Incoherent Trapping Research Articles

Coherent and incoherent trapping of a diffusion-assisted system in the presence of an external periodic field

Field induced trapping on a line segment of a diffusion-driven system is studied with an aim to gain an insight into the occurrence of coherent stochastic resonance, which is thoroughly explored as synchronized mean-free passages to the traps. Synchronization (coherence) between the external bias, the noise in the system and the temporal trapping events is found to attain an optimum value by increasing the forcing frequency towards the relevant resonant frequency, revealing a minimum in the nonmonotonic mean-free-passage time (MFPT) to trapping. The MFPT at a given forcing frequency, is also nonmonotonic when considered as a function of the diffusion coefficient of the medium, and reveals a maximum exhibiting the least synchronization effect (incoherent trapping).

The effects of impurity scattering and transport topology on trapping in quasi-one-dimensional systems: Application to excitons in molecular crystals

A model is developed which permits a detailed examination of the effects of impurity scattering and deviations from a strictly one-dimensional transport topology on the trapping rate and on the mean-square displacement of mobile species in systems which are nearly one dimensional in their transport characteristics, i.e., quasi-one-dimensional. The model is applied specifically to Frenkel excitons in molecular crystals, but may be readily adapted to other types of systems. Both coherent (wavelike) and incoherent (diffusive) microscopic modes of exciton transport are considered. In the strictly one-dimensional limit following pulsed optical excitation, a time-dependent trapping rate function is obtained as opposed to the commonly employed trapping rate constant. It is demonstrated that the coherent and incoherent trapping rate functions have identical dependencies on time and on impurity concentration and the macroscopic rate of transport can be calculated. To treat deviations from strictly one-dimensional transport topology, it is necessary to consider anisotropic walks on the system’s ’’superlattice,’’ i.e., the array of molecular chain segments which are bounded by scattering impurities. Montroll’s Green function formalism is employed to obtain solutions to various walk topologies which have not previously been reported in the literature. In the quasi-one-dimensional case, a time-dependent trapping rate function is also obtained unless the walk on the superlattice is nearly isotropic. The various trapping rate functions are then employed to provide explicit expressions for the time-dependent trap population which is proportional to the intensity of time resolved optical emission, a widely used experimental observable. Finally, the exciton mean-square displacement for a quasi-one-dimensional system with scattering impurities is shown to remain basically one dimensional even in situations where exciton trapping behaves in a manner best described in terms of a three-dimensional topology.

An Experimental Investigation of Strong Turbulence in a Sheared Magnetoplasma Column

An experimental investigation of low frequency (ω < Ω1) strong turbulence in a transversely sheared magnetoplasma column is reported. A single large amplitude mode and a broad band spectrum are observed in the radial region of sheared azimuthal velocity. The large amplitude mode, previously identified as Kelvin–Helmholtz turbulence, is studied with respect to coherence, amplitude saturation, and temporal evolution. General agreement is found with Dupree's strong turbulence theory. However, both coherent and incoherent trapping appear to be important for amplitude saturation and coherence. Spatial spreading of the mode is also observed during temporal evolution indicating a more complex wave particle interaction than is currently assumed in nonlinear theory. Using correlation techniques, the broad band spectrum is found to consist of azimuthally propagating clumps of 'ballistic modes' which produce an [Formula: see text] spectrum. The clumps have a scale size of the order of a Larmor radius, propagate with the average rotation velocity, appear to be amplitude limited by trapping, and diffuse spatially as they decay. This observation of clumps represents the first experimental identification of 'ballistic modes' in a magnetoplasma column.