192 research outputs found
Electronic transport through a quantum dot network
The conductance through a finite quantum dot network is studied as a function
of inter-dot coupling. As the coupling is reduced, the system undergoes a
transition from the antidot regime to the tight binding limit, where Coulomb
resonances with on average increasing charging energies are observed.
Percolation models are used to describe the conduction in the open and closed
regime and contributions from different blockaded regions can be identified. A
strong negative average magnetoresistance in the Coulomb blockade regime is in
good quantitative agreement with theoretical predictions for magnetotunneling
between individual quantum dots.Comment: 5 pages, 5 figure
Calculation and spectroscopy of the Landau band structure at a thin and atomically precise tunneling barrier
Two laterally adjacent quantum Hall systems separated by an extended barrier
of a thickness on the order of the magnetic length possess a complex Landau
band structure in the vicinity of the line junction. The energy dispersion is
obtained from an exact quantum-mechanical calculation of the single electron
eigenstates for the coupled system by representing the wave functions as a
superposition of parabolic cylinder functions. For orbit centers approaching
the barrier, the separation of two subsequent Landau levels is reduced from the
cyclotron energy to gaps which are much smaller. The position of the
anticrossings increases on the scale of the cyclotron energy as the magnetic
field is raised. In order to experimentally investigate a particular gap at
different field strengths but under constant filling factor, a GaAs/AlGaAs
heterostructure with a 52 Angstrom thick tunneling barrier and a gate electrode
for inducing the two-dimensional electron systems was fabricated by the cleaved
edge overgrowth method. The shift of the gaps is observed as a displacement of
the conductance peaks on the scale of the filling factor. Besides this effect,
which is explained within the picture of Landau level mixing for an ideal
barrier, we report on signatures of quantum interferences at imperfections of
the barrier which act as tunneling centers. The main features of the recent
experiment of Yang, Kang et al. are reproduced and discussed for different gate
voltages. Quasiperiodic oscillations, similar to the Aharonov Bohm effect at
the quenched peak, are revealed for low magnetic fields before the onset of the
regular conductance peaks.Comment: 8 pages, 10 figures, 1 tabl
Shock Waves in Nanomechanical Resonators
The dream of every surfer is an extremely steep wave propagating at the
highest speed possible. The best waves for this would be shock waves, but are
very hard to surf. In the nanoscopic world the same is true: the surfers in
this case are electrons riding through nanomechanical devices on acoustic waves
[1]. Naturally, this has a broad range of applications in sensor technology and
for communication electronics for which the combination of an electronic and a
mechanical degree of freedom is essential. But this is also of interest for
fundamental aspects of nano-electromechanical systems (NEMS), when it comes to
quantum limited displacement detection [2] and the control of phonon number
states [3]. Here, we study the formation of shock waves in a NEMS resonator
with an embedded two-dimensional electron gas using surface acoustic waves. The
mechanical displacement of the nano-resonator is read out via the induced
acoustoelectric current. Applying acoustical standing waves we are able to
determine the anomalous acoustocurrent. This current is only found in the
regime of shock wave formation. We ontain very good agreement with model
calculations.Comment: 14 Pages including 4 figure
Charged Excitons in the Quantum Hall Regime
We review our recent optical experiments on two-dimensional electron systems at temperatures below 1 K and under high magnetic fields. The two-dimensional electron systems are realized in modulation-doped GaAs-AlGaAs single quantum wells. Via gate electrodes the carrier density of the two-dimensional electron systems can be tuned in a quite broad range between about 1×10^{10} cm^{-2} and 2×10^{11} cm^{-2}. In dilute two-dimensional electron systems, at very low electron densities, we observe the formation of negatively charged excitons in photoluminescence experiments. In this contribution we report about the observation of a dark triplet exciton, which is observable at temperatures below 1 K and for electron filling factors <1/3, i.e., in the fractional quantum Hall regime only. In experiments where we have increased the density of the two-dimensional electron systems so that a uniform two-dimensional electron system starts to form, we have found a strong energy anomaly of the charged excitons in the vicinity of filling factor 1/3. This anomaly was found to exist in a very narrow parameter range of the density and temperature, only. We propose a model where we assume that localized charged excitons and a uniform Laughlin liquid coexist. The localized charged exciton in close proximity to the Laughlin liquid leads to the creation of a fractionally-charged quasihole in the liquid, which can account for the experimentally observed anomaly
Local scale-invariance in ageing phenomena
Many materials quenched into their ordered phase undergo ageing and there
show dynamical scaling. For any given dynamical exponent z, this can be
extended to a new form of local scale-invariance which acts as a dynamical
symmetry. The scaling functions of the two-time correlation and response
functions of ferromagnets with a non-conserved order parameter are determined.
These results are in agreement with analytical and numerical studies of various
models, especially the kinetic Glauber-Ising model in 2 and 3 dimensions.Comment: Invited talk; spring meeting of the german physical society,
Regensburg the 8th of March 2004, 12 pages, style file
Dynamic Rabi Oscillations in a Quantum Dot Embedded in a Nanobridge in the Presence of Surface Acoustic Waves
A quantum dot is created within a suspended nanobridge containing a two-dimensional electron gas. The electron current through this dot exhibits well-pronounced Coulomb blockade oscillations. When surface acoustic waves (SAW) are driven through the nanobridge, Coulomb blockade peaks are shifted. To explain this feature, we derive the expressions for the quantum dot level populations and electron currents through these levels and show that SAW-induced Rabi oscillations lead to the observed phenomenology
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