Poiseuille flow in a nanochannel – use of different thermostats

Poiseuille flow of a liquid in a nano-channel is simulated by molecular dynamics by embedding the fluid particles in a uniform force field. The channel is periodic in y and z directions and along x direction it is bounded by atomic walls. The imposition of the body force generates heat in the system leading to shear heating and a non-uniform temperature rise across the channel. In this nonequilibrium system, one can attempt to control temperature in different ways: velocity rescaling, thermostats or wall-fluid coupling. We evaluate and compare different methods critically by analyzing the fluctuations and time averaged quantities from various simulations. When particles will be inserted into the flow, it is expected that the dynamics will depend on the thermostat chosen. First observations show little influence of the thermostats on single tracer particles – this needs further study

On the Stability of Coherent States for Pais-Uhlenbeck Oscillator

We have constructed coherent states for the higher derivative Pais-Uhlenbeck Oscillator. In the process we have suggested a novel way to construct coherent states for the oscillator having only negative energy levels. These coherent states have negative energies in general but their coordinate and momentum expectation values and dispersions behave in an identical manner as that of normal (positive energy) oscillator. The coherent states for the Pais-Uhlenbeck Oscillator have constant dispersions and a modified Heisenberg Uncertainty Relation. Moreover, under reasonable assumptions on parameters these coherent states can have positive energies.Comment: Title changed, modified version with no major change in results and conclusions, to appear in Mod.Phys.Lett.

Realistic theory of electromagnetically-induced transparency and slow light in a hot vapor of atoms undergoing collisions

We present a realistic theoretical treatment of a three-level $\Lambda$ system in a hot atomic vapor interacting with a coupling and a probe field of arbitrary strengths, leading to electromagnetically-induced transparency and slow light under the two-photon resonance condition. We take into account all the relevant decoherence processes including col5Blisions. Velocity-changing collisions (VCCs) are modeled in the strong collision limit effectively, which helps in achieving optical pumping by the coupling beam across the entire Doppler profile. The steady-state expressions for the atomic density-matrix elements are numerically evaluated to yield the experimentally measured response characteristics. The predictions, taking into account a dynamic rate of influx of atoms in the two lower levels of the $\Lambda$, are in excellent agreement with the reported experimental results for $^4$He*. The role played by the VCC parameter is seen to be distinct from that by the transit time or Raman coherence decay rate

Dynamical fluctuations in biochemical reactions and cycles

We develop theory for the dynamics and fluctuations in some cyclic and linear biochemical reactions. We use the approach of maximum caliber, which computes the ensemble of paths taken by the system, given a few experimental observables. This approach may be useful for interpreting single-molecule or few-particle experiments on molecular motors, enzyme reactions, ion-channels, and phosphorylation-driven biological clocks. We consider cycles where all biochemical states are observable. Our method shows how: (1) the noise in cycles increases with cycle size and decreases with the driving force that spins the cycle and (2) provides a recipe for estimating small-number features, such as probability of backward spin in small cycles, from experimental data. The back-spin probability diminishes exponentially with the deviation from equilibrium. We believe this method may also be useful for other few-particle nonequilibrium biochemical reaction systems

Observation of the Faraday effect via beam deflection in a longitudinal magnetic field

We report the observation of the magnetic field induced circular differential deflection of light at the interface of a Faraday medium. The difference in the angles of refraction or reflection between the two circular polarization components is a function of the magnetic field strength and the Verdet constant. The reported phenomena permit the observation of the Faraday effect not via polarization rotation in transmission, but via changes in the propagation direction in refraction or in reflection. An unpolarized light beam is predicted to split into its two circular polarization components. The light deflection arises within a few wavelengths at the interface and is therefore independent of pathlength