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## Observation of hexatic liquid vortex matter in YBa2Cu3O7

Hauglin, Harald; Woodard, Nathan G.; Lafyatis, Gregory P.
Tipo: Artigo de Revista Científica
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An atomic beam probe is used to study the structure and dynamics of quantized supercurrent vortex lines in YBa_{2}Cu_{3}O_{7-?} at temperatures up to within 0.7 K below T_{C}. Here we report the direct observation of a vortex configuration with sample wide bond-orientational order but only short range translational correlation. The data imply the existence of an intermediate hexatic' vortex line liquid phase. We find that the hexatic liquid is in thermal equilibrium over a narrow temperature range below T_{C} and is quenched into an immobile hexatic glass at low temperatures.; Comment: Submitted to PRL, 3 figures

## 2D versus 3D Freezing of a Lennard-Jones Fluid in a Slit Pore: A Molecular Dynamics Study

Gribova, N.; Maleki, H.; Arnold, A.; Holm, C.; Schilling, T.
Tipo: Artigo de Revista Científica
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We present a computer simulation study of a (6,12)-Lennard-Jones fluid confined to a slit pore, formed by two uniform planes. These interact via (3,9)-Lennard-Jones potential with the fluid particles. When the fluid approaches the liquid-to-solid transition we first observe layering parallel to the walls. In order to investigate the nature of the freezing transition we performed a detailed analysis of the bond-orientational order parameter in the layers. We found no signs of hexatic order which would indicate a melting scenario of the Kosterlitz-Thouless type. An analysis of the mean-square displacement shows that the particles can easily move between the layers, making the crystallization a 3d-like process. This is consistent with the fact that we observe a considerable hysteresis in the heating-freezing curves, showing that the crystallization transition proceeds as an activated process.; Comment: submitter to J. Phys.: Condens. Matter

## Potential-tuning molecular dynamics studies of fusion, and the question of ideal glassformers: (I) The Gay-Berne model

Kapko, Vitaliy; Matyushov, Dmitry V.; Angell, C. Austen
Tipo: Artigo de Revista Científica
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The ability of some liquids to vitrify during supercooling is usually seen as a consequence of the rates of crystal nucleation (and/or crystal growth) becoming small- thus a matter of kinetics. However there is evidence, dating back to the empirics of coal briquetting for maximum trucking efficiency, that ellipsoids pack efficiently when disordered. Noting that key studies of non-spherical object packing have never been followed from hard ellipsoids or spherocylinders (diatomics excepted) into the world of molecules with attractive forces, we have made a molecular dynamics MD study of crystal melting and glass formation on the Gay- Berne (G-B) model of ellipsoidal objects across the aspect ratio range of the hard ellipsoid studies. Here we report that, in the aspect ratio range of maximum ellipsoid packing efficiency, various G-B crystalline states, that cannot be obtained directly from the liquid, disorder spontaneously near 0 K and transform to liquids without any detectable enthalpy of fusion. Without claiming to have proved the existence of single component examples, we use the present observations, together with our knowledge of non-ideal mixing effects, to discuss the probable existence of "ideal glassformers" - single or multicomponent liquids that vitrify before ever becoming metastable with respect to crystals. The existence of crystal-free routes to the glassy state removes any precrystalline fluctuation perspective from the "glass problem". Unexpectedly we find that liquids with aspect ratios in the "crystallophobic" range also behave in an unusual (non-hysteritic) way during temperature cycling through the glass transition. We link this to the highly volume fraction-sensitive ("fragile") behavior observed in recent hard dumbbell studies at similar length/diameter ratios.

## MR Imaging of Reynolds Dilatancy in the Bulk of Smooth Granular Flows

Sakaie, Ken; Fenistein, Denis; Carroll, Timothy J.; van Hecke, Martin; Umbanhowar, Paul
Tipo: Artigo de Revista Científica
Relevância na Pesquisa
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Dense granular matter has to expand in order to flow, a phenomenon known as dilatancy. Here we perform, by means of Magnetic Resonance Imaging, direct measurements of the evolution of the local packing density of a slow and smooth granular shear flow generated in a split-bottomed geometry. The dilatancy is found to be surprisingly strong. The dilated zone follow the region of large strain rate and slowly spreads as a function of time. This suggests that the local packing density is governed by the total amount of local strain experienced since the start of the experiment.; Comment: 4 pages, 3 figs, submitted to PRE

## Lateral correlation of multivalent counterions is the universal mechanism of charge inversion

Nguyen, T. T.; Grosberg, A. Yu.; Shklovskii, B. I.
Tipo: Artigo de Revista Científica
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We review works on screening of a macroion, such as colloidal particle or double helix DNA, by multivalent counterions. Multivalent metal ions, charged micelles, short or long polyelectrolytes can play the role of multivalent counterions. Due to the strong Coulomb repulsion such multivalent counterions form a strongly correlated liquid resembling a Wigner crystal at the surface of the macroion. Even if the macroion is neutralized by this liquid, a newly arriving counterion creates in the liquid a correlation hole or image which attracts the ion to the surface. As a result, total charge of adsorbed counterions can substantially exceed the bare macroion charge, so that the macroion net charge inverts its sign. We discuss two previously suggested explanations of charge inversion and show that physics underlying both of them is intrinsically that of correlations, so that correlation is the universal force driving charge inversion.; Comment: In Proceedings of Les Houches school on Electrostatic effects in Soft Matter and Biophysics, October 2000. Many corrections

## Spontaneous motion in hierarchically assembled active matter

Sanchez, Tim; Chen, Daniel T. N.; DeCamp, Stephen J.; Heymann, Michael; Dogic, Zvonimir
Tipo: Artigo de Revista Científica
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With exquisite precision and reproducibility, cells orchestrate the cooperative action of thousands of nanometer-sized molecular motors to carry out mechanical tasks at much larger length scales, such as cell motility, division and replication. Besides their biological importance, such inherently non-equilibrium processes are an inspiration for developing biomimetic active materials from microscopic components that consume energy to generate continuous motion. Being actively driven, these materials are not constrained by the laws of equilibrium statistical mechanics and can thus exhibit highly sought-after properties such as autonomous motility, internally generated flows and self-organized beating. Starting from extensile microtubule bundles, we hierarchically assemble active analogs of conventional polymer gels, liquid crystals and emulsions. At high enough concentration, microtubules form a percolating active network characterized by internally driven chaotic flows, hydrodynamic instabilities, enhanced transport and fluid mixing. When confined to emulsion droplets, 3D networks spontaneously adsorb onto the droplet surfaces to produce highly active 2D nematic liquid crystals whose streaming flows are controlled by internally generated fractures and self-healing...

## Twisting and Bending Stress in DNA Minicircles

Zoli, Marco
Tipo: Artigo de Revista Científica
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The interplay between bending of the molecule axis and appearance of disruptions in circular DNA molecules, with $\sim 100$ base pairs, is addressed. Three minicircles with different radii and almost equal content of AT and GC pairs are investigated. The DNA sequences are modeled by a mesoscopic Hamiltonian which describes the essential interactions in the helix at the level of the base pair and incorporates twisting and bending degrees of freedom. Helix unwinding and bubble formation patterns are consistently computed by a path integral method that sums over a large number of molecule configurations compatible with the model potential. The path ensembles are determined, as a function of temperature, by minimizing the free energy of the system. Fluctuational openings appear along the helix to release the stress due to the bending of the molecule backbone. In agreement with the experimental findings, base pair disruptions are found with larger probability in the smallest minicircle of \textit{66-bps} whose bending angle is $\sim 6^{o}$. For this minicircle, a sizeable untwisting is obtained with the helical repeat showing a step-like increase at $\tau =\,315K$. The method can be generalized to determine the bubble probability profiles of open ends linear sequences.; Comment: Soft Matter (2014)...

## Comparison of mean-field theories for vortices in trapped Bose-Einstein condensates

Virtanen, S. M. M.; Simula, T. P.; Salomaa, M. M.
Tipo: Artigo de Revista Científica
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We compute structures of vortex configurations in a harmonically trapped Bose-Einstein condensed atom gas within three different gapless self-consistent mean-field theories. Outside the vortex core region, the density profiles for the condensate and the thermal gas are found to differ only by a few percent between the Hartree-Fock-Bogoliubov-Popov theory and two of its recently proposed gapless extensions. In the core region, however, the differences in the density profiles are substantial. The structural differences are reflected in the energies of the quasiparticle states localized near the vortex core. Especially, the predictions for the energy of the lowest quasiparticle excitation differ considerably between the theoretical models investigated.; Comment: 4 pages, 2 figures

## Shear shocks in fragile networks

Ulrich, Stephan; Upadhyaya, Nitin; van Opheusden, Bas; Vitelli, Vincenzo
Tipo: Artigo de Revista Científica
Relevância na Pesquisa
55.53%
A minimal model for studying the mechanical properties of amorphous solids is a disordered network of point masses connected by unbreakable springs. At a critical value of its mean connectivity, such a network becomes fragile: it undergoes a rigidity transition signaled by a vanishing shear modulus and transverse sound speed. We investigate analytically and numerically the linear and non-linear visco-elastic response of these fragile solids by probing how shear fronts propagate through them. Our approach, that we tentatively label shear front rheology, provides an alternative route to standard oscillatory rheology. In the linear regime, we observe at late times a diffusive broadening of the fronts controlled by an effective shear viscosity that diverges at the critical point. No matter how small the microscopic coefficient of dissipation, strongly disordered networks behave as if they were over-damped because energy is irreversibly leaked into diverging non-affine fluctuations. Close to the transition, the regime of linear response becomes vanishingly small: the tiniest shear strains generate strongly non-linear shear shock waves qualitatively different from their compressional counterparts in granular media. The inherent non-linearities trigger an energy cascade from low to high frequency components that keep the network away from attaining the quasi-static limit. This mechanism...

## The Jamming point street-lamp in the world of granular media

Coulais, Corentin; Behringer, Robert P.; Dauchot, Olivier
Tipo: Artigo de Revista Científica
Relevância na Pesquisa
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The Jamming of soft spheres at zero temperature, the J-point, has been extensively studied both numerically and theoretically and can now be considered as a safe location in the space of models, where a street lamp has been lit up. However, a recent work by Ikeda et al, 2013 reveals that, in the Temperature/Packing fraction parameter space, experiments on colloids are actually rather far away from the scaling regime illuminated by this lamp. Is it that the J-point has little to say about real system? What about granular media? Such a-thermal, frictional, systems are a-priori even further away from the idealized case of thermal soft spheres. In the past ten years, we have systematically investigated horizontally shaken grains in the vicinity of the Jamming transition. We discuss the above issue in the light of very recent experimental results. First, we demonstrate that the contact network exhibits a remarkable dynamics, with strong heterogeneities, which are maximum at a packing fraction phi star, distinct and smaller than the packing fraction phi dagger, where the average number of contact per particle starts to increase. The two cross-overs converge at point J in the zero mechanical excitation limit. Second, a careful analysis of the dynamics on time scales ranging from a minute fraction of the vibration cycle to several thousands of cycles allows us to map the behaviors of this shaken granular system onto those observed for thermal soft spheres and demonstrate that some light of the J-point street-lamp indeed reaches the granular universe.; Comment: 20 pages...

## Monte Carlo and Molecular Dynamics Simulation of the Glass Transition of Polymers

Binder, K.; Baschnagel, J.; Bennemann, C.; Paul, W.
Tipo: Artigo de Revista Científica
Relevância na Pesquisa
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Two coarse-grained models for polymer chains in dense glass-forming polymer melts are studied by computer simulation: the bond-fluctuation model on a simple cubic lattice, where a bond-length potential favors long bonds, is treated by dynamic Monte Carlo methods, and a bead-spring model in the continuum with a Lennard-Jones potential between the beads is treated by Molecular Dynamics. While the dynamics of both models differ for short length scales and associated time scales, on mesoscopic spatial and temporal scales both models behave similarly. In particular, the mode coupling theory of the glass transition can be used to interpret the slowing down of the undercooled polymer melt. For the off-lattice model, the approach to the critical point of mode coupling is both studied for constant pressure and for constant volume. The lattice model allows a test of the Gibbs-Di Marzio entropy theory of the glass transition, and our finding is that although the entropy does decrease significantly, there is no `entropy catastrophe'', where the fluid entropy would turn negative. Finally, an outlook on confinement effects on the glass transition in thin film geometry is given.; Comment: 11 pages of REVTeX, 6 PostScript figures, submitted to J. Phys.: Condens. Matter

## Depinning and dynamics of vortices confined in mesoscopic flow channels

Besseling, R.; Kes, P. H.; Droese, T.; Vinokur, V. M.
Tipo: Artigo de Revista Científica
Relevância na Pesquisa
55.53%
We study the behavior of vortex matter in artificial flow channels confined by pinned vortices in the channel edges (CE's). The critical current $J_s$ is governed by the interaction with static vortices in the CE's. We study structural changes associated with (in)commensurability between the channel width $w$ and the natural row spacing $b_0$, and their effect on $J_s$. The behavior depends crucially on the presence of disorder in the CE arrays. For ordered CE's, maxima in $J_s$ occur at matching $w=nb_0$ ($n$ integer), while for $w\neq nb_0$ defects along the CE's cause a vanishing $J_s$. For weak CE disorder, the sharp peaks in $J_s$ at $w=nb_0$ become smeared via nucleation and pinning of defects. The corresponding quasi-1D $n$ row configurations can be described by a (disordered)sine-Gordon model. For larger disorder and $w\simeq nb_0$, $J_s$ levels at $\sim 30 %$ of the ideal lattice strength $J_s^0$. Around 'half filling' ($w/b_0 \simeq n\pm 1/2$), disorder causes new features, namely {\it misaligned} defects and coexistence of $n$ and $n \pm 1$ rows in the channel. This causes a {\it maximum} in $J_s$ around mismatch, while $J_s$ smoothly decreases towards matching due to annealing of the misaligned regions. We study the evolution of static and dynamic structures on changing $w/b_0$...

## From Vulcanization to Isotropic and Nematic Rubber Elasticity

Xing, Xiangjun; Mukhopadhyay, Swagatam; Goldbart, Paul M.; Zippelius, Annette
Tipo: Artigo de Revista Científica
Relevância na Pesquisa
55.53%
A Landau theory is constructed for the vulcanization transition in cross-linked polymer systems with spontaneous nematic ordering. The neo-classical theory of the elasticity of nematic elastomers is derived via the minimization of this Landau free energy; this neo-classical theory contains the classical theory of rubber elasticity as its isotropic limit. Our work not only reveals the statistical-mechanical roots of these elasticity theories, but also demonstrates that they are applicable to a wide class of random solids. It also constitutes a starting-point for the investigation of sample-to-sample fluctuations in various forms of vulcanized matter.; Comment: 7 pages, 1 eps figure, submited to Europhys. Lett

## Integration through transients for Brownian particles under steady shear

Fuchs, Matthias; Cates, Michael E.
Tipo: Artigo de Revista Científica
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55.53%
Starting from the microscopic Smoluchowski equation for interacting Brownian particles under stationary shearing, exact expressions for shear-dependent steady-state averages, correlation and structure functions, and susceptibilities are obtained, which take the form of generalized Green-Kubo relations. They require integration of transient dynamics. Equations of motion with memory effects for transient density fluctuation functions are derived from the same microscopic starting point. We argue that the derived formal expressions provide useful starting points for approximations in order to describe the stationary non-equilibrium state of steadily sheared dense colloidal dispersions.; Comment: 17 pages, Submitted to J. Phys.: Condens. Matter; revised version with minor corrections

## Dynamics of Self-Threading Ring Polymers in a Gel

Michieletto, Davide; Marenduzzo, Davide; Orlandini, Enzo; Alexander, Gareth P.; Turner, Matthew S.
Tipo: Artigo de Revista Científica
Relevância na Pesquisa
55.53%
We study of the dynamics of ring polymers confined to diffuse in a background gel at low concentrations. We do this in order to probe the inter-play between topology and dynamics in ring polymers. We develop an algorithm that takes into account the possibility that the rings hinder their own motion by passing through themselves, i.e. "self-threading". Our results suggest that the number of self-threadings scales extensively with the length of the rings and that this is substantially independent of the details of the model. The slowing down of the rings' dynamics is found to be related to the fraction of segments that can contribute to the motion. Our results give a novel perspective on the motion of ring polymers in gel, for which a complete theory is still lacking, and may help us to understand the irreversible trapping of ring polymers in gel electrophoresis experiments.; Comment: 11 pages,14 figures, Soft Matter 2014

## Gel-forming patchy colloids and network glass formers: Thermodynamic and Dynamic analogies

Sciortino, Francesco
Tipo: Artigo de Revista Científica
Relevância na Pesquisa
55.53%
This article discusses recent attempts to provide a deeper understanding of the thermoreversible "gel" state of colloidal matter and to unravel the analogies between gels at the colloidal level and gels at the molecular level, commonly known as network-forming strong liquids. The connection between gel-forming patchy colloids and strong liquids is provided by the limited valence of the inter-particle interactions, i.e. by the presence of a limit in the number of bonded nearest neighbors.; Comment: Proceeding of Stat-Phys 2007. EPJB in press

## Momentum-Transfer to and Elementary-Excitations of a Bose-Einstein Condensate by a Time-Dependent Optical Potential

Band, Y. B.; Sokuler, M.
Tipo: Artigo de Revista Científica
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55.53%
We present results of calculations on Bose-Einstein condensed $^{87}$Rb atoms subjected to a moving standing-wave light-potential of the form $V_L(z,t) = V_0(t) \cos(q z-\omega t)$. We calculate the mean-field dynamics (the order paramter) of the condensate and determine the resulting condensate momentum in the $z$ direction, $P_z(q,\omega,V_0,t_p)$, where $V_0$ is the peak optical potential strength and $t_p$ is the pulse duration. Although the local density approximation for the Bogoliubov excitation spectral distribution is a good approximation for very low optical intensities, long pulse duration and sufficiently large values of the wavevector $q$ of the light-potential, for small $q$, short duration pulses, or for not-so-low intensities, the local density perturbative description of the excitation spectrum breaks down badly, as shown by our results.; Comment: 8 pages, 7 figures

## Molecular Structure and Electrophysical Properties of Pentacene Thioderivatives

Gorishnyi, M. P.
Tipo: Artigo de Revista Científica
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55.53%
Possible conformations of the thioderivatives of pentacene (Pn) have been considered. The absorption spectra of polythiopentacene (PTPn) solutions and films have been studied. PTPn is revealed to be a mixture of Pn thioderivatives with different numbers of S atoms. After this mixture having been condensed in vacuum onto quartz substrates, its main components are tetrathiopentacene (TTPn) and hexathiopentacene (HTPn). The position of the maximum in the long-wave absorption bands of Pn thioderivatives is a linear function of the number of valence electrons in S atoms, which take part in the conjugation with the {\pi}-system of the pentacene frame of PTPn molecules. The analysis of the photocurrent and capacitor photovoltage (CPV) spectra in the range of the first electron transitions in PTPn has shown that the photoconductivity is of the hole type and is caused by the dissociation of excitons at the electron capture centers. The frontal CPV is caused by the Dember photovoltage ({\phi}D), and the back one by the surface-barrier photovoltage ({\phi}b).

## Theory of ice premelting in porous media

Hansen-Goos, Hendrik; Wettlaufer, J. S.
Tipo: Artigo de Revista Científica
Relevância na Pesquisa
55.53%
Premelting describes the confluence of phenomena that are responsible for the stable existence of the liquid phase of matter in the solid region of its bulk phase diagram. Here we develop a theoretical description of the premelting of water ice contained in a porous matrix, made of a material with a melting temperature substantially larger than ice itself, to predict the amount of liquid water in the matrix at temperatures below its bulk freezing point. Our theory combines the interfacial premelting of ice in contact with the matrix, grain boundary melting in the ice, and impurity and curvature induced premelting, the latter occurring in regions which force the ice-liquid interface into a high curvature configuration. These regions are typically found at points where the matrix surface is concave, along contact lines of a grain boundary with the matrix, and in liquid veins. Both interfacial premelting and curvature induced premelting depend on the concentration of impurities in the liquid, which, due to the small segregation coefficient of impurities in ice are treated as homogeneously distributed in the premelted liquid. Our principal result is an equation for the fraction of liquid in the porous medium as a function of the undercooling...

## Structural diversity and the role of particle shape and dense fluid behavior in assemblies of hard polyhedra

Damasceno, Pablo F.; Engel, Michael; Glotzer, Sharon C.