Página 18 dos resultados de 179249 itens digitais encontrados em 0.403 segundos

## Quantum Criticality in Electron-doped BaFe_{2-x}Ni_xAs_2

Zhou, R.; Li, Z.; Yang, J.; Sun, D. L.; Lin, C. T.; Zheng, Guo-qing
Tipo: Artigo de Revista Científica
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A quantum critical point (QCP) is a point in a system's phase diagram at which an order is completely suppressed at absolute zero temperature (T). The presence of a quantum critical point manifests itself in the finite-T physical properties, and often gives rise to new states of matter. Superconductivity in the cuprates and in heavy fermion materials is believed by many to be mediated by fluctuations associated with a quantum critical point. In the recently-discovered iron-pnictide high temperature superconductors, it is unknown whether a QCP exists or not in a carrier-doped system. Here we report transport and nuclear magnetic resonance (NMR) measurements on BaFe_{2-x}Ni_xAs_2 (0 =< x =< 0.17). We find two critical points at x_{c1} = 0.10 and x_{c2} = 0.14. The electrical resistivity follows \rho = \rho_0 + A*T^n, with n = 1 around x_{c1} and another minimal n = 1.1 at x_{c2}. By NMR measurements, we identity x_{c1} to be a magnetic QCP and suggest that x_{c2} is a new type of QCP associated with a nematic structural phase transition. Our results suggest that the superconductivity in carrier-doped pnictides is closely linked to the quantum criticality.; Comment: 21 pages, 9 figures

## A molecular dynamics study of chemical gelation in a patchy particle model

Corezzi, Silvia; De Michele, Cristiano; Zaccarelli, Emanuela; Fioretto, Daniele; Sciortino, Francesco
Tipo: Artigo de Revista Científica
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We report event-driven molecular dynamics simulations of the irreversible gelation of hard ellipsoids of revolution containing several associating groups, characterizing how the cluster size distribution evolves as a function of the extent of reaction, both below and above the gel point. We find that in a very large interval of values of the extent of reaction, parameter-free mean-field predictions are extremely accurate, providing evidence that in this model the Ginzburg zone near the gel point, where non-mean field effects are important, is very limited. We also find that the Flory's hypothesis for the post-gelation regime properly describes the connectivity of the clusters even if the long-time limit of the extent of reaction does not reach the fully reacted state. This study shows that irreversibly aggregating asymmetric hard-core patchy particles may provide a close realization of the mean-field model, for which available theoretical predictions may help control the structure and the connectivity of the gel state. Besides chemical gels, the model is relevant to network-forming soft materials like systems with bioselective interactions, functionalized molecules and patchy colloids.; Comment: 6 pages, 4 figures, to be published in Soft Matter

## Through the Eye of the Needle: Recent Advances in Understanding Biopolymer Translocation

Panja, Debabrata; Barkema, Gerard T.; Kolomeisky, Anatoly B.
Tipo: Artigo de Revista Científica
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In recent years polymer translocation, i.e., transport of polymeric molecules through nanometer-sized pores and channels embedded in membranes, has witnessed strong advances. It is now possible to observe single-molecule polymer dynamics during the motion through channels with unprecedented spatial and temporal resolution. These striking experimental studies have stimulated many theoretical developments. In this short theory-experiment review, we discuss recent progress in this field with a strong focus on non-equilibrium aspects of polymer dynamics during the translocation process.; Comment: 29 pages, 6 figures, 3 tables, to appear in J. Phys.: Condens. Matter as a Topical Review

## Oscillatory settling in wormlike-micelle solutions: bursts and a long time scale

Kumar, Nitin; Majumdar, Sayantan; Sood, Aditya; Govindarajan, Rama; Ramaswamy, Sriram; Sood, A. K.
Tipo: Artigo de Revista Científica
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We study the dynamics of a spherical steel ball falling freely through a solution of entangled wormlike-micelles. If the sphere diameter is larger than a threshold value, the settling velocity shows repeated short oscillatory bursts separated by long periods of relative quiescence. We propose a model incorporating the interplay of settling-induced flow, viscoelastic stress and, as in M. E. Cates, D. A. Head and A. Ajdari, Phys. Rev. E, 2002, 66, 025202(R) and A. Aradian and M. E. Cates, Phys. Rev. E, 2006, 73, 041508, a slow structural variable for which our experiments offer independent evidence.; Comment: To appear in Soft Matter

## Recent high-magnetic-field studies of unusual groundstates in quasi-two-dimensional crystalline organic metals and superconductors

Singleton, J.; Harrison, N.; McDonald, R.; Goddard, P. A.; Bangura, A.; Coldea, A.; Montgomery, L. K.; Chi, X.
Tipo: Artigo de Revista Científica
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After a brief introduction to crystalline organic superconductors and metals, we shall describe two recently-observed exotic phases that occur only in high magnetic fields. The first involves measurements of the non-linear electrical resistance of single crystals of the charge-density-wave (CDW) system (Per)$_2$Au(mnt)$_2$ in static magnetic fields of up to 45 T and temperatures as low as 25 mK. The presence of a fully gapped CDW state with typical CDW electrodynamics at fields higher that the Pauli paramagnetic limit of 34 T suggests the existence of a modulated CDW phase analogous to the Fulde-Ferrell-Larkin-Ovchinnikov state. Secondly, measurements of the Hall potential of single crystals of $\alpha$-(BEDT-TTF)$_2$KHg(SCN)$_4$, made using a variant of the Corbino geometry in quasistatic magnetic fields, show persistent current effects that are similar to those observed in conventional superconductors. The longevity of the currents, large Hall angle, flux quantization and confinement of the reactive component of the Hall potential to the edge of the sample are all consistent with the realization of a new state of matter in CDW systems with significant orbital quantization effects in strong magnetic fields.; Comment: SNS 2004 Conference presentation

## Determination of thermal and optical parameters of melanins by photopyroelectric spectroscopy

de Albuquerque, J. E.; Giacomantonio, C.; White, A.; Meredith, P.
Tipo: Artigo de Revista Científica
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Photopyroelectric spectroscopy (PPE) was used to study the thermal and optical properties of electropolymerized melanins. The photopyroelectric intensity signal and its phase were independently measured as a function of wavelength, as well as a function of chopping frequency for a given wavelength in the saturation part of the PPE spectrum. Equations for both the intensity and the phase of the PPE signal were used to fit the experimental results. From the fittings we obtained for the first time, with great accuracy, the thermal diffusivity coefficient, the thermal conductivity and the specific heat of the samples, as well as a value for the condensed phase optical gap, which we found to be 1.70 eV.; Comment: 4 Page Letter

## Coarse-graining dynamics for convection-diffusion of colloids: Taylor dispersion

Sané, Jimaan; Louis, Ard A.; Padding, Johan
Tipo: Artigo de Revista Científica
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By applying a hybrid Molecular dynamics and mesoscopic simulation technique, we study the classic convection-diffusion problem of Taylor dispersion for colloidal discs in confined flow. We carefully consider the time and length-scales of the underlying colloidal system. These are, by computational necessity, altered in the coarse-grained simulation method, but as long as this is carefully managed, the underlying physics can be correctly interpreted. We find that when the disc diameter becomes non-negligible compared to the diameter of the pipe, there are important corrections to the original Taylor picture. For example, the colloids can flow more rapidly than the underlying fluid, and their Taylor dispersion coefficient is decreased. The long-time tails in the velocity autocorrelation functions are altered by the Poiseuille flow. Some of the conclusions about coarse-graining the dynamics of colloidal suspensions are relevant for a wider range of complex fluids.; Comment: submitted to Faraday Discussions for FD144 - Multiscale Modelling of Soft Matter

## Optical excitations in electroluminescent polymers: poly(para-phenylenevinylene) family

Harigaya, Kikuo
Tipo: Artigo de Revista Científica
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55.53%
Component of photoexcited states with large spatial extent is investigated for optical absorption spectra of the electroluminescent conjugated polymers by using the intermediate exciton theory. We calculate the ratio of oscillator strengths due to long-range excitons with respect to sum of all the oscillator strengths of the absorption as a function of the monomer number. The oscillator strengths of the long-range excitons in poly(para-phenylene) are smaller than those in poly(para-phenylenevinylene), and those of poly(para-phenylenedivinylene) are larger than those in poly(para-phenylenevinylene) Such relative variations are explained by the differences of the number of vinylene units. The oscillator strengths of long-range excitons in poly(di-para-phenylenevinylene) are much larger than those of the above three polymers, due to the increase of number of phenyl rings. We also find that the energy position of the almost localized exciton is neary the same in the four polymers.; Comment: 15 pages; 9 figures; Accepted for publication in J. Phys.: Condens. Matter

## Disentangling the role of structure and friction in shear jamming

Vinutha, H. A.; Sastry, Srikanth
Tipo: Artigo de Revista Científica
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Amorphous packings of spheres have been intensely investigated in order to understand the mechanical and flow behaviour of dense granular matter, and to explore universal aspects of the transition from fluid to structurally arrested or jammed states. Considerable attention has recently been focussed on anisotropic packings of frictional grains generated by shear deformation leading to shear jamming, which occurs below the jamming density for isotropic packings of frictionless grains. With the aim of disentangling the role of shear deformation induced structures and friction in generating shear jamming, we study sheared assemblies of frictionless spheres computationally, over a wide range of densities, extending far below the isotropic jamming point. We demonstrate the emergence of a variety of geometric features characteristic of jammed packings with the increase of shear strain. The average contact number and the distributions of contact forces suggest the presence of a threshold density, well below the isotropic jamming point, above which a qualitative change occurs in the jamming behaviour of sheared configurations. We show that above this threshold density, friction stabilizes the sheared configurations we generate. Our results thus reveal the emergence of geometric features characteristic of jammed states as a result of shear deformation alone...

## Smectic Polymer Vesicles

Jia, Lin; Cao, Amin; Levy, Daniel; Xu, Bing; Albouy, Pierre-Antoine; Xing, Xiangjun; Bowick, Mark J.; Li, Min-Hui
Tipo: Artigo de Revista Científica
Relevância na Pesquisa
55.53%
Polymer vesicles are stable robust vesicles made from block copolymer amphiphiles. Recent progress in the chemical design of block copolymers opens up the exciting possibility of creating a wide variety of polymer vesicles with varying fine structure, functionality and geometry. Polymer vesicles not only constitute useful systems for drug delivery and micro/nano-reactors but also provide an invaluable arena for exploring the ordering of matter on curved surfaces embedded in three dimensions. By choosing suitable liquid-crystalline polymers for one of the copolymer components one can create vesicles with smectic stripes. Smectic order on shapes of spherical topology inevitably possesses topological defects (disclinations) that are themselves distinguished regions for potential chemical functionalization and nucleators of vesicle budding. Here we report on glassy striped polymer vesicles formed from amphiphilic block copolymers in which the hydrophobic block is a smectic liquid crystal polymer containing cholesteryl-based mesogens. The vesicles exhibit two-dimensional smectic order and are ellipsoidal in shape with defects, or possible additional budding into isotropic vesicles, at the poles.; Comment: 17 pages, 4 figures, pdf

## Inferring the effective thickness of polyelectrolytes from stretching measurements at various ionic strengths: applications to DNA and RNA

Toan, Ngo Minh; Micheletti, Cristian
Tipo: Artigo de Revista Científica
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By resorting to the thick-chain model we discuss how the stretching response of a polymer is influenced by the self-avoidance entailed by its finite thickness. The characterization of the force versus extension curve for a thick chain is carried out through extensive stochastic simulations. The computational results are captured by an analytic expression that is used to fit experimental stretching measurements carried out on DNA and single-stranded RNA (poly-U) in various solutions. This strategy allows us to infer the apparent diameter of two biologically-relevant polyelectrolytes, namely DNA and poly-U, for different ionic strengths. Due to the very different degree of flexibility of the two molecules, the results provide insight into how the apparent diameter is influenced by the interplay between the (solution-dependent) Debye screening length and the polymers' bare'' thickness. For DNA, the electrostatic contribution to the effective radius, $\Delta$, is found to be about 5 times larger than the Debye screening length, consistently with previous theoretical predictions for highly-charged stiff rods. For the more flexible poly-U chains the electrostatic contribution to $\Delta$ is found to be significantly smaller than the Debye screening length.; Comment: iopart...

## Frustrated Polyelectrolyte Bundles and T=0 Josephson-Junction Arrays

Grason, Gregory M.; Bruinsma, Robijn F.
Tipo: Artigo de Revista Científica
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We establish a one-to-one mapping between a model for hexagonal polyelectrolyte bundles and a model for two-dimensional, frustrated Josephson-junction arrays. We find that the T=0 insulator-to-superconductor transition of the {\it quantum} system corresponds to a continuous liquid-to-solid transition of the condensed charge in the finite temperature {\it classical} system. We find that the role of the vector potential in the quantum system is played by elastic strain in the classical system. Exploiting this correspondence we show that the transition is accompanied by a spontaneous breaking of chiral symmetry and that at the transition the polyelectrolyte bundle adopts a universal response to shear.; Comment: 4 pages, 2 figures, 1 table minor changes to text, reference added

## Melting and freezing of argon in a granular packing of linear mesopore arrays

Schaefer, Christof; Hofmann, Tommy; Wallacher, Dirk; Huber, Patrick; Knorr, Klaus
Tipo: Artigo de Revista Científica
Relevância na Pesquisa
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Freezing and melting of Ar condensed in a granular packing of template-grown arrays of linear mesopores (SBA-15, mean pore diameter 8 nanometer) has been studied by specific heat measurements C as a function of fractional filling of the pores. While interfacial melting leads to a single melting peak in C, homogeneous and heterogeneous freezing along with a delayering transition for partial fillings of the pores result in a complex freezing mechanism explainable only by a consideration of regular adsorption sites (in the cylindrical mesopores) and irregular adsorption sites (in niches of the rough external surfaces of the grains, and at points of mutual contact of the powder grains). The tensile pressure release upon reaching bulk liquid/vapor coexistence quantitatively accounts for an upward shift of the melting/freeezing temperature observed while overfilling the mesopores.; Comment: 4 pages, 4 figures, to appear as a Letter in Physical Review Letters

## Nonlinear Elasticity, Fluctuations and Heterogeneity of Nematic Elastomers

Tipo: Artigo de Revista Científica
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55.53%
Liquid crystal elastomers realize a fascinating new form of soft matter that is a composite of a conventional crosslinked polymer gel (rubber) and a liquid crystal. These {\em solid} liquid crystal amalgams, quite similarly to their (conventional, fluid) liquid crystal counterparts, can spontaneously partially break translational and/or orientational symmetries, accompanied by novel soft Goldstone modes. As a consequence, these materials can exhibit unconventional elasticity characterized by symmetry-enforced vanishing of some elastic moduli. Thus, a proper description of such solids requires an essential modification of the classical elasticity theory. In this work, we develop a {\em rotationally invariant}, {\em nonlinear} theory of elasticity for the nematic phase of ideal liquid crystal elastomers. We show that it is characterized by soft modes, corresponding to a combination of long wavelength shear deformations of the solid network and rotations of the nematic director field. We study thermal fluctuations of these soft modes in the presence of network heterogeneities and show that they lead to a large variety of anomalous elastic properties, such as singular length-scale dependent shear elastic moduli, a divergent elastic constant for splay distortion of the nematic director...

## First-order phase transitions in two-dimensional off-lattice liquid crystals

Wensink, H. H.; Vink, R. L. C.
Tipo: Artigo de Revista Científica
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We consider an off-lattice liquid crystal pair potential in strictly two dimensions. The potential is purely repulsive and short-ranged. Nevertheless, by means of a single parameter in the potential, the system is shown to undergo a first-order phase transition. The transition is studied using mean-field density functional theory, and shown to be of the isotropic-to-nematic kind. In addition, the theory predicts a large density gap between the two coexisting phases. The first-order nature of the transition is confirmed using computer simulation and finite-size scaling. Also presented is an analysis of the interface between the coexisting domains, including estimates of the line tension, as well as an investigation of anchoring effects.; Comment: 12 pages, 17 figures, submitted to J. Phys.: Condens. Matter

## Transport properties controlled by a thermostat: An extended dissipative particle dynamics thermostat

Junghans, Christoph; Praprotnik, Matej; Kremer, Kurt
Tipo: Artigo de Revista Científica
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55.53%
We introduce a variation of the dissipative particle dynamics (DPD) thermostat that allows for controlling transport properties of molecular fluids. The standard DPD thermostat acts only on a relative velocity along the interatomic axis. Our extension includes the damping of the perpendicular components of the relative velocity, yet keeping the advantages of conserving Galilei invariance and within our error bar also hydrodynamics. This leads to a second friction parameter for tuning the transport properties of the system. Numerical simulations of a simple Lennard-Jones fluid and liquid water demonstrate a very sensitive behaviour of the transport properties, e.g., viscosity, on the strength of the new friction parameter. We envisage that the new thermostat will be very useful for the coarse-grained and adaptive resolution simulations of soft matter, where the diffusion constants and viscosity of the coarse-grained models are typically too high/low, respectively, compared to all-atom simulations.; Comment: 6 pages, 4 figures

## Stress phase space for static granular matter

Tipo: Artigo de Revista Científica
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55.53%
This paper proposes a phase space to compare the static packings of a granular system compatible to a macrostate that is set by the external stress. The nature of this phase space is analyzed, showing that the consideration of the allowed and forbidden regions and the internal degrees of freedom of every configuration (i.e. geometrical pattern) could be a relevant factor for the establishment of its probability and, therefore, of the expected properties of the sample. This is due to the fact that many combinations of forces acting on a particle can keep it in static equilibrium. Every set of forces can be considered equivalent to a microscopic stress field, but the kind of interaction and the geometrical restrictions mean that not all stress states can be represented by any set, whereas others can be represented by many sets. Consequently the points of the phase space are degenerate, and the density of states of each configuration strongly determines the most probable statistical distribution. It is shown how these functions just depend on the deviatoric stress. A first analysis of two-dimensional (2D) arrangements is included to clarify this assertion.; Comment: 8 pages, 10 figures

## Shear-induced structuration of confined carbon black gels: Steady-state features of vorticity-aligned flocs

Grenard, Vincent; Taberlet, Nicolas; Manneville, Sebastien
Tipo: Artigo de Revista Científica
Relevância na Pesquisa
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Various dispersions of attractive particles are known to aggregate into patterns of vorticity-aligned stripes when sheared in confined geometries. We report a thorough experimental investigation of such shear-induced vorticity alignment through direct visualization of carbon black gels in both simple plane shear and rotational shear cells. Control parameters such as the gap width, the strain rate, and the gel concentration are systematically varied. It is shown that in steady states the wavelength of the striped pattern depends linearly on the gap width $h$ while being insensitive to both the gel concentration $C$ and the shear rate $\gp$. The width of the vorticity-aligned flocs coincides with the gap width and is also independent of $C$ and $\gp$, which hints to a simple picture in terms of compressible cylindrical flocs. Finally, we show that there exists a critical shear rate $\gp_c$ above which structuration does not occur and that $\gp_c$ scales as $h^{-\alpha}$ with $\alpha=1.4\pm 0.1$ independently of $C$. This extensive data set should open the way to quantitative modelling of the vorticity alignment phenomenon in attractive colloidal systems.; Comment: 10 pages, 14 figures, submitted to Soft Matter

## Theory for the phase behaviour of a colloidal fluid with competing interactions

Archer, A. J.; Ionescu, C.; Pini, D.; Reatto, L.
Tipo: Artigo de Revista Científica
Adsorption of polymers to surfaces is crucial for understanding many fundamental processes in nature. Recent experimental studies indicate that the adsorption dynamics is dominated by non-equilibrium effects. We investigate the adsorption of a single polymer of length $N$ to a planar solid surface in the absence of hydrodynamic interactions. We find that for weak adsorption energies the adsorption time scales $\sim N^{(1+2\nu)/(1+\nu)}$, where $\nu$ is the Flory exponent for the polymer. We argue that in this regime the single chain adsorption is closely related to a field-driven polymer translocation through narrow pores. Surprisingly, for high adsorption energies the adsorption time becomes longer, as it scales $\sim N^{(1+\nu)}$, which is explained by strong stretching of the unadsorbed part of the polymer close to the adsorbing surface. These two dynamic regimes are separated by an energy scale that is characterised by non-equilibrium contributions during the adsorption process.; Comment: 11 pages, 3 figures, 1 table, substantial added material, to appear in J. Phys.: Condens. Matter as a Fast Track Communication