104 research outputs found
Charge transfer and coherence dynamics of tunnelling system coupled to a harmonic oscillator
We study the transition probability and coherence of a two-site system,
interacting with an oscillator. Both properties depend on the initial
preparation. The oscillator is prepared in a thermal state and, even though it
cannot be considered as an extended bath, it produces decoherence because of
the large number of states involved in the dynamics. In the case in which the
oscillator is intially displaced a coherent dynamics of change entangled with
oscillator modes takes place. Coherency is however degraded as far as the
oscillator mass increases producing a increasingly large recoherence time.
Calculations are carried on by exact diagonalization and compared with two
semiclassical approximations. The role of the quantum effects are highlighted
in the long-time dynamics, where semiclassical approaches give rise to a
dissipative behaviour. Moreover, we find that the oscillator dynamics has to be
taken into account, even in a semiclassical approximation, in order to
reproduce a thermally activated enhancement of the transition probability
Ab initio and nuclear inelastic scattering studies of FeSi/GaAs heterostructures
The structure and dynamical properties of the FeSi/GaAs(001) interface
are investigated by density functional theory and nuclear inelastic scattering
measurements. The stability of four different atomic configurations of the
FeSi/GaAs multilayers is analyzed by calculating the formation energies and
phonon dispersion curves. The differences in charge density, magnetization, and
electronic density of states between the configurations are examined. Our
calculations unveil that magnetic moments of the Fe atoms tend to align in a
plane parallel to the interface, along the [110] direction of the FeSi
crystallographic unit cell. In some configurations, the spin polarization of
interface layers is larger than that of bulk FeSi. The effect of the
interface on element-specific and layer-resolved phonon density of states is
discussed. The Fe-partial phonon density of states measured for the FeSi
layer thickness of three monolayers is compared with theoretical results
obtained for each interface atomic configuration. The best agreement is found
for one of the configurations with a mixed Fe-Si interface layer, which
reproduces the anomalous enhancement of the phonon density of states below 10
meVComment: 14 pages, 9 figures, 4 table
Nonlinear Dynamics of Composite Fermions in Nanostructures
We outline a theory describing the quasi-classical dynamics of composite
fermions in the fractional quantum Hall regime in the potentials of arbitrary
nanostructures. By an appropriate parametrization of time we show that their
trajectories are independent of their mass and dispersion. This allows to study
the dynamics in terms of an effective Hamiltonian although the actual
dispersion is as yet unknown. The applicability of the theory is verified in
the case of antidot arrays where it explains details of magnetoresistance
measurements and thus confirms the existence of these quasiparticles.Comment: submitted to Europhys. Lett., 4 pages, postscrip
Surface morphology of low temperature grown GaAs on singular and vicinal substrates
Abstract The evolution of the surface morphology of epitaxial GaAs layers grown at low substrate temperatures (LT-GaAs) on singular and vicinal (001) GaAs substrates is studied by means of kinetic Monte-Carlo simulations. The simulation model includes the effects of Ehrlich-Schwoebel barriers at step-edges as well as anisotropic surface diffusion. We find that the surface morphology is dominated by a pattern of elongated growth mounds, which are organized into columns parallel to [1( 10]. The formation of this pattern is gradually suppressed on vicinal substrates as the misorientation angle increases. Simulated surface morphologies are compared to atomic force microscopy measurements on LT-GaAs epilayers grown on singular GaAs(001) substrates at different temperatures and good quantitative agreement is found. We propose to use vicinal substrates for LT-GaAs growth in order to overcome the known problem of epitaxial breakdown above a certain epitaxial thickness
Individual scatterers as microscopic origin of equilibration between spin- polarized edge channels in the quantum Hall regime
The equilibration length between spin-polarized edge states in the Quantum
Hall regime is measured as a function of a gate voltage applied to an electrode
on top of the edge channels. Reproducible fluctuations in the coupling are
observed and interpreted as a mesoscopic fingerprint of single spin-flip
scatterers which are turned on and off. A model to analyze macroscopic edge
state coupling in terms of individual scatterers is developed, and
characteristic values for these scatterers in our samples are extracted. For
all samples investigated, the distance between spin-flip scatterers lies
between the Drude and the quantum scattering length.Comment: 4 pages, 2 figure
Free-carrier absorption in Be-and C-doped GaAs epilayers and far infrared detector applications
Far infrared ͑FIR͒ absorption, reflection, and transmission in heavily doped p-GaAs multilayer structures have been measured for wavelengths 20-200 m and compared with the calculated results. Both Be ͑in the range 3ϫ10 18 -2.6ϫ10 19 cm Ϫ3 ͒ and C (1.8ϫ10 18 -4.7ϫ10 19 cm Ϫ3 )-doped structures were studied. It is found that the observed absorption, reflection, and transmission are explained correctly by the model with a dominant role of free-carrier absorption in highly doped regions. High reflection from heavily doped thick layers is attractive for the resonant cavity enhanced FIR detectors
A comparison of mass flow rate estimates based on point scale and integral measurements
- …