# This Week at Physics

## This Week at Physics |
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### Events on Thursday, September 26th, 2013

**R. G. Herb Condensed Matter Seminar****Cavity-mediated near-critical dissipative dynamics of a driven condensate****Time:**10:00 am**Place:**Chamberlin 5310**Speaker:**Dr. Manas Kulkarni, Princeton University**Abstract:**We investigate the near-critical dynamics of atomic density fluctuations in the non-equilibrium self-organization transition of an optically driven quantum gas coupled to a single mode of a cavity. In this system cavity-mediated long-range 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 two-mode approximation for the collective motional degrees of freedom of the atomic condensate which results in an effective open-system Dicke model. Here, including the full spectrum of atomic modes we find a finite lifetime for a roton-like mode in the Bogoliubov excitation spectrum that is strongly pump-dependent. The corresponding decay rate and critical exponents for the phase-transition are calculated explaining the non-monotonic pump-dependent atomic damping rate observed in recent experiments. We compute the near-critical behavior of the intra-cavity field fluctuations, that has been previously shown to be enhanced with respect to the equilibrium Dicke model in a two-mode 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- Add this event to your calendar
**Astronomy Colloquium****"Cosmic magnetogenesis: From spontaneously emitted aperiodic turbulent to ordered equipartition fields"****Time:**3:30 pm**Place:**4421 Sterling Hall**Speaker:**Reinhard Schlickeiser, Ruhr-University Bochum, Germany**Abstract:**

Nonmagnetized fully-ionized 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 frozen-in 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- Add this event to your calendar