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http://hdl.handle.net/2117/222
2015-03-03T15:12:46ZDiffraction control of reflected beam by chirped mirror
http://hdl.handle.net/2117/22805
Title: Diffraction control of reflected beam by chirped mirror
Authors: Cheng, Yu Chieh; Peckus, Martynas; Kicas, S.; Trull Silvestre, José Francisco; Cojocaru, Crina; Vilaseca Alavedra, Ramon; Drazdys, R.; Staliunas, Kestutis
Abstract: We propose and demonstrate a novel functionality of chirped mirror for monochromatic light beams: a diffraction control in reflections resulting in focusing or imaging of beams. The chirped mirrors, commonly used for manipulating temporal profiles of pulses, here are applied for manipulating the spatial dispersion of a monochromatic beam. By penetrating into dielectric layers of chirped mirror, the monochromatic beam experiences the negative diffraction, therefore the beam diverge propagating in front and behind the structure in normal diffraction region can be compensated inside this structure with negative diffraction. The result is focusing or imaging of the reflected beam from a flat interface of chirped mirror without optical axis.Numerical bifurcation methods and their application to fluid dynamics: analysis beyond simulation
http://hdl.handle.net/2117/21331
Title: Numerical bifurcation methods and their application to fluid dynamics: analysis beyond simulation
Authors: Dijkstra, Hendrik; Wubs, Fred W.; Cliffe, Andrew K.; Doedel, Eusebius J.; Dragomirescu, Ioana Florica; Eckhardt, Bruno; Gelfgat, Alexander Yu; Hazel, Andrew L.; Lucarini, Valerio; Salinger, Andrew G.; Phipps, Erik T.; Sánchez Umbría, Juan; Schuttelaars, Henk M.; Tuckerman, Laurette S.; Thiele, Uwe
Abstract: We provide an overview of current techniques and typical applications of numerical bifurcation analysis in fluid dynamical problems. Many of these problems are characterized by high-dimensional dynamical systems which undergo transitions as parameters are changed. The computation of the critical conditions associated with these transitions, popularly referred to as 'tipping points', is important for understanding the transition mechanisms. We describe the two basic classes of methods of numerical bifurcation analysis, which differ in the explicit or implicit use of the Jacobian matrix of the dynamical system. The numerical challenges involved in both methods are mentioned and possible solutions to current bottlenecks are given. To demonstrate that numerical bifurcation techniques are not restricted to relatively low-dimensional dynamical systems, we provide several examples of the application of the modern techniques to a diverse set of fluid mechanical problems.Silicon-Chip-Based Dielectric Spectroscopy for Conductivity and Molecular Dynamics Studies of Organic Films
http://hdl.handle.net/2117/26553
Title: Silicon-Chip-Based Dielectric Spectroscopy for Conductivity and Molecular Dynamics Studies of Organic Films
Authors: Tripathi, Pragya; Gonzalo Ruiz, Javier; Mitsari, Efstratia; Zachariah, Manesh; Romanini, Michela; Tamarit Mur, José Luis; Muñoz Berbel, F. Xavier; Macovez, Roberto
Abstract: Interdigital electrodes fabricated by standard lithography on silicon chips are employed to probe the dipolar molecular dynamics and electric conduction properties of thin rhodamine films grown with two different methods. The conductivity is due to electronic charge carriers, and at around room-temperature, it is higher by 1 order of magnitude in solution-deposited films than in thermally evaporated ones. The organic material exhibits two intrinsic dynamic processes, of which the one at higher temperature is due to the orientational motion of the dipole moment of the rhodamine units, while the one at lower temperature is due to the motion of a local dipole associated with the chlorine counterions and is absent in thermally evaporated films. Our results show that thin-film dielectric spectroscopy is an easily implementable and versatile tool to extract valuable information on thin organic films.2015-03-02T11:17:38ZInteraction network based early-warning indicators of vegetation transitions
http://hdl.handle.net/2117/26550
Title: Interaction network based early-warning indicators of vegetation transitions
Authors: Tirabassi, Giulio; Viebahn, Jan P.; Dakos, Vasilis; Dijkstra, Hendrik; Masoller Alonso, Cristina; Rietkerk, Max; Dekker, Stefan C.
Abstract: Changes in vegetation patterns in semi-arid regions can precede the abrupt transition to bare soil. Here, complex network techniques are used to develop novel early-warning indicators for these desertification transitions. These indicators are applied to results from a local positive feedback vegetation model and are compared to classical indicators, such as the autocorrelation and variance of biomass time series. A quantitative measure is also introduced to evaluate the quality of the early-warning indicators. Based on this measure, the network-based indicators are superior to the classical ones, being more sensitive to the presence of the transition point.2015-03-02T09:52:19ZITER safety studies: the effect of two simultaneous perturbations during a loss of plasma control transient
http://hdl.handle.net/2117/26549
Title: ITER safety studies: the effect of two simultaneous perturbations during a loss of plasma control transient
Authors: Rivas Reguera, José Carlos; Dies Llovera, Javier
Abstract: The loss of plasma control events in ITER are safety cases investigated to give an upper bound of the worse effects foreseeable from a total failure of the plasma control function. Conservative analyses based on simple OD models for plasma balance equations and 1D models for wall heat transfer are used to determine the effects of such transients on wall integrity from a thermal point of view. In this contribution, progress in a2015-03-02T08:49:53ZGinzburg-Landau modelling of precursor nanoscale textures in ferroelastic materials
http://hdl.handle.net/2117/26521
Title: Ginzburg-Landau modelling of precursor nanoscale textures in ferroelastic materials
Authors: Planes, Antoni; Lloveras Muntané, Pol Marcel; Castán, Teresa; Saxena, Avadh; Porta, Marcel
Abstract: A Ginzburg–Landau free-energy model is proposed to study spatially inhomogeneous states that often occur as precursors of ferroelastic/martensitic transitions. Disorder is included in the harmonic coefficient of the free-energy density which gives rise to a spatial distribution of transition temperatures, and lattice integrity is imposed through Saint-Vénant compatibility conditions which lead to a long-range anisotropic elastic interaction. We show that precursor textures are a result of the competition between elastic anisotropy and disorder. Cross-hatched modulations (tweed patterns) take place for temperatures above the martensitic phase in the limit of high anisotropy and/or low disorder while a nano-cluster phase-separated state occurs at low anisotropies or high disorder. In the latter case, nanoscale inhomogeneities give rise to glassy behaviour while the structural transition is inhibited. Interestingly, in this case, the ferroelastic system also displays a large thermo-mechanical response so that the low-symmetry structure can be easily induced by the application of relatively small stresses within a broad temperature range.2015-02-26T12:03:36ZDynamic structure function of a cold Fermi gas at unitarity
http://hdl.handle.net/2117/26519
Title: Dynamic structure function of a cold Fermi gas at unitarity
Authors: Astrakharchik, Grigori; Boronat Medico, Jordi; Krotscheck, Eckhard; Lichtenegger, Thomas
Abstract: We present a theoretical study of the dynamic structure function of a resonantly interacting two-component Fermi gas at zero temperature. Our approach is based on dynamic many-body theory able to describe excitations in strongly correlated Fermi systems. The fixed-node diffusion Monte Carlo method is used to produce the ground-state correlation functions which are used as an input for the excitation theory. Our approach reproduces recent Bragg scattering data in both the density and the spin channel. In the BCS regime, the response is close to that of the ideal Fermi gas. On the BEC side, the Bose peak associated with the formation of dimers dominates the density channel of the dynamic response. When the fraction of dimers is large our theory departs from the experimental data, mainly in the spin channel.2015-02-26T08:58:44ZRole of frequency mismatch in neuronal communication through coherence
http://hdl.handle.net/2117/26516
Title: Role of frequency mismatch in neuronal communication through coherence
Authors: Sancristóbal Alonso, Belén de; Vicente, Raul; García Ojalvo, Jordi
Abstract: Neuronal gamma oscillations have been described in local field potentials of different brain regions of multiple species. Gamma oscillations are thought to reflect rhythmic synaptic activity organized by inhibitory interneurons. While several aspects of gamma rhythmogenesis are relatively well understood, we have much less solid evidence about how gamma oscillations contribute to information processing in neuronal circuits. One popular hypothesis states that a flexible routing of information between distant populations occurs via the control of the phase or coherence between their respective oscillations. Here, we investigate how a mismatch between the frequencies of gamma oscillations from two populations affects their interaction. In particular, we explore a biophysical model of the reciprocal interaction between two cortical areas displaying gamma oscillations at different frequencies, and quantify their phase coherence and communication efficiency. We observed that a moderate excitatory coupling between the two areas leads to a decrease in their frequency detuning, up to similar to 6 Hz, with no frequency locking arising between the gamma peaks. Importantly, for similar gamma peak frequencies a zero phase difference emerges for both LFP and MUA despite small axonal delays. For increasing frequency detunings we found a significant decrease in the phase coherence (at non-zero phase lag) between the MUAs but not the LFPs of the two areas. Such difference between LFPs and MUAs behavior is due to the misalignment between the arrival of afferent synaptic currents and the local excitability windows. To test the efficiency of communication we evaluated the success of transferring rate-modulations between the two areas. Our results indicate that once two populations lock their peak frequencies, an optimal phase relation for communication appears. However, the sensitivity of locking to frequency mismatch suggests that only a precise and active control of gamma frequency could enable the selection of communication channels and their directionality.2015-02-25T16:11:41ZCross-frequency transfer in a stochastically driven mesoscopic neuronal model
http://hdl.handle.net/2117/26462
Title: Cross-frequency transfer in a stochastically driven mesoscopic neuronal model
Authors: Jedynak, Maciej; Pons Rivero, Antonio Javier; García Ojalvo, Jordi
Abstract: The brain is known to operate in multiple coexisting frequency bands. Increasing experimental evidence suggests that interactions between those distinct bands play a crucial role in brain processes, but the dynamical mechanisms underlying this cross-frequency coupling are still under investigation. Two approaches have been proposed to address this issue. In the first one distinct nonlinear oscillators representing the brain rhythms involved are coupled actively (bidirectionally), whereas in the second one the oscillators are coupled unidirectionally and thus the driving between them is passive. Here we elaborate the latter approach by implementing a stochastically driven network of coupled neural mass models that operate in the alpha range. This model exhibits a broadband power spectrum with 1/fb form, similar to those observed experimentally. Our results show that such a model is able to reproduce recent experimental observations on the effect of slow rocking on the alpha activity associated with sleep. This suggests that passive driving can account for cross-frequency transfer in the brain, as a result of the complex nonlinear dynamics of its underlying oscillators.2015-02-23T11:51:17ZUniversality in molecular halo clusters
http://hdl.handle.net/2117/26455
Title: Universality in molecular halo clusters
Authors: Stipanovic, P.; Markic, L. Vranjes; Beslic, Ivana; Boronat Medico, Jordi
Abstract: The ground state of weakly bound dimers and trimers with a radius extending well into the classically forbidden region is explored, with the goal to test the predicted universality of quantum halo states. The focus of the study is molecules consisting of T down arrow, D down arrow, He-3, He-4, and alkali atoms, where the interaction between particles is much better known than in the case of nuclei, which are traditional examples of quantum halos. The study of realistic systems is supplemented by model calculations in order to analyze how low-energy properties depend on the interaction potential. The use of variational and diffusion Monte Carlo methods enabled a very precise calculation of both the size and binding energy of the trimers. In the quantum halo regime, and for large values of scaled binding energies, all clusters follow almost the same universal line. As the scaled binding energy decreases, Borromean states separate from tango trimers.2015-02-20T16:17:53ZFirst-principles modeling of quantum nuclear effects and atomic interactions in solid He-4 at high pressure
http://hdl.handle.net/2117/26453
Title: First-principles modeling of quantum nuclear effects and atomic interactions in solid He-4 at high pressure
Authors: Cazorla Silva, Claudio; Boronat Medico, Jordi
Abstract: We present a first-principles computational study of solid He-4 at T = 0 K and pressures up to similar to 160 GPa. Our computational strategy consists in using van der Waals density functional theory (DFT-vdW) to describe the electronic degrees of freedom in this material, and the diffusion Monte Carlo (DMC) method to solve the Schrodinger equation describing the behavior of the quantum nuclei. For this, we construct an analytical interaction function based on the pairwise Aziz potential that closely matches the volume variation of the cohesive energy calculated with DFT-vdW in dense helium. Interestingly, we find that the kinetic energy of solid He-4 does not increase appreciably with compression for P >= 85 GPa. Also, we show that the Lindemann ratio in dense solid He-4 amounts to 0.10 almost independently of pressure. The reliability of customary quasiharmonic DFT (QH DFT) approaches in describing quantum nuclear effects in solids is also studied. We find that QH DFT simulations, although provide a reasonable equation of state in agreement with experiments, are not able to reproduce correctly these critical effects in compressed He-4. In particular, we disclose huge discrepancies of at least similar to 50% in the calculated He-4 kinetic energies using both the QH DFT and present DFT-DMC methods.2015-02-20T15:30:21ZPrograma de desarrollo y diseminación de sistemas micro eólicos de generación de energía en países en desarrollo
http://hdl.handle.net/2117/26426
Title: Programa de desarrollo y diseminación de sistemas micro eólicos de generación de energía en países en desarrollo
Authors: Velo García, Enrique; Batet Miracle, Lluís; Clos Costa, Daniel; Ferrer Martí, Laia2015-02-19T11:47:30ZMesoscopic Segregation of Excitation and Inhibition in a Brain Network Model
http://hdl.handle.net/2117/26402
Title: Mesoscopic Segregation of Excitation and Inhibition in a Brain Network Model
Authors: Malagarriga Guasch, Daniel; Villa, Alessandro; García Ojalvo, Jordi; Pons Rivero, Antonio Javier
Abstract: Neurons in the brain are known to operate under a careful balance of excitation and inhibition, which maintains neural microcircuits within the proper operational range. How this balance is played out at the mesoscopic level of neuronal populations is, however, less clear. In order to address this issue, here we use a coupled neural mass model to study
computationally the dynamics of a network of cortical macrocolumns operating in a partially synchronized, irregular regime. The topology of the network is heterogeneous, with a few of the nodes acting as connector hubs while the rest are relatively poorly connected. Our results show that in this type of mesoscopic network excitation and inhibition spontaneously segregate, with some columns actingmainly in an excitatory manner while some others have predominantly an inhibitory effect on their neighbors.We characterize the conditions under which this segregation arises, and relate the character of the different columns with their to- pological role within the network. In particular, we show that the connector hubs are preferentially inhibitory, the more so the larger the node's connectivity. These results suggest a potential mesoscale organization of the excitation-inhibition balance in brain networks.2015-02-18T08:06:30ZBeam focalization in reflection from flat dielectric subwavelength gratings
http://hdl.handle.net/2117/26401
Title: Beam focalization in reflection from flat dielectric subwavelength gratings
Authors: Cheng, Yu Chieh; Zeng, H; Trullàs Simó, Joaquim; Cojocaru, Crina; Malinauskas, Mangirdas; Jukna, T; Wiersma, D.S.; Staliunas, Kestutis
Abstract: We experimentally demonstrate the recently predicted effect of near-field focusing for light beams from flat dielectric subwavelength gratings (SWGs). This SWGs were designed for visible light 532 nm and fabricated by direct laser writing in a negative photoresist, with the refractive index n = 1.5 and the period d = 314 nm. The laterally invariant gratings can focus light beams without any optical axis to achieve the transversal invariance. We show that focal distances can be obtained up to 13 mu m at normal reflection for TE polarization.2015-02-18T07:43:19ZModeling large scale shoreline sand waves under oblique wave incidence
http://hdl.handle.net/2117/26393
Title: Modeling large scale shoreline sand waves under oblique wave incidence
Authors: Van den Berg, Niels; Falqués Serra, Albert; Ribas Prats, Francesca2015-02-17T13:50:34Z