Assessment of the ocean circulation in the Azores region as predicted by a numerical model assimilating altimeter data from Topex/Poseidon and ERS-1 satellites

International audienceTwo years of altimetric data from Topex/Poseidon (October 1992-September 1994) and ERS-1 (October 1992-December 1993) were assimilated into a numerical model of the North Atlantic. The results of these simulations are analysed in the Azores region to assess the performance of our model in this particular region. Maps of instantaneous dynamic topography and transports show that the model performs well in reproducing the velocities and transports of the Azores Front. Drifter data from the Semaphore experiment are also used to study the correlation between the drifter velocities and the corresponding model velocities. Some interesting oceanographic results are also obtained by examining the seasonal and interannual variability of the circulation and the influence of bathymetry on the variability of the Azores Front. Thus, on the basis of our two year experiment, it is possible to confirm the circulation patterns proposed by previous studies regarding the seasonal variations in the origin of the Azores Current. Moreover, it is shown that the Azores Current is quite narrow in the first year of assimilation (1992-1993), but becomes much wider in the second year (1993-1994). The role of the bathymetry appears important in this area since the mesoscale activity is shown to be strongly related to the presence of topographic slopes. Finally, spectral analyses of sea-level changes over time and space are used to identify two types of wave already noticed in other studies: a wave with (300 km)-1 wave number and (120 days)-1 frequency, which is characteristic of mesoscale undulation, and a wave with (600 km)-1 wave number and (250 days)-1 frequency which probably corresponds to a Rossby wave generated in the east of the basin

Energy Relaxation in the Integer Quantum Hall Regime

We investigate the energy exchanges along an electronic quantum channel realized in the integer quantum Hall regime at filling factor $\nu_L=2$. One of the two edge channels is driven out-of-equilibrium and the resulting electronic energy distribution is measured in the outer channel, after several propagation lengths $0.8~\mu$m$\leq L\leq30~\mu$m. Whereas there are no discernable energy transfers toward thermalized states, we find efficient energy redistribution between the two channels without particle exchanges. At long distances $L\geq10~\mu$m, the measured energy distribution is a hot Fermi function whose temperature is lower than expected for two interacting channels, which suggests the contribution of extra degrees of freedom. The observed short energy relaxation length challenges the usual description of quantum Hall excitations as quasiparticles localized in one edge channel.Comment: To be published in PRL, 10 pages including supplementary materia

Scaling of the low temperature dephasing rate in Kondo systems

We present phase coherence time measurements in quasi-one-dimensional Ag wires doped with Fe Kondo impurities of different concentrations $n_s$. Due to the relatively high Kondo temperature $T_{K}\approx 4.3K$ of this system, we are able to explore a temperature range from above $T_{K}$ down to below $0.01 T_{K}$. We show that the magnetic contribution to the dephasing rate $\gamma_m$ per impurity is described by a single, universal curve when plotted as a function of $(T/T_K)$. For $T>0.1 T_K$, the dephasing rate is remarkably well described by recent numerical results for spin $S=1/2$ impurities. At lower temperature, we observe deviations from this theory. Based on a comparison with theoretical calculations for $S>1/2$, we discuss possible explanations for the observed deviations.Comment: 4 pages, 3 figures, submitted to Phys. Rev. Let

Detection of Coulomb Charging around an Antidot

We have detected oscillations of the charge around a potential hill (antidot) in a two-dimensional electron gas as a function of a perpendicular magnetic field B. The field confines electrons around the antidot in closed orbits, the areas of which are quantised through the Aharonov-Bohm effect. Increasing B reduces each state's area, pushing electrons closer to the centre, until enough charge builds up for an electron to tunnel out. This is a new form of the Coulomb blockade seen in electrostatically confined dots. We have also studied h/2e oscillations and found evidence for coupling of opposite spin states of the lowest Landau level.Comment: 3 pages, 3 Postscript figures, submitted to the proceedings of EP2DS-1

Transport through side-coupled double quantum dots: from weak to strong interdot coupling

We report low-temperature transport measurements through a double quantum dot device in a configuration where one of the quantum dots is coupled directly to the source and drain electrodes, and a second (side-coupled) quantum dot interacts electrostatically and via tunneling to the first one. As the interdot coupling increases, a crossover from weak to strong interdot tunneling is observed in the charge stability diagrams that present a complex pattern with mergings and apparent crossings of Coulomb blockade peaks. While the weak coupling regime can be understood by considering a single level on each dot, in the intermediate and strong coupling regimes, the multi-level nature of the quantum dots needs to be taken into account. Surprisingly, both in the strong and weak coupling regimes, the double quantum dot states are mainly localized on each dot for most values of the parameters. Only in an intermediate coupling regime the device presents a single dot-like molecular behavior as the molecular wavefunctions weight is evenly distributed between the quantum dots. At temperatures larger than the interdot coupling energy scale, a loss of coherence of the molecular states is observed.Comment: 9 pages, 5 figure

Order-disorder transition in nanoscopic semiconductor quantum rings

Using the path integral Monte Carlo technique we show that semiconductor quantum rings with up to six electrons exhibit a temperature, ring diameter, and particle number dependent transition between spin ordered and disordered Wigner crystals. Due to the small number of particles the transition extends over a broad temperature range and is clearly identifiable from the electron pair correlation functions.Comment: 4 pages, 5 figures, For recent information on physics of small systems see http://www.smallsystems.d

Channel Interference in a Quasi Ballistic Aharonov-Bohm Experiment

New experiments are presented on the transmission of electron waves through a 2DEG (2 dimensional electron gas) ring with a gate on top of one of the branches. Magnetoconductance oscillations are observed, and the phase of the Aharanov-Bohm signal alternates between 0 and pi as the gate voltage is scanned. A Fourier transform of the data reveals a dominant period in the voltage which corresponds to the energy spacing between successive transverse modes.A theoretical model including random phase shifts between successive modes reproduces the essential features of the experiment.Comment: 4 pages, 6 Postscript figures, TEX, submitted to Physical Review Letter

Effect of Disorder on the Quantum Coherence in Mesoscopic Wires

We present phase coherence time measurements in quasi-one-dimensional mesoscopic wires made from high mobility two-dimensional electron gas. By implanting gallium ions into a GaAs/AlGaAs heterojunction we are able to vary the diffusion coefficient over 2 orders of magnitude. We show that in the diffusive limit, the decoherence time follows a power law as a function of diffusion coefficient as expected by theory. When the disorder is low enough so that the samples are semi-ballistic, we observe a new and unexpected regime in which the phase coherence time is independent of disorder. In addition, for all samples the temperature dependence of the phase coherence time follows a power law down to the lowest temperatures without any sign of saturation and strongly suggests that the frequently observed low temperature saturation is not intrinsic.Comment: 4 pages, 4 figure

Generation of energy selective excitations in quantum Hall edge states

We operate an on-demand source of single electrons in high perpendicular magnetic fields up to 30T, corresponding to a filling factor below 1/3. The device extracts and emits single charges at a tunable energy from and to a two-dimensional electron gas, brought into well defined integer and fractional quantum Hall (QH) states. It can therefore be used for sensitive electrical transport studies, e.g. of excitations and relaxation processes in QH edge states