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Generation and Evolution of High-Mach Number, Laser-Driven Magnetized Collisionless Shocks in the Laboratory
http://cds.cern.ch/record/2226780
Shocks act to convert incoming supersonic flows to heat, and in collisionless plasmas the shock layer forms on kinetic plasma scales through collective electromagnetic effects. These collisionless shocks have been observed in many space and astrophysical systems [Smith 1975, Smith 1980, Burlaga 2008, Sulaiman 2015], and are believed to accelerate particles, including cosmic rays, to extremely high energies [Kazanas 1986, Loeb 2000, Bamba 2003, Masters 2013, Ackermann 2013]. Of particular importance are the class of high-Mach number, supercritical shocks [Balogh 2013] ($M_A\gtrsim4$), which must reflect significant numbers of particles back into the upstream to accommodate entropy production, and in doing so seed proposed particle acceleration mechanisms [Blandford 1978, McClements 2001, Caprioli 2014, Matsumoto 2015]. Here we present the first laboratory generation of high-Mach number magnetized collisionless shocks created through the interaction of an expanding laser-driven plasma with a magnetized ambient plasma. Time-resolved, two-dimensional imaging of plasma density and magnetic fields shows the formation and evolution of a supercritical shock propagating at magnetosonic Mach number $M_{ms}\approx12$. Particle-in-cell simulations constrained by experimental data show in detail the shock formation, separate reflection dynamics of C$^{+6}$ and H$^{+1}$ ions in the multi-species ambient plasma, and density and magnetic field compressions and overshoots in the shock layer. The development of this experimental platform complements present remote sensing and spacecraft observations, and opens the way for controlled laboratory investigations of high-Mach number collisionless shocks, including the mechanisms and efficiency of particle acceleration.Schaeffer, DerekFox, WillHaberberger, DanFiksel, GennadyBhattacharjee, AmitavaBarnak, DanielHu, SuxingGermaschewski, KaiSat, 22 Oct 2016 05:30:57 GMT20 Oct 2016arXiv:1610.06533http://cds.cern.ch/record/2226780['arXiv:1610.06533']arXiv:1610.06533Continuum Kinetic and Multi-Fluid Simulations of Classical Sheaths
http://cds.cern.ch/record/2226779
The kinetic study of plasma sheaths is critical, among other things, to understand the deposition of heat on walls, the effect of sputtering, and contamination of the plasma with detrimental impurities. The plasma sheath also provides a boundary condition and can often have a significant global impact on the bulk plasma. In this paper, kinetic studies of classical sheaths are performed with the continuum code, Gkeyll, that directly solves the Vlasov-Poisson/Maxwell equations. The code uses a novel version of the finite-element discontinuous Galerkin (DG) scheme that conserves energy in the continuous-time limit. The electrostatic field is computed using the Poisson equation. Ionization and scattering collisions are included, however, surface effects are neglected. The aim of this work is to introduce the continuum-kinetic method and compare its results to those obtained from an already established finite-volume multi-fluid model also implemented in Gkeyll. Novel boundary conditions on the fluids allow the sheath to form without specifying wall fluxes, so the fluids and fields adjust self-consistently at the wall. The work presented here demonstrates that the kinetic and fluid results are in agreement for the momentum flux, showing that in certain regimes, a multi-fluid model can be a useful approximation for simulating the plasma boundary. There are differences in the electrostatic potential between the fluid and kinetic results. Further, the direct solutions of the distribution function presented here highlight the non-Maxwellian distribution of electrons in the sheath, emphasizing the need for a kinetic model.Cagas, PetrHakim, AmmarJuno, JamesSrinivasan, BhuvanaSat, 22 Oct 2016 05:30:57 GMT20 Oct 2016arXiv:1610.06529http://cds.cern.ch/record/2226779['arXiv:1610.06529']arXiv:1610.06529New Zealand's size and isolation can stimulate new science
http://cds.cern.ch/record/2226778
It is conventionally thought that New Zealand's distance from the large, northern hemisphere centres of learning, and our relatively small population and wealth, are detrimental to the contribution we can make to the advancement of scientific knowledge. Here the reverse point of view is argued.Yock, PhilipSat, 22 Oct 2016 05:30:57 GMT20 Oct 2016arXiv:1610.06518http://cds.cern.ch/record/2226778['arXiv:1610.06518']arXiv:1610.06518Community Extraction in Multilayer Networks with Heterogeneous Community Structure
http://cds.cern.ch/record/2226777
Multilayer networks are a useful way to capture and model multiple, binary relationships among a fixed group of objects. While community detection has proven to be a useful exploratory technique for the analysis of single-layer networks, the development of community detection methods for multilayer networks is still in its infancy. We propose and investigate a procedure, called Multilayer Extraction, that identifies densely connected vertex-layer sets in multilayer networks. Multilayer Extraction makes use of a significance based score that quantifies the connectivity of an observed vertex-layer set by comparison with a multilayer fixed degree random graph model. Unlike existing detection methods, Multilayer Extraction handles networks with heterogeneous layers where community structure may be different from layer to layer. The procedure is able to capture overlapping communities, and it identifies background vertex-layer pairs that do not belong to any community. We establish large-graph consistency of the vertex-layer set optimizer of our proposed multilayer score under the multilayer stochastic block model. We investigate the performance of Multilayer Extraction empirically on three applications, as well as a test bed of simulations. Our theoretical and numerical evaluations suggest that Multilayer Extraction is an effective exploratory tool for analyzing complex multilayer networks. Publicly available R software for Multilayer Extraction is available at https://github.com/jdwilson4/MultilayerExtraction.Wilson, James DPalowitch, JohnBhamidi, ShankarNobel, Andrew BSat, 22 Oct 2016 05:30:57 GMT20 Oct 2016arXiv:1610.06511http://cds.cern.ch/record/2226777['arXiv:1610.06511']arXiv:1610.06511Information Overload in Group Communication: From Conversation to Cacophony in the Twitch Chat
http://cds.cern.ch/record/2226776
Online communication channels, especially social web platforms, are rapidly replacing traditional ones. Online platforms allow users to overcome physical barriers, enabling worldwide participation. However, the power of online communication bears an important negative consequence --- we are exposed to too much information to process. Too many participants, for example, can turn online public spaces into noisy, overcrowded fora where no meaningful conversation can be held. Here we analyze a large dataset of public chat logs from Twitch, a popular video streaming platform, in order to examine how information overload affects online group communication. We measure structural and textual features of conversations such as user output, interaction, and information content per message across a wide range of information loads. Our analysis reveals the existence of a transition from a conversational state to a cacophony --- a state of overload with lower user participation, more copy-pasted messages, and less information per message. These results hold both on average and at the individual level for the majority of users. This study provides a quantitative basis for further studies of the social effects of information overload, and may guide the design of more resilient online communication systems.Nematzadeh, AzadehCiampaglia, Giovanni LucaAhn, Yong-YeolFlammini, AlessandroSat, 22 Oct 2016 05:30:57 GMT20 Oct 2016arXiv:1610.06497http://cds.cern.ch/record/2226776['arXiv:1610.06497']arXiv:1610.06497Chromatic Effect for THz Generation in a Novel Wave-front Tilt Scheme
http://cds.cern.ch/record/2226775
Deriving single or few cycle terahertz pulse (THz) by intense femtosecond laser through cascaded optical rectification in electro-optic crystals is a crucial technique in cutting-edge time-resolved spectroscopy to characterize micro-scale structures and ultrafast dynamics. In the past decade, lithium niobate (LN) crystal implementation of wave-front tilt scheme has been prevalently used, while painstaking efforts have been invested in order to achieve higher THz conversion efficiency. In this research we developed a brand new type of LN crystal possessing dual-face-cut and Brewster coupling, and conducted experimental and simulative investigation systematically to optimize the multi-dimensionally entangled parameters in THz generation, predicting the extreme conversion efficiency of 10% is potentially promising at the THz absorption coefficient of 0.5cm-1. More remarkably, we first discovered that the chirp of the driving laser pulse plays a decisive role in the wave-front tilt scheme, and the THz generation efficiency could be enhanced tremendously by applying an appropriate chirp.Li, BinZhang, WenyanLiu, XiaoqingChen, JianhuiDeng, HaixiaoFeng, ChaoFeng, LieLan, TaiheLiu, BoLiu, JiaWang, DongWang, XingtaoZeng, ZhinanZhang, lijianZhang, TongZhao, ZhentangSat, 22 Oct 2016 05:30:57 GMT20 Oct 2016arXiv:1610.06493http://cds.cern.ch/record/2226775['arXiv:1610.06493']arXiv:1610.06493Spatio-temporal extreme events in a laser with a saturable absorber
http://cds.cern.ch/record/2226774
We study extreme events occurring in the transverse $(x,y)$ section of the field emitted by a broad-area semiconductor laser with a saturable absorber. The spatio-temporal events on which we perform the statistical analysis are identified as maxima of the field intensity in the 3D space $(x,y,t)$. We identify regions in the parameter space where extreme events are more likely to occur and we study the connection of those extreme events with the cavity solitons that are known to exist in the same system, both stationary and self-pulsing.Rimoldi, CristinaBarland, StéphanePrati, FrancoTissoni, GiovannaSat, 22 Oct 2016 05:30:57 GMT20 Oct 2016arXiv:1610.06452http://cds.cern.ch/record/2226774['arXiv:1610.06452']arXiv:1610.06452Dynamic accessibility using Big Data: The role of the changing conditions of network congestion and destination attractiveness
http://cds.cern.ch/record/2226773
This paper analyses urban accessibility using a fully dynamic approach.Moya-Gómez, BorjaSalas-Olmedo, María HenarGarcía-Palomares, Juan CarlosGutiérrez, JavierSat, 22 Oct 2016 05:30:57 GMT20 Oct 2016arXiv:1610.06450http://cds.cern.ch/record/2226773['arXiv:1610.06450']arXiv:1610.06450Properties of Surface Plasmon Polaritons on lossy materials: Lifetimes, periods and excitation conditions
http://cds.cern.ch/record/2226772
The possibility to excite Surface Plasmon Polaritons (SPPs) at the interface between two media depends on the optical properties of both media and geometrical aspects. Specific conditions allowing the coupling of light with a plasmon-active interface must be satisfied. Plasmonic effects are well described in noble metals where the imaginary part of the dielectric permittivity is often neglected ("perfect medium approximation"). However, some systems exist for which such approximation cannot be applied, hence requiring a refinement of the common SPP theory. In this context, several properties of SPPs such as excitation conditions, period of the electromagnetic field modulation and SPP lifetime then may strongly deviate from that of the perfect medium approximation. In this paper, calculations taking into account the imaginary part of the dielectric permittivities are presented. The model identifies analytical terms which should not be neglected in the mathematical description of SPPs on lossy materials. These calculations are applied to numerous material combinations resulting in a prediction of the corresponding SPP features. A list of plasmon-active interfaces is provided along with a quantification of the above mentioned SPP properties in the regime where the perfect medium approximation is not applicable.Derrien, Thibault J -YKrüger, JörgBonse, JörnSat, 22 Oct 2016 05:30:56 GMT20 Oct 2016arXiv:1610.06442http://cds.cern.ch/record/2226772['arXiv:1610.06442']arXiv:1610.06442Conical-Shaped Titania Nanotubes for Optimized Light Management in DSSCs Reach Back-side Illumination Efficiencies > 8%
http://cds.cern.ch/record/2226771
In the present work, we introduce the anodic growth of conical shaped TiO2 nanotube arrays. These titania nanocones provide a scaffold for dye-sensitized solar cell (DSSC) structures with significantly improved photon management, providing an optimized absorption profile compared with conventional cylindrical nanotube arrays. Finite difference time domain (FDTD) modelling demonstrates a drastically changed power-absorption characteristic over the tube length. When used in a back-side illumination DSSC configuration, nanocone structures can reach over 60 % higher solar cell conversion efficiency than conventional tubes. The resulting {\eta} of ca. 8 % represents one of the highest reported values for Graetzel type DSSCs used under back-side illumination.So, SeulgiKriesch, ArianPeschel, UlfSchmuki, PatrikSat, 22 Oct 2016 05:30:56 GMT19 Oct 2016arXiv:1610.06437http://cds.cern.ch/record/2226771['arXiv:1610.06437']arXiv:1610.06437A Quantum Cellular Automata architecture with nearest neighbor interactions using one quantum gate type
http://cds.cern.ch/record/2226770
We propose an architecture based on Quantum cellular Automata which allows the use of only one type of quantum gates per computational step in order to perform nearest neighbor interactions. The model is built in partial steps, each one of them analyzed using nearest neighbor interactions, starting with single qubit operations and continuing with two qubit ones. The effectiveness of the model is tested and valuated by developing a quantum circuit implementing the Quantum Fourier Transform. The important outcome of this validation was that the operations are performed in a local and controlled manner thus reducing the error rate of each computational step.Ntalaperas, DKonofaos, NSat, 22 Oct 2016 05:30:56 GMT20 Oct 2016arXiv:1610.06426http://cds.cern.ch/record/2226770['arXiv:1610.06426']arXiv:1610.06426Back to the Origins. Using Matrix Functions of H\"uckel Hamiltonian for Quantum Interference
http://cds.cern.ch/record/2226769
A book chapter where a new matrix function of the HMO Hamiltonian is proposed to deal with the Quantum Interference phenomenon in conjugated organic molecules.Estrada, ErnestoSat, 22 Oct 2016 05:30:56 GMT18 Oct 2016arXiv:1610.06422http://cds.cern.ch/record/2226769['arXiv:1610.06422']arXiv:1610.06422Imaging the potential distribution of charged adsorbates on graphene by low-energy electron holography
http://cds.cern.ch/record/2226768
While imaging individual atoms can routinely be achieved in high resolution transmission electron microscopy, visualizing the potential distribution of individually charged adsorbates leading to a phase shift of the probing electron wave is still a challenging task. Since low-energy electrons are sensitive to localized potential gradients, we employed this tool in the 30 eV kinetic energy range to visualize the potential distribution of localized charged adsorbates present on free-standing graphene.Latychevskaia, TatianaWicki, FlavioEscher, ConradFink, Hans-WernerSat, 22 Oct 2016 05:30:56 GMT20 Oct 2016arXiv:1610.06420http://cds.cern.ch/record/2226768['arXiv:1610.06420']arXiv:1610.06420High Energy Effects of Noncommutative Spacetime Geometry
http://cds.cern.ch/record/2226767
In this paper, we endeavour to obtain a modified form of the Foldy-Wouthuysen and Cini-Toushek transformations by resorting to the noncommutative nature of space-time geometry, starting from the Klein-Gordon equation. Also, we obtain a shift in the energy levels due to noncommutativity and from these results a limit for the Lorentz factor in the ultra-relativistic case has been derived in conformity with observationsSidharth, Burra GDas, AbhishekSat, 22 Oct 2016 05:30:56 GMT13 Oct 2016arXiv:1610.06415http://cds.cern.ch/record/2226767['arXiv:1610.06415']arXiv:1610.06415Towards a new generalized space expansion dynamics applied to the rotation of galaxies and Tully Fisher law
http://cds.cern.ch/record/2226766
Up to now, the rotational velocities of galaxies are not clearly understood and the experimental Tully Fisher rule, linking the total galactic mass to the fourth power of the velocity, through an acceleration coefficient of about 10-10 m/s2 has not found a deep theoretical explanation. Tentative proposals (MOND theory of a modified Newton s law and extraneous dark matter) do not bring a definite clarification. We propose here a new approach to this problem, without exotic matter and using the classical Newton force. But we introduce a new additional universal acceleration, which could represent a universal expansion law valid at the scale level of a galaxy. We show that this hypothesis leads to a good description of the observed variations of the galactic transverse velocity. It can be considered as a consequence of the Scale Expansion Cosmos theory (SEC) introduced by J. Masreliez, but we postulate that the space expansion acceleration universally applies at any scale. We obtain a formal derivation of the Tully Fisher law, linking the constant galactic transverse velocity to its total mass, via the universal minimum acceleration. We derive a good estimate of the TF acceleration coefficient and show that expansion should be proportional to the square root of the local volumic mass density. Our conjecture is in fact a new dynamics principle which could be applied to many other physical problems at different scales. Applying it to the range of the solar planet system confirms the well known Kepler laws, at least as a valid approximation for the order of magnitude of the solar system.Fleuret, JacquesSat, 22 Oct 2016 05:30:56 GMT14 Oct 2016arXiv:1610.06414http://cds.cern.ch/record/2226766['arXiv:1610.06414']arXiv:1610.06414Ultra-bright GeV photon source via controlled electromagnetic cascades in laser-dipole waves
http://cds.cern.ch/record/2226765
One aim of upcoming high-intensity laser facilities is to provide new high-flux gamma-ray sources. Electromagnetic cascades may serve for this, but are known to limit both field strengths and particle energies, restricting efficient production of photons to sub-GeV energies. Here we show how to create a directed GeV photon source, enabled by a controlled interplay between the cascade and anomalous radiative trapping. Using advanced 3D QED particle-in-cell (PIC) simulations and analytic estimates, we show that the concept is feasible for planned peak powers of 10 PW level. A higher peak power of 40 PW can provide $10^9$ photons with GeV energies in a well-collimated 3 fs beam, achieving peak brilliance ${9 \times 10^{24}}$ ph s$^{-1}$mrad$^{-2}$mm$^{-2}$/0.1${\%}$BW. Such a source would be a powerful tool for studying fundamental electromagnetic and nuclear processes.Gonoskov, ABashinov, ABastrakov, SEfimenko, EIlderton, AKim, AMarklund, MMeyerov, IMuraviev, ASergeev, ASat, 22 Oct 2016 05:30:56 GMT20 Oct 2016arXiv:1610.06404http://cds.cern.ch/record/2226765['arXiv:1610.06404']arXiv:1610.06404An analytical model of iceberg drift
http://cds.cern.ch/record/2226764
Iceberg drift and decay and the associated freshwater release are increasingly seen as important processes in Earth's climate system, yet a detailed understanding of their dynamics has remained elusive. Here, an idealized model of iceberg drift is presented. The model is designed to include the most salient physical processes that determine iceberg motion while remaining sufficiently simple to facilitate physical insight into iceberg drift dynamics. We derive an analytical solution of the model, which helps build understanding and also enables the rapid computation of large numbers of iceberg trajectories. The long-standing empirical rule of thumb that icebergs drift at 2% of the wind velocity, relative to the ocean current, is derived here from physical first principles, and it is shown that this relation only holds in the limit of strong winds or small icebergs, which approximately applies for typical icebergs in the Arctic. It is demonstrated that the opposite limit of weak winds or large icebergs approximately applies for typical Antarctic tabular icebergs, and that in this case the icebergs simply move with the ocean surface current. It is furthermore found that when winds are strong, wind drag drives icebergs in the direction the wind blows, whereas weak winds drive icebergs at a 90 degree angle to the wind direction.Wagner, Till J WDell, Rebecca WEisenman, IanSat, 22 Oct 2016 05:30:56 GMT20 Oct 2016arXiv:1610.06403http://cds.cern.ch/record/2226764['arXiv:1610.06403']arXiv:1610.06403In-beam measurement of the hydrogen hyperfine splitting - towards antihydrogen spectroscopy
http://cds.cern.ch/record/2226763
Antihydrogen, the lightest atom consisting purely of antimatter, is an ideal laboratory to study the CPT symmetry by comparison to hydrogen. With respect to absolute precision, transitions within the ground-state hyperfine structure (GS-HFS) are most appealing by virtue of their small energy separation. ASACUSA proposed employing a beam of cold antihydrogen atoms in a Rabi-type experiment to determine the GS-HFS in a field-free region. Here we present a measurement of the zero-field hydrogen GS-HFS using the spectroscopy apparatus of ASACUSA's antihydrogen experiment. The measured value of $\nu_\mathrm{HF}$=$1~420~405~748.4(3.4)(1.6)~\textrm{Hz}$ with a relative precision of $\Delta$$\nu_\mathrm{HF}$/$\nu_\mathrm{HF}$=$2.7\times10^{-9}$ constitutes the most precise determination of this quantity in a beam and verifies the developed spectroscopy methods for the antihydrogen HFS experiment to the ppb level. Together with the recently presented observation of antihydrogen atoms $2.7~\textrm{m}$ downstream of the production region, the prerequisites for a measurement with antihydrogen are now available within the ASACUSA collaboration.Diermaier, MJepsen, C BKolbinger, BMalbrunot, CMassiczek, OSauerzopf, CSimon, M CZmeskal, JWidmann, ESat, 22 Oct 2016 05:30:56 GMT20 Oct 2016arXiv:1610.06392http://cds.cern.ch/record/2226763['arXiv:1610.06392']arXiv:1610.06392Single photon superradiance and cooperative Lamb shift in an optoelectronic device
http://cds.cern.ch/record/2226762
Single photon superradiance is a strong enhancement of spontaneous emission appearing when a single excitation is shared between a large number of two-level systems. This enhanced rate can be accompanied by a shift of the emission frequency, the cooperative Lamb shift, issued from the exchange of virtual photons between the emitters. In this work we present a semiconductor optoelectronic device allowing the observation of these two phenomena at room temperature. We demonstrate experimentally and theoretically that plasma oscillations in spatially separated quantum wells interact through real and virtual photon exchange. This gives rise to a superradiant mode displaying a large cooperative Lamb shift.Frucci, GiuliaHuppert, SimonVasanelli, AngelaDailly, BaptisteTodorov, YankoSirtori, CarloBeaudoin, GrégoireSagnes, IsabelleSat, 22 Oct 2016 05:30:56 GMT20 Oct 2016arXiv:1610.06391http://cds.cern.ch/record/2226762['arXiv:1610.06391']arXiv:1610.06391Multiparticle collision dynamics in porous media
http://cds.cern.ch/record/2226761
We adopt the multiparticle collision dynamics method to simulate fluid flows in porous media. For this, the particle-level drag force is introduced into the original algorithm. The force hinder the flow resulting in global resistance and decrease of permeability. The extended algorithm is validated in the flow through fully porous channel with analytical solution. Basic properties of the solver are investigated including its dependency of permeability on model parameters.Matyka, MaciejSat, 22 Oct 2016 05:30:56 GMT20 Oct 2016arXiv:1610.06383http://cds.cern.ch/record/2226761['arXiv:1610.06383']arXiv:1610.06383An automated image analysis framework for segmentation and division plane detection of single live Staphylococcus aureus cells which can operate at millisecond sampling time scales using bespoke Slimfield microscopy
http://cds.cern.ch/record/2226760
Staphylococcus aureus is an important pathogen, giving rise to antimicrobial resistance in cell strains such as Methicillin Resistant S. aureus (MRSA). Here we report an image analysis framework for automated detection and image segmentation of cells in S. aureus cell clusters, and explicit identification of their cell division planes. We use a new combination of several existing analytical tools of image analysis to detect cellular and subcellular morphological features relevant to cell division from millisecond time scale sampled images of live pathogens at a detection precision of single molecules. We demonstrate this approach using a fluorescent reporter GFP fused to the protein EzrA that localises to a mid-cell plane during division and is involved in regulation of cell size and division. This image analysis framework presents a valuable platform from which to study candidate new antimicrobials which target the cell division machinery, but may also have more general application in detecting morphologically complex structures of fluorescently labelled proteins present in clusters of other types of cells.Wollman, Adam J MMiller, HelenFoster, SimonLeake, Mark CSat, 22 Oct 2016 05:30:56 GMT20 Oct 2016arXiv:1610.06361http://cds.cern.ch/record/2226760['arXiv:1610.06361']arXiv:1610.06361Quantitative Test of the Evolution of Geant4 Electron Backscattering Simulation
http://cds.cern.ch/record/2226759
Evolutions of Geant4 code have affected the simulation of electron backscattering with respect to previously published results. Their effects are quantified by analyzing the compatibility of the simulated electron backscattering fraction with a large collection of experimental data for a wide set of physics configuration options available in Geant4. Special emphasis is placed on two electron scattering implementations first released in Geant4 version 10.2: the Goudsmit-Saunderson multiple scattering model and a single Coulomb scattering model based on Mott cross section calculation. The new Goudsmit-Saunderson multiple scattering model appears to perform equally or less accurately than the model implemented in previous Geant4 versions, depending on the electron energy. The new Coulomb scattering model was flawed from a physics point of view, but computationally fast in Geant4 version 10.2; the physics correction released in Geant4 version 10.2p01 severely degrades its computational performance. Evolutions in the Geant4 geometry domain have addressed physics problems observed in electron backscattering simulation in previous publications.Basaglia, TullioHan, Min CheolHoff, GabrielaKim, Chan HyeongKim, Sung HunPia, Maria GraziaSaracco, PaoloSat, 22 Oct 2016 05:30:55 GMT20 Oct 2016arXiv:1610.06349http://cds.cern.ch/record/2226759['arXiv:1610.06349']arXiv:1610.06349Capturing Structural Dynamics in Crystalline Silicon Using Chirped Electrons from a Laser Wakefield Accelerator
http://cds.cern.ch/record/2226758
Recent progress in laser wakefield acceleration has led to the emergence of a new generation of electron and X-ray sources that may have enormous benefits for ultrafast science. These novel sources promise to become indispensable tools for the investigation of structural dynamics on the femtosecond time scale, with spatial resolution on the atomic scale. Here, we demonstrate the use of laser-wakefield-accelerated electron bunches for time-resolved electron diffraction measurements of the structural dynamics of single-crystal silicon nano-membranes pumped by an ultrafast laser pulse. In our proof-of-concept study, we resolve the silicon lattice dynamics on a picosecond time scale by deflecting the momentum-time correlated electrons in the diffraction peaks with a static magnetic field to obtain the time-dependent diffraction efficiency. Further improvements may lead to femtosecond temporal resolution, with negligible pump-probe jitter being possible with future laser-wakefield-accelerator ultrafast-electron-diffraction schemes.He, Z -HBeaurepaire, BNees, J AGallé, GScott, S APérez, J R SanchezLagally, M GKrushelnick, KThomas, A G RFaure, JSat, 22 Oct 2016 05:30:55 GMT20 Oct 2016arXiv:1610.06334http://cds.cern.ch/record/2226758['arXiv:1610.06334']arXiv:1610.06334Are some packings more equal than others? A direct test of the Edwards conjecture
http://cds.cern.ch/record/2226757
In the late 1980s, Sir Sam Edwards proposed a possible statistical-mechanical framework to describe the properties of disordered granular materials. A key assumption underlying the theory was that all jammed packings are equally likely. In the intervening years it has never been possible to test this bold hypothesis directly. Here we present simulations that provide direct evidence that at the unjamming point, all packings of soft repulsive particles are equally likely, even though generically, jammed packings are not. Our results therefore support Edwards' original conjecture. We also present evidence that at unjamming the configurational entropy of the system is maximal.Martiniani, StefanoSchrenk, K JulianRamola, KabirChakraborty, BulbulFrenkel, DaanSat, 22 Oct 2016 05:30:55 GMT20 Oct 2016arXiv:1610.06328http://cds.cern.ch/record/2226757['arXiv:1610.06328']arXiv:1610.06328Detection of Zak phases and topological invariants in a chiral photonic quantum walk
http://cds.cern.ch/record/2226756
Topological insulators are fascinating states of matter exhibiting protected edge states and robust quantized features in their bulk. Here, we propose and validate experimentally a method to detect topological properties in the bulk of one-dimensional chiral systems. We first introduce the mean chiral displacement, and we show that it rapidly approaches a multiple of the Zak phase in the long time limit. Then we measure the Zak phase in a photonic quantum walk, by direct observation of the mean chiral displacement in its bulk. Next, we measure the Zak phase in an alternative, inequivalent timeframe, and combine the two windings to characterize the full phase diagram of this Floquet system. Finally, we prove the robustness of the measure by introducing dynamical disorder in the system. This detection method is extremely general, as it can be applied to all one-dimensional platforms simulating static or Floquet chiral systems.Cardano, FD'Errico, ADauphin, AMaffei, MPiccirillo, Bde Lisio, CDe Filippis, GCataudella, VSantamato, EMarrucci, LLewenstein, MMassignan, PSat, 22 Oct 2016 05:30:55 GMT20 Oct 2016arXiv:1610.06322http://cds.cern.ch/record/2226756['arXiv:1610.06322']arXiv:1610.06322