36,351 research outputs found
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
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 , are in excellent
agreement with the reported experimental results for 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
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