http://www.bibsonomy.org/burst/user/statphys23/selfBibSonomy BuRST Feed for /user/statphys23/self2008-08-21T04:12:12+02:00http://www.bibsonomy.org/bibtex/23899376256836d33b6709156c11fee7f/statphys23statphys232007-06-20T10:16:09+02:00fluctuations criticality universal disorder turbulence transitions self topic-3 organized avalanches statphys23 <span style="color:#555555;">S.C. <a href="http://www.bibsonomy.org/author/Chapman">Chapman</a> and N.W. <a href="http://www.bibsonomy.org/author/Watkins">Watkins</a> </span><em>Abstract Book of the XXIII IUPAP International Conference on Statistical Physics, </em><em>Genova, Italy, </em><em>9-13 July2007. </em>Wed Jun 20 10:16:09 CEST 2007Genova, ItalyAbstract Book of the XXIII IUPAP International Conference on Statistical Physics9-13 JulyOrder parameters in avalanching systems and in fluid turbulence2007fluctuations criticality universal disorder turbulence transitions self topic-3 organized avalanches statphys23 We apply dimensional analysis (the Buckingham $\Pi$ theorem) to obtain the parameters for a generic finite-sized avalanching system. The order parameters that we identify relevant to the discussion here are essentially the driving rate, normalised to the rates of transport and dissipation respectively. A mean field approach has previously identified the self organized critical (SOC) state to be in the limit of slowly driven, interaction dominated transport (SDIDT). The order parameters that we identify are just those taken to zero in the SDIDT limit. Importantly, we find that critical behaviour can persist for an intermediate range of the drive, provided the drive is spatially local and the pile is sufficiently large. Intermediate, local driving simply corresponds to a spatial coarse-graining of the pile. Our result follows from dimensional analysis and is therefore quite general, the only requirement being that the system is driven and dissipating, and that the dynamics are such that propagation occurs when a locally defined threshold for instability is exceeded. This is consistent with numerical results [1] and suggests that SDIDT is part of a wider class of avalanching systems, For SOC, strong driving ultimately brings the system away from criticality to laminar flow. This is in contrast to fluid turbulence, where increasing the drive (i.e. the order parameter, here the Reynolds number) brings the system away from laminar flow towards disorder. Parallels between the Reynolds number and the order parameters for SOC that we identify here can succinctly identify the similarities, and differences, between SOC and turbulence. That these order parameters apply to a range of systems, encompassing SOC and turbulence, also follows from an analytical treatment of 'universal fluctuations' [2], previously suggested [3] to be a ubiquitous feature of highly correlated systems such as models for SOC, forest fires, granular media and turbulence. 1) Corral and Paczuski, Phys. Rev. Lett., 83, 572, 1999.\\ 2) Chapman et al, J. Phys. A (Math. Gen.), 38, 2289, 2005.\\ 3) Bramwell et al, Nature, 396, 552, 1998; Bramwell et al, Phys. Rev. Lett., 87, 118902, 2001.http://www.bibsonomy.org/bibtex/24cbd87ff4096b8bf7cc161f65f06399d/statphys23statphys232007-06-20T10:16:09+02:00asymmetric morphological films instabilities deposition growth nitride topic-4 alloys statphys23 assembly self coupled semiconductors. thin instability nano-structures epitaxial <span style="color:#555555;">A. <a href="http://www.bibsonomy.org/author/Chatterji">Chatterji</a> and N. <a href="http://www.bibsonomy.org/author/Yang">Yang</a> and R.C. <a href="http://www.bibsonomy.org/author/Desai">Desai</a> </span><em>Abstract Book of the XXIII IUPAP International Conference on Statistical Physics, </em><em>Genova, Italy, </em><em>9-13 July2007. </em>Wed Jun 20 10:16:09 CEST 2007Genova, ItalyAbstract Book of the XXIII IUPAP International Conference on Statistical Physics9-13 JulyEpitaxial Growth in coherent, strained, asymmetric alloy films$^2$2007asymmetric morphological films instabilities deposition growth nitride topic-4 alloys statphys23 assembly self coupled semiconductors. thin instability nano-structures epitaxial We examine the linear stability/ instability of an epitaxially grown coherent thin alloy film. Misfit stresses produce morphological instability in a dislocation-free thin film. The stability of the film's surface is affected by the coupling between the morphological and alloy segregation instabilities if the temperature and composition of the alloy is within the spinodal. An earlier$^3$ nonequilibrium continuum model is extended to nonsymmetric binary alloys. In the model, film- substrate misfit strain $\epsilon$, solute expansion coefficient $\eta$, growth velocity $V$ and growth temperature $T$ are important parameters that determine the film stability. The asymmetry of the binary alloy, as represented by the mean order parameter $\phi_o$ ($\phi_o = 2 c_o - 1$, $c_o$ is mean concentration) and the dependence of elastic moduli $E$ and $\mu$ on the fluctuations $\delta \phi$ are also important. While $\delta \phi(\vec{x}, \tau)$ is a measure of the segregation instability, the morphological instability is monitored through the fluctuations of the film thickness $\delta h(\vec{x}, \tau)$ around its mean value $\bar h(\tau) = V \tau$. We find that for $T < T_c$ where $T_c$ is the mean field critical temperature, the system is linearly unstable at all values of $\phi_0$ (for $GaAsN$ films grown on $GaAs$ substrate, $T=600K$ and $T_c=12990K$). The morphological instability is affected by the growth velocity even for small $\epsilon$ and the system remains linearly unstable as $\phi_0$ moves from the spinodal to the metastable region of the mean field phase diagram. For $\epsilon =0$, the morphological instability is absent and high values of $V$ suppress compositional instability as a deposited layer is buried before the surface diffusion has a chance to segregate the two components. However for $\epsilon \ne 0$, large $V$ enhances the coupled instability. The surface diffusion in solid alloy films is an activated process and the time scale for diffusion becomes exponentially large at lower temperatures. Since surface diffusion plays a central role in the coupled phase segregation and morphological instability, the time scales for the evolution of $\delta \phi(\vec{x}, \tau)$ and $\delta h({\vec x}, \tau)$ can become very large in some systems, enabling an experimentalist to grow relatively flat films of uniform composition $\phi_0$ at low $T$. We determine the growth velcity $V_o (\phi_o)$ for which the coupled instability is minimised. For $V=V_o$ the phase segregation instability is just suppressed by the deposition. Our results in the multi-parameter space could provide some guidelines for growing flat films and also to exploit the coupled instability in the alloy film to form self assembled nano-structures. \\ $^2$~Supported by NSERC of Canada. \\ $^3$ F. L\'{e}onard and R. C. Desai, Phys. Rev. B{\bf 57}, 4805 (1998); Appl. Phys. Lett. {\bf 74}, 40 (1999). Z.-F. Huang and R. C. Desai, Phys. Rev. B{\bf 65}, 195421 and 205419 (2002).http://www.bibsonomy.org/bibtex/23cd570cceb77974fe3208ab122414ed0/statphys23statphys232007-06-20T10:16:09+02:00particles formation self rheology topic-10 propelled active statphys23 matter cytoskeleton cell pattern <span style="color:#555555;">M.C. <a href="http://www.bibsonomy.org/author/Marchetti">Marchetti</a> </span><em>Abstract Book of the XXIII IUPAP International Conference on Statistical Physics, </em><em>Genova, Italy, </em><em>9-13 July2007. </em>Wed Jun 20 10:16:09 CEST 2007Genova, ItalyAbstract Book of the XXIII IUPAP International Conference on Statistical Physics9-13 JulyHydrodynamics and rheology of soft active matter: from the cytoskeleton to bacterial swarms2007particles formation self rheology topic-10 propelled active statphys23 matter cytoskeleton cell pattern The cytoskeleton is a viscoelastic material with highly unusual properties. Its mechanical and rheological behavior influences a wide variety of cell functions, from crawling to division. Our work focuses of the physical aspects of this behavior and models the cytoskeleton as an active material. Specifically, I will describe the effect of motor activity on the rheological properties of suspensions of cytoskeletal filaments and active crosslinkers. Contractile activity is responsible for a spectacular difference in the viscoelastic properties on opposite sides of the order-disorder transition. In the nematic phase activity also yields nonvanishing stresses proportional to the rate of ATP consumption even in the absence of mechanical deformation. Analogies and differences between the behavior of active gels and those of collections of self-propelled entities, such as bacteria, will also be discussed.http://www.bibsonomy.org/bibtex/2e45d6733f1e038b3440628291008161b/statphys23statphys232007-06-20T10:16:09+02:00boundary topic-3 rayleigh-benard statphys23 convection layers similarity self <span style="color:#555555;">F. <a href="http://www.bibsonomy.org/author/Fontenele Araujo">Fontenele Araujo</a> and G. <a href="http://www.bibsonomy.org/author/Ahlers">Ahlers</a> and E. <a href="http://www.bibsonomy.org/author/Brown">Brown</a> and D. <a href="http://www.bibsonomy.org/author/Funfschilling">Funfschilling</a> and S. <a href="http://www.bibsonomy.org/author/Grossmann">Grossmann</a> and D. <a href="http://www.bibsonomy.org/author/Lohse">Lohse</a> </span><em>Abstract Book of the XXIII IUPAP International Conference on Statistical Physics, </em><em>Genova, Italy, </em><em>9-13 July2007. </em>Wed Jun 20 10:16:09 CEST 2007Genova, ItalyAbstract Book of the XXIII IUPAP International Conference on Statistical Physics9-13 JulyThermal convection with variable transport properties2007boundary topic-3 rayleigh-benard statphys23 convection layers similarity self Thermally driven fluid motion usually involves significant variations in transport properties like viscosity and thermal conductivity. In the context of Rayleigh-B\'enard convection (flow in a container heated from below and cooled from above), such variations can break the top-down symmetries of the velocity, temperature, and density profiles. This symmetry breaking is indicated by the difference $T_{c} - T_{m}$, where $T_{c}$ is the temperature in the center of the convection container and $T_m = (T_{b} + T_{t})/2$ is the mean temperature between the bottom ($T_{b}$) and top ($T_{t}$) plates. On the basis of boundary-layer equations with variable transport properties [1-2], we compute $T_{c} - T_{m}$ as function of $\Delta = T_{b} - T_{t}$. Two different fluids are considered: gaseous ethane close to its critical point [1] and water [2]. The latter exhibits $T_{c} > T_{m}$ for increasing $\Delta$, meaning that the top boundary-layer becomes thicker than its counterpart at the bottom plate. In contrast, when working fluid is gaseous ethane, the opposite symmetry breaking is observed. In both cases, our theoretical results are in reasonable agreement with experimental measurements (see figure 1). References 1) G. Ahlers, F. Fontenele Araujo, D. Funfschilling, S. Grossmann, and D. Lohse. Physical Review Letters 98, 054501 (2007).\\ 2) G. Ahlers, E. Brown, F. Fontenele Araujo, D. Funfschilling, S. Grossmann, and D. Lohse. Journal of Fluid Mechanics 569, 409 (2006)