165 research outputs found
Spatial Pattern Formation in External Noise: Theory and Simulation
Spatial pattern formation in excitable fluctuating media was researched
analytically from the point of view of the order parameters concept. The
reaction-diffusion system in external noise is considered as a model of such
medium. Stochastic equations for the unstable mode amplitudes (order
parameters), dispersion equations for the unstable mode averaged amplitudes,
and the Fokker-Planck equation for the order parameters have been obtained. The
developed theory makes it possible to analyze different noise-induced effects,
including the variation of boundaries of ordering and disordering phase
transitions depending on the parameters of external noiseComment: 22 pages, 10 figure
Steady and Unsteady Current Modes and Thermal Runaway Conditions of High-Tc Composite Superconductors
Sub- and overcritical stable states of composite high-Tc superconductors with different E(J) dependences and their unavoidable overheating
Electron correlation effects in electron-hole recombination in organic light-emitting diodes
We develop a general theory of electron--hole recombination in organic light
emitting diodes that leads to formation of emissive singlet excitons and
nonemissive triplet excitons. We briefly review other existing theories and
show how our approach is substantively different from these theories. Using an
exact time-dependent approach to the interchain/intermolecular charge-transfer
within a long-range interacting model we find that, (i) the relative yield of
the singlet exciton in polymers is considerably larger than the 25% predicted
from statistical considerations, (ii) the singlet exciton yield increases with
chain length in oligomers, and, (iii) in small molecules containing nitrogen
heteroatoms, the relative yield of the singlet exciton is considerably smaller
and may be even close to 25%. The above results are independent of whether or
not the bond-charge repulsion, X_perp, is included in the interchain part of
the Hamiltonian for the two-chain system. The larger (smaller) yield of the
singlet (triplet) exciton in carbon-based long-chain polymers is a consequence
of both its ionic (covalent) nature and smaller (larger) binding energy. In
nitrogen containing monomers, wavefunctions are closer to the noninteracting
limit, and this decreases (increases) the relative yield of the singlet
(triplet) exciton. Our results are in qualitative agreement with
electroluminescence experiments involving both molecular and polymeric light
emitters. The time-dependent approach developed here for describing
intermolecular charge-transfer processes is completely general and may be
applied to many other such processes.Comment: 19 pages, 11 figure
About magnetic relaxation of partially penetrated screening current in superconductors with various models of flux creep
Magnetic relaxation of the partially penetrated screening current during flux creep has been studied. A theoretical analysis was based on the macroscopic description of the flux creep using the power and exponential equations for current-voltage characteristics of superconductors. The analytical flux-creep solutions were written and compared with numerical simulations of corresponding problems. Equations describing the relaxation of the electromagnetic field, magnetic moment, moving penetration boundary are derived. It is shown that peculiarities of relaxation phenomena are determined by dynamics of the electric field on the surface of the superconductor. The performed analysis allows to formulate nontrivial conjugation conditions that take place on moving boundary of screening current. In accordance with these conditions the electromagnetic field induced inside a superconductor by external perturbations smoothly approaches its undisturbed values. The essential role of the low electric field area of current-voltage characteristics in the flux relaxation and primarily in high-temperature superconductors is shown
Kinetics of the irreversible propagation of a thermal instability in the presence of a nonuniform temperature distribution over the cross section of a superconducting composite
Identifiability in the large of a linear heat-conduction equation subject to cauchy boundary conditions
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