Showing papers by "Jožef Stefan Institute published in 2016"

Production of Highly Monolayer Enriched Dispersions of Liquid-Exfoliated Nanosheets by Liquid Cascade Centrifugation

Abstract: While liquid exfoliation is a powerful technique to produce defect-free nanosheets in large quantities, its usefulness is limited by broad nanosheet thickness distributions and low monolayer contents. Here we demonstrate liquid processing techniques, based on iterative centrifugation cascades, which can be designed to achieve either highly efficient nanosheet size-selection and/or monolayer enrichment. The resultant size-selected dispersions were used to establish quantitative metrics to determine monolayer volume fraction, as well as mean nanosheet size and thickness, from standard spectroscopic measurements. Such metrics allowed us to design and optimize centrifugation cascades to enrich liquid exfoliated WS2 dispersions up to monolayer contents of 75%. Monolayer-rich dispersions show relatively bright photoluminescence with narrow line widths (<35 meV) indicating the high quality of the nanosheets. The enriched dispersions display extinction spectra with distinct features, which also allow the direct e...

Materials perspective on Casimir and van der Waals interactions

Abstract: Electromagnetic fluctuation-induced interactions known as van der Waals, Casimir, and Casimir-Polder forces are an active and exciting area of research. This review summarizes recent progress in this field with emphasis on theoretical and computational developments and their applications to materials including molecular structures, Dirac-like systems, optical metamaterials, composites with nontrivial boundary conditions, and biological matter.

Ultrafast optical spectroscopy of strongly correlated materials and high-temperature superconductors: a non-equilibrium approach

Abstract: In the last two decades non-equilibrium spectroscopies have evolved from avant-garde studies to crucial tools for expanding our understanding of the physics of strongly correlated materials. The possibility of obtaining simultaneously spectroscopic and temporal information has led to insights that are complementary to (and in several cases beyond) those attainable by studying the matter at equilibrium. From this perspective, multiple phase transitions and new orders arising from competing interactions are benchmark examples where the interplay among electrons, lattice and spin dynamics can be disentangled because of the different timescales that characterize the recovery of the initial ground state. For example, the nature of the broken-symmetry phases and of the bosonic excitations that mediate the electronic interactions, eventually leading to superconductivity or other exotic states, can be revealed by observing the sub-picosecond dynamics of impulsively excited states. Furthermore, recent experimental...

Leptoquark model to explain the B -physics anomalies, R K and R D

Abstract: We show that a model with a scalar leptoquark of hypercharge $Y=1/6$ which includes light right-handed neutrinos can successfully describe both of the $B$-physics anomalies, ${R}_{K}^{\mathrm{exp}}l{R}_{K}^{\mathrm{SM}}$ and ${R}_{D}^{\mathrm{exp}}g{R}_{D}^{\mathrm{SM}}$. We discuss the corresponding low energy effective theory and, after using the known experimental data as constraints, we show that the model is viable and that it offers several predictions which can be tested experimentally.

Ultrafast optical spectroscopy of strongly correlated materials and high-temperature superconductors: a non-equilibrium approach

Abstract: In the last two decades, non-equilibrium spectroscopies have evolved from avant-garde studies to crucial tools for expanding our understanding of the physics of strongly correlated materials. The possibility of obtaining simultaneously spectroscopic and temporal information has led to insights that are complementary to (and in several cases beyond) those attainable by studying the matter at equilibrium. Multiple phase transitions and new orders arising from competing interactions are benchmark examples where the interplay among electrons, lattice, and spin dynamics can be disentangled because of the different timescales that characterize the recovery of the initial ground state. The nature of the broken-symmetry phases and of the bosonic excitations that mediate the electronic interactions, eventually leading to superconductivity or other exotic states, can be revealed by observing the sub-picosecond dynamics of impulsively excited states. Recent experimental developments have made possible to monitor the time-evolution of both the single-particle and collective excitations under extreme conditions, such as those arising from strong and selective photo-stimulation. Here, we review the most recent achievements in the experimental and theoretical studies of the non-equilibrium electronic, optical, structural and magnetic properties of correlated materials. The focus will be mainly on the prototypical case of correlated oxides that exhibit unconventional superconductivity or other exotic phases, even though the discussion will extend also to other topical systems. The necessity of extending the actual experimental capabilities and the numerical and analytic tools to microscopically treat the non-equilibrium phenomena beyond the simple phenomenological approaches represents one of the most challenging new frontier in physics.

Palatable leptoquark scenarios for lepton flavor violation in exclusive b → sℓ 1 ℓ 2 modes

Abstract: We examine various scenarios that involve a light $$ \mathcal{O} $$ (1 TeV) leptoquark state and select those which are compatible with the current experimental values for ℬ(B s → μμ), $$ \mathrm{\mathcal{B}}{\left(B\to K\mu \mu \right)}_{\mathrm{large}-{q}^2},{R}_K={\mathrm{\mathcal{B}}}^{\prime}\left(B\to K\mu \mu \right)/{\mathrm{\mathcal{B}}}^{\prime}\left(B\to Kee\right) $$ , and which lead to predictions consistent with other experimental data. We show that two such scenarios are phenomenologically plausible, namely the one with a doublet of scalar leptoquarks of hypercharge 1/6, and the one with a triplet of vector leptoquarks of hypercharge 2/3. We also argue that a model with a singlet scalar leptoquark of hypercharge 1/3 is not viable. Using the present experimental data as constraints, it is shown that the exclusive lepton flavor violating decays, ℬ(B s → μτ), ℬ(B → Kμτ) and ℬ(B → K ∗ μτ), can be as large as $$ \mathcal{O}\left(1{0}^{-5}\right) $$ .

Variations in MHC Class II Antigen Processing and Presentation in Health and Disease.

Abstract: MHC class II (MHC-II) molecules are critical in the control of many immune responses. They are also involved in most autoimmune diseases and other pathologies. Here, we describe the biology of MHC-II and MHC-II variations that affect immune responses. We discuss the classic cell biology of MHC-II and various perturbations. Proteolysis is a major process in the biology of MHC-II, and we describe the various components forming and controlling this endosomal proteolytic machinery. This process ultimately determines the MHC-II-presented peptidome, including cryptic peptides, modified peptides, and other peptides that are relevant in autoimmune responses. MHC-II also variable in expression, glycosylation, and turnover. We illustrate that MHC-II is variable not only in amino acids (polymorphic) but also in its biology, with consequences for both health and disease.

Physics of leptoquarks in precision experiments and at particle colliders

Abstract: We present a comprehensive review of physics effects generated by leptoquarks (LQs), i.e., hypothetical particles that can turn quarks into leptons and vice versa, of either scalar or vector nature. These considerations include discussion of possible completions of the Standard Model that contain LQ fields. The main focus of the review is on those LQ scenarios that are not problematic with regard to proton stability. We accordingly concentrate on the phenomenology of light leptoquarks that is relevant for precision experiments and particle colliders. Important constraints on LQ interactions with matter are derived from precision low-energy observables such as electric dipole moments, (g-2) of charged leptons, atomic parity violation, neutral meson mixing, Kaon, B, and D meson decays, etc. We provide a general analysis of indirect constraints on the strength of LQ interactions with the quarks and leptons to make statements that are as model independent as possible. We address complementary constraints that originate from electroweak precision measurements, top, and Higgs physics. The Higgs physics analysis we present covers not only the most recent but also expected results from the Large Hadron Collider (LHC). We finally discuss direct LQ searches. Current experimental situation is summarized and self-consistency of assumptions that go into existing accelerator-based searches is discussed. A progress in making next-to-leading order predictions for both pair and single LQ productions at colliders is also outlined.

Multilingual Twitter Sentiment Classification: The Role of Human Annotators

Abstract: What are the limits of automated Twitter sentiment classification? We analyze a large set of manually labeled tweets in different languages, use them as training data, and construct automated classification models. It turns out that the quality of classification models depends much more on the quality and size of training data than on the type of the model trained. Experimental results indicate that there is no statistically significant difference between the performance of the top classification models. We quantify the quality of training data by applying various annotator agreement measures, and identify the weakest points of different datasets. We show that the model performance approaches the inter-annotator agreement when the size of the training set is sufficiently large. However, it is crucial to regularly monitor the self- and inter-annotator agreements since this improves the training datasets and consequently the model performance. Finally, we show that there is strong evidence that humans perceive the sentiment classes (negative, neutral, and positive) as ordered.

Fast electronic resistance switching involving hidden charge density wave states.

Abstract: The functionality of computer memory elements is currently based on multi-stability, driven either by locally manipulating the density of electrons in transistors or by switching magnetic or ferroelectric order. Another possibility is switching between metallic and insulating phases by the motion of ions, but their speed is limited by slow nucleation and inhomogeneous percolative growth. Here we demonstrate fast resistance switching in a charge density wave system caused by pulsed current injection. As a charge pulse travels through the material, it converts a commensurately ordered polaronic Mott insulating state in 1T–TaS2 to a metastable electronic state with textured domain walls, accompanied with a conversion of polarons to band states, and concurrent rapid switching from an insulator to a metal. The large resistance change, high switching speed (30 ps) and ultralow energy per bit opens the way to new concepts in non-volatile memory devices manipulating all-electronic states. The control of a material's state via external stimuli is the basis of modern information storage technology. Here, the authors use pulsed currents to induce fast switching between Mott insulator and metallic states in the charge density wave system 1T-TaS2, presenting an all-electronic storage mechanism.

Continuous stress detection using a wrist device: in laboratory and real life

Abstract: Continuous exposure to stress is harmful for mental and physical health, but to combat stress, one should first detect it. In this paper we propose a method for continuous detection of stressful events using data provided from a commercial wrist device. The method consists of three machine-learning components: a laboratory stress detector that detects short-term stress every 2 minutes; an activity recognizer that continuously recognizes user's activity and thus provides context information; and a context-based stress detector that exploits the output of the laboratory stress detector and the user's context in order to provide the final decision on 20 minutes interval. The method was evaluated in a laboratory and a real-life setting. The accuracy on 55 days of real-life data, for a 2-class problem, was 92%. The method is currently being integrated in a smartphone application for managing mental health and well-being.

Adaptive Control of Exoskeleton Robots for Periodic Assistive Behaviours Based on EMG Feedback Minimisation.

Abstract: In this paper we propose an exoskeleton control method for adaptive learning of assistive joint torque profiles in periodic tasks. We use human muscle activity as feedback to adapt the assistive joint torque behaviour in a way that the muscle activity is minimised. The user can then relax while the exoskeleton takes over the task execution. If the task is altered and the existing assistive behaviour becomes inadequate, the exoskeleton gradually adapts to the new task execution so that the increased muscle activity caused by the new desired task can be reduced. The advantage of the proposed method is that it does not require biomechanical or dynamical models. Our proposed learning system uses Dynamical Movement Primitives (DMPs) as a trajectory generator and parameters of DMPs are modulated using Locally Weighted Regression. Then, the learning system is combined with adaptive oscillators that determine the phase and frequency of motion according to measured Electromyography (EMG) signals. We tested the method with real robot experiments where subjects wearing an elbow exoskeleton had to move an object of an unknown mass according to a predefined reference motion. We further evaluated the proposed approach on a whole-arm exoskeleton to show that it is able to adaptively derive assistive torques even for multiple-joint motion.

Measurement of the Radiation Energy in the Radio Signal of Extensive Air Showers as a Universal Estimator of Cosmic-Ray Energy

Abstract: We measure the energy emitted by extensive air showers in the form of radio emission in the frequency range from 30 to 80 MHz. Exploiting the accurate energy scale of the Pierre Auger Observatory, we obtain a radiation energy of 15.8 \pm 0.7 (stat) \pm 6.7 (sys) MeV for cosmic rays with an energy of 1 EeV arriving perpendicularly to a geomagnetic field of 0.24 G, scaling quadratically with the cosmic-ray energy. A comparison with predictions from state-of-the-art first-principle calculations shows agreement with our measurement. The radiation energy provides direct access to the calorimetric energy in the electromagnetic cascade of extensive air showers. Comparison with our result thus allows the direct calibration of any cosmic-ray radio detector against the well-established energy scale of the Pierre Auger Observatory.

The CC domain structure from the wheat stem rust resistance protein Sr33 challenges paradigms for dimerization in plant NLR proteins.

Abstract: Plants use intracellular immunity receptors, known as nucleotide-binding oligomerization domain-like receptors (NLRs), to recognize specific pathogen effector proteins and induce immune responses. These proteins provide resistance to many of the world’s most destructive plant pathogens, yet we have a limited understanding of the molecular mechanisms that lead to defense signaling. We examined the wheat NLR protein, Sr33, which is responsible for strain-specific resistance to the wheat stem rust pathogen, Puccinia graminis f. sp. tritici. We present the solution structure of a coiled-coil (CC) fragment from Sr33, which adopts a four-helix bundle conformation. Unexpectedly, this structure differs from the published dimeric crystal structure of the equivalent region from the orthologous barley powdery mildew resistance protein, MLA10, but is similar to the structure of the distantly related potato NLR protein, Rx. We demonstrate that these regions are, in fact, largely monomeric and adopt similar folds in solution in all three proteins, suggesting that the CC domains from plant NLRs adopt a conserved fold. However, larger C-terminal fragments of Sr33 and MLA10 can self-associate both in vitro and in planta, and this self-association correlates with their cell death signaling activity. The minimal region of the CC domain required for both cell death signaling and self-association extends to amino acid 142, thus including 22 residues absent from previous biochemical and structural protein studies. These data suggest that self-association of the minimal CC domain is necessary for signaling but is likely to involve a different structural basis than previously suggested by the MLA10 crystallographic dimer.

Selective Plasma Etching of Polymeric Substrates for Advanced Applications.

Abstract: In today’s nanoworld, there is a strong need to manipulate and process materials on an atom-by-atom scale with new tools such as reactive plasma, which in some states enables high selectivity of interaction between plasma species and materials. These interactions first involve preferential interactions with precise bonds in materials and later cause etching. This typically occurs based on material stability, which leads to preferential etching of one material over other. This process is especially interesting for polymeric substrates with increasing complexity and a “zoo” of bonds, which are used in numerous applications. In this comprehensive summary, we encompass the complete selective etching of polymers and polymer matrix micro-/nanocomposites with plasma and unravel the mechanisms behind the scenes, which ultimately leads to the enhancement of surface properties and device performance.

Therapeutic Use of Exercising in Hypoxia: Promises and Limitations

Abstract: It is well-established that different altitude training modalities can improve convective oxygen (O2) transport capacity and physical fitness of athletes (Millet et al., 2010). Exercising in hypoxia also induces specific muscular adaptations including increased oxidative enzymes (e.g., citrate synthase) activity, mitochondrial density, capillary-to-fiber ratio, and fiber cross-sectional area (Hoppeler et al., 2008). These changes with hypoxic training are mostly modulated via hypoxia-inducible factor 1α (HIF-1α) signaling cascade, which is not activated to the same extent when training is performed in normoxia or by passive hypoxic exposure. Indeed, large body of literature shows that, compared to hypoxic exercise, passive exposure to hypoxia does not provoke similar acute responses. In healthy individuals, both systemic (e.g., performance enhancement), cardiovascular (e.g., maximal O2 uptake, VO2max) or transcriptional muscular responses are minimal with intermittent passive exposures at moderate altitude. On the other hand, there are clear evidences that when hypoxia is combined with exercise, it triggers specific responses, not observed following similar exercise in normoxia (Bartsch et al., 2008; Lundby et al., 2009). In addition, greater specific adaptations have been reported in high-intensity vs. moderate-intensity hypoxic intervention (Faiss et al., 2013) (e.g., improvements in muscle O2 homeostasis and tissue perfusion induced by enhanced mitochondrial efficiency, control of mitochondrial respiration, angiogenesis, and muscle buffering capacity). It seems that the main underlying mechanism is the larger hypoxemia resulting from the combination of muscle deoxygenation (high-intensity exercise) and systemic desaturation (moderate hypoxia). In patients or elderly individuals, altitude is generally associated with increased health risks through enhanced sympathetic vasoconstrictor activation (Blitzer et al., 1996), obstructive sleep apneas (Nespoulet et al., 2012), hypoxemia (Levine et al., 1997), pulmonary hypertension (Valencia-Flores et al., 2004), arrhythmias (Kujanik et al., 2000), and alterations of postural control (Degache et al., 2012). However, several studies have investigated the therapeutic benefits of exercising in mild hypoxia on the blood pressure regulation and the influence of different hypoxic modalities in healthy individuals (Bailey et al., 2001; Wang et al., 2007; Haufe et al., 2008; Nishiwaki et al., 2011; Morishima et al., 2014; Shi et al., 2014) or in patients with different cardiovascular and respiratory risk factors such as chronic obstructive pulmonary disease (COPD) (Haider et al., 2009), obesity (Wiesner et al., 2010), coronary artery disease (Burtscher et al., 2004). Recent studies (Haufe et al., 2008; Wiesner et al., 2010) have also reported that sustained hypoxia may be of benefit to weight management programs of obese patients (Urdampilleta et al., 2012; Kayser and Verges, 2013). Both exercise (Williams et al., 2002) and/or intermittent hypoxia (Burtscher et al., 2004; Shatilo et al., 2008) have been suggested to positively influence age-related alterations in elderly individuals. Finally, living at altitude seems to have contradictory effects on different mortality risk factors. Therefore, this essay summarizes recent evidences suggesting that exercising in hypoxia might be a valuable and viable “therapeutic strategy.” We discuss the benefits and risks/limitations in (i) hypertensive (ii) obese, (iii) elderly individuals. Since the benefits of being active have been extensively investigated in these three groups of individuals (see respective reviews on the effects of physical activity in Cherubini et al., 1998; Baillot et al., 2014; Borjesson et al., 2016), the present article focus on the potential additional health benefits provided by hypoxic exercise, when compared to normoxic exercise. For safety and practical reasons, patients cannot access high altitude (even by using hypoxic devices) and preferably stay at moderate altitude (1800–3000 m). In this setting, exercise is used to increase the overall hypoxia-induced metabolic stress and thereby provide benefits beyond those achievable by normoxic therapeutic training modalities.

Spontaneous liquid crystal and ferromagnetic ordering of colloidal magnetic nanoplates.

Abstract: Ferrofluids are familiar as colloidal suspensions of ferromagnetic nanoparticles in aqueous or organic solvents. The dispersed particles are randomly oriented but their moments become aligned if a magnetic field is applied, producing a variety of exotic and useful magnetomechanical effects. A longstanding interest and challenge has been to make such suspensions macroscopically ferromagnetic, that is having uniform magnetic alignment in the absence of a field. Here we report a fluid suspension of magnetic nanoplates that spontaneously aligns into an equilibrium nematic liquid crystal phase that is also macroscopically ferromagnetic. Its zero-field magnetization produces distinctive magnetic self-interaction effects, including liquid crystal textures of fluid block domains arranged in closed flux loops, and makes this phase highly sensitive, with it dramatically changing shape even in the Earth's magnetic field.

Absence of full many-body localization in the disordered Hubbard chain

Abstract: We present numerical results within the one-dimensional disordered Hubbard model for several characteristic indicators of the many-body localization (MBL). Considering traditionally studied charge disorder (i.e., the same disorder strength for both spin orientations) we find that even at strong disorder all signatures consistently show that while charge degree of freedom is nonergodic, the spin is delocalized and ergodic. This indicates the absence of the full MBL in the model that has been simulated in recent cold-atom experiments. Full localization can be restored if spin-dependent disorder is used instead.