1,886 research outputs found

    An Exact Solution to O(26) Sigma Model coupled to 2-D Gravity

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    By a mapping to the bosonic string theory, we present an exact solution to the O(26) sigma model coupled to 2-D quantum gravity. In particular, we obtain the exact gravitational dressing to the various matter operators classified by the irreducible representations of O(26). We also derive the exact form of the gravitationally modified beta function for the original coupling constant e2e^2. The relation between our exact solution and the asymptotic solution given in ref[3] is discussed in various aspects.Comment: 10 pages, pupt-144

    Melting of Polydisperse Hard Disks

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    The melting of a polydisperse hard disk system is investigated by Monte Carlo simulations in the semigrand canonical ensemble. This is done in the context of possible continuous melting by a dislocation unbinding mechanism, as an extension of the 2D hard disk melting problem. We find that while there is pronounced fractionation in polydispersity, the apparent density-polydispersity gap does not increase in width, contrary to 3D polydisperse hard spheres. The point where the Young's modulus is low enough for the dislocation unbinding to occur moves with the apparent melting point, but stays within the density gap, just like for the monodisperse hard disk system. Additionally, we find that throughout the accessible polydispersity range, the bound dislocation-pair concentration is high enough to affect the dislocation unbinding melting as predicted by Kosterlitz, Thouless, Halperin, Nelson and Young.Comment: 6 pages, 6 figure

    Internal thermal noise in the LIGO test masses : a direct approach

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    The internal thermal noise in LIGO's test masses is analyzed by a new technique, a direct application of the Fluctuation-Dissipation Theorem to LIGO's readout observable, x(t)=x(t)=(longitudinal position of test-mass face, weighted by laser beam's Gaussian profile). Previous analyses, which relied on a normal-mode decomposition of the test-mass motion, were valid only if the dissipation is uniformally distributed over the test-mass interior, and they converged reliably to a final answer only when the beam size was a non-negligible fraction of the test-mass cross section. This paper's direct analysis, by contrast, can handle inhomogeneous dissipation and arbitrary beam sizes. In the domain of validity of the previous analysis, the two methods give the same answer for Sx(f)S_x(f), the spectral density of thermal noise, to within expected accuracy. The new analysis predicts that thermal noise due to dissipation concentrated in the test mass's front face (e.g. due to mirror coating) scales as 1/r021/r_0^2, by contrast with homogeneous dissipation, which scales as 1/r01/r_0 (r0r_0 is the beam radius); so surface dissipation could become significant for small beam sizes.Comment: 6 pages, RevTex, 1 figur

    A crude model to study radio frequency induced density modification close to launchers

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    The interplay between radio frequency (RF) waves and the density is discussed by adopting the general framework of a 2-time-scale multi-fluid treatment, allowing to separate the dynamics on the RF time scale from that on the time scale on which macroscopic density and flows vary as a result of the presence of electromagnetic and/or electrostatic fields. The focus is on regions close to launchers where charge neutrality is incomplete and waves are commonly evanescent. The fast time scale dynamics influences the slow time scale behavior via quasilinear terms (the Ponderomotive force for the case of the equation of motion). Electrons and ions are treated on the same footing. Also, both fast and slow waves are retained in the wave description. Although this work is meant as a subtopic of a large study-the wave induced "convective cell" physics at hand is of a 2- or 3-dimensional nature while this paper limits itself to a single dimension-a few tentative examples are presented

    Maximal work extraction from quantum systems

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    Thermodynamics teaches that if a system initially off-equilibrium is coupled to work sources, the maximum work that it may yield is governed by its energy and entropy. For finite systems this bound is usually not reachable. The maximum extractable work compatible with quantum mechanics (``ergotropy'') is derived and expressed in terms of the density matrix and the Hamiltonian. It is related to the property of majorization: more major states can provide more work. Scenarios of work extraction that contrast the thermodynamic intuition are discussed, e.g. a state with larger entropy than another may produce more work, while correlations may increase or reduce the ergotropy.Comment: 5 pages, 0 figures, revtex

    Chemical Potential and the Nature of the Dark Energy: The case of phantom

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    The influence of a possible non zero chemical potential μ\mu on the nature of dark energy is investigated by assuming that the dark energy is a relativistic perfect simple fluid obeying the equation of state (EoS), p=ωρp=\omega \rho (ω<0,constant\omega <0, constant). The entropy condition, S0S \geq 0, implies that the possible values of ω\omega are heavily dependent on the magnitude, as well as on the sign of the chemical potential. For μ>0\mu >0, the ω\omega-parameter must be greater than -1 (vacuum is forbidden) while for μ<0\mu < 0 not only the vacuum but even a phantomlike behavior (ω<1\omega <-1) is allowed. In any case, the ratio between the chemical potential and temperature remains constant, that is, μ/T=μ0/T0\mu/T=\mu_0/T_0. Assuming that the dark energy constituents have either a bosonic or fermionic nature, the general form of the spectrum is also proposed. For bosons μ\mu is always negative and the extended Wien's law allows only a dark component with ω<1/2\omega < -1/2 which includes vacuum and the phantomlike cases. The same happens in the fermionic branch for μ0\mu 0 are permmited only if 1<ω<1/2-1 < \omega < -1/2. The thermodynamics and statistical arguments constrain the EoS parameter to be ω<1/2\omega < -1/2, a result surprisingly close to the maximal value required to accelerate a FRW type universe dominated by matter and dark energy (ω10/21\omega \lesssim -10/21).Comment: 7 pages, 5 figure

    Efficiency at maximum power of low dissipation Carnot engines

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    We study the efficiency at maximum power, η\eta^*, of engines performing finite-time Carnot cycles between a hot and a cold reservoir at temperatures ThT_h and TcT_c, respectively. For engines reaching Carnot efficiency ηC=1Tc/Th\eta_C=1-T_c/T_h in the reversible limit (long cycle time, zero dissipation), we find in the limit of low dissipation that η\eta^* is bounded from above by ηC/(2ηC)\eta_C/(2-\eta_C) and from below by ηC/2\eta_C/2. These bounds are reached when the ratio of the dissipation during the cold and hot isothermal phases tend respectively to zero or infinity. For symmetric dissipation (ratio one) the Curzon-Ahlborn efficiency ηCA=1Tc/Th\eta_{CA}=1-\sqrt{T_c/T_h} is recovered.Comment: 4 pages, 1 figure, 1 tabl

    Demixing in a single-peak distributed polydisperse mixture of hard spheres

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    An analytic derivation of the spinodal of a polydisperse mixture is presented. It holds for fluids whose excess free energy can be accurately described by a function of a few moments of the size distribution. It is shown that one such mixture of hard spheres in the Percus-Yevick approximation never demixes, despite its size distribution. In the Boublik-Mansoori-Carnahan-Starling-Leland approximation, though, it demixes for a sufficiently wide log-normal size distribution. The importance of this result is twofold: first, this distribution is unimodal, and yet it phase separates; and second, log-normal size distributions appear in many experimental contexts. The same phenomenon is shown to occur for the fluid of parallel hard cubes.Comment: 4 pages, 2 figures, needs revtex, multicol, epsfig and amstex style file

    Holevo's bound from a general quantum fluctuation theorem

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    We give a novel derivation of Holevo's bound using an important result from nonequilibrium statistical physics, the fluctuation theorem. To do so we develop a general formalism of quantum fluctuation theorems for two-time measurements, which explicitly accounts for the back action of quantum measurements as well as possibly non-unitary time evolution. For a specific choice of observables this fluctuation theorem yields a measurement-dependent correction to the Holevo bound, leading to a tighter inequality. We conclude by analyzing equality conditions for the improved bound.Comment: 5 page

    Influence of rotational force fields on the determination of the work done on a driven Brownian particle

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    For a Brownian system the evolution of thermodynamic quantities is a stochastic process. In particular, the work performed on a driven colloidal particle held in an optical trap changes for each realization of the experimental manipulation, even though the manipulation protocol remains unchanged. Nevertheless, the work distribution is governed by established laws. Here, we show how the measurement of the work distribution is influenced by the presence of rotational, i.e. nonconservative, radiation forces. Experiments on particles of different materials show that the rotational radiation forces, and therefore their effect on the work distributions, increase with the particle refractive index.Comment: 12 pages, 4 figure
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