Search results for "Diffusion"
showing 10 items of 1615 documents
Self-consistent calculation of the flux-flow conductivity in diffusive superconductors
2017
In the framework of Keldysh-Usadel kinetic theory, we study the temperature dependence of flux-flow conductivity (FFC) in diffusive superconductors. By using self-consistent vortex solutions we find the exact values of dimensionless parameters that determine the diffusion-controlled FFC both in the limit of the low temperatures and close to the critical one. Taking into account the electron-phonon scattering we study the transition between flux-flow regimes controlled either by the diffusion or the inelastic relaxation of non-equilibrium quasiparticles. We demonstrate that the inelastic electron-phonon relaxation leads to the strong suppression of FFC as compared to the previous estimates m…
Memory expansion for diffusion coefficients
1998
We present a memory expansion for macroscopic transport coefficients such as the collective and tracer diffusion coefficients ${D}_{C}$ and ${D}_{T},$ respectively. The successive terms in this expansion for ${D}_{C}$ describe rapidly decaying memory effects of the center-of-mass motion, leading to fast convergence when evaluated numerically. For ${D}_{T},$ one obtains an expansion of similar form that contains terms describing memory effects in single-particle motion. As an example we evaluate ${D}_{C}$ and ${D}_{T}$ for three strongly interacting surface systems through Monte Carlo simulations, and for a simple model diffusion system via molecular dynamics calculations. We show that the n…
A dynamical mean field theory for the study of surface diffusion constants
1997
We present a combined analytical and numerical approach based on the Mori projection operator formalism and Monte Carlo simulations to study surface diffusion within the lattice-gas model. In the present theory, the average jump rate and the susceptibility factor appearing are evaluated through Monte Carlo simulations, while the memory functions are approximated by the known results for a Langmuir gas model. This leads to a dynamical mean field theory (DMF) for collective diffusion, while approximate correlation effects beyond DMF are included for tracer diffusion. We apply our formalism to three very different strongly interacting systems and compare the results of the new approach with th…
Dynamical mean field theory: an efficient method to study surface diffusion coefficients
1998
Abstract We test the accuracy of the dynamical mean field theory (DMF) developed recently for the collective and tracer diffusion coefficients D C and D T , respectively, by Monte Carlo simulations of two very strongly interacting model systems. The deviation of the DMF results from the true hydrodynamic diffusion coefficients is a measure of memory effects, which are not fully accounted for in DMF. In the cases studied here, DMF predicts the behavior of both D C and D T accurately, while the memory effects are found to be most pronounced at low temperatures, and at high coverages and stronger interactions. Nevertheless, the computational cost of DMF is just a fraction of what is needed for…
Kinetic model for surface reconstruction
2002
Institut fu ¨r Physikalische und Theoretische Chemie, Technische Universitat Braunschweig, Hans-Sommer-Strase 10,38106 Braunschweig, Germany~Received 7 December 2001; published 25 July 2002!A microscopic kinetic model for the ab @e.g., hex131 for Pt~100! and 132131 for Pt~110!#surface reconstruction is investigated by means of the mean field approximation and Monte Carlo simulations.It considers homogeneous phase nucleation that induces small surface phase defects. These defects can grow ordecline via phase border propagation in dependence on the chemical coverage by an adsorbate A ~CO!.Anasymmetry in the adsorbate surface diffusion from one surface phase to the other gives rise to two criti…
Turbulent mixing and dispersion mechanisms over flexible and dense vegetation
2019
The present study investigates flow turbulence and dispersion processes in the presence of flexible and dense vegetation on the bed. The turbulent dispersion coefficients and the terms of the turbulent kinetic energy equation are determined by using data collected in a straight laboratory channel with living vegetation on the bed. Results show that the turbulent integral lengths assume an order of magnitude comparable to the stems’ characteristic dimension independently by the direction and the turbulence assumes an isotropic behavior. The coefficients of dispersion have a trend similar to that of the turbulent lengths and assume low values in the longitudinal, transversal and vertical dire…
Contour detection based on nonlinear discrete diffusion in a cellular nonlinear network
2001
International audience; A contour detection based on a diffusive cellular nonlinear network is proposed. It is shown that there exists a particular nonlinear function for which, numerically, the obtained contour is satisfactory. Furthermore, this nonlinear function can be achieved using analog components.
STABILITY AND SPECTRA OF SMALL 3He-4He CLUSTERS
2005
Diffusion Monte Carlo calculations have been systematically performed to analyze the stability of small mixed 3 He -4 He clusters, as well as their excitation spectra. The picture that emerges is that of systems with strong shell effects whose binding and excitation energies are essentially determined by the monopole properties of an effective Hamiltonian.
Ortho-H2 and the age of prestellar cores
2013
Prestellar cores form from the contraction of cold gas and dust material in dark clouds before they collapse to form protostars. Several concurrent theories exist to describe this contraction but they are currently difficult to distinguish. One major difference is the timescale involved in forming the prestellar cores: some theories advocate nearly free-fall speed via, e.g., rapid turbulence decay, while others can accommodate much longer periods to let the gas accumulate via, e.g., ambipolar diffusion. To tell the difference between these theories, measuring the age of prestellar cores could greatly help. However, no reliable clock currently exists. We present a simple chemical clock based…