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.

Down-conversion process in Tb3+-Yb3+ co-doped Calibo glasses

The down-conversion process in Tb3+-Yb3+ co-doped Calibo glasses was studied. The emission, excitation and time-resolved measurements indicated the existence of an energy conversion through the excitation of Tb3+ ions to near-infrared emission by Yb3+ ions. The emission intensity dependence on excitation power confirms that the one-photon process is responsible for the Yb3+ emission. An enhanced Yb3+ emission was observed with Yb3+ doping and an optimal energy transfer efficiency of 32% was obtained before reaching near-infrared emission quenching. The mechanism of the non-resonant energy transfer from Tb3+ to Yb3+ is discussed in terms of the Tb3+-Yb3+ cross-relaxation and multiphonon decay processes. (C) 2012 Elsevier B.V. All rights reserved.

Lanthanum carbonate, like sevelamer-HCl, retards the progression of vascular calcification and atherosclerosis in uremic apolipoprotein E-deficient mice

Atherosclerosis and vascular calcification (VC) progression in chronic kidney disease is favored by disturbances of mineral metabolism. We compared the effect of phosphate binder lanthanum (La) carbonate with sevelamer-HCl on atherosclerosis, VC and bone structure and function in mice with chronic renal failure (CRF). Apolipoprotein E-deficient (apoE(-/-)) mice were randomized to one non-CRF and three CRF groups, fed with standard diet (one non-CRF and one CRF) or diet supplemented with either 3% lanthanum carbonate (La3%) or 3% sevelamer-HCl (Sev3%). Both La3% and Sev3% supplemented CRF mice displayed a decrease of serum phosphorus, calcification at both intimal and medial aortic sites and atherosclerosis. This was associated with a reduction of plaque Type I collagen expression by both binders and of positive nitrotyrosine staining in response to sevelamer-HCl only. Increased mineral apposition and bone formation rates in unsupplemented CRF mice were reduced by Sev3% but not by La3%. The beneficial effects of La carbonate and sevelamer-HCl on the progression of VC and atherosclerosis in CRF mice could be mainly due to a decrease in phosphate retention and likewise a reduction of arterial Type I collagen expression. The effect of La carbonate differed from that of sevelamer-HCl in that it did not appear to exert its vascular effects via changes in oxidative stress or bone remodeling in the present model.

Near-infrared quantum cutting through a three-step energy transfer process in Nd3+-Yb3+ co-doped fluoroindogallate glasses

The Nd3+-Yb3+ couple was investigated in fluoroindogallate glasses using optical spectroscopy to elucidate the energy transfer mechanisms involved in the downconversion (DC) process. Upon excitation of a Nd3+ ion by an ultraviolet photon, DC through a three-step energy transfer process occurs, in which the energy of the ultraviolet photon absorbed by the Nd3+ ion is converted into three infrared photons emitted by Yb3+ ions, i.e. quantum cutting (QC). In addition, with excitation in the visible, our results confirm that the DC process occurs through a one-step energy transfer process, in which the energy of a visible photon absorbed by the Nd3+ ion is converted into only one infrared photon emitted by an Yb3+ ion. Time-resolved measurements enabled the estimation of the efficiencies of the cross-relaxation processes between Nd3+ and Yb3+ ions.

Femtosecond Laser in Polymeric Materials: Microfabrication of Doped Structures and Micromachining

The use of laser light to modify the material's surface or bulk as well as to induce changes in the volume through a chemical reaction has received great attention in the last few years, due to the possibility of tailoring the material's properties aiming at technological applications. Here, we report on recent progress of microstructuring and microfabrication in polymeric materials by using femtosecond lasers. In the first part, we describe how polymeric materials' micromachining, either on the surface or bulk, can be employed to change their optical and chemical properties promising for fabricating waveguides, resonators, and self-cleaning surfaces. In the second part, we discuss how two-photon absorption polymerization can be used to fabricate active microstructures by doping the basic resin with molecules presenting biological and optical properties of interest. Such microstructures can be used to fabricate devices with applications in optics, such as microLED, waveguides, and also in medicine, such as scaffolds for tissue growth.

Densification and enhancement of the grain boundary conductivity of gadolinium-doped barium cerate by ultra fast flash grain welding

Doped barium cerate is a promising solid electrolyte for intermediate temperature fuel cells as a protonic conductor. However, it is difficult to sinter it to high density at a reasonable temperature. Moreover, it presents a high grain boundary resistivity at intermediate temperatures. Flash grain welding was applied to compacted samples, starting from a temperature of 910 degrees C and applying, for a short time, an ac electric polarization of 40 V, 1000 Hz. At that frequency, the resulting current flows through the grain boundaries promoting a welding via a local Joule heating. A large decrease of the grain boundary resistivity was observed by impedance spectroscopy. Scanning electron microscopy observations of polished and etched surfaces revealed highly sintered regions. Attempts were also made to combine flash grain welding with conventional sintering. (C) 2012 Elsevier Ltd. All rights reserved.

Surface Segregation in Chromium-Doped Nanocrystalline Tin Dioxide Pigments

Surface properties play an important role in understanding and controlling nanocrystalline materials. The accumulation of dopants on the surface, caused by surface segregation, can therefore significantly affect nanomaterials properties at low doping levels, offering a way to intentionally control nanoparticles features. In this work, we studied the distribution of chromium ions in SnO2 nanoparticles prepared by a liquid precursor route at moderate temperatures (500 degrees C). The powders were characterized by infrared spectroscopy, X-ray diffraction, (scanning) transmission electron microscopy, Electron Energy Loss Spectroscopy, and Mossbauer spectroscopy. We showed that this synthesis method induces a limited solid solution of chromium into SnO2 and a segregation of chromium to the surface. The s-electron density and symmetry of Sn located on the surface were significantly affected by the doping, while Sn located in the bulk remained unchanged. Chromium ions located on the surface and in the bulk showed distinct oxidation states, giving rise to the intense violet color of the nanoparticles suitable for pigment application.

Local order and electronic structure of Pb1-xLaxZr0.40Ti0.60O3 materials and its relation with ferroelectric properties

The knowledge of electronic and local structures is a fundamental step towards understanding the properties of ferroelectric ceramics. X-ray absorption near-edge structure (XANES) of Pb1-xLaxZr0.40Ti0.60O3 ferroelectric samples was measured in order to know how the local order and electronic structure are related to their ferroelectric property, which was tailored by the substitution of lead by lanthanum atoms. The analysis of XANES spectra collected at Ti K- and L-edges XANES showed that the substitution of Pb by La leads to a decrement of local distortion around Ti atoms on the TiO6 octahedron. The analysis of O K-edge XANES spectra showed that the hybridization between O 2p and Pb 6sp states is related to the displacement of Ti atoms in the TiO6 octahedra. Based on these results, it is possible to determine that the degree of ferroelectricity in these samples and the manifestation of relaxor behavior are directly related to the weakening of O 2p and Pb 6sp hybridization. (C) 2012 American Institute of Physics. [http://dx.doi.org/10.1063/1.4720472]

Effect of the vanadium(V) concentration on the spectroscopic properties of nanosized europium-doped yttrium phosphates

Nanosized rare earth phosphovanadate phosphors (Y(P,V)O-4:Eu3+) have been prepared by applying the organic-inorganic polymeric precursors methodology. Luminescent powders with tetragonal structure and different vanadate concentrations (0%, 1%, 5%, 10%, 20%, 50%, and 100%, with regard to the phosphate content) were then obtained for evaluation of their structural and spectroscopic properties. The solids were characterized by scanning electron microscopy, X-ray diffractometry, vibrational spectroscopy (Raman and infrared), and electronic spectroscopy (emission, excitation, luminescence lifetimes, chromaticity, quantum efficiencies, and Judd-Ofelt intensity parameters). The solids exhibited very intense D-5(0) -> F-7(J) Eu3+ transitions, and it was possible to control the luminescent characteristics, such as excitation maximum, lifetime and emission colour, through the vanadium(V) concentration. The observed luminescent properties correlated to the characteristics of the chemical environments around the Eu3+ ions with respect to the composition of the phosphovanadates. The Eu3+ luminescence spectroscopy results indicated that the presence of larger vanadium(V) amounts in the phosphate host lattice led to more covalent and polarizable chemical environments. So, besides allowing for control of the luminescent properties of the solids, the variation in the vanadate concentration in the obtained YPO4:Eu3+ phosphors enabled the establishment of a strict correlation between the observable spectroscopic features and the chemical characteristics of the powders.

Study of the vertical transport in p-doped superlattices based on group III-V semiconductors

The electrical conductivity σ has been calculated for p-doped GaAs/Al0.3Ga0.7As and cubic GaN/Al0.3Ga0.7N thin superlattices (SLs). The calculations are done within a self-consistent approach to the k → ⋅ p → theory by means of a full six-band Luttinger-Kohn Hamiltonian, together with the Poisson equation in a plane wave representation, including exchange correlation effects within the local density approximation. It was also assumed that transport in the SL occurs through extended minibands states for each carrier, and the conductivity is calculated at zero temperature and in low-field ohmic limits by the quasi-chemical Boltzmann kinetic equation. It was shown that the particular minibands structure of the p-doped SLs leads to a plateau-like behavior in the conductivity as a function of the donor concentration and/or the Fermi level energy. In addition, it is shown that the Coulomb and exchange-correlation effects play an important role in these systems, since they determine the bending potential.

Luminescence quantum efficiency at 1.5 µm of Er3+- doped tellurite glass determined by thermal lens spectroscopy

Erbium doped tellurite glasses (TeO2 + Li2O + TiO2) were prepared by conventional melt-quenching method to study the influence of the Er3+ concentration on the luminescence quantum efficiency (η) at 1.5 µm. Absorption and luminescence data were used to characterize the samples, and the η parameter was measured using the well-known thermal lens spectroscopy. For low Er3+ concentration, the measured values are around 76%, and the concentration behavior of η shows Er-Er and Er-OH- interactions, which agreed with the measured lifetime values.

Concentration quenching and thermal activation of the luminescence from terbium-doped a-SiC:H and c-AlN thin films

The effect of terbium (Tb) doping on the photoluminescence (PL) of crystalline aluminum nitride (c-AlN) and amorphous hydrogenated silicon carbide (a-SiC:H) thin films has been investigated for different Tb atomic concentrations. The samples were prepared by DC and RF magnetron reactive sputtering techniques covering the concentration range of Tb from 0.5 to 11 at.%. The Tb-related light emission versus the Tb concentration is reported for annealing temperatures of 450 °C, 750 °C and 1000 °C. In the low concentration region the intensity exhibits a linear increase and its slope is enhanced with the annealing temperature giving an activation energy of 0.106 eV in an Arrhenius plot. In the high concentration region an exponential decay is recorded which is almost independent on the host material, its structure and the annealing process.

Dye doped, core / shell silica nanoparticles: synthesis, characterization, & biotechnological applications

The aim of this thesis was to design, synthesize and develop a nanoparticle based system to be used as a chemosensor or as a label in bioanalytical applications. A versatile fluorescent functionalizable nanoarchitecture has been effectively produced based on the hydrolysis and condensation of TEOS in direct micelles of Pluronic® F 127, obtaining highly monodisperse silica - core / PEG - shell nanoparticles with a diameter of about 20 nm. Surface functionalized nanoparticles have been obtained in a one-pot procedure by chemical modification of the hydroxyl terminal groups of the surfactant. To make them fluorescent, a whole library of triethoxysilane fluorophores (mainly BODIPY based), encompassing the whole visible spectrum has been synthesized: this derivatization allows a high degree of doping, but the close proximity of the molecules inside the silica matrix leads to the development of self - quenching processes at high doping levels, with the concomitant fall of the fluorescence signal intensity. In order to bypass this parasite phenomenon, multichromophoric systems have been prepared, where highly efficient FRET processes occur, showing that this energy pathway is faster than self - quenching, recovering the fluorescence signal. The FRET efficiency remains very high even four dye nanoparticles, increasing the pseudo Stokes shift of the system, attractive feature for multiplexing analysis. These optimized nanoparticles have been successfully exploited in molecular imaging applications such as in vitro, in vivo and ex vivo imaging, proving themselves superior to conventional molecular fluorophores as signaling units.

Titanate-based paraelectric glass-ceramics for applications in GHz electronics

Das Gebiet der drahtlosen Kommunikationsanwendungen befindet sich in einem permanenten Entwicklungsprozess (Mobilfunkstandards: GSM/UMTS/LTE/5G, glo-bale Navigationssatellitensysteme (GNSS): GPS, GLONASS, Galileo, Beidou) zu immer höheren Datenraten und zunehmender Miniaturisierung, woraus ein hoher Bedarf für neue, optimierte Hochfrequenzmaterialien resultiert. Diese Entwicklung zeigt sich besonders in den letzten Jahren in der zunehmenden Entwicklung und Anzahl von Smartphones, welche verschiedene Technologien mit unterschiedlichen Arbeitsfrequenzen innerhalb eines Geräts kombinieren (data: 1G-4G, GPS, WLAN, Bluetooth). Die für zukünftige Technologien (z.B. 5G) benötigte Performance-steigerung kann durch die Verwendung von auf MIMO basierenden Antennensystemen realisiert werden (multiple-input & multiple-output, gesteuerte Kombination von mehreren Antennen) für welche auf dielectric Loading basierende Technologien als eine der vielversprechendsten Implementierungslösungen angesehen werden. rnDas Ziel dieser Arbeit war die Entwicklung einer geeigneten paraelektrischen Glaskeramik ($varepsilon_{r}$ > 20, $Qf$ > 5000 GHz, |$tau_f$| < 20 ppm/K; im GHz Frequenzbe-reich) im $mathrm{La_{2}O_{3}}$-$mathrm{TiO_{2}}$-$mathrm{SiO_{2}}$-$mathrm{B_{2}O_{3}}$-System für auf dielectric Loading basierende Mobilfunkkommunikationstechnologien als Alternative zu existierenden kommerziell genutzten Sinterkeramiken. Der Fokus lag hierbei auf der Frage, wie die makroskopi-schen dielektrischen Eigenschaften der Glaskeramik mit ihrer Mikrostruktur korreliert bzw. modifiziert werden können. Es konnte gezeigt werden, dass die dielektrischen Materialanforderungen durch das untersuchte System erfüllt werden und dass auf Glaskeramik basierende Dielektrika weitere vorteilhafte nichtelektro-nische Eigenschaften gegenüber gesinterten Keramiken besitzen, womit dielektrische Glaskeramiken durchaus als geeignete Alternative angesehen werden können. rnEin stabiles Grünglas mit minimalen Glasbildneranteil wurde entwickelt und die chemische Zusammensetzung bezüglich Entglasung und Redoxinstabilitäten optimiert. Geeignete Dotierungen für dielektrisch verlustarme $mathrm{TiO_{2}}$-haltige Glaskeramiken wurden identifiziert.rnDer Einfluss der Schmelzbedingungen auf die Keimbildung wurde untersucht und der Keramisierungsprozess auf einen maximalen Anteil der gewünschten Kristallphasen optimiert um optimale dielektrische Eigenschaften zu erhalten. Die mikroskopische Struktur der Glaskeramiken wurde analysiert und ihr Einfluss auf die makroskopischen dielektrischen Eigenschaften bestimmt. Die Hochfrequenzverlustmechanismen wurden untersucht und Antennen-Prototypenserien wurden analysiert um die Eignung von auf Glaskeramik basierenden Dielektrika für die Verwendung in dielectric Loading Anwendungen zu zeigen.