Bright solitons and soliton trains in a fermion-fermion mixture

We use a time-dependent dynamical mean-field-hydrodynamic model to predict and study bright solitons in a degenerate fermion-fermion mixture in a quasi-one-dimensional cigar-shaped geometry using variational and numerical methods. Due to a strong Pauli-blocking repulsion among identical spin-polarized fermions at short distances there cannot be bright solitons for repulsive interspecies fermion-fermion interactions. However, stable bright solitons can be formed for a sufficiently attractive interspecies interaction. We perform a numerical stability analysis of these solitons and also demonstrate the formation of soliton trains. These fermionic solitons can be formed and studied in laboratory with present technology.

Study of a degenerate dipolar Fermi gas of Dy-161 atoms

Fundação de Amparo à Pesquisa do Estado de São Paulo (FAPESP)

Understanding the origin of photoluminescence in disordered Ca0.60Sr0.40WO4: An experimental and first-principles study

Visible photoluminescence (PL) was observed for the first time at room temperature in structurally disordered calcium strontium tungstate powder, Ca0.60Sr0.40WO4 (CSW), obtained by the polymeric precursor method. The PL behavior of CSW powders has been analyzed as a function of the disorder rate, based on experimental and theoretical studies. Quantum mechanical theory based on density functional theory at the B3LYP level has been employed to study the electronic structure of two periodic models representing both crystalline and disordered powders. Their electronic structures have been analyzed in terms of density of states, band dispersion and charge densities. The calculations indicate a break in symmetry when passing from crystalline to disordered models, creating localized electronic levels above the valence band. Moreover, a negative charge transfer process takes place from the [WO3] cluster to the [WO4] cluster. The polarization induced by the break in symmetry and the existence of localized levels favors the creation of trapped holes and electrons, originating the PL phenomenon. (c) 2007 Elsevier B.V. All rights reserved.

The change in magnetic properties of Fe3Al compound due to substitution of Fe by Co

The magnetic moment using self-consistent spin-polarized energy band calculations of Fe3Al and Fe2CoAl Heusler phases are presented. These results are compared with the experimental values obtained from the magnetization curves of these materials. (C) 2004 Elsevier B.V. All rights reserved.

Correlation among order-disorder, electronic levels, and photoluminescence in amorphous CT : Sm

Ca0.95Sm0.05TiO3 (CT:Sm) powder was prepared by the polymeric precursor method (PPM). Order-disorder at short and long range has been investigated by means of Raman spectroscopy, X-ray diffraction (XRD), and photoluminescence emission (PL) experimental techniques. The broad PL band and the Sm emission spectrum measured at room temperature indicate the increase of structural order with annealing temperature. The measured PL emission reveals that the PL intensity changes with the degree of disorder in the CT: Sm. The electronic structures were performed by the ab initio periodic method in the DFT level with the hybrid nonlocal B3LYP approximation. Theoretical results are analyzed in terms of DOS, charge densities, and Mulliken charges. Localized levels into the band gap of the CT: Sm material favor the creation of the electron-hole pair, supporting the observed room-temperature PL phenomenon.

Self-bound droplet of Bose and Fermi atoms in one dimension: Collective properties in mean-field and Tonks-Girardeau regimes

We investigate a dilute mixture of bosons and spin-polarized fermions in one dimension. With an attractive Bose-Fermi scattering length the ground state is a self-bound droplet, i.e., a Bose-Fermi bright soliton where the Bose and Fermi clouds are superimposed. We find that the quantum fluctuations stabilize the Bose-Fermi soliton such that the one-dimensional bright soliton exists for any finite attractive Bose-Fermi scattering length. We study density profile and collective excitations of the atomic bright soliton showing that they depend on the bosonic regime involved: mean-field or Tonks-Girardeau.

Conditions giving rise to intense visible room temperature photoluminescence in SrWO4 thin films: the role of disorder

The nature of intense visible photoluminescence at room temperature of SrWO4 (SWO) non-crystalline thin films is discussed in the light of experimental results and theoretical calculations. The SWO thin films were synthesized by the polymeric precursors method. Their structural properties have been obtained by X-ray diffraction data and the corresponding photoluminescence (PL) spectra have been measured. The UV-vis optical spectra measurements suggest the creation of localized states in the disordered structure. The photoluminescence measurements reveal that the PL changes with the degree of disorder in the SWO thin film. To understand the origin of visible PL at room temperature in disordered SWO, we performed quantum-mechanical calculations on crystalline and disordered SWO periodic models. Their electronic structures are analyzed in terms of DOS, hand dispersion and charge densities. We used DFT method with the hybrid non-local B3LYP approximation. The polarization induced by the symmetry break and the existence of localized levels favors the creation of trapped holes and electrons, giving origin to the room temperature photoluminescence phenomenon in the SWO thin films. (c) 2004 Elsevier B.V. All rights reserved.

The use of pulsed electric acoustic technique for measuring electric field profiles in ferroelectric ceramic

The pulsed electric acoustic technique, PEA, have been usually applied to probe space charge profiles in polymers. In this work we show preliminary results obtained with lead zirconate-titanate and niobium, PZTN, ferroelectric ceramic samples. Experiments showed that induced charge densities on sample electrodes are mainly due to the ferroelectric polarization of the sample. We present results of the typical PEA response and the procedure to deconvolute the signal in order to obtain the charge densities and the electric field profiles. The PEA setup allows us to show a non-uniform polarization during ferroelectric switching.

Tetragonal to monoclinic phase transition observed during Zr anodisation

Plasma electrolytic oxidation (PEO) is a coating procedure that utilises anodic oxidation in aqueous electrolytes above the dielectric breakdown voltage to produce oxide coatings that have specific properties. These conditions facilitate oxide formation under localised high temperatures and pressures that originate from short-lived microdischarges at sites over the metal surface and have fast oxide volume expansion. Anodic ZrO2 films were prepared by subjecting metallic zirconium to PEO in acid solutions (H2C 2O4 and H3PO4) using a galvanostatic DC regime. The ZrO2 microstructure was investigated in films that were prepared at different charge densities. During the anodic breakdown, an important change in the amplitude of the voltage oscillations at a specific charge density was observed (i.e., the transition charge density (Q T)). We verified that this transition charge is a monotonic function of both the current density and temperature applied during the anodisation, which indicated that Q T is an intrinsic response of this system. The oxide morphology and microstructure were characterised using SEM and X-ray diffraction experiments (XRD) techniques. X-ray diffraction analysis revealed that the change in voltage oscillation was correlated with oxide microstructure changes during the breakdown process. © 2012 Springer-Verlag Berlin Heidelberg.

Adsorption of pH-responsive polyelectrolyte chains onto spherical macroions

Conselho Nacional de Desenvolvimento Científico e Tecnológico (CNPq)

Robust isothermal electric control of exchange bias at room temperature

Voltage-controlled spin electronics is crucial for continued progress in information technology. It aims at reduced power consumption, increased integration density and enhanced functionality where non-volatile memory is combined with highspeed logical processing. Promising spintronic device concepts use the electric control of interface and surface magnetization. From the combination of magnetometry, spin-polarized photoemission spectroscopy, symmetry arguments and first-principles calculations, we show that the (0001) surface of magnetoelectric Cr2O3 has a roughness-insensitive, electrically switchable magnetization. Using a ferromagnetic Pd/Co multilayer deposited on the (0001) surface of a Cr2O3 single crystal, we achieve reversible, room-temperature isothermal switching of the exchange-bias field between positive and negative values by reversing the electric field while maintaining a permanent magnetic field. This effect reflects the switching of the bulk antiferromagnetic domain state and the interface magnetization coupled to it. The switchable exchange bias sets in exactly at the bulk Néel temperature.

Electronic and magnetic properties of SnO2/CrO2 thin superlattices

In this article, using first-principles electronic structure calculations within the spin density functional theory, alternated magnetic and non-magnetic layers of rutile-CrO2 and rutile-SnO2 respectively, in a (CrO2) n (SnO2) n superlattice (SL) configuration, with n being the number of monolayers which are considered equal to 1, 2, ..., 10 are studied. A half-metallic behavior is observed for the (CrO2) n (SnO2) n SLs for all values of n. The ground state is found to be FM with a magnetic moment of 2 μB per chromium atom, and this result does not depend on the number of monolayers n. As the FM rutile-CrO2 is unstable at ambient temperature, and known to be stabilized when on top of SnO2, the authors suggest that (CrO2) n (SnO2) n SLs may be applied to spintronic technologies since they provide efficient spin-polarized carriers.

Darstellung und Charakterisierung perlenkettenartiger und zylindrischer Polyelektrolyte

Zusammenfassung: Es sollten zum einen strukturell fixierte, perlenkettenartige Polyelektrolyte dargestellt werden. Dazu wurde lineares Poly-2-vinylpyridin(PVP) mit vinylfunktionalisierten, hydrophoben Quaternisierungsagentien zu einer Polyseife umgesetzt. Bei der Quaternisierungsreaktion ließ sich der Gehalt an hydrophoben Gruppen variieren, wodurch Polyseifen mit unterschiedlichen Ladungsdichten zugänglich wurden. Trotz vielfältiger Versuche war es dennoch nicht möglich, eine polymerisationsfähige Polyseife herzustellen, welche in wäßriger Lösung intramolekular micellisiert und die Überstruktur einer Perlenkette annimmt. Durch die Herstellung hochreiner PVP-Makromonomere konnten zylindrische Bürsten hergestellt werden. Durch Umsetzung der PVP-Bürsten mit Methyltosylat sind unter milden Reaktionsbedingungen nahezu vollständig umgesetzte positiv geladene Polyelektrolyte zugänglich. Durch eine Sulfonierung von Polystyrol-Polymakromonomeren wurden negativ geladene zylindrische Polyelektrolyte erhalten.Das Verhalten dieser Polyelektrolyte in verdünnter wäßriger Lösung wurde mit der statischen und der dynamischen Lichtstreuung untersucht. Dabei deuten die statischen Messungen darauf hin, daß deren Verhalten in verdünnter wäßriger Lösung maßgeblich durch die osmotische Aktivität der Gegenionen bestimmt wird.Durch eine Quaternisierung der PVP-Bürsten mit langkettigen Reagentien konnten hochverzweigte Polyelektrolytarchitekturen hergestellt werden. Dabei läßt sich die Tatsache, daß eine Quaternisierung mit solchen Reagentien einen nur unwesentlichen Einfluß auf die Struktur der Bürste hat, nicht durch einfache Überlegungen erklären. Dennoch scheinen die langkettigen Seitenketten die Ausbildung geordneter Strukturen innerhalb von Domänen an der Oberfläche zu induzieren.

Existence of solutions and asymtptotic limits of the Euler-Poisson and the quantum drift-diffusion systems

My work concerns two different systems of equations used in the mathematical modeling of semiconductors and plasmas: the Euler-Poisson system and the quantum drift-diffusion system. The first is given by the Euler equations for the conservation of mass and momentum, with a Poisson equation for the electrostatic potential. The second one takes into account the physical effects due to the smallness of the devices (quantum effects). It is a simple extension of the classical drift-diffusion model which consists of two continuity equations for the charge densities, with a Poisson equation for the electrostatic potential. Using an asymptotic expansion method, we study (in the steady-state case for a potential flow) the limit to zero of the three physical parameters which arise in the Euler-Poisson system: the electron mass, the relaxation time and the Debye length. For each limit, we prove the existence and uniqueness of profiles to the asymptotic expansion and some error estimates. For a vanishing electron mass or a vanishing relaxation time, this method gives us a new approach in the convergence of the Euler-Poisson system to the incompressible Euler equations. For a vanishing Debye length (also called quasineutral limit), we obtain a new approach in the existence of solutions when boundary layers can appear (i.e. when no compatibility condition is assumed). Moreover, using an iterative method, and a finite volume scheme or a penalized mixed finite volume scheme, we numerically show the smallness condition on the electron mass needed in the existence of solutions to the system, condition which has already been shown in the literature. In the quantum drift-diffusion model for the transient bipolar case in one-space dimension, we show, by using a time discretization and energy estimates, the existence of solutions (for a general doping profile). We also prove rigorously the quasineutral limit (for a vanishing doping profile). Finally, using a new time discretization and an algorithmic construction of entropies, we prove some regularity properties for the solutions of the equation obtained in the quasineutral limit (for a vanishing pressure). This new regularity permits us to prove the positivity of solutions to this equation for at least times large enough.

Methodische und technische Weiterentwicklung der 3-He-MRT im Hinblick auf erweiterte lungendiagnostische Anwendungsmöglichkeiten

Die vorliegende Arbeit untersucht die Möglichkeiten und Limitierungen der MR-Lungenbildgebung mit hyperpolarisiertem 3-He bei gegenüber üblichen Magnetfeldstärken reduzierten magnetischen Führungsfeldern. Dabei werden insbesondere auch die funktionellen Bildgebungstechniken (dynamische Bildgebung, diffusionsgewichtete Bildgebung und Bestimmung des Sauerstoff-Partialdrucks) berücksichtigt. Experimentell geschieht dies durch in vivo Messungen an einem 0.1 T-Ganzkörpertomographen. Zur systematischen Untersuchung der MR-Bildgebung unter dem Einfluss diffundierender Kontrastmittel werden analytische Simulationen, Monte-Carlo-Studien und Referenzexperimente durchgeführt. Hier wird das Augenmerk besonders auf den Einfluss von Diffusions- und Suszeptibilitätsartefakten auf die morphologische und die diffusionsgewichtete Bildgebung gerichtet. Die Entwicklung und der Vergleich verschiedener Konzepte zur Erzeugung von MR-Führungsmagnetfeldern führt zur Erfindung eines neuartigen Prinzips zur weiträumigen Homogenisierung von Magnetfeldern. Die Umsetzung dieses Prinzips erfolgt in Form eines besonders kompakten Transportbehälters für kernspinpolarisierte Edelgase. Die Arbeit beinhaltet eine ausführliche Diskussion der MR-Bildgebungstechnik in Theorie und Praxis, um die Anknüpfungspunkte an die angestellten Untersuchungen herauszuarbeiten. Teile dieser Studien wurden von der Europäischen Raumfahrtorganisation ESA finanziert (Contract No.15308/01/NL/PA).