We analyze collisional decoherence of atoms or molecules prepared in a coherent superposition of nondegenerate internal states at ultralow temperatures and placed in an ultracold buffer gas. Our analysis is applicable for an arbitrary bath-particle to tracer-particle mass ratio. Both elastic and inelastic collisions contribute to decoherence. We obtain an expression relating the observable decoherence rate to pairwise scattering properties, specifically the scattering lengths and low-temperature scattering amplitudes. We consider the dependence on the bath-particle to tracer-particle mass ratio for the case of light bath and heavy tracer particles. The expressions obtained may be useful in low-temperature applications where accurate estimates of decoherence rates are needed. The results suggest a method for determining the scattering lengths of atoms and molecules in different internal states by measuring decoherence-induced damping of coherent oscillations.
Bridge phases associated with a phase transition between two liquid phases occur when a two-component liquid mixture is confined between chemically patterned walls. In the bulk the liquid mixture with components A, B undergoes phase separation into an A-rich phase and a B-rich phase. The walls bear stripes attractive to A. In the bridge phase A-rich and B-rich regions alternate. Grand canonical Monte Carlo studies are performed with the alignment between stripes on opposite walls varied. Misalignment of the stripes places the nanoscopic liquid bridges under shear strain. The bridges exert a Hookean restoring force on the walls for small displacements from equilibrium. As the strain increases there are deviations from Hooke's law. Eventually there is an abrupt yielding of the bridges. Molecular dynamics simulations show the bridges form or disintegrate on time scales which are fast compared to wall motion and transport of molecules into or from the confined space. Some interesting possible applications of the phenomena are discussed.
The dynamics of spatiotemporal patterns in oscillatory reaction-diffusion systems subject to periodic forcing with a spatially random forcing amplitude field are investigated. Quenched disorder is studied using the resonantly forced complex Ginzburg-Landau equation in the 3:1 resonance regime. Front roughening and spontaneous nucleation of target patterns are observed and characterized. Time dependent spatially varying forcing fields are studied in the 3:1 forced FitzHugh-Nagumo system. The periodic variation of the spatially random forcing amplitude breaks the symmetry among the three quasi-homogeneous states of the system, making the three types of fronts separating phases inequivalent. The resulting inequality in the front velocities leads to the formation of "compound fronts" with velocities lying between those of the individual component fronts, and "pulses" which are analogous structures arising from the combination of three fronts. Spiral wave dynamics is studied in systems with compound fronts. (c) 2000 American Institute of Physics.
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