Showing papers by "Jožef Stefan Institute published in 2010"

Snapshots of cooperative atomic motions in the optical suppression of charge density waves

Abstract: The development of tabletop femtosecond electron diffraction sources has provided an alternative way of observing atomic motions in crystalline materials. This technique has now been applied to the charge-density-wave material 1T-TaS2, in which modulation of the electron density is accompanied by a periodic lattice distortion. Previous time-resolved studies have revealed the dynamics of the electronic charge density wave, but until now the dynamics of the lattice system has been only indirectly inferred. In this new experiment, atomic motions were observed in response to a 140 femtosecond optical pulse. Periodic lattice distortion was seen to collapse on an exceptionally fast timescale (about 250 fs), indicative of an electronically driven process involving a high degree of cooperativity. The surprisingly high degree of cooperation in the observed structural dynamics between the electronic and lattice system illustrates the potential for the technique in studies of strongly correlated systems.

Task-Specific Generalization of Discrete and Periodic Dynamic Movement Primitives

Abstract: Acquisition of new sensorimotor knowledge by imitation is a promising paradigm for robot learning. To be effective, action learning should not be limited to direct replication of movements obtained during training but must also enable the generation of actions in situations a robot has never encountered before. This paper describes a methodology that enables the generalization of the available sensorimotor knowledge. New actions are synthesized by the application of statistical methods, where the goal and other characteristics of an action are utilized as queries to create a suitable control policy, taking into account the current state of the world. Nonlinear dynamic systems are employed as a motor representation. The proposed approach enables the generation of a wide range of policies without requiring an expert to modify the underlying representations to account for different task-specific features and perceptual feedback. The paper also demonstrates that the proposed methodology can be integrated with an active vision system of a humanoid robot. 3-D vision data are used to provide query points for statistical generalization. While 3-D vision on humanoid robots with complex oculomotor systems is often difficult due to the modeling uncertainties, we show that these uncertainties can be accounted for by the proposed approach.

3D microlasers from self-assembled cholesteric liquid-crystal microdroplets.

Abstract: We demonstrate a tunable and omnidirectional microlaser in the form of a microdroplet of a dye-doped, cholesteric liquid crystal in a carrier fluid. The cholesteric forms a Bragg-onion optical microcavity and the omnidirectional 3D lasing is due to the stimulated emission of light from the dye molecules in the liquid crystal. The lasing wavelength depends solely on the natural helical period of the cholesteric and can be tuned by varying the temperature. Millions of microlasers can be formed simply by mixing a liquid crystal, a laser dye and a carrier fluid, thus providing microlasers for soft-matter photonic devices.

What Determines the Inhibition Effectiveness of ATA, BTAH, and BTAOH Corrosion Inhibitors on Copper?

Abstract: Three corrosion inhibitors for copper-3-amino-1,2,4-triazole (ATA), benzotriazole (BTAH), and 1-hydroxybenzotriazole (BTAOH)-were investigated by corrosion experiments and atomistic computer simulations. The trend of corrosion inhibition effectiveness of the three inhibitors on copper in near-neutral chloride solution is determined experimentally as BTAH ≳ ATA ≫ BTAOH. A careful analysis of the results of computer simulations based on density functional theory allowed to pinpoint the superior inhibiting action of BTAH and ATA as a result of their ability to form strong N-Cu chemical bonds in deprotonated form. While these bonds are not as strong as the Cl-Cu bonds, the presence of solvent favors the adsorption of inhibitor molecules onto the surface due to stronger solvation of the Cl(-) anions. Moreover, benzotriazole displays the largest affinity among the three inhibitors to form intermolecular aggregates, such as [BTA-Cu](n) polymeric complex. This is another factor contributing to the stability of the protective inhibitor film on the surface, thus making benzotriazole an outstanding corrosion inhibitor for copper. These findings cannot be anticipated on the basis of inhibitors' molecular electronic properties alone, thus emphasizing the importance of a rigorous modeling of the interactions between the components of the corrosion system in corrosion inhibition studies.

Self-assembled artificial cilia

Abstract: Due to their small dimensions, microfluidic devices operate in the low Reynolds number regime. In this case, the hydrodynamics is governed by the viscosity rather than inertia and special elements have to be introduced into the system for mixing and pumping of fluids. Here we report on the realization of an effective pumping device that mimics a ciliated surface and imitates its motion to generate fluid flow. The artificial biomimetic cilia are constructed as long chains of spherical superparamagnetic particles, which self-assemble in an external magnetic field. Magnetic field is also used to actuate the cilia in a simple nonreciprocal manner, resulting in a fluid flow. We prove the concept by measuring the velocity of a cilia-pumped fluid as a function of height above the ciliated surface and investigate the influence of the beating asymmetry on the pumping performance. A numerical simulation was carried out that successfully reproduced the experimentally obtained data.

An evaluation of biomass co-firing in Europe.

Abstract: Reduction of the emissions of greenhouses gases, increasing the share of renewable energy sources (RES) in the energy balance, increasing electricity production from renewable energy sources and decreasing energy dependency represent the main goals of all current strategies in Europe. Biomass co-firing in large coal-based thermal power plants provides a considerable opportunity to increase the share of RES in the primary energy balance and the share of electricity from RES in gross electricity consumption in a country. Biomass-coal co-firing means reducing CO 2 and SO 2 , emissions and it may also reduce NO x emissions, and also represents a near-term, low-risk, low-cost and sustainable energy development. Biomass-coal co-firing is the most effective measure to reduce CO 2 emissions, because it substitutes coal, which has the most intensive CO 2 emissions per kWh electricity production, by biomass, with a zero net emission of CO 2 . Biomass co-firing experience worldwide are reviewed in this paper. Biomass co-firing has been successfully demonstrated in over 150 installations worldwide for most combinations of fuels and boiler types in the range of 50–700 MWe, although a number of very small plants have also been involved. More than a hundred of these have been in Europe. A key indicator for the assessment of biomass co-firing is intrduced and used to evaluate all available biomass co-firing technologies.

Trigger and aperture of the surface detector array of the pierre auger observatory

Abstract: The surface detector array of the Pierre Auger Observatory consists of 1600 water-Cherenkov detectors, for the study of extensive air showers (EAS) generated by ultra-high-energy cosmic rays. We describe the trigger hierarchy, from the identification of candidate showers at the level of a single detector, amongst a large background (mainly random single cosmic ray muons), up to the selection of real events and the rejection of random coincidences. Such trigger makes the surface detector array fully efficient for the detection of EAS with energy above 3 x 10(18) eV, for all zenith angles between 0 degrees and 60 degrees, independently of the position of the impact point and of the mass of the primary particle. In these range of energies and angles, the exposure of the surface array can be determined purely on the basis of the geometrical acceptance. (C) 2009 Elsevier B.V. All rights reserved.

Coherent dynamics of macroscopic electronic order through a symmetry breaking transition

Abstract: The speed with which symmetry breaking transitions occur in the solid state makes them difficult to study in the time domain. State-of-the-art pump–probe measurements of the dynamics of charge-density waves in terbium telluride enable the evolution of the symmetry breaking charge-order transition of this system to be studied with unprecedented temporal resolution.

Predicting gene function using hierarchical multi-label decision tree ensembles

Abstract: S. cerevisiae, A. thaliana and M. musculus are well-studied organisms in biology and the sequencing of their genomes was completed many years ago. It is still a challenge, however, to develop methods that assign biological functions to the ORFs in these genomes automatically. Different machine learning methods have been proposed to this end, but it remains unclear which method is to be preferred in terms of predictive performance, efficiency and usability. We study the use of decision tree based models for predicting the multiple functions of ORFs. First, we describe an algorithm for learning hierarchical multi-label decision trees. These can simultaneously predict all the functions of an ORF, while respecting a given hierarchy of gene functions (such as FunCat or GO). We present new results obtained with this algorithm, showing that the trees found by it exhibit clearly better predictive performance than the trees found by previously described methods. Nevertheless, the predictive performance of individual trees is lower than that of some recently proposed statistical learning methods. We show that ensembles of such trees are more accurate than single trees and are competitive with state-of-the-art statistical learning and functional linkage methods. Moreover, the ensemble method is computationally efficient and easy to use. Our results suggest that decision tree based methods are a state-of-the-art, efficient and easy-to-use approach to ORF function prediction.

Polymorphism control of superconductivity and magnetism in Cs 3 C 60 close to the Mott transition

Abstract: The crystal structure of a solid controls the interactions between the electronically active units and thus its electronic properties. In the high-temperature superconducting copper oxides, only one spatial arrangement of the electronically active Cu(2+) units-a two-dimensional square lattice-is available to study the competition between the cooperative electronic states of magnetic order and superconductivity. Crystals of the spherical molecular C(60)(3-) anion support both superconductivity and magnetism but can consist of fundamentally distinct three-dimensional arrangements of the anions. Superconductivity in the A(3)C(60) (A = alkali metal) fullerides has been exclusively associated with face-centred cubic (f.c.c.) packing of C(60)(3-) (refs 2, 3), but recently the most expanded (and thus having the highest superconducting transition temperature, T(c); ref. 4) composition Cs(3)C(60) has been isolated as a body-centred cubic (b.c.c.) packing, which supports both superconductivity and magnetic order. Here we isolate the f.c.c. polymorph of Cs(3)C(60) to show how the spatial arrangement of the electronically active units controls the competing superconducting and magnetic electronic ground states. Unlike all the other f.c.c. A(3)C(60) fullerides, f.c.c. Cs(3)C(60) is not a superconductor but a magnetic insulator at ambient pressure, and becomes superconducting under pressure. The magnetic ordering occurs at an order of magnitude lower temperature in the geometrically frustrated f.c.c. polymorph (Neel temperature T(N) = 2.2 K) than in the b.c.c.-based packing (T(N) = 46 K). The different lattice packings of C(60)(3-) change T(c) from 38 K in b.c.c. Cs(3)C(60) to 35 K in f.c.c. Cs(3)C(60) (the highest found in the f.c.c. A(3)C(60) family). The existence of two superconducting packings of the same electronically active unit reveals that T(c) scales universally in a structure-independent dome-like relationship with proximity to the Mott metal-insulator transition, which is governed by the role of electron correlations characteristic of high-temperature superconducting materials other than fullerides.

An agent-based approach to care in independent living

Abstract: This paper presents a multi-agent system for the care of elderly people living at home on their own, with the aim to prolong their independence. The system is composed of seven groups of agents providing a reliable, robust and flexible monitoring by sensing the user in the environment, reconstructing the position and posture to create the physical awareness of the user in the environment, reacting to critical situations, calling for help in the case of an emergency, and issuing warnings if unusual behavior is detected. The system has been tested during several on-line demonstrations.

Compositional range and electrical properties of the morphotropic phase boundary in the Na0.5Bi0.5TiO3–K0.5Bi0.5TiO3 system

Abstract: We have investigated the Na 0.5 Bi 0.5 TiO 3 –K 0.5 Bi 0.5 TiO 3 (NBT–KBT) system, with its complex perovskite structure, as a promising material for piezoelectric applications. The NBT–KBT samples were synthesized using a solid-state reaction method and characterized with XRD and SEM. Room-temperature XRD showed a gradual change in the crystal structure from tetragonal in the KBT to rhombohedral in the NBT, with the presence of an intermediate morphotropic region in the samples with a compositional fraction x between 0.17 and 0.25. The fitted perovskite lattice parameters confirmed an increase in the size of the crystal lattice from NBT towards KBT, which coincides with an increase in the ionic radii. Electrical measurements on the samples showed that the maximum values of the dielectric constant, the remanent polarization and the piezoelectric coefficient are reached at the morphotropic phase boundary (MPB) ( ɛ = 1140 at 1 MHz; P r = 40 μC/cm 2 ; d 33 = 134 pC/N).

The Hunt for New Physics at the Large Hadron Collider

Abstract: The Large Hadron Collider presents an unprecedented opportunity to probe the realm of new physics in the TeV region and shed light on some of the core unresolved issues of particle physics. These include the nature of electroweak symmetry breaking, the origin of mass, the possible constituent of cold dark matter, new sources of CP violation needed to explain the baryon excess in the universe, the possible existence of extra gauge groups and extra matter, and importantly the path Nature chooses to resolve the hierarchy problem - is it supersymmetry or extra dimensions. Many models of new physics beyond the standard model contain a hidden sector which can be probed at the LHC. Additionally, the LHC will be a top factory and accurate measurements of the properties of the top and its rare decays will provide a window to new physics. Further, the LHC could shed light on the origin of neutralino masses if the new physics associated with their generation lies in the TeV region. Finally, the LHC is also a laboratory to test the hypothesis of TeV scale strings and D-brane models. An overview of these possibilities is presented in the spirit that it will serve as a companion to the Technical Design Reports (TDRs) by the particle detector groups ATLAS and CMS to facilitate the test of the new theoretical ideas at the LHC. Which of these ideas stands the test of the LHC data will govern the course of particle physics in the subsequent decades.

Investigation of Irradiated Silicon Detectors by Edge-TCT

Abstract: A Transient Current Technique (TCT) utilizing an IR laser with 100 ps pulse width and beam diameter of FWHM = 8 μm was used to evaluate non-irradiated and irradiated p-type silicon micro-strip detectors. The beam was parallel with the surface and perpendicular to the strips (Edge-TCT) so that the electron hole pairs were created at known depth in the detector. Induced current pulses were measured in one of the strips. The pulse shapes were analyzed in a new way, which does not require the knowledge of effective trapping times, to determine drift velocity, charge collection and electric field profiles in heavily irradiated silicon detectors. The profiles were studied at different laser beam positions (depth of carrier generation), voltages and fluences up to 5·1015 neutrons cm-2. A strong evidence for charge multiplication at high voltages was found with the detector irradiated to the highest fluence.

From nucleation to nanowires: a single-step process in reactive plasmas

Abstract: This feature article introduces a deterministic approach for the rapid, single-step, direct synthesis of metal oxide nanowires. This approach is based on the exposure of thin metal samples to reactive oxygen plasmas and does not require any intervening processing or external substrate heating. The critical roles of the reactive oxygen plasmas, surface processes, and plasma-surface interactions that enable this growth are critically examined by using a deterministic viewpoint. The essentials of the experimental procedures and reactor design are presented and related to the key process requirements. The nucleation and growth kinetics is discussed for typical solid–liquid–solid and vapor–solid–solid mechanisms related to the synthesis of the oxide nanowires of metals with low (Ga, Cd) and high (Fe) melting points, respectively. Numerical simulations are focused on the possibility to predict the nanowire nucleation points through the interaction of the plasma radicals and ions with the nanoscale morphological features on the surface, as well as to control the localized ‘hot spots’ that in turn determine the nanowire size and shape. This generic approach can be applied to virtually any oxide nanoscale system and further confirms the applicability of the plasma nanoscience approaches for deterministic nanoscale synthesis and processing.

Electron-Phonon Coupling in High-Temperature Cuprate Superconductors Determined from Electron Relaxation Rates

Abstract: We determined electronic relaxation times via pump-probe optical spectroscopy using sub-15 fs pulses for the normal state of two different cuprate superconductors. We show that the primary relaxation process is the electron-phonon interaction and extract a measure of its strength, the second moment of the Eliashberg function λ[ω2] = 800 ± 200 meV2 for La(1.85)Sr(0.15)CuO4 and λ[ω2] = 400 ± 100 meV2 for YBa(2)Cu(3)O(6.5). These values suggest a possible fundamental role of the electron-phonon interaction in the superconducting pairing mechanism.

Density functional theory study of ATA, BTAH, and BTAOH as copper corrosion inhibitors: adsorption onto Cu(111) from gas phase.

Abstract: A low-coverage gas-phase adsorption of three corrosion inhibitors-3-amino-1,2,4-triazole (ATA), benzotriazole (BTAH), and 1-hydroxybenzotriazole (BTAOH)-on perfect Cu(111) surface has been studied and characterized using density functional theory calculations. We find that the molecules in neutral form chemisorb weakly to the perfect surface in an upright geometry. The strength of the chemisorption increases in the order BTAH < BTAOH < ATA with adsorption energies of -0.40, -0.53, and -0.60 eV, respectively. The molecules bond to the surface with triazole nitrogen atoms and also through X-H···Metal hydrogen bonds (X = N or O). In addition to chemisorption, BTAH and BTAOH can also physisorb with the molecular plane being nearly parallel to the surface and the energies of the physisorption are -0.72 and -0.97 eV, respectively, hence being more exothermic than the corresponding chemisorption energies. On the other hand, the molecules in dehydrogenated form chemisorb strongly to the surface and the strength of the chemisorption increases in the order BTAO· < ATA· < BTA· with the adsorption energies of -1.65, -2.22, and -2.78 eV, respectively. This order is compatible with the trend of experimentally observed corrosion inhibition effectiveness on copper in near-neutral chloride solutions. Although the calculations are performed at the metal/vacuum interface, they provide enough insight to rationalize why in some experiments the BTAH was observed to be adsorbed with an upright geometry and in the others with parallel geometry.

Co-Doped ZnO nanoparticles: Minireview

Abstract: Diluted magnetic semiconductors with a Curie temperature exceeding 300 K are promising candidates for spintronic devices and spin-based electronic technologies We review recent achievements in the field of one of them: Co-doped ZnO at the nanoparticulate scale

Increased Cellular Uptake of Biocompatible Superparamagnetic Iron Oxide Nanoparticles into Malignant Cells by an External Magnetic Field

Abstract: Superparamagnetic iron oxide nanoparticles (SPIONs) are used as delivery systems for different therapeutics including nucleic acids for magnetofection-mediated gene therapy. The aim of our study was to evaluate physicochemical properties, biocompatibility, cellular uptake and trafficking pathways of the custom-synthesized SPIONs for their potential use in magnetofection. Custom-synthesized SPIONs were tested for size, shape, crystalline composition and magnetic behavior using a transmission electron microscope, X-ray diffractometer and magnetometer. SPIONs were dispersed in different aqueous media to obtain ferrofluids, which were tested for pH and stability using a pH meter and zetameter. Cytotoxicity was determined using the MTS and clonogenic assays. Cellular uptake and trafficking pathways were qualitatively evaluated by transmission electron microscopy and quantitatively by inductively coupled plasma atomic emission spectrometry. SPIONs were composed of an iron oxide core with a diameter of 8-9 nm, coated with a 2-nm-thick layer of silica. SPIONs, dispersed in 0.9% NaCl solution, resulted in a stable ferrofluid at physiological pH for several months. SPIONs were not cytotoxic in a broad range of concentrations and were readily internalized into different cells by endocytosis. Exposure to neodymium-iron-boron magnets significantly increased the cellular uptake of SPIONs, predominantly into malignant cells. The prepared SPIONs displayed adequate physicochemical and biomedical properties for potential use in magnetofection. Their cellular uptake was dependent on the cell type, and their accumulation within the cells was dependent on the duration of exposure to an external magnetic field.

Light colored scalars from grand unification and the forward-backward asymmetry in tt̄ production

Abstract: The experimental results on the $t\overline{t}$ production cross section at the Tevatron are well described by the QCD contributions within the standard model, while the recently measured forward-backward asymmetry is larger than predicted within this framework. We consider light colored scalars appearing in a particular $SU(5)$ grand unified theory model within the 45-dimensional Higgs representation. A virtue of the model is that it connects the presence of a light colored $SU(2)$ singlet (${\ensuremath{\Delta}}_{6}$) and a color octet weak doublet (${\ensuremath{\Delta}}_{1}$) with bounds on the proton lifetime, which constrain the parameter space of both scalars. We find that both the total $t\overline{t}$ production cross section and the forward-backward asymmetry can be accommodated simultaneously within this model. The experimental results prefer a region for the mass of ${\ensuremath{\Delta}}_{6}$ around 400 GeV, while ${\ensuremath{\Delta}}_{1}$ is then constrained to have a mass around the TeV scale as well. We analyze possible experimental signatures and find that ${\ensuremath{\Delta}}_{6}$ associated top production could be probed in the $t\overline{t}+\mathrm{\text{jets}}$ final states at Tevatron and the LHC.

Influence of synthesis method on structural and magnetic properties of cobalt ferrite nanoparticles

Abstract: The Co–ferrite nanoparticles having a relatively uniform size distribution around 8 nm were synthesized by three different methods. A simple co-precipitation from aqueous solutions and a co-precipitation in an environment of microemulsions are low temperature methods (50 °C), whereas a thermal decomposition of organo-metallic complexes was performed at elevated temperature of 290 °C. The X-ray diffractometry (XRD) showed spinel structure, and the high-resolution transmission electron microscopy (HRTEM) a good crystallinity of all the nanoparticles. Energy-dispersive X-ray spectroscopy (EDS) showed the composition close to stoichiometric (~CoFe2O4) for both co-precipitated nanoparticles, whereas the nanoparticles prepared by the thermal decomposition were Co-deficient (~Co0.6Fe2.4O4). The X-ray absorption near-edge structure (XANES) analysis showed Co valence of 2+ in all the samples, Fe valence 3+ in both co-precipitated samples, but average Fe valence of 2.7+ in the sample synthesized by thermal decomposition. The variations in cation distribution within the spinel lattice were observed by structural refinement of X-ray absorption fine structure (EXAFS). Like the bulk CoFe2O4, the nanoparticles synthesized at elevated temperature using thermal decomposition displayed inverse spinel structure with the Co ions occupying predominantly octahedral lattice sites, whereas co-precipitated samples showed considerable proportion of cobalt ions occupying tetrahedral sites (nearly 1/3 for the nanoparticles synthesized by co-precipitation from aqueous solutions and almost 1/4 for the nanoparticles synthesized in microemulsions). Magnetic measurements performed at room temperature and at 10 K were in good agreement with the nanoparticles’ composition and the cation distribution in their structure. The presented study clearly shows that the distribution of the cations within the spinel lattice of the ferrite nanoparticles, and consequently their magnetic properties are strongly affected by the synthesis method used.

Femtosecond response of quasiparticles and phonons in superconducting YBa(2)Cu(3)O(7-δ) studied by wideband terahertz spectroscopy.

Abstract: We measure the anisotropic midinfrared response of electrons and phonons in bulk YBa(2)Cu(3)O(7-δ) after femtosecond photoexcitation. A line shape analysis of specific lattice modes reveals their transient occupation and coupling to the superconducting condensate. The apex oxygen vibration is strongly excited within 150 fs, demonstrating that the lattice absorbs a major portion of the pump energy before the quasiparticles are thermalized. Our results attest to substantial electron-phonon scattering and introduce a powerful concept probing electron-lattice interactions in a variety of complex materials.

Collider signatures for the heavy lepton triplet in the type I+III seesaw mechanism

Abstract: The minimal SU(5) theory augmented by the fermionic adjoint representation restores the coupling constant unification and gives realistic neutrino masses and mixing through the hybrid Type I and Type III seesaw. The crucial prediction of the theory is an SU(2) lepton triplet with the mass below TeV. We study the signature of these heavy leptons at the hadron and lepton colliders. The smoking gun evidence of the theory, as in general seesaw mechanisms, is $\ensuremath{\Delta}L=2$ lepton-number violation through events of a pair of like-sign leptons plus four jets without significant missing energy at hadron colliders. We find that via this unique channel the heavy lepton can be searched for up to a mass of 200 GeV at the Tevatron with $8\text{ }\text{ }{\mathrm{fb}}^{\ensuremath{-}1}$, and up to 450 (700) GeV at the LHC of 14 TeV C.M. energy with $10(100)\text{ }\text{ }{\mathrm{fb}}^{\ensuremath{-}1}$. The 7 TeV LHC run of $1\text{ }\text{ }{\mathrm{fb}}^{\ensuremath{-}1}$ is expected to probe a mass window of 110--200 GeV. We also comment on how to distinguish this theory from other models with similar heavy leptons. Finally, we compare the production rates and angular distributions of heavy leptons in ${e}^{+}{e}^{\ensuremath{-}}$ collisions for various models.