm and several 100 nm, with those of the corresponding homogeneous surfaces. 2PPE was induced employing femtosecond laser radiation from a frequency-doubled Ti:sapphire laser in the spectral range between 375 nm and 425 nm with a pulse width of 200 fs and a repetition rate of 80 MHz. The use of a pulsed radiation source allowed us to use a high-resolution photoemission electron microscope as imaging time-of-flight spectrometer, and thus to obtain spectroscopic information about the laterally resolved electron signal. Ag nanoparticle films have been deposited on Si(111) by electron-beam evaporation, a technique leading to hemispherically-shaped Ag clusters. Isolated Cu nanoparticles have been generated by prolonged heating of a polycrystalline Cu sample. If compared to the spectra of the corresponding homogeneous surfaces, the Cu and Ag nanoparticle spectra are characterized by a strongly enhanced total 2PPE yield (enhancement factor up to 70), by a shift (about 0.1 eV) of the Fermi level onset towards lower final state energies, by a reduction of the work function (typically by 0.2 eV) and by a much steeper increase of the 2PPE yield towards lower final state energies. The shift of the Fermi level onset in the nanoparticle spectra has been explained by a positive unit charge (localized photohole) residing on the particle during the time-scale relevant for the 2PPE process (few femtoseconds). The total 2PPE yield enhancement and the different overall shape of the spectra have been explained by considering that the laser frequency was close to the localized surface plasmon resonance of the Cu and Ag nanoparticles. The synchronous oscillations induced by the laser in the metal electrons enhance the near-zone (NZ) field, defined as the linear superposition of the laser field and the field produced in the vicinity of the particles by the forced charge oscillations. From the present measurements it is clear that the NZ field behavior is responsible for the 2PPE enhancement and affects the 2PPE spatial and energy distribution and its dynamics. In particular, its strong spatial dependence allows indirect transitions through real intermediate states to take place in the metal clusters. Such transitions are forbidden by momentum conservation arguments and are thus experimentally much less probable on homogeneous surfaces. Further, we investigated specially tailored moon-shaped small metal nanostructures, whose NZ field was theoretically predicted, and compared the calculation with the laterally resolved 2PPE signal. We could show that the 2PPE signal gives a clear fingerprint of the theoretically predicted spatial dependence of the NZ field. This potential of our method is highly attractive in the novel field of plasmonics.

On The Timing of Default Losses: When and Why?

At the light of what happened in 2010 and 2011, a lot of European countries founded themselves in a difficult position where all the credit rating agencies were downgrading debt states. Problem of solvency and guarantees on the states' bond were perceived as too risky for a Monetary Union as Europe is. Fear of a contagion from Greece as well was threatening the other countries as Italy, Spain, Portugal and Ireland; while Germany and France asked for a division between risky and riskless bond in order to feel more safe. Our paper gets inspiration by Roch and Uhlig (2011), it refers to the Argentinian case examined by Arellano (2008) and examine possible interventions as monetization or bailout as proposed by Cole and Kehoe (2000). We propose a model in which a state defaults and cannot repay a fraction of the old bond; but contrary to Roch and Uhlig that where considering a one-time cost of default we consider default as an accumulation of losses, perceived as unpaid fractions of the old debts. Our contributions to literature is that default immediately imply that economy faces a bad period and, accumulating losses, government will be worse-off. We studied a function for this accumulation of debt period by period, in order to get an idea of the magnitude of this waste of resources that economy will face when experiences a default. Our thesis is that bailouts just postpone the day of reckoning (Roch, Uhlig); so it's better to default before accumulate a lot of debts. What Europe need now is the introduction of new reforms in a controlled default where the Eurozone will be saved in its whole integrity and a state could fail with the future promise of a resurrection. As experience show us, governments are not interested into reducing debts since there are ECB interventions. That clearly create a distortion between countries in the same monetary union, giving to the states just an illusion about their future debtor position.

Timing per l'inserimento in lista dei pazienti affetti da fibrosi cistica che necessitano di trapianto polmonare: casistica di un centro regionale di riferimento

Timing of waiting list entrance for patients with cystic fibrosis in need of pulmonary transplant: the experience of a regional referral centre Objective: Evaluation of parameters that can predict a rapid decay of general conditions of patients affected by Cystic Fibrosis (CF) with no specific criteria to be candidate to pulmonary transplant. Material and methods: Fifteen patients with CF who died for complications and 8 who underwent lung transplantation in the 2000-2010 decade, were enrolled. Clinical data 2 years before the event (body max index, FEV1%, number of EV antibiotic treatments per year, colonization with Methicillin-resistant Staphylococcus aureus (MRSA), pseudomonas aeruginosa mucosus, burkholderia cepacia, pulmonary allergic aspergilosis) were compared among the 2 groups. Results: Mean FEV1% was significantly higher and mean number of antibiotic treatment was lower in deceased than in the transplanted patients (p<0.002 and p<0.001 respectively). Although in patients who died there were no including criteria to enter the transplant list 2 years before the exitus, suggestive findings such as low BMI (17.3), high incidence of hepatic pathology (33.3%), diabetes (50%), and infections with MRSA infection (25%), Pseudomonas aeruginosa (83.3%) and burkholderia cepacia (8.3%) were found with no statistical difference with transplanted patients, suggesting those patients were at risk of severe prognosis. In patients who died, females were double than males. Conclusion: While evaluating patients with CF, negative prognostic factors such as the ones investigated in this study, should be considered to select individuals with high mortality risk who need stricter therapeutical approach and follow up. Inclusion of those patients in the transplant waiting list should be taken into account.

Size-selective investigations of spectral and emission properties of small particles and nanotubes

In dieser Arbeit wurde die Elektronenemission von Nanopartikeln auf Oberflächen mittels spektroskopischen Photoelektronenmikroskopie untersucht. Speziell wurden metallische Nanocluster untersucht, als selbstorganisierte Ensembles auf Silizium oder Glassubstraten, sowie ferner ein Metall-Chalcogenid (MoS2) Nanoröhren-Prototyp auf Silizium. Der Hauptteil der Untersuchungen war auf die Wechselwirkung von fs-Laserstrahlung mit den Nanopartikeln konzentriert. Die Energie der Lichtquanten war kleiner als die Austrittsarbeit der untersuchten Proben, so dass Ein-Photonen-Photoemission ausgeschlossen werden konnte. Unsere Untersuchungen zeigten, dass ausgehend von einem kontinuierlichen Metallfilm bis hin zu Clusterfilmen ein anderer Emissionsmechanismus konkurrierend zur Multiphotonen-Photoemission auftritt und für kleine Cluster zu dominieren beginnt. Die Natur dieses neuen Mechanismus` wurde durch verschiedenartige Experimente untersucht. Der Übergang von einem kontinuierlichen zu einem Nanopartikelfilm ist begleitet von einer Zunahme des Emissionsstroms von mehr als eine Größenordnung. Die Photoemissions-Intensität wächst mit abnehmender zeitlicher Breite des Laserpulses, aber diese Abhängigkeit wird weniger steil mit sinkender Partikelgröße. Die experimentellen Resultate wurden durch verschiedene Elektronenemissions-Mechanismen erklärt, z.B. Multiphotonen-Photoemission (nPPE), thermionische Emission und thermisch unterstützte nPPE sowie optische Feldemission. Der erste Mechanismus überwiegt für kontinuierliche Filme und Partikel mit Größen oberhalb von mehreren zehn Nanometern, der zweite und dritte für Filme von Nanopartikeln von einer Größe von wenigen Nanometern. Die mikrospektroskopischen Messungen bestätigten den 2PPE-Emissionsmechanismus von dünnen Silberfilmen bei „blauer“ Laseranregung (hν=375-425nm). Das Einsetzen des Ferminiveaus ist relativ scharf und verschiebt sich um 2hν, wenn die Quantenenergie erhöht wird, wogegen es bei „roter“ Laseranregung (hν=750-850nm) deutlich verbreitert ist. Es zeigte sich, dass mit zunehmender Laserleistung die Ausbeute von niederenergetischen Elektronen schwächer zunimmt als die Ausbeute von höherenergetischen Elektronen nahe der Fermikante in einem Spektrum. Das ist ein klarer Hinweis auf eine Koexistenz verschiedener Emissionsmechanismen in einem Spektrum. Um die Größenabhängigkeit des Emissionsverhaltens theoretisch zu verstehen, wurde ein statistischer Zugang zur Lichtabsorption kleiner Metallpartikel abgeleitet und diskutiert. Die Elektronenemissionseigenschaften bei Laseranregung wurden in zusätzlichen Untersuchungen mit einer anderen Anregungsart verglichen, der Passage eines Tunnelstroms durch einen Metall-Clusterfilm nahe der Perkolationsschwelle. Die elektrischen und Emissionseigenschaften von stromtragenden Silberclusterfilmen, welche in einer schmalen Lücke (5-25 µm Breite) zwischen Silberkontakten auf einem Isolator hergestellt wurden, wurden zum ersten Mal mit einem Emissions-Elektronenmikroskop (EEM) untersucht. Die Elektronenemission beginnt im nicht-Ohmschen Bereich der Leitungsstrom-Spannungskurve des Clusterfilms. Wir untersuchten das Verhalten eines einzigen Emissionszentrums im EEM. Es zeigte sich, dass die Emissionszentren in einem stromleitenden Silberclusterfilm Punktquellen für Elektronen sind, welche hohe Emissions-Stromdichten (mehr als 100 A/cm2) tragen können. Die Breite der Energieverteilung der Elektronen von einem einzelnen Emissionszentrum wurde auf etwa 0.5-0.6 eV abgeschätzt. Als Emissionsmechanismus wird die thermionische Emission von dem „steady-state“ heißen Elektronengas in stromdurchflossenen metallischen Partikeln vorgeschlagen. Größenselektierte, einzelne auf Si-Substraten deponierte MoS2-Nanoröhren wurden mit einer Flugzeit-basierten Zweiphotonen-Photoemissions-Spektromikroskopie untersucht. Die Nanoröhren-Spektren wiesen bei fs-Laser Anregung eine erstaunlich hohe Emissionsintensität auf, deutlich höher als die SiOx Substratoberfläche. Dagegen waren die Röhren unsichtbar bei VUV-Anregung bei hν=21.2 eV. Eine ab-initio-Rechnung für einen MoS2-Slab erklärt die hohe Intensität durch eine hohe Dichte freier intermediärer Zustände beim Zweiphotonen-Übergang bei hν=3.1 eV.

Concept and realization of the A4 compton backscattering polarimeter at MAMI

The main concern of the A4 parity violation experiment at the Mainzer Microtron accelerator facility is to study the electric and magnetic contributions of strange quarks to the charge and magnetism of the nucleons at the low momentum transfer region. More precisely, the A4 collaboration investigates the strange quarks' contribution to the electric and magnetic vector form factors of the nucleons. Thus, it is important that the A4 experiment uses an adequate and precise non-destructive online monitoring tool for the electron beam polarization when measuring single spin asymmetries in elastic scattering of polarized electrons from unpolarized nucleons. As a consequence, the A4 Compton backscattering polarimeter was designed and installed such that we can take the absolute measurement of the electron beam polarization without interruption to the parity violation experiment. The present study shows the development of an electron beam line that is called the chicane for the A4 Compton backscattering polarimeter. The chicane is an electron beam transport line and provides an interaction region where the electron beam and the laser beam overlap. After studying the properties of beam line components carefully, we developed an electron beam control system that makes a beam overlap between the electron beam and the laser beam. Using the system, we can easily achieve the beam overlap in a short time. The electron control system, of which the performance is outstanding, is being used in production beam times. And the study presents the development of a scintillating fiber electron detector that reduces the statistical error in the electron polarization measurement. We totally redesigned the scintillating fiber detector. The data that were taken during a 2008 beam time shows a huge background suppression, approximately 80 percent, while leaving the Compton spectra almost unchanged when a coincidence between the fiber detector and the photon detector is used. Thus, the statistical error of the polarization measurement is reduced by about 40 percent in the preliminary result. They are the significant progress in measuring a degree of polarization of the electron beam.

Electron spectroscopy of novel charge transfer systems based on polycyclic aromatic hydrocarbons

Organische Ladungstransfersysteme weisen eine Vielfalt von konkurrierenden Wechselwirkungen zwischen Ladungs-, Spin- und Gitterfreiheitsgraden auf. Dies führt zu interessanten physikalischen Eigenschaften, wie metallische Leitfähigkeit, Supraleitung und Magnetismus. Diese Dissertation beschäftigt sich mit der elektronischen Struktur von organischen Ladungstransfersalzen aus drei Material-Familien. Dabei kamen unterschiedliche Photoemissions- und Röntgenspektroskopietechniken zum Einsatz. Die untersuchten Moleküle wurden z.T. im MPI für Polymerforschung synthetisiert. Sie stammen aus der Familie der Coronene (Donor Hexamethoxycoronen HMC und Akzeptor Coronen-hexaon COHON) und Pyrene (Donor Tetra- und Hexamethoxypyren TMP und HMP) im Komplex mit dem klassischen starken Akzeptor Tetracyanoquinodimethan (TCNQ). Als dritte Familie wurden Ladungstransfersalze der k-(BEDT-TTF)2X Familie (X ist ein monovalentes Anion) untersucht. Diese Materialien liegen nahe bei einem Bandbreite-kontrollierten Mottübergang im Phasendiagramm.rnFür Untersuchungen mittels Ultraviolett-Photoelektronenspektroskopie (UPS) wurden UHV-deponierte dünne Filme erzeugt. Dabei kam ein neuer Doppelverdampfer zum Einsatz, welcher speziell für Milligramm-Materialmengen entwickelt wurde. Diese Methode wies im Ladungstransferkomplex im Vergleich mit der reinen Donor- und Akzeptorspezies energetische Verschiebungen von Valenzzuständen im Bereich weniger 100meV nach. Ein wichtiger Aspekt der UPS-Messungen lag im direkten Vergleich mit ab-initio Rechnungen.rnDas Problem der unvermeidbaren Oberflächenverunreinigungen von lösungsgezüchteten 3D-Kristallen wurde durch die Methode Hard-X-ray Photoelectron Spectroscopy (HAXPES) bei Photonenenergien um 6 keV (am Elektronenspeicherring PETRA III in Hamburg) überwunden. Die große mittlere freie Weglänge der Photoelektronen im Bereich von 15 nm resultiert in echter Volumensensitivität. Die ersten HAXPES Experimente an Ladungstransferkomplexen weltweit zeigten große chemische Verschiebungen (mehrere eV). In der Verbindung HMPx-TCNQy ist die N1s-Linie ein Fingerabdruck der Cyanogruppe im TCNQ und zeigt eine Aufspaltung und einen Shift zu höheren Bindungsenergien von bis zu 6 eV mit zunehmendem HMP-Gehalt. Umgekehrt ist die O1s-Linie ein Fingerabdruck der Methoxygruppe in HMP und zeigt eine markante Aufspaltung und eine Verschiebung zu geringeren Bindungsenergien (bis zu etwa 2,5eV chemischer Verschiebung), d.h. eine Größenordnung größer als die im Valenzbereich.rnAls weitere synchrotronstrahlungsbasierte Technik wurde Near-Edge-X-ray-Absorption Fine Structure (NEXAFS) Spektroskopie am Speicherring ANKA Karlsruhe intensiv genutzt. Die mittlere freie Weglänge der niederenergetischen Sekundärelektronen (um 5 nm). Starke Intensitätsvariationen von bestimmten Vorkanten-Resonanzen (als Signatur der unbesetzte Zustandsdichte) zeigen unmittelbar die Änderung der Besetzungszahlen der beteiligten Orbitale in der unmittelbaren Umgebung des angeregten Atoms. Damit war es möglich, präzise die Beteiligung spezifischer Orbitale im Ladungstransfermechanismus nachzuweisen. Im genannten Komplex wird Ladung von den Methoxy-Orbitalen 2e(Pi*) und 6a1(σ*) zu den Cyano-Orbitalen b3g und au(Pi*) und – in geringerem Maße – zum b1g und b2u(σ*) der Cyanogruppe transferiert. Zusätzlich treten kleine energetische Shifts mit unterschiedlichem Vorzeichen für die Donor- und Akzeptor-Resonanzen auf, vergleichbar mit den in UPS beobachteten Shifts.rn

Femtosecond laser-based investigations of magnetic circular dichroism in near-threshold photoemission

During the last decades magnetic circular dichroism (MCD) has attracted much interest and evolved into various experimental methods for the investigation of magnetic thin films. For example, synchrotron-based X-ray magnetic circular dichroism (XMCD) displays the absolute values of spin and orbital magnetic moments. It thereby benefits from large asymmetry values of more than 30% due to the excitation of atomic core-levels. Similarly large values are also expected for threshold photoemission magnetic circular dichroism (TPMCD). Using lasers with photon energies in the range of the sample work function this method gives access to the occupied electronic structure close to the Fermi level. However, except for the case of Ni(001) there exist only few studies on TPMCD moreover revealing much smaller asymmetries than XMCD-measurements. Also the basic physical mechanisms of TPMCD are not satisfactorily understood. In this work we therefore investigate TPMCD in one- and two-photon photoemission (1PPE and 2PPE) for ferromagnetic Heusler alloys and ultrathin Co films using ultrashort pulsed laser light. The observed dichroism is explained by a non-conventional photoemission model using spin-resolved band-structure calculations and linear response theory. For the two Heusler alloys Ni2MnGa and Co2FeSi we give first evidence of TPMCD in the regime of two-photon photoemission. Systematic investigations concerning general properties of TPMCD in 1PPE and 2PPE are carried out at ultrathin Co films grown on Pt(111). Here, photon-energy dependent measurements reveal asymmetries of 1.9% in 1PPE and 11.7% in 2PPE. TPMCD measurements at decreased work function even yield larger asymmetries of 6.2% (1PPE) and 17% (2PPE), respectively. This demonstrates that enlarged asymmetries are also attainable for the TPMCD effect on Co(111). Furthermore, we find that the TPMCD asymmetry is bulk-sensitive for 1PPE and 2PPE. This means that the basic mechanism leading to the observed dichroism must be connected to Co bulk properties; surface effects do not play a crucial role. Finally, the enhanced TPMCD asymmetries in 2PPE compared to the 1PPE case are traced back to the dominant influence of the first excitation step and the existence of a real intermediate state. The observed TPMCD asymmetries cannot be interpreted by conventional photoemission theory which only considers direct interband transitions in the direction of observation (Γ-L). For Co(111), these transitions lead to evanescent final states. The excitation to such states, however, is incompatible with the measured bulk-sensitivity of the asymmetry. Therefore, we generalize this model by proposing the TPMCD signal to arise mostly from direct interband transitions in crystallographic directions other than (Γ-L). The necessary additional momentum transfer to the excited electrons is most probably provided by electron-phonon or -magnon scattering processes. Corresponding calculations on the basis of this model are in reasonable agreement with the experimental results so that this approach represents a promising tool for a quantitative description of the TPMCD effect. The present findings encourage an implementation of our experimental technique to time- and spatially-resolved photoemission electron microscopy, thereby enabling a real time imaging of magnetization dynamics of single excited states in a ferromagnetic material on a femtosecond timescale.

Operating System Contribution to Composable Timing Behaviour in High-Integrity Real-Time Systems

The development of High-Integrity Real-Time Systems has a high footprint in terms of human, material and schedule costs. Factoring functional, reusable logic in the application favors incremental development and contains costs. Yet, achieving incrementality in the timing behavior is a much harder problem. Complex features at all levels of the execution stack, aimed to boost average-case performance, exhibit timing behavior highly dependent on execution history, which wrecks time composability and incrementaility with it. Our goal here is to restitute time composability to the execution stack, working bottom up across it. We first characterize time composability without making assumptions on the system architecture or the software deployment to it. Later, we focus on the role played by the real-time operating system in our pursuit. Initially we consider single-core processors and, becoming less permissive on the admissible hardware features, we devise solutions that restore a convincing degree of time composability. To show what can be done for real, we developed TiCOS, an ARINC-compliant kernel, and re-designed ORK+, a kernel for Ada Ravenscar runtimes. In that work, we added support for limited-preemption to ORK+, an absolute premiere in the landscape of real-word kernels. Our implementation allows resource sharing to co-exist with limited-preemptive scheduling, which extends state of the art. We then turn our attention to multicore architectures, first considering partitioned systems, for which we achieve results close to those obtained for single-core processors. Subsequently, we shy away from the over-provision of those systems and consider less restrictive uses of homogeneous multiprocessors, where the scheduling algorithm is key to high schedulable utilization. To that end we single out RUN, a promising baseline, and extend it to SPRINT, which supports sporadic task sets, hence matches real-world industrial needs better. To corroborate our results we present findings from real-world case studies from avionic industry.

Band structure of Heusler compounds studied by photoemission and tunneling spectroscopy

Heusler compounds are key materials for spintronic applications. They have attracted a lot of interest due to their half-metallic properties predicted by band structure calculations.rnThe aim of this work is to evaluate experimentally the validity of the predictions of half metallicity by band structure calculations for two specific Heusler compounds, Co2FeAl0.3Si0.7 and Co2MnGa. Two different spectroscopy methods for the analysis of the electronic properties were used: Angular Resolved Ultra-violet Photoemission Spectroscopy (ARUPS) and Tunneling Spectroscopy.rnHeusler compounds are prepared as thin films by RF-sputtering in an ultra-high vacuum system. rnFor the characterization of the samples, bulk and surface crystallographic and magnetic properties of Co2FeAl0.3Si0.7 and Co2MnGa are studied. X-ray and electron diffraction reveal a bulk and surface crossover between two different types of sublattice order (from B2 to L21) with increasing annealing temperature. X-ray magnetic circular dichroism results show that the magnetic properties in the surface and bulk are identical, although the magnetic moments obtained are 5% below from the theoretically predicted.rnBy ARUPS evidence for the validity of the predicted total bulk density of states (DOS) was demonstrated for both Heusler compounds. Additional ARUPS intensity contributions close to the Fermi energy indicates the presence of a specific surface DOS. Moreover, it is demonstrated that the crystallographic order, controlled by annealing, plays an important role on brodening effects of DOS features. Improving order resulted in better defined ARUPS features.rnTunneling magnetoresistance measurements of Co2FeAl0.3Si0.7 and Co2MnGa based MTJ’s result in a Co2FeAl0.3Si0.7 spin polarization of 44%, which is the highest experimentally obtained value for this compound, although it is lower than the 100% predicted. For Co2MnGa no high TMR was achieved.rnUnpolarized tunneling spectroscopy reveals contribution of interface states close to the Fermi energy. Additionally magnon excitations due to magnetic impurities at the interface are observed. Such contributions can be the reason of a reduced TMR compared to the theoretical predictions. Nevertheless, for energies close to the Fermi energy and for Co2MnGa, the validity of the band structure calculations is demonstrated with this technique as well.

Timing attack di Paul C. Kocher: attacco al sistema di sicurezza RSA mediante strumenti di statistica

In questa tesi ho voluto descrivere il Timing Attack al sistema crittografico RSA, il suo funzionamento, la teoria su cui si basa, i suoi punti di forza e i punti deboli. Questo particolare tipo di attacco informatico fu presentato per la prima volta da Paul C. Kocher nel 1996 all’“RSA Data Security and CRYPTO conferences”. Nel suo articolo “Timing Attacks on Implementations of Diffie-Hellman, RSA, DSS, and Other Systems” l’autore svela una nuova possibile falla nel sistema RSA, che non dipende da debolezze del crittosistema puramente matematiche, ma da un aspetto su cui nessuno prima di allora si era mai soffermato: il tempo di esecuzione delle operazioni crittografiche. Il concetto è tanto semplice quanto geniale: ogni operazione in un computer ha una certa durata. Le variazioni dei tempi impiegati per svolgere le operazioni dal computer infatti, necessariamente dipendono dal tipo di algoritmo e quindi dalle chiavi private e dal particolare input che si è fornito. In questo modo, misurando le variazioni di tempo e usando solamente strumenti statistici, Kocher mostra che è possibile ottenere informazioni sull’implementazione del crittosistema e quindi forzare RSA e altri sistemi di sicurezza, senza neppure andare a toccare l’aspetto matematico dell’algoritmo. Di centrale importanza per questa teoria diventa quindi la statistica. Questo perché entrano in gioco molte variabili che possono influire sul tempo di calcolo nella fase di decifrazione: - La progettazione del sistema crittografico - Quanto impiega la CPU ad eseguire il processo - L’algoritmo utilizzato e il tipo di implementazione - La precisione delle misurazioni - Ecc. Per avere più possibilità di successo nell’attaccare il sistema occorre quindi fare prove ripetute utilizzando la stessa chiave e input differenti per effettuare analisi di correlazione statistica delle informazioni di temporizzazione, fino al punto di recuperare completamente la chiave privata. Ecco cosa asserisce Kocher: “Against a vulnerable system, the attack is computationally inexpensive and often requires only known ciphertext.”, cioè, contro sistemi vulnerabili, l’attacco è computazionalmente poco costoso e spesso richiede solo di conoscere testi cifrati e di ottenere i tempi necessari per la loro decifrazione.

Hard x-ray photoelectron spectroscopy of bulk and thin films of Heusler compounds

X-ray photoemission spectroscopy (XPS) is one of the most universal and powerful tools for investigation of chemical states and electronic structures of materials. The application of hard x-rays increases the inelastic mean free path of the emitted electrons within the solid and thus makes hard x-ray photoelectron spectroscopy (HAXPES) a bulk sensitive probe for solid state research and especially a very effective nondestructive technique to study buried layers.rnThis thesis focuses on the investigation of multilayer structures, used in magnetic tunnel junctions (MTJs), by a number of techniques applying HAXPES. MTJs are the most important components of novel nanoscale devices employed in spintronics. rnThe investigation and deep understanding of the mechanisms responsible for the high performance of such devices and properties of employed magnetic materials that are, in turn, defined by their electronic structure becomes feasible applying HAXPES. Thus the process of B diffusion in CoFeB-based MTJs was investigated with respect to the annealing temperature and its influence on the changes in the electronic structure of CoFeB electrodes that clarify the behaviour and huge TMR ratio values obtained in such devices. These results are presented in chapter 6. The results of investigation of the changes in the valence states of buried off-stoichiometric Co2MnSi electrodes were investigated with respect to the Mn content α and its influence on the observed TMR ratio are described in chapter 7.rnrnMagnetoelectronic properties such as exchange splitting in ferromagnetic materials as well as the macroscopic magnetic ordering can be studied by magnetic circular dichroism in photoemission (MCDAD). It is characterized by the appearance of an asymmetry in the photoemission spectra taken either from the magnetized sample with the reversal of the photon helicity or by reversal of magnetization direction of the sample when the photon helicity direction is fixed. Though recently it has been widely applied for the characterization of surfaces using low energy photons, the bulk properties have stayed inaccessible. Therefore in this work this method was integrated to HAXPES to provide an access to exploration of magnetic phenomena in the buried layers of the complex multilayer structures. Chapter 8 contains the results of the MCDAD measurements employing hard x-rays for exploration of magnetic properties of the common CoFe-based band-ferromagnets as well as half-metallic ferromagnet Co2FeAl-based MTJs.rnrnInasmuch as the magnetoresistive characteristics in spintronic devices are fully defined by the electron spins of ferromagnetic materials their direct measurements always attracted much attention but up to date have been limited by the surface sensitivity of the developed techniques. Chapter 9 presents the results on the successfully performed spin-resolved HAXPES experiment using a spin polarimeter of the SPLEED-type on a buried Co2FeAl0.5Si0.5 magnetic layer. The measurements prove that a spin polarization of about 50 % is retained during the transmission of the photoelectrons emitted from the Fe 2p3/2 state through a 3-nm-thick oxide capping layer.rn

Studio e realizzazione di un "timing circuit" e "output circuit" per pacemaker

Nell'elaborato è stato svolto uno studio su più livelli degli elementi essenziali del pacemaker asincrono secondo la realizzazione circuitale proposta da Wilson Greatbatch nel 1960. Un primo livello ha riguardato l’analisi teorica del circuito. Un secondo livello ha riguardato un’analisi svolta con LTSPICE. Con questo stesso programma, si è analizzato il segnale di temporizzazione e la forma d’onda sul carico al variare del valore di alcuni componenti chiave del circuito. Infine, si è proceduto alla sua realizzazione su breadboard.