323 research outputs found
Monte-Carlo simulation of the coherent backscattering of electrons in a ballistic system
We study weak localization effects in the ballistic regime as induced by
man-made scatterers. Specular reflection of the electrons off these scatterers
results into backscattered trajectories which interfere with their
time-reversed path resulting in weak localization corrections to the
resistance. Using a semi-classical theory, we calculate the change in
resistance due to these backscattered trajectories. We found that the inclusion
of the exact shape of the scatterers is very important in order to explain the
experimental results of Katine et al. [Superlattices and Microstructures 20,
337 (1996)].Comment: 3 pages, 7 ps figures, accepted for publication in Superlattices and
Microstructure
Technological learning for innovating towards sustainable cultivation practices: the Vietnamese smallholder rose sector
Deregulation and globalisation has altered the views of public involvement in development and led to strategies focusing on private sector participation. An implicit assumption seems to be that these linkages will enhance the technological capacity of smallholder producers by way of more cost-efficient technologies trickling down through the value chain or by quality requirements inducing best practices. The argument put forward in this paper is that sustainable non traditional agricultural chain development requires more purposeful actions and institutional transitions, both in the public and private spheres, targeting improved upstream innovative capacities. Empirical findings from a Dutch-Vietnamese partnership on sustainable floriculture development are used. Research revealed that the pest and disease control solutions applied by smallholder rose growers were incremental adaptations of experiences obtained in former food crop cultivation practices. Floriculture however may require more drastic changes in cultivation practices to make the sector more environmentally benign. In the case of smallholder Vietnamese flower producers, this implies adaptation of knowledge and skills currently not present. An important hindrance in promoting this knowledge and skills appears to be the weak vertical linkages between flower growers and public and private research and development organizations
Dynamics of the superconducting condensate in the presence of a magnetic field. Channelling of vortices in superconducting strips at high currents
On the basis of the time-dependent Ginzburg-Landau equation we studied the
dynamics of the superconducting condensate in a wide two-dimensional sample in
the presence of a perpendicular magnetic field and applied current. We could
identify two critical currents: the current at which the pure superconducting
state becomes unstable ( \cite{self1}) and the current at which the
system transits from the resistive state to the superconducting state
(). The current decreases monotonically with external
magnetic field, while exhibits a maximum at . For sufficient
large magnetic fields the hysteresis disappears and . In
this high magnetic field region and for currents close to the voltage
appears as a result of the motion of separate vortices. With increasing current
the moving vortices form 'channels' with suppressed order parameter along which
the vortices can move very fast. This leads to a sharp increase of the voltage.
These 'channels' resemble in some respect the phase slip lines which occur at
zero magnetic field.Comment: 5 pages, 4 figures, Proceedings of Third European Conference on
Vortex Matter in Superconductor
Landau levels and magnetopolaron effect in dilute GaAs:N
The magnetic-field dependence of the energy spectrum of GaAs doped with
nitrogen impurities is investigated. Our theoretical model is based on the
phenomenological Band Anticrossing Model (BAC) which we extended in order to
include magnetic field and electron - phonon interaction. Due to the highly
localized nature of the nitrogen state, we find that the energy levels are very
different from those of pure GaAs. The polaron correction results in a lower
cyclotron resonance energy as compared to pure GaAs. The magneto-absorption
spectrum exhibits series of asymmetric peaks close to the cyclotron energy
Magnetic coupling between mesoscopic superconducting rings
Using the nonlinear Ginzburg-Landau theory we investigated the dependence of
the magnetic coupling between two concentric mesoscopic superconducting rings
on their thickness. The size of this magnetic coupling increases with the
thickness of the rings.Comment: 3 pages text, 3 ps figures, to be published in Physica C (Proceedings
of the 2nd European Conference in School Format "Vortex Matter in
Superconductors"
Exact broken-symmetry states and Hartree-Fock solutions for quantum dots at high magnetic fields
Wigner molecules formed at high magnetic fields in circular and elliptic
quantum dots are studied by exact diagonalization (ED) and unrestricted
Hartree-Fock (UHF) methods with multicenter basis of displaced lowest Landau
level wave functions. The broken symmetry states with semi-classical charge
density constructed from superpositions of the ED solutions are compared to the
UHF results. UHF overlooks the dependence of the few-electron wave function on
the actual relative positions of electrons localized in different charge
puddles and partially compensates for this neglect by an exaggerated separation
of charge islands which are more strongly localized than in the exact
broken-symmetry states.Comment: QD2004 proceedings under press in Physica
Wigner crystallization in the two electron quantum dot
Wigner crystallization can be induced in a quantum dot by increasing the
effective electron-electron interaction through a decrease of the electron
density or by the application of a strong magnetic field. We show that the
ground state in both cases is very similar but the energy scales are very
different and therefore also the dynamics.Comment: 4 pages, 4 figure
Mesoscopic superconducting disks
Using the non-linear Ginzburg-Landau (GL) eqs. type I superconducting disks
of finite radius () and thickness () are studied in a perpendicular
magnetic field. Depending on and , first or second order phase
transitions are found for the normal to superconducting state. For sufficiently
large several transitions in the superconducting phase are found
corresponding to different angular momentum giant vortex states. In increasing
magnetic field the superconductor is in its ground state, while in field down
sweep it is possible to drive the system into metastable states. We also
present a quantitative analysis of the relation between the detector output and
the sample magnetization. The latter, and the incorporation of the finite
thickness of the disks, are essential in order to obtain quantitative agreement
with experiment.Comment: A brief review with new result
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