44 research outputs found
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
Phase coherence in the inelastic cotunneling regime
Two quantum dots with tunable mutual tunnel coupling have been embedded in a
two-terminal Aharonov-Bohm geometry. Aharonov-Bohm oscillations are
investigated in the cotunneling regime. Visibilities of more than 0.8 are
measured indicating that phase-coherent processes are involved in the elastic
and inelastic cotunneling. An oscillation-phase change of pi is detected as a
function of bias voltage at the inelastic cotunneling onset.Comment: 4 pages, 4 figure
Coherent probing of excited quantum dot states in an interferometer
Measurements of elastic and inelastic cotunneling currents are presented on a
two-terminal Aharonov--Bohm interferometer with a Coulomb blockaded quantum dot
embedded in each arm. Coherent current contributions, even in magnetic field,
are found in the nonlinear regime of inelastic cotunneling at finite bias
voltage. The phase of the Aharonov--Bohm oscillations in the current exhibits
phase jumps of at the onsets of inelastic processes. We suggest that
additional coherent elastic processes occur via the excited state. Our
measurement technique allows the detection of such processes on a background of
other inelastic current contributions and contains information about the
excited state occupation probability and the inelastic relaxation rates
Spatial mapping and manipulation of two tunnel-coupled quantum dots
The metallic tip of a scanning force microscope operated at 300 mK is used to
locally induce a potential in a fully controllable double quantum dot defined
via local anodic oxidation in a GaAs/AlGaAs heterostructure. Using scanning
gate techniques we record spatial images of the current through the sample for
different numbers of electrons on the quantum dots (i.e., for different quantum
states). Owing to the spatial resolution of current maps, we are able to
determine the spatial position of the individual quantum dots, and investigate
their apparent relative shifts due to the voltage applied to a single gate
Measurement Back-Action in Quantum Point-Contact Charge Sensing
Charge sensing with quantum point-contacts (QPCs) is a technique widely used in semiconductor quantum-dot research. Understanding the physics of this measurement process, as well as finding ways of suppressing unwanted measurement back-action, are therefore both desirable. In this article, we present experimental studies targeting these two goals. Firstly, we measure the effect of a QPC on electron tunneling between two InAs quantum dots, and show that a model based on the QPC’s shot-noise can account for it. Secondly, we discuss the possibility of lowering the measurement current (and thus the back-action) used for charge sensing by correlating the signals of two independent measurement channels. The performance of this method is tested in a typical experimental setup.Swiss National Science Foundatio