Higgs Mode and Magnon Interactions in 2D Quantum Antiferromagnets from Raman Scattering

We present a theory for Raman scattering on 2D quantum antiferromagnets. The microscopic Fleury-Loudon Hamiltonian is expressed in terms of an effective $O(3)$ - model. Well within the N\'eel ordered phase, the Raman spectrum contains a two-magnon and a two-Higgs contribution, which are calculated diagramatically. The vertex functions for both the Higgs and magnon contributions are determined from a numerical solution of the corresponding Bethe-Salpeter equation. Due to the momentum dependence of the Raman vertex in the relevant $B_{1g}+E_{2g}$ symmetry, the contribution from the Higgs mode is strongly suppressed. Except for intermediate values of the Higgs mass, it does not show up as separate peak in the spectrum but gives rise to a broad continuum above the dominant contribution from two-magnon excitations. The latter give rise to a broad, asymmetric peak at $\omega\simeq 2.44\, J$, which is a result of magnon-magnon interactions mediated by the Higgs mode. The full Raman spectrum is determined completely by the antiferromagnetic exchange coupling $J$ and a dimensionless Higgs mass. Experimental Raman spectra of undoped cuprates turn out to be in very good agreement with the theory only with inclusion of the Higgs contribution. They thus provide a clear signature of the presence of a Higgs mode in spin one-half 2D quantum antiferromagnets.Comment: 12 pages, 15 figure

Modelling the steady state spectral energy distribution of the BL-Lac Object PKS 2155-304 using a selfconsistent SSC model

In this paper we present a fully selfconsistent SSC model with particle acceleration due to shock and stochastic acceleration (Fermi-I and Fermi-II-Processes respectively) to model the quiescent spectral energy distribution (SED) observed from PKS 2155. The simultaneous August/September 2008 multiwavelength data of H.E.S.S., Fermi, RXTE, SWIFT and ATOM give new constraints to the high-energy peak in the SED concerning its curvature. We find that, in our model, a monoenergetic injection of electrons at $\gamma_0 = 910$ into the model region, which are accelerated by Fermi-I- and Fermi-II-processes while suffering synchrotron and inverse Compton losses, finally leads to the observed SED of PKS 2155-30.4 shown in H.E.S.S. and Fermi-LAT collaborations (2009). In contrast to other SSC models our parameters arise from the jet's microphysics and the spectrum is evolving selfconsistently from diffusion and acceleration. The $\gamma_0$-factor can be interpreted as two counterstreaming plasmas due to the motion of the blob at a bulk factor of $\Gamma = 58$ and opposed moving upstream electrons at moderate Lorentz factors with an average of $\gamma_u \approx 8$.Comment: 4 figure

Exposures and exposure hedging in exchange rate risk management

Corporations are affected by increasing volatilities on foreign exchange markets. A response to this development was the creation of financial instruments, so called derivatives, in order to protect corporations from the effects of flexible exchange rates. To understand the included risks and to take correct decisions it is necessary to get a fundamental insight into exchange rate risk management. First it is the aim of this paper to systemize the possibilities of determining exchange rate risk as well as objectives of exchange rate risk management. In the second part of the paper a model to determine the optimal hedge ratio in the case of hedging transaction risks with forwards is described. --Currency Risk,Transaction Risk,Currency Forwards,Optimal Hedging

Government Spending in the Czech Republic and Slovakia after 1993

Import 22/07/2015Analýza vývoje veřejných výdajů je aktuálním problémem řady zemí. Hlavním důvodem je růst objemu veřejných výdajů, rostoucí váha veřejných výdajů na HDP, zvyšující se podíl transferů na celkových veřejných výdajích. Po stručném teoretickém vymezení porovnává práce vládní výdaje v České a Slovenské republice. Komparace byla provedena dle metodiky Evropského systému národních a regionálních účtů (ESA 95) a klasifikace funkcí vládních institucí (COFOG).The analysis of public expenditure development is the current problem of many countries. The main reason is the growth of public expenditure, increasing the weight of public expenditure in GDP, increasing the share of transfers in total public expenditure. A brief theoretical definition compares the work of government spending in the Czech and Slovak Republics. The comparison was performed according to the methodology of the European System of Accounts (ESA 95) and classification of functions of government (COFOG).156 - Katedra národohospodářskávelmi dobř

Modelling the variability of 1ES1218+30.4

The blazar 1ES1218+30.4 has been previously detected by the VERITAS and MAGIC telescopes in the very high energies. The new detection of VERITAS from December 2008 to April 2009 proves that 1ES1218+30.4 is not static, but shows short-time variability. We show that the time variability may be explained in the context of a self-consistent synchrotron-self Compton model, while the long time observation do not necessarily require a time-resolved treatment. The kinetic equations for electrons and photons in a plasma blob are solved numerically including Fermi acceleration for electrons as well as synchrotron radiation and Compton scattering. The light curve observed by VERITAS can be reproduced in our model by assuming a changing level of electron injection compared to the constant state of 1ES1218+30.4. The multiwavelength behaviour during an outburst becomes comprehensible by the model. The long time measurements of VERITAS are still explainable via a constant emission in the SSC context, but the short outbursts each require a time-resolved treatment.Comment: 4 pages, 3 figures; accepted in A&A; AA/2010/1429

Finite-ranged interactions and multiband effects in quantum point contacts

Seit ihrer Entdeckung im Jahre 1996, hat die 0.7-Anomalie in Quantenpunktkontakten (QP- Cs) große experimentelle und theoretische Aufmerksamkeit erregt. Dabei hat sich heraus gestellt, dass die 0.7-Anomalie, neben ihrer zunächst entdeckten Ausprägung als schulter- ähnliche Struktur im Leitwert bei endlicher Temperatur, praktisch auch in allen anderen QPC Observablen, wie z.B. Thermopower, Schrottrauschen, etc. zu beobachten ist. Die damit assozierten anomalen Effekte werden mittlerweile zusammengefasst als 0.7-Physik bezeichnet. Trotz ihrer umfangreichen experimentellen Untersuchung steht die Ursache der 0.7-Anomalie nach wie vor zur Diskussion. Es existieren zahlreiche theoretische Erklärungsversuche, denen unterschiedliche Mechanismen zu Grunde liegen und die sich zum Teil auf unterschiedliche Aspekte der 0.7-Physik konzentrieren. In 2013 hat die von Delft Gruppe die sog. van-Hove ridge Erklärung vorgestellt. Diese erlaubt eine intuitive Erklärung der 0.7-Physik, basierend auf den Eigenschaften der nichtwechselwirkenden lokalen Zustandsdichte (LDOS), welche durch die Geometrie des QPCs festgelegt ist. Zur Untermauerung und dem weiteren Ausbau dieser Idee wurde die funktionale Renormalisierungsgruppe (fRG) verwendet, um zahlreiche QPC Observablen zu berechnen. Diesen Berechnungen wurde ein mikroskopisches Model, bestehend aus einem Hüpfterm, einer QPC Barriere und einem Onsite Wechselwirkungsterm zu Grunde gelegt. Wichtige 0.7-Aspekte, wie z.B. die asymmetrische Magnetfeldabhängigkeit des Leitwerts, wurden mittels dieser Berechnungen untersucht. Nichtsdestotrotz konnte das Markenzeichen der 0.7-Anomalie, nämlich eine ausgeprägte Schulter im Leitwert bei endlicher Temperatur, bislang nicht reproduziert werden. In dieser Arbeit untersuchen wir QPC Modelle, die anstelle reiner Onsite Wechselwirkun- gen auch Wechselwirkungen mit endlicher Reichweite enthalten. Dies ist besonders interessant, da es die Zahl der möglichen Wechselwirkungsprozesse drastisch erhöht, und daher womöglich das fehlende Bindeglied dastellt, um die charakteristische 0.7-Schulter im Leitwert aus einem mikroskopischen Model zu erhalten. Neben der unmittelbaren Anwendung im Rahmen von Standard QPCs, stellt die Berücksichtigung von Wechselwirkungen mit endlicher Reichweite auch einen wichtigen Schritt zur Behandlung von längeren QPCs und dem Übergang zu Quantendrähten dar. Für diese ausgedehnteren Systeme ist die endliche Reichweite der Wech- selwirkung, wegen der reduzierten Abschirmung in Bereichen von niedriger Elektrondichte, besonders relevant. Zur Untersuchung endlicher Wechselwirkungsreichweiten erweitern wir die existierende fRG Approximationsmethode der gekoppelte Leiter (CLA) durch eine Ausweitung des räum- lichen Feedbacks zwischen unterschiedlichen fRG Kanälen. In einer Reihe von Publikationen wenden wir diese neue, erweiterte CLA Methode (eCLA) in Implementationen von wachsender Komplexität auf QPC- und Quantenpunktmodelle (QD Modelle) an. Den Höhepunkt dieser Arbeiten bildet die semi-dynamische Behandlung von Wechselwirkungen endlicher Reichweite in QPCs. Trotz Problemen unserer Methode bezüglich der Verletzung von Ward Identitäten haben wir vielversprechende Anzeichen dafür gefunden, dass eine endliche Wechselwirkungs- reichweite – vergleichbar mit der charakteristischen QPC Länge – ein Schlüsselbestandteil einer ausgeprägten 0.7-Schulter im Leitwert sein könnte. Neben der Behandlung von QPCs mit endlicher Wechselwirkungsreichweite haben wir unsere verbesserte eCLA Methode auch auf einige weitere Problemstellungen angewendet, von denen zwei besonders erwähnenswert sind. (i) Wir haben bemerkt, dass die erweiterte Rückfütterung zwischen den fRG Kanälen den fRG Fluss stabilisiert und dadurch die Untersuchung von physikalischen Parameterbereichen ermöglicht, die mit vorhergehenden fRG Methoden nicht zugänglich waren. So waren wir beispielsweise in der Lage den Leitwert eines Quanten- punkts zu berechnen, der nur einige wenige Elektronen nahe dem chemischen Potential enthält. (ii) Die Fähigkeit Wechselwirkungen mit endlicher Reichweite zu behandeln ermöglicht es auch (mittels einer geeigneten Abbildung) QPCs mit mehreren wechselwirkenden Bändern zu behandeln. Wir nutzen dies zur Untersuchung des 0.7-Analogs, der bei großem externen Magnetfeld am Schnittpunkt von Subbändern unterschiedlicher Spinspezies auftritt. Anhand unserer fRG Berechnungen waren wir in der Lage die Magnetfeldabhängigkeit des Analogs zu reproduzieren und konnten, darauf aufbauend, die auftretende Formasymmetrie des Leitwerts – je nachdem ob man sich dem Leitwert von größeren oder kleineren Magnetfeldern nähert – mittels eines intuitiven Hartree Arguments erklären. Zusätzlich zu unseren eCLA Arbeiten haben wir mit der einfacheren CLA Vorgänger- methode eindimensionale ungeordnete Systeme betrachtet und Anzeichen eines möglichen Vielteilchenlokalisierungsübergangs (MBL-Übergangs) untersucht.Since its discovery in 1996, the 0.7-anomaly in quantum point contacts (QPCs) has drawn a lot of experimental and theoretical attention. Initially found as the development of a shoulder-like structure in the conductance with increasing temperature, it soon became clear that virtually all QPC observables like thermal power, shot-noise, etc., show anomalous behavior in the 0.7-region, constituting a whole set of phenomena collectively known as 0.7-physics. Although experimentally well established, the origin of the 0.7-anomaly is still under debate. There exist numerous theoretical explanation attempts, evoking different mechanisms and partially focusing on different aspects of the 0.7-physics. In 2013, the von Delft group presented the so called van-Hove ridge explanation, which provides an intuitive explanation of the 0.7-physics, tracing its root back to the properties of the non-interacting local density of states (LDOS) which is defined by the geometry of the QPC. To develop and corroborate this explanation, the functional renormalization group (fRG) was used to compute various QPC observables starting from a microscopic one-dimensional model comprised of hopping term, QPC barrier and onsite interaction term. Important 0.7-features like the asymmetric magnetic field dependence of the conductance could be studied using those calculations. However, the trademark feature of the 0.7-anomaly – the pronounced shoulder in the finite temperature conductance had not yet been reproduced. In this thesis, we investigate QPC models that in addition to onsite interactions also include a finite interaction range. This is an interesting endeavor, since it increases the number of interaction processes tremendously, possibly contributing the final step to reproduce the trademark 0.7-shoulder from a microscopic model. Besides this immediate application to standard QPCs, it also constitutes an important step towards the treatment of longer QPCs and the transition to quantum wires. For these longer systems, the finite-ranged character of the interactions is especially significant due to reduced screening in the extended spatial regions of low electron density. In order to achieve the goal of treating finite interaction ranges, we extend a previously used coupled ladder fRG approximation (CLA) scheme, by extending the spatial feedback between different fRG channels. In a series of publications, we apply this new extended CLA (eCLA) scheme in rising levels of sophistication to QPC and quantum dot (QD) models, culminating in a semi-dynamic treatment of finite-ranged interactions in QPCs. Despite methodological problems arising from the violation of Ward identities, we indeed find promising evidence that a finite interaction range, comparable to the characteristic QPC length, is a key ingredient in obtaining a pronounced 0.7-shoulder in the conductance. Alongside this main application, we also applied our improved eCLA method to various other problems, of which two are especially mention-worthy. (i) We noticed that the extended feedback between fRG channels stabilizes the fRG flow, enabling the study of physical parameter regimes that were not accessible with the previously existing fRG methods. As a prime example, we were able to calculate the conductance of a quantum dot, containing only a few electrons close to the chemical potential. (ii) Being able to treat finite-ranged interactions also enables us (via a suitable mapping) to treat QPC models with multiple interacting bands. We use this to study the 0.7-analog occuring at the intersection of different subbands with opposite spin at large magnetic fields. Using eCLA calculations, we could reproduce the magnetic field dependence of the analog, and devise an intuitive Hartree explanation that explains the occurring shape-asymmetry depending on whether the 0.7-analog is approached from lower or higher magnetic fields. Additionally, we investigated one-dimensional disordered systems with the previous CLA method and searched for signs of a possible many-body localization (MBL) transition

Should We Flip the Script?: A Literature Review of Deficit-Based Perspectives on First-Year Undergraduate Students’ Information Literacy

This mixed method systematic review considers recent literature on the information literacy (IL) skills of first-year undergraduate students. The review uncovers the following themes: faculty and librarians perceive first-year students as lacking IL skills; students have varying perceptions of their IL skills; assessment studies yield conflicting findings on first-year students\u27 IL; communication between high school and college librarians is challenging; and some IL researchers emphasise and leverage first-year students\u27 prior knowledge and experience in IL instruction. These themes emerge from extensive searches in four research databases for scholarly and professional articles written in English within the past ten years. With the exception of a few articles, studies reviewed consistently express their findings in terms of students’ gaps or deficits. We question whether this is the most productive basis for developing effective IL programs. Instead, we call for further investigation of students’ existing knowledge and skills as a basis for implementing constructivist and strengths-based pedagogies

Self-assembly of photo-crosslinkable block copolymers and their application in 3D printing

Natural materials are composed of a limited number of molecular building blocks, e.g. amino acids, carbohydrates), and their exceptional properties are governed by their intricate hierarchical structure on multiple length scales. While 3D printing has emerged as the standard method for the precise fabrication of minute devices, this level of precision is unattainable with current state-of-the-art materials for 3D printing. A common method of obtaining such nanoscale structure in systematic systems exploits the potential for polymers to self-assemble under certain conditions. One important class of polymers which has the ability to form self-assembled structures at a scale of 5- 50 nm are amphiphilic block copolymers. They are tunable over a broad variety of morphologies, ranging from micelles and vesicles to continuous network structures, which can form in both undiluted melt or solution. While these properties have been extensively investigated in 2D films, they have not yet been exploited to generate 3D structures entailing high resolution features, complex geometries and a controlled nanostructure. To that end, in the current PhD thesis, new self-assembled printable materials based on block copolymers (BCPs) that enable precise control of the nanostructure in 3D are investigated. In particular, well-defined BCPs consisting of a poly(styrene) block and a poly(methacrylate)-based copolymer decorated with printable units are selected as suitable self-assembling materials. A broad library of BCPs with different compositions and molecular weight is synthesized using controlled radical polymerization. A subsequent extensive investigation of the phase behavior before and after the functionalization is performed using SAXS, SEM, and SNOM. Lamellar, cylindrical and gyroid morphologies are observed dependent on the composition as well as the molecular weight, allowing the phase diagram of the system to be generated. The dependency of the domain spacing d on the molecular weight of the polymer is found to be described by a power law, which is in accordance with that published for other systems both experimentally as well as in theory. The synthesized library of BCPs is then utilized to create printable formulations for the fabrication of complex 3D microstructures using two-photon laser printing. By fine-tuning the BCP composition and solvent in the formulations, the fabrication of precise 3D nano-ordered structures is demonstrated for the first time. Hereby, the key achievement is a controlled nano-order within the entirety of the 3D structures. To show this, imaging of the cross-sections of the 3D printed samples is performed, enabling visualization also from the inside. A detailed view of both lamellar as well as cylindrical morphology, dependent on the polymer design, is presented. The morphologies are fitting well with those found in the respective bulk polymer analysis, as well as SAXS measurements of the printing ink formulation