200 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
Few electron double quantum dot in an isotopically purified Si quantum well
We present a few electron double quantum dot (QD) device defined in an
isotopically purified Si quantum well (QW). An electron mobility of is observed in the QW which is the highest mobility
ever reported for a 2D electron system in Si. The residual concentration
of Si nuclei in the Si QW is lower than , at the
verge where the hyperfine interaction is theoretically no longer expected to
dominantly limit the spin dephasing time. We also demonstrate a
complete suppression of hysteretic gate behavior and charge noise using a
negatively biased global top gate.Comment: 4 pages, 3 figure
Electrical control of inter-dot electron tunneling in a quantum dot molecule
We employ ultrafast pump-probe spectroscopy to directly monitor electron
tunneling between discrete orbital states in a pair of spatially separated
quantum dots. Immediately after excitation, several peaks are observed in the
pump-probe spectrum due to Coulomb interactions between the photo-generated
charge carriers. By tuning the relative energy of the orbital states in the two
dots and monitoring the temporal evolution of the pump-probe spectra the
electron and hole tunneling times are separately measured and resonant
tunneling between the two dots is shown to be mediated both by elastic and
inelastic processes. Ultrafast (< 5 ps) inter-dot tunneling is shown to occur
over a surprisingly wide bandwidth, up to ~8 meV, reflecting the spectrum of
exciton-acoustic phonon coupling in the system
Optical Properties of InAs Quantum Dot Array Ensembles with Predetermined Lateral Sizes from 20 to 40 nm
Cleaved edge overgrowth and selective area epitaxy were combined for the synthesis of InAs quantum dot (QD) arrays with lateral sizes from 20 to 40 nm. The optical properties were locally assessed by confocal photoluminescence spectroscopy experiments at liquid helium temperature. The emission lines redshift as the lateral size of the QDs is increased. In agreement with a narrow size distribution, significantly narrow emission lines are observed for measurements in QD ensembles. Excitation power dependent luminescence measurements were realized on QD ensembles. A shell filling behavior was observed. The same measurements realized on single QDs led to the observation of multiple excitonic effects. Polarization dependent luminescence measurements indicate the existence of in-plane optical anisotropy, which strictly follows in-plane morphological anisotropy of the QDs. These results are encouraging for the use of quantum dot arrays in quantum information science and technology, as well as for new device concepts. (C) 2010 The Japan Society of Applied Physic
Recent advances in exciton based quantum information processing in quantum dot nanostructures
Recent experimental developments in the field of semiconductor quantum dot
spectroscopy will be discussed. First we report about single quantum dot
exciton two-level systems and their coherent properties in terms of single
qubit manipulations. In the second part we report on coherent quantum coupling
in a prototype "two-qubit" system consisting of a vertically stacked pair of
quantum dots. The interaction can be tuned in such quantum dot molecule devices
using an applied voltage as external parameter.Comment: 37 pages, 15 figures, submitted to New Journal of Physics, focus
issue on Solid State Quantum Information, added reference
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