6,872 research outputs found
Theory for the nonequilibrium dynamics of flexible chain molecules: relaxation to equilibrium of pentadecane from an all-trans conformation
We extend to nonequilibrium processes our recent theory for the long time
dynamics of flexible chain molecules. While the previous theory describes the
equilibrium motions for any bond or interatomic separation in (bio)polymers by
time correlation functions, the present extension of the theory enables the
prediction of the nonequilibrium relaxation that occurs in processes, such as
T-jump experiments, where there are sudden transitions between, for example,
different equilibrium states. As a test of the theory, we consider the
``unfolding'' of pentadecane when it is transported from a constrained
all-trans conformation to a random-coil state at thermal equilibrium. The time
evolution of the mean-square end-to-end distance after release of the
constraint is computed both from the theory and from Brownian dynamics (BD)
simulations. The predictions of the theory agree very well with the BD
simulations. Furthermore, the theory produces enormous savings in computer
time. This work is a starting point for the application of the new method to
nonequilibrium processes with biological importance such as the helix-coil
transition and protein folding.Comment: 11 pages total, including 2 Postscript figures; submitted to Journal
of Chemical Physic
Generation of electron spin polarization in disordered organic semiconductors
The generation mechanisms of electron spin polarization (ESP) of charge
carriers (electrons and holes, called "doublets") in doublet-doublet
recombination and triplet-doublet quenching in disordered organic
semiconductors are analyzed in detail. The ESP is assumed to result from
quantum transitions between the states of the spin Hamiltonian of the pair of
interacting particles. The value of the ESP is essentially determined by the
mechanism of relative motion of particles. In our work we have considered the
cage and free diffusion models. The effect of possible attractive
spin-independent interactions between particles is also analyzed. Estimation
with obtained formulas shows that the proposed mechanisms can lead to a fairly
strong ESP much larger than the thermal one (at room temperatures)Comment: 10 pages, 3 figure
Phase and Scaling Properties of Determinants Arising in Topological Field Theories
In topological field theories determinants of maps with negative as well as
positive eigenvalues arise. We give a generalisation of the zeta-regularisation
technique to derive expressions for the phase and scaling-dependence of these
determinants. For theories on odd-dimensional manifolds a simple formula for
the scaling dependence is obtained in terms of the dimensions of certain
cohomology spaces. This enables a non-perturbative feature of Chern-Simons
gauge theory to be reproduced by path-integral methods.Comment: 12 pages, Latex. To appear in Physics Letters
Thermoviscoplastic analysis of fibrous periodic composites using triangular subvolumes
The nonlinear viscoplastic behavior of fibrous periodic composites is analyzed by discretizing the unit cell into triangular subvolumes. A set of these subvolumes can be configured by the analyst to construct a representation for the unit cell of a periodic composite. In each step of the loading history, the total strain increment at any point is governed by an integral equation which applies to the entire composite. A Fourier series approximation allows the incremental stresses and strains to be determined within a unit cell of the periodic lattice. The nonlinearity arising from the viscoplastic behavior of the constituent materials comprising the composite is treated as fictitious body force in the governing integral equation. Specific numerical examples showing the stress distributions in the unit cell of a fibrous tungsten/copper metal matrix composite under viscoplastic loading conditions are given. The stress distribution resulting in the unit cell when the composite material is subjected to an overall transverse stress loading history perpendicular to the fibers is found to be highly heterogeneous, and typical homogenization techniques based on treating the stress and strain distributions within the constituent phases as homogeneous result in large errors under inelastic loading conditions
Dynamics of Diblock Copolymers in Dilute Solutions
We consider the dynamics of freely translating and rotating diblock (A-B),
Gaussian copolymers, in dilute solutions. Using the multiple scattering
technique, we have computed the diffusion and the friction coefficients D_AB
and Zeta_AB, and the change Eta_AB in the viscosity of the solution as
functions of x = N_A/N and t = l_B/l_A, where N_A, N are the number of segments
of the A block and of the whole copolymer, respectively, and l_A, l_B are the
Kuhn lengths of the A and B blocks. Specific regimes that maximize the
efficiency of separation of copolymers with distinct "t" values, have been
identified.Comment: 20 pages Revtex, 7 eps figures, needs epsf.tex and amssymb.sty,
submitted to Macromolecule
Magnetic field effects on electron-hole recombination in disordered organic semiconductors
Characteristic properties of magnetic field effects on spin selective
geminate and bulk electron-hole polaron pair (PP) recombination are analyzed in
detail within the approach based on the stochastic Liouville equation. Simple
expressions for the magnetic field (B) dependence of recombination yield and
rate are derived within two models of relative PP motion: free diffusion and
diffusion in the presence of well (cage). The spin evolution of PPs is
described taking in account the relaxation induced by hyperfine interaction,
anisotropic part of the Zeeman interaction induced, as well as -mechanism. A large variety of the -dependences of the recombination yield
and rate is obtained depending on the relative weights of
above-mentioned mechanisms. The proposed general method and derived particular
formulas are shown to be quite useful for the analysis of recent experimental
results.Comment: 12 pages, 3 figure
Critical behavior of long straight rigid rods on two-dimensional lattices: Theory and Monte Carlo simulations
The critical behavior of long straight rigid rods of length (-mers) on
square and triangular lattices at intermediate density has been studied. A
nematic phase, characterized by a big domain of parallel -mers, was found.
This ordered phase is separated from the isotropic state by a continuous
transition occurring at a intermediate density . Two analytical
techniques were combined with Monte Carlo simulations to predict the dependence
of on , being . The first involves
simple geometrical arguments, while the second is based on entropy
considerations. Our analysis allowed us also to determine the minimum value of
(), which allows the formation of a nematic phase on a
triangular lattice.Comment: 23 pages, 5 figures, to appear in The Journal of Chemical Physic
Gravitational quantum states of neutrons in a rough waveguide
A theory of gravitational quantum states of ultracold neutrons in waveguides
with absorbing/scattering walls is presented. The theory covers recent
experiments in which the ultracold neutrons were beamed between a mirror and a
rough scatterer/absorber. The analysis is based on a recently developed theory
of quantum transport along random rough walls which is modified in order to
include leaky (absorbing) interfaces and, more importantly, the low-amplitude
high-aperture roughness. The calculations are focused on a regime when the
direct transitions into the continuous spectrum above the absorption threshold
dominate the depletion of neutrons from the gravitational states and are more
efficient than the processes involving the intermediate states. The theoretical
results for the neutron count are sensitive to the correlation radius (lateral
size) of surface inhomogeneities and to the ratio of the particle energy to the
absorption threshold in a weak roughness limit. The main impediment for
observation of the higher gravitational states is the "overhang" of the
particle wave functions which can be overcome only by use scatterers with
strong roughness. In general, the strong roughness with high amplitude is
preferable if one wants just to detect the individual gravitational states,
while the strong roughness experiments with small amplitude and high aperture
are preferable for the quantitative analysis of the data. We also discuss the
ways to further improve the accuracy of calculations and to optimize the
experimental regime.Comment: 48 pages, 14 figure
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