28,038 research outputs found
Relaxation processes in harmonic glasses?
A relaxation process, with the associated phenomenology of sound attenuation
and sound velocity dispersion, is found in a simulated harmonic Lennard-Jones
glass. We propose to identify this process with the so called microscopic (or
instantaneous) relaxation process observed in real glasses and supercooled
liquids. A model based on the memory function approach accounts for the
observation, and allows to relate to each others: 1) the characteristic time
and strength of this process, 2) the low frequency limit of the dynamic
structure factor of the glass, and 3) the high frequency sound attenuation
coefficient, with its observed quadratic dependence on the momentum transfer.Comment: 11 pages, 3 figure
Electron-Electron Relaxation Effect on Auger Recombination in Direct Band Semiconductors
Influence of electron-electron relaxation processes on Auger recombination
rate in direct band semiconductors is investigated. Comparison between
carrier-carrier and carrier-phonon relaxation processes is provided. It is
shown that relaxation processes are essential if the free path length of
carriers doesn't exceed a certain critical value, which exponentially increases
with temperature. For illustration of obtained results a typical InGaAsP
compound is used
On relaxation processes in collisionless mergers
We analyze N-body simulations of halo mergers to investigate the mechanisms
responsible for driving mixing in phase-space and the evolution to dynamical
equilibrium. We focus on mixing in energy and angular momentum and show that
mixing occurs in step-like fashion following pericenter passages of the halos.
This makes mixing during a merger unlike other well known mixing processes such
as phase mixing and chaotic mixing whose rates scale with local dynamical time.
We conclude that the mixing process that drives the system to equilibrium is
primarily a response to energy and angular momentum redistribution that occurs
due to impulsive tidal shocking and dynamical friction rather than a result of
chaotic mixing in a continuously changing potential. We also analyze the merger
remnants to determine the degree of mixing at various radii by monitoring
changes in radius, energy and angular momentum of particles. We confirm
previous findings that show that the majority of particles retain strong memory
of their original kinetic energies and angular momenta but do experience
changes in their potential energies owing to the tidal shocks they experience
during pericenter passages. Finally, we show that a significant fraction of
mass (~ 40%) in the merger remnant lies outside its formal virial radius and
that this matter is ejected roughly uniformly from all radii outside the inner
regions. This highlights the fact that mass, in its standard virial definition,
is not additive in mergers. We discuss the implications of these results for
our understanding of relaxation in collisionless dynamical systems.Comment: Version accepted for Publication in Astrophysical Journal, March 20,
2007, v685. Minor changes, latex, 14 figure
Effect of molecular relaxation processes on travelling wave solutions of sonic boom waveforms
Asymptotic and numerical analyses are presented for the travelling wave solution of the one-dimensional acoustic wave associated with the sonic boom, subject to thermoviscous dissipation and two molecular relaxation processes. Examination of how these relaxation processes affect the propagation of a weak shock is discussed in detail
Atomic relaxation processes near conducting and superconducting surfaces
The aim of this thesis is to investigate the interaction of neutral atoms with conducting and superconducting surfaces. Experimental advances in the magnetic confinement of ultracold atoms have shown that they can act as a powerful tool in a wide range of phenomena such as electric and magnetic field imaging and matter wave interferometry. Coherent manipulation of atoms and ever smaller magnetic traps are essential elements in the implementation of integrated quantum devices for fundamental research, quantum information processing and precision measurements. This thesis considers main influences on atoms placed within three different environments which are useful in achieving miniaturization and efficient control in atomic magnetic traps: carbon nanotubes, dielectric surfaces and superconducting thin films. The possibility of holding atoms near the outside of a carbon nanotubes will be addressed. In order to give a qualitative analysis of the atom-nanotube interaction, thermally induced spin-flips and the Casimir-Polder potential have been considered. The comparison between these two effects is presented in this thesis. It indicates that the Casimir-Polder force is the dominant loss mechanism and an estimation of the minimum trapping distance is given based on its effect. Secondly, a first-principles derivation of spatial atomic-sublevel decoherence near dielectric and metallic surfaces will be presented. The rate obtained for the decay of spatial coherence has dual implications, on one hand, it can be considered as a measure of the coherence length of the fluctuations of the electromagnetic field arising from a given substrate. On the other hand, it turns out to be relevant for quantum information encoding in double well potentials. Finally, the known spin-flip transition rate will be linked to the flux noise spectrum in superconducting thin films showing the feasibility of using cold atomic clouds in the investigation of vortex dynamics.Imperial Users onl
Power-law decay in first-order relaxation processes
Starting from a simple definition of stationary regime in first-order
relaxation processes, we obtain that experimental results are to be fitted to a
power-law when approaching the stationary limit. On the basis of this result we
propose a graphical representation that allows the discrimination between
power-law and stretched exponential time decays. Examples of fittings of
magnetic, dielectric and simulated relaxation data support the results.Comment: to appear in Phys. Rev. B; 4 figure
Elastic Spin Relaxation Processes in Semiconductor Quantum Dots
Electron spin decoherence caused by elastic spin-phonon processes is
investigated comprehensively in a zero-dimensional environment. Specifically, a
theoretical treatment is developed for the processes associated with the
fluctuations in the phonon potential as well as in the electron procession
frequency through the spin-orbit and hyperfine interactions in the
semiconductor quantum dots. The analysis identifies the conditions (magnetic
field, temperature, etc.) in which the elastic spin-phonon processes can
dominate over the inelastic counterparts with the electron spin-flip
transitions. Particularly, the calculation results illustrate the potential
significance of an elastic decoherence mechanism originating from the
intervalley transitions in semiconductor quantum dots with multiple equivalent
energy minima (e.g., the X valleys in SiGe). The role of lattice anharmonicity
and phonon decay in spin relaxation is also examined along with that of the
local effective field fluctuations caused by the stochastic electronic
transitions between the orbital states. Numerical estimations are provided for
typical GaAs and Si-based quantum dots.Comment: 57 pages, 14 figure
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