39,769 research outputs found

    Massive Fields and the 2D String

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    The first massive level of closed bosonic string theory is studied. Free-field equations are derived by imposing Weyl invariance on the world sheet. A two-parameter solution to the equation of motion and constraints is found in two dimensions with a flat linear-dilaton background. One-to-one tachyon scattering is studied in this background. The results support Dhar, Mandal and Wadia's proposal that 2D critical string theory corresponds to the c=1 matrix model in which both sides of the Fermi sea are excited.Comment: 17 pages, Latex. V2: One ref added, minor rephrasing of the first paragraph in Sec.3.1, typos in (56) and (57) correcte

    Condensation Transitions in a One-Dimensional Zero-Range Process with a Single Defect Site

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    Condensation occurs in nonequilibrium steady states when a finite fraction of particles in the system occupies a single lattice site. We study condensation transitions in a one-dimensional zero-range process with a single defect site. The system is analysed in the grand canonical and canonical ensembles and the two are contrasted. Two distinct condensation mechanisms are found in the grand canonical ensemble. Discrepancies between the infinite and large but finite systems' particle current versus particle density diagrams are investigated and an explanation for how the finite current goes above a maximum value predicted for infinite systems is found in the canonical ensemble.Comment: 18 pages, 4 figures, revtex

    Correlation function algebra for inhomogeneous fluids

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    We consider variational (density functional) models of fluids confined in parallel-plate geometries (with walls situated in the planes z=0 and z=L respectively) and focus on the structure of the pair correlation function G(r_1,r_2). We show that for local variational models there exist two non-trivial identities relating both the transverse Fourier transform G(z_\mu, z_\nu;q) and the zeroth moment G_0(z_\mu,z_\nu) at different positions z_1, z_2 and z_3. These relations form an algebra which severely restricts the possible form of the function G_0(z_\mu,z_\nu). For the common situations in which the equilibrium one-body (magnetization/number density) profile m_0(z) exhibits an odd or even reflection symmetry in the z=L/2 plane the algebra simplifies considerably and is used to relate the correlation function to the finite-size excess free-energy \gamma(L). We rederive non-trivial scaling expressions for the finite-size contribution to the free-energy at bulk criticality and for systems where large scale interfacial fluctuations are present. Extensions to non-planar geometries are also considered.Comment: 15 pages, RevTex, 4 eps figures. To appear in J.Phys.Condens.Matte

    Spontaneous Jamming in One-Dimensional Systems

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    We study the phenomenon of jamming in driven diffusive systems. We introduce a simple microscopic model in which jamming of a conserved driven species is mediated by the presence of a non-conserved quantity, causing an effective long range interaction of the driven species. We study the model analytically and numerically, providing strong evidence that jamming occurs; however, this proceeds via a strict phase transition (with spontaneous symmetry breaking) only in a prescribed limit. Outside this limit, the nearby transition (characterised by an essential singularity) induces sharp crossovers and transient coarsening phenomena. We discuss the relevance of the model to two physical situations: the clustering of buses, and the clogging of a suspension forced along a pipe.Comment: 8 pages, 4 figures, uses epsfig. Submitted to Europhysics Letter

    Alternating steady state in one-dimensional flocking

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    We study flocking in one dimension, introducing a lattice model in which particles can move either left or right. We find that the model exhibits a continuous nonequilibrium phase transition from a condensed phase, in which a single `flock' contains a finite fraction of the particles, to a homogeneous phase; we study the transition using numerical finite-size scaling. Surprisingly, in the condensed phase the steady state is alternating, with the mean direction of motion of particles reversing stochastically on a timescale proportional to the logarithm of the system size. We present a simple argument to explain this logarithmic dependence. We argue that the reversals are essential to the survival of the condensate. Thus, the discrete directional symmetry is not spontaneously broken.Comment: 8 pages LaTeX2e, 5 figures. Uses epsfig and IOP style. Submitted to J. Phys. A (Math. Gen.

    Gender differences in public perceptions on National Health Insurance

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    Background. Implementation of National Health Insurance (NHI) commenced recently. With the promise of addressing drastic inequalities in the health sector, NHI has the potential to positively transform the health system. In particular, NHI could have a significant positive impact on females, who are disadvantaged under the current system, with higher rates of poor health and lower rates of medical scheme membership compared with males. Despite NHI’s transformative potential, however, the public discourse on NHI as portrayed in the mediasuggests that it is an unpopular policy. The evidence presented in this paper is to the contrary.Objectives. To assess the general public’s opinion on NHI and to explore gender differences in perceptions.Methods. This paper reports on findings from a 2010 cross-sectional nationally representative survey of the South African population that assessed social attitudes, including perceptions on NHI. Sex-disaggregated data were analysed in SPSS version 20.Results and conclusions. There is broad public acceptance of NHI, indicating that an overwhelming majority of South Africans would prefer an NHI system to the current two-tiered system. Support for NHI has increased since similar studies in 2005 and 2008, with the simultaneous growth of public discourse on the policy. More females than males support NHI, reflecting the potential of the NHI system to have a positive impact on gender equality and the health of women and girls

    Periodically driven stochastic un- and refolding transitions of biopolymers

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    Mechanical single molecule experiments probe the energy profile of biomolecules. We show that in the case of a profile with two minima (like folded/unfolded) periodic driving leads to a stochastic resonance-like phenomenon. We demonstrate that the analysis of such data can be used to extract four basic parameters of such a transition and discuss the statistical requirements of the data acquisition. As advantages of the proposed scheme, a polymeric linker is explicitly included and thermal fluctuations within each well need not to be resolved.Comment: 7 pages, 5 figures, submitted to EP
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