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Tagged particle diffusion in one-dimensional gas with Hamiltonian dynamics-II
http://hdl.handle.net/2289/6315
Title: Tagged particle diffusion in one-dimensional gas with Hamiltonian dynamics-II<br/><br/>Authors: Roy, Anjan; Dhar, Abhishek; Narayan, Onuttom; Sabhapandit, Sanjib<br/><br/>Abstract: We study various temporal correlation functions of a tagged particle in one-dimensional systems of interacting point particles evolving with Hamiltonian dynamics. Initial conditions of the particles are chosen from the canonical thermal distribution. The correlation functions are studied in finite systems, and their forms examined at short and long times. Various one-dimensional systems are studied. Results of numerical simulations for the Fermi–Pasta–Ulam chain are qualitatively similar to results for the harmonic chain, and agree unexpectedly well with a simple description in terms of linearized equations for damped fluctuating sound waves. Simulation results for the alternate mass hard particle gas reveal that—in contradiction to our earlier results (Roy et al. in J Stat Phys 150(5):851–866, 2013) with smaller system sizes—the diffusion constant slowly converges to a constant value, in a manner consistent with mode coupling theories. Our simulations also show that the behaviour of the Lennard–Jones gas depends on its density. At low densities, it behaves like a hard-particle gas, and at high densities like an anharmonic chain. In all the systems studied, the tagged particle was found to show normal diffusion asymptotically, with convergence times depending on the system under study. Finite size effects show up at time scales larger than sound traversal times, their nature being system-specific.<br/><br/>Description: Restricted Access. An open-access version is available at arXiv.org (one of the alternative locations)A quantum kinematics for asymptotically flat gravity
http://hdl.handle.net/2289/6299
Title: A quantum kinematics for asymptotically flat gravity<br/><br/>Authors: Campiglia, Miguel; Varadarajan, Madhavan<br/><br/>Abstract: We construct a quantum kinematics for asymptotically flat gravity based on the Koslowski–Sahlmann (KS) representation. The KS representation is a generalization of the representation underlying loop quantum gravity (LQG) which supports, in addition to the usual LQG operators, the action of 'background exponential operators', which are connection dependent operators labelled by 'background' su(2) electric fields. KS states have, in addition to the LQG state label corresponding to one dimensional excitations of the triad, a label corresponding to a 'background' electric field that describes three dimensional excitations of the triad. Asymptotic behaviour in quantum theory is controlled through asymptotic conditions on the background electric fields that label the states and the background electric fields that label the operators. Asymptotic conditions on the triad are imposed as conditions on the background electric field state label while confining the LQG spin net graph labels to compact sets. We show that KS states can be realised as wave functions on a quantum configuration space of generalized connections and that the asymptotic behaviour of each such generalized connection is determined by that of the background electric fields which label the background exponential operators. Similar to the spatially compact case, the Gauss law and diffeomorphism constraints are then imposed through group averaging techniques to obtain a large sector of gauge invariant states. It is shown that this sector supports a unitary action of the group of asymptotic rotations and translations and that, as anticipated by Friedman and Sorkin, for appropriate spatial topology, this sector contains states that display fermionic behaviour under $2\pi $ rotations<br/><br/>Description: Open AccessDirected search for gravitational waves from Scorpius X-1 with initial LIGO data
http://hdl.handle.net/2289/6272
Title: Directed search for gravitational waves from Scorpius X-1 with initial LIGO data<br/><br/>Authors: Aasi, J.; Abbott, B.P.; Abbott, R.; Iyer, B.R.; LIGO Scientific Collaboration and Virgo Collaboration; +250 Co authors<br/><br/>Abstract: We present results of a search for continuously emitted gravitational radiation, directed at the brightest low-mass x-ray binary, Scorpius X-1. Our semicoherent analysis covers 10 days of LIGO S5 data ranging from 50–550 Hz, and performs an incoherent sum of coherent F-statistic power distributed amongst frequency-modulated orbital sidebands. All candidates not removed at the veto stage were found to be consistent with noise at a 1% false alarm rate. We present Bayesian 95% confidence upper limits on gravitational-wave strain amplitude using two different prior distributions: a standard one, with no a priori assumptions about the orientation of Scorpius X-1; and an angle-restricted one, using a prior derived from electromagnetic observations. Median strain upper limits of 1.3×10−24 and 8×10−25 are reported at 150 Hz for the standard and angle-restricted searches respectively. This proof-of-principle analysis was limited to a short observation time by unknown effects of accretion on the intrinsic spin frequency of the neutron star, but improves upon previous upper limits by factors of ∼1.4 for the standard, and 2.3 for the angle-restricted search at the sensitive region of the detector.<br/><br/>Description: Open AccessLive cell plasma membranes do not exhibit a miscibility phase transition over a wide range of temperatures
http://hdl.handle.net/2289/6256
Title: Live cell plasma membranes do not exhibit a miscibility phase transition over a wide range of temperatures<br/><br/>Authors: Lee, Hyung; Saha, Suvrajit; Polley, Anirban; Hunag, Hector; Mayor, Satyajit; Rao, Madan; Groves, Jay T.<br/><br/>Abstract: Lipid/cholesterol mixtures derived from cell membranes as well as their synthetic reconstitutions exhibit well-defined miscibility phase transitions and critical phenomena near physiological temperatures. This suggests that lipid/cholesterol-mediated phase separation plays a role in the organization of live cell membranes. However, macroscopic lipid-phase separation is not generally observed in cell membranes, and the degree to which properties of isolated lipid mixtures are preserved in the cell membrane remain unknown. A fundamental property of phase transitions is that the variation of tagged particle diffusion with temperature exhibits an abrupt change as the system passes through the transition, even when the two phases are distributed in a nanometer-scale emulsion. We support this using a variety of Monte Carlo and atomistic simulations on model lipid membrane systems. However, temperature-dependent fluorescence correlation spectroscopy of labeled lipids and membrane-anchored proteins in live cell membranes shows a consistently smooth increase in the diffusion coefficient as a function of temperature. We find no evidence of a discrete miscibility phase transition throughout a wide range of temperatures: 14–37 °C. This contrasts the behavior of giant plasma membrane vesicles (GPMVs) blebbed from the same cells, which do exhibit phase transitions and macroscopic phase separation. Fluorescence lifetime analysis of a DiI probe in both cases reveals a significant environmental difference between the live cell and the GPMV. Taken together, these data suggest the live cell membrane may avoid the miscibility phase transition inherent to its lipid constituents by actively regulating physical parameters, such as tension, in the membrane.<br/><br/>Description: Restricted Access.