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Driven inelastic Maxwell gases
http://hdl.handle.net/2289/6209
Title: Driven inelastic Maxwell gases<br/><br/>Authors: Prasad, V.V.; Sabhapandit, Sanjib; Dhar, Abhishek<br/><br/>Abstract: We consider the inelastic Maxwell model, which consists of a collection of particles that are characterized by only their velocities and evolving through binary collisions and external driving. At any instant, a particle is equally likely to collide with any of the remaining particles. The system evolves in continuous time with mutual collisions and driving taken to be point processes with rates τ−1c and τ−1w, respectively. The mutual collisions conserve momentum and are inelastic, with a coefficient of restitution r. The velocity change of a particle with velocity v, due to driving, is taken to be Δv=−(1+rw)v+η, where rw∈[−1,1] and η is Gaussian white noise. For rw∈(0,1], this driving mechanism mimics the collision with a randomly moving wall, where rw is the coefficient of restitution. Another special limit of this driving is the so-called Ornstein-Uhlenbeck process given by dvdt=−Γv+η. We show that while the equations for the n-particle velocity distribution functions (n=1,2,...) do not close, the joint evolution equations of the variance and the two-particle velocity correlation functions close. With the exact formula for the variance we find that, for rw≠−1, the system goes to a steady state. Also we obtain the exact tail of the velocity distribution in the steady state. On the other hand, for rw=−1, the system does not have a steady state. Similarly, the system goes to a steady state for the Ornstein-Uhlenbeck driving with Γ≠0, whereas for the purely diffusive driving (Γ=0), the system does not have a steady state.<br/><br/>Description: Open AccessTorsional instanton effects in quantum gravity
http://hdl.handle.net/2289/6208
Title: Torsional instanton effects in quantum gravity<br/><br/>Authors: Kaul, Romesh K.; Sengupta, Sandipan<br/><br/>Abstract: We show that in the first-order gravity theory coupled to axions the instanton number of the Giddings-Strominger wormhole can be interpreted as the Nieh-Yan topological index. The axion charge of the baby universes is quantized in terms of the Nieh-Yan integers. Tunneling between universes of different Nieh-Yan charges implies a nonperturbative vacuum state. The associated topological vacuum angle can be identified with the Barbero-Immirzi parameter.<br/><br/>Description: Open AccessEffects of temperature and ground-state coherence decay on enhancement and amplification in a delta atomic system
http://hdl.handle.net/2289/6207
Title: Effects of temperature and ground-state coherence decay on enhancement and amplification in a delta atomic system<br/><br/>Authors: Manjappa, Manukumara; Undurti, Satya Sainadh; Karigowda, Asha; Narayanan, Andal; Sanders, Barry C<br/><br/>Abstract: We study phase-sensitive amplification of electromagnetically induced transparency in a warm Rb85 vapor wherein a microwave driving field couples the two lower-energy states of a Λ energy-level system thereby transforming into a Δ system. Our theoretical description includes effects of ground-state coherence decay and temperature effects. In particular, we demonstrate that driving-field-enhanced electromagnetically induced transparency is robust against significant loss of coherence between ground states. We also show that for specific field intensities, a threshold rate of ground-state coherence decay exists at every temperature. This threshold separates the probe-transmittance behavior into two regimes: probe amplification vs probe attenuation. Thus, electromagnetically induced transparency plus amplification is possible at any temperature in a delta system.<br/><br/>Description: Open AccessAcceleration of neutrals in a nanosecond laser produced nickel plasma
http://hdl.handle.net/2289/6206
Title: Acceleration of neutrals in a nanosecond laser produced nickel plasma<br/><br/>Authors: Smijesh, N.; Chandrasekharan, K.; Philip, Reji<br/><br/>Abstract: Time of flight dynamics of slow neutrals, fast neutrals, and ions from a nanosecond laser produced nickel (Ni) plasma are investigated. Species arrival times confirm the hypothesis that fast neutrals are formed by the recombination of fast ions with free electrons. Both neutrals and ions are found to accelerate for a short interval immediately after ablation, which is attributed to internal Coulomb forces which create electrostatic potentials resulting in the charged particle acceleration. This process is further enhanced by laser-plasma energy coupling. Emission from neutrals could be measured for longer axial distances in the plume compared to that of ions confirming that the ions recombine to form neutrals as they move away from the target surface.<br/><br/>Description: Open Access