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Events on Thursday, September 26th, 2013
 R. G. Herb Condensed Matter Seminar
 Cavitymediated nearcritical dissipative dynamics of a driven condensate
 Time: 10:00 am
 Place: Chamberlin 5310
 Speaker: Dr. Manas Kulkarni, Princeton University
 Abstract: We investigate the nearcritical dynamics of atomic density fluctuations in the nonequilibrium selforganization transition of an optically driven quantum gas coupled to a single mode of a cavity. In this system cavitymediated longrange interactions between atoms, tunable by the drive strength, lead to softening of an excitation mode recently observed in experiments. This phenomenon has previously been studied within a twomode approximation for the collective motional degrees of freedom of the atomic condensate which results in an effective opensystem Dicke model. Here, including the full spectrum of atomic modes we find a finite lifetime for a rotonlike mode in the Bogoliubov excitation spectrum that is strongly pumpdependent. The corresponding decay rate and critical exponents for the phasetransition are calculated explaining the nonmonotonic pumpdependent atomic damping rate observed in recent experiments. We compute the nearcritical behavior of the intracavity field fluctuations, that has been previously shown to be enhanced with respect to the equilibrium Dicke model in a twomode approximation. We highlight the role of the finite size of the system in the suppression of it below the expectations of the open Dicke model.
 Host: Maxim Vavilov
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 Astronomy Colloquium
 "Cosmic magnetogenesis: From spontaneously emitted aperiodic turbulent to ordered equipartition fields"
 Time: 3:30 pm
 Place: 4421 Sterling Hall
 Speaker: Reinhard Schlickeiser, RuhrUniversity Bochum, Germany
 Abstract:
Nonmagnetized fullyionized plasmas spontaneously emit aperiodic turbulent magnetic field fluctuations. Its fluctuation intensities are dominated by the contribution from a recently found collective, damped mode, which modifies the earlier estimate of the total magnetic field strength to $delta B=24beta _e^{1/4}(gn_em_ec^2)^{1/2}$ G in the case of no collisional damping, where $g$ denotes the plasma parameter, $beta _e$ the thermal electron velocity in units of $c$ and $n_e$ the electron density. Accounting for simultaneous viscous damping reduces the estimate to $delta B=2305g(n_em_ec^2)^{1/2}$ G. For the unmagnetized intergalactic medium, immediately after the reionization onset, the field strengths from this mechanism are about $6.8cdot 10^{13}$ G for no collisional damping and $1.5cdot 10^{16}$ G for viscous damping. Maximum spatial scales of $10^{15}$ cm of the emitted aperiodic fluctuations are possible. These guaranteed magnetic fields in the form of randomly distributed fluctuations, produced by the spontaneous emission of the isotropic, thermal IGM plasma, may serve as seed fields for possible amplification by later possible plasma instabilities from anisotropic plasma particle distributions functions, MHD instabilities and/or the MHD dynamo process. Because of the high turbulent plasma beta, the seed fields are tied passively to the highly conducting IGM plasma as frozenin magnetic fluxes, and therefore are subject to subsequent hydrodynamical shear or compression of the IGM medium from the shock waves of the supernova explosions of the first stars at the end of their lifetimes, or from supersonic stellar and galactic winds.
 Host: Prof Alexander Lazarian
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