arXiv subjects

P. Sharma

Publications and source records attributed to P. Sharma.

41 records · Page 3Linked to original sources

arXiv2005

Plasma instabilities and magnetic-field growth in clusters of galaxies

We show that under very general conditions, cluster plasmas threaded by weak magnetic fields are subject to very fast growing plasma instabilities driven by the anisotropy of the plasma pressure (viscous stress) with respect to the local direction of the magnetic field. Such an anisotropy will naturally arise in any weakly magnetized plasma that has low collisionality and is subject to stirring. The magnetic field must be sufficiently weak for the instabilities to occur, viz., beta>Re^{1/2}. The instabilities are captured by the extended MHD model with Braginskii viscosity. However, their growth rates are proportional to the wavenumber down to the ion gyroscale, so MHD equations with Braginskii viscosity are not well posed and a fully kinetic treatment is necessary. The instabilities can lead to magnetic fields in clusters being amplified from seed strength of ~10^{-18} G to dynamically important strengths of ~10 microG on cosmologically trivial time scales (~10^8 yr). The fields produced during the amplification stage are at scales much smaller than observed. Predicting the saturated field scale and structure will require a kinetic theory of magnetized cluster turbulence.

astro-ph↗

arXiv2004

Magnetised plasma turbulence in clusters of galaxies

Cluster plasmas are magnetised already at very low magnetic field strength. Low collisionality implies that conservation of the first adiabatic invariant results in an anisotropic viscous stress (Braginskii viscosity) or, equivalently, anisotropic plasma pressure. This triggers firehose and mirror instabilities, which have growth rates proportional to the wavenumber down to scales of the order of ion Larmor radius. This means that MHD equations with Braginskii viscosity are not well posed and fully kinetic description is necessary. In this paper, we review the basic picture of small-scale dynamo in the cluster plasma and attempt to reconcile it with the existence of plasma instabilities at collisionless scales.

astro-ph↗

arXiv2004

Magnetization and Spin-Diffusion of Liquid 3He in Aerogel

We report theoretical calculations of the normal-state spin diffusion coefficient of 3He in aerogel, including both elastic and inelastic scattering of 3He quasiparticles, and compare these results with experimental data for 3He in 98% porous silica aerogel. This analysis provides a determination of the elastic mean free path within the aerogel. Measurements of the magnetization of the superfluid phase provide a test of the theory of pairbreaking and magnetic response of low-energy excitations in the ``dirty'' B-phase of 3He in aerogel. A consistent interpretation of the data for the spin-diffusion coefficient, magnetization and superfluid transition temperature is obtained by including correlation effects in the aerogel density.

cond-mat.supr-con↗

arXiv2004

Specific Heat of Disordered Superfluid $^{3}$He

The specific heat of superfluid $^{3}$He, disordered by a silica aerogel, is found to have a sharp discontinuity marking the thermodynamic transition to superfluidity at a temperature reduced from that of bulk $^{3}$He. The magnitude of the discontinuity is also suppressed. This disorder effect can be understood from the Ginzburg-Landau theory which takes into account elastic quasiparticle scattering suppressing both the transition temperature and the amplitude of the order parameter. We infer that the limiting temperature dependence of the specific heat is linear at low temperatures in the disordered superfluid state, consistent with predictions of gapless excitations everywhere on the Fermi surface.

cond-mat.supr-con↗

arXiv2003

All-optical switching with bacteriorhodopsin

All-optical, mirrorless switching and bistability is demonstrated with bacteriorhodopsin (bR). A low-power, 532 nm laser beam modulates the transmission of a cw laser beam at 635 nm that corresponds to peak absorption of the O-excited state in the bR photocycle. The switch has features like a NOT-gate; its switching contrast depends on the pulse width and average power of the modulating laser. The phase of the switching characteristics can be controlled by varying the pulse width and frequency of the modulating laser. Simulations based on a rate equation approach considering a six-state model of the bR photocycle successfully reproduce the experimental results.

physics.optics↗