Oxygen vacancies in strained SrTiO3_{3} thin films: formation enthalpy and manipulation

We report the enthalpy of oxygen vacancy formation in thin films of electron-doped SrTiO$_{3}$, under different degrees of epitaxial stress. We demonstrate that both compressive and tensile strain decrease this energy at a very similar rate, and promote the formation of stable doubly ionized oxygen vacancies. Moreover, we also show that unintentional cationic vacancies introduced under typical growth conditions, produce a characteristic rotation pattern of TiO$_6$ octahedra. The local concentration of oxygen vacancies can be modulated by an electric field with an AFM tip, changing not only the local electrical potential, but also producing a non-volatile mechanical response whose sign (up/down) can be reversed by the electric field.Comment: Physical Review B (accepted for publication

Electron degeneracy and intrinsic magnetic properties of epitaxial Nb:SrTiO3_3 thin-films controlled by defects

We report thermoelectric power experiments in e-doped thin films of SrTiO$_3$ (STO) which demonstrate that the electronic band degeneracy can be lifted through defect management during growth. We show that even small amounts of cationic vacancies, combined with epitaxial stress, produce a homogeneous tetragonal distortion of the films, resulting in a Kondo-like resistance upturn at low temperature, large anisotropic magnetoresistance, and non-linear Hall effect. Ab-initio calculations confirm a different occupation of each band depending on the degree of tetragonal distortion. The phenomenology reported in this paper for tetragonally distorted e-doped STO thin films, is similarto that observed in LaAlO$_3$/STO interfaces and magnetic STO quantum wells.Comment: 5 pages, 5 figure

Reduction of thermal conductivity in ferroelectric SrTiO3 thin films

Bulk SrTiO3 is a quantum paraelectric in which an antiferrodistortive distortion below approximate to 105 K and quantum fluctuations at low temperature preclude the stabilization of a long-range ferroelectric state. However, biaxial mechanical stress, impurity doping, and Sr nonstoichiometry, among other mechanisms, are able to stabilize a ferroelectric or relaxor ferroelectric state at room temperature, which develops into a longer-range ferroelectric state below 250 K. In this paper, we show that epitaxial SrTiO3 thin films grown under tensile strain on DyScO3 exhibit a large reduction of thermal conductivity, approximate to 60% of at room temperature, with respect to identical strain-free or compressed films. The thermal conductivity shows a further reduction below 250 K, a temperature concurrent with the peak in the dielectric constant [J. H. Haeni et al., Nature (London) 430, 758 (2004)]. These results suggest that strain gradients in the relaxor and ferroelectric phase of SrTiO3 are very effective phonon scatterers, limiting the thermal transport in this material

Efficient Triangle Counting in Large Graphs via Degree-based Vertex Partitioning

The number of triangles is a computationally expensive graph statistic which is frequently used in complex network analysis (e.g., transitivity ratio), in various random graph models (e.g., exponential random graph model) and in important real world applications such as spam detection, uncovering of the hidden thematic structure of the Web and link recommendation. Counting triangles in graphs with millions and billions of edges requires algorithms which run fast, use small amount of space, provide accurate estimates of the number of triangles and preferably are parallelizable. In this paper we present an efficient triangle counting algorithm which can be adapted to the semistreaming model. The key idea of our algorithm is to combine the sampling algorithm of Tsourakakis et al. and the partitioning of the set of vertices into a high degree and a low degree subset respectively as in the Alon, Yuster and Zwick work treating each set appropriately. We obtain a running time $O \left(m + \frac{m^{3/2} \Delta \log{n}}{t \epsilon^2} \right)$ and an $\epsilon$ approximation (multiplicative error), where $n$ is the number of vertices, $m$ the number of edges and $\Delta$ the maximum number of triangles an edge is contained. Furthermore, we show how this algorithm can be adapted to the semistreaming model with space usage $O\left(m^{1/2}\log{n} + \frac{m^{3/2} \Delta \log{n}}{t \epsilon^2} \right)$ and a constant number of passes (three) over the graph stream. We apply our methods in various networks with several millions of edges and we obtain excellent results. Finally, we propose a random projection based method for triangle counting and provide a sufficient condition to obtain an estimate with low variance.Comment: 1) 12 pages 2) To appear in the 7th Workshop on Algorithms and Models for the Web Graph (WAW 2010

Quantitative parameters for the examination of InGaN QW multilayers by low-loss EELS

We present a detailed examination of a multiple InxGa1-xN quantum well (QW) structure for optoelectronic applications. The characterization is carried out using scanning transmission electron microscopy (STEM), combining high-angle annular dark field (HAADF) imaging and electron energy loss spectroscopy (EELS). Fluctuations in the QW thickness and composition are observed in atomic resolution images. The impact of these small changes on the electronic properties of the semiconductor material is measured through spatially localized low-loss EELS, obtaining band gap and plasmon energy values. Because of the small size of the InGaN QW layers additional effects hinder the analysis. Hence, additional parameters were explored, which can be assessed using the same EELS data and give further information. For instance, plasmon width was studied using a model-based fit approach to the plasmon peak; observing a broadening of this peak can be related to the chemical and structural inhomogeneity in the InGaN QW layers. Additionally, Kramers-Kronig analysis (KKA) was used to calculate the complex dielectric function (CDF) from the EELS spectrum images (SIs). After this analysis, the electron effective mass and the sample absolute thickness were obtained, and an alternative method for the assessment of plasmon energy was demonstrated. Also after KKA, the normalization of the energy-loss spectrum allows us to analyze the Ga 3d transition, which provides additional chemical information at great spatial resolution. Each one of these methods is presented in this work together with a critical discussion of their advantages and drawbacks

Chemical solution synthesis and ferromagnetic resonance of epitaxial thin films of yttrium iron garnet

We report the fabrication of epitaxial Y3Fe5O12 (YIG) thin films on Gd3Ga5O12 (111) using a chemical solution method. Cubic YIG is a ferrimagnetic material at room temperature, with excellent magneto-optical properties, high electrical resistivity, and a very narrow ferromagnetic resonance, which makes it particularly suitable for applications in filters and resonators at microwave frequencies. But these properties depend on the precise stoichiometry and distribution of Fe3+ ions among the octahedral/tetrahedral sites of a complex structure, which hampered the production of high-quality YIG thin films by affordable chemical methods. Here we report the chemical solution synthesis of YIG thin films, with excellent chemical, crystalline, and magnetic homogeneity. The films show a very narrow ferromagnetic resonance (long spin relaxation time), comparable to that obtained from high-vacuum physical deposition methods. These results demonstrate that chemical methods can compete to develop nanometer-thick YIG films with the quality required for spintronic devices and other high-frequency applications

Critical magnetic behavior in [Ag8/Co0.5]x64, [Ag8/Co1]x32 and [Ag16/Co1]x32 epitaxial multilayers

We investigate the low temperature magnetic behavior of three epitaxial Co/Ag multilayers, grown onto MgO (001) substrates, with a nominal content per period of either half a monolayer or one monolayer of Co, and either 8 or 16 Ag monolayers. The samples were studied by X-ray reflectivity and diffraction, transmission electron microscopy, magnetometry and ac susceptometry. The results indicated a well defined stacking sequence in the growth direction, the number of periods and of Ag monolayers per period being coincident with the nominal values for each sample. The Co layers were found to be discontinuous and corresponded to a quasi-monodisperse in-plane distribution of Co nanoparticles embedded in a Ag(001) matrix. The zero-field cooled and field cooled temperature variations of the low field magnetization indicated the presence of irreversibilities at temperatures below 20 K. The ac field frequency (f) and temperature (T) dependencies of the real part of the susceptibility (¿') corresponded to a Vogel-Fulcher behavior in the three samples, and indicated a frequency shift parameter (G) of the order of 4 x 10-2. For each sample, the experimental data corresponding to the variations of the imaginary part of the ac susceptibility (¿¿) with f and T were found to collapse into a single curve according to the dynamic scaling law. Taken together, these results allow us to conclude that the three multilayers experience a phase transition of the paramagnetic to superspin glass type, driven by the dipolar interactions between the Co nanoparticles. Regarding the influence of the multilayer features, we found a clear dependence of the order parameter of the transition on the nominal number of Co monolayers per period

First-line therapy in atypical hemolytic uremic syndrome: consideration on infants with a poor prognosis.

BackgroundAtypical hemolytic uremic syndrome (aHUS) is a rare and heterogeneous disorder. The first line treatment of aHUS is plasma therapy, but in the past few years, the recommendations have changed greatly with the advent of eculizumab, a humanized monoclonal anti C5-antibody. Although recent recommendations suggest using it as a primary treatment for aHUS, important questions have arisen about the necessity of immediate use of eculizumab in all cases. We aimed to draw attention to a specific subgroup of aHUS patients with rapid disease progression and high mortality, in whom plasma therapy may not be feasible.MethodsWe present three pediatric patients of acute complement-mediated HUS with a fatal outcome. Classical and alternative complement pathway activity, levels of complement factors C3, C4, H, B and I, as well as of anti-factor H autoantibody and of ADAMTS13 activity were determined. The coding regions of CFH, CFI, CD46, THBD, CFB and C3 genes were sequenced and the copy number of CFI, CD46, CFH and related genes were analyzed.ResultsWe found severe activation and consumption of complement components in these patients, furthermore, in one patient we identified a previously not reported mutation in CFH (Ser722Stop), supporting the diagnosis of complement-mediated HUS. These patients were not responsive to the FFP therapy, and all cases had fatal outcome.ConclusionTaking the heterogeneity and the variable prognosis of atypical HUS into account, we suggest that the immediate use of eculizumab should be considered as first-line therapy in certain small children with complement dysregulation

Novel therapeutic strategies targeting HIV integrase

Integration of the viral genome into host cell chromatin is a pivotal and unique step in the replication cycle of retroviruses, including HIV. Inhibiting HIV replication by specifically blocking the viral integrase enzyme that mediates this step is an obvious and attractive therapeutic strategy. After concerted efforts, the first viable integrase inhibitors were developed in the early 2000s, ultimately leading to the clinical licensure of the first integrase strand transfer inhibitor, raltegravir. Similarly structured compounds and derivative second generation integrase strand transfer inhibitors, such as elvitegravir and dolutegravir, are now in various stages of clinical development. Furthermore, other mechanisms aimed at the inhibition of viral integration are being explored in numerous preclinical studies, which include inhibition of 3' processing and chromatin targeting. The development of new clinically useful compounds will be aided by the characterization of the retroviral intasome crystal structure. This review considers the history of the clinical development of HIV integrase inhibitors, the development of antiviral drug resistance and the need for new antiviral compounds