351 research outputs found

    A double-dot quantum ratchet driven by an independently biased quantum point contact

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    We study a double quantum dot (DQD) coupled to a strongly biased quantum point contact (QPC), each embedded in independent electric circuits. For weak interdot tunnelling we observe a finite current flowing through the unbiased Coulomb blockaded DQD in response to a strong bias on the QPC. The direction of the current through the DQD is determined by the relative detuning of the energy levels of the two quantum dots. The results are interpreted in terms of a quantum ratchet phenomenon in a DQD energized by a nearby QPC.Comment: revised versio

    Dynamic photoconductive gain effect in shallow-etched AlGaAs/GaAs quantum wires

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    We report on a dynamic photoconductive gain effect in quantum wires which are lithographically fabricated in an AlGaAs/GaAs quantum well via a shallow-etch technique. The effect allows resolving the one-dimensional subbands of the quantum wires as maxima in the photoresponse across the quantum wires. We interpret the results by optically induced holes in the valence band of the quantum well which shift the chemical potential of the quantum wire. The non-linear current-voltage characteristics of the quantum wires also allow detecting the photoresponse effect of excess charge carriers in the conduction band of the quantum well. The dynamics of the photoconductive gain are limited by the recombination time of both electrons and holes

    Direct control of the tunnel splitting in a one-electron double quantum dot

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    Quasi-static transport measurements are employed on a laterally defined tunnel-coupled double quantum dot. A nearby quantum point contact allows us to track the charge as added to the device. If charged with only up to one electron, the low-energy spectrum of the double quantum dot is characterized by its quantum mechanical interdot tunnel splitting. We directly measure its magnitude by utilizing particular anticrossing features in the stability diagram at finite source-drain bias. By modification of gate voltages defining the confinement potential as well as by variation of a perpendicular magnetic field we demonstrate the tunability of the coherent tunnel coupling.Comment: High resolution pdf file available at http://www2.nano.physik.uni-muenchen.de/~huettel/research/anticrossing.pd

    Molecular states in a one-electron double quantum dot

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    The transport spectrum of a strongly tunnel-coupled one-electron double quantum dot electrostatically defined in a GaAs/AlGaAs heterostructure is studied. At finite source-drain-voltage we demonstrate the unambiguous identification of the symmetric ground state and the antisymmetric excited state of the double well potential by means of differential conductance measurements. A sizable magnetic field, perpendicular to the two-dimensional electron gas, reduces the extent of the electronic wave-function and thereby decreases the tunnel coupling. A perpendicular magnetic field also modulates the orbital excitation energies in each individual dot. By additionally tuning the asymmetry of the double well potential we can align the chemical potentials of an excited state of one of the quantum dots and the ground state of the other quantum dot. This results in a second anticrossing with a much larger tunnel splitting than the anticrossing involving the two electronic ground states.Comment: 4 pages, 4 figures; EP2DS-16 conference contributio

    Long exciton spin memory in coupled quantum wells

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    Spatially indirect excitons in a coupled quantum well structure were studied by means of polarization and time resolved photoluminescence. A strong degree of circular polarization (> 50%) in emission was achieved when the excitation energy was tuned into resonance with the direct exciton state. The indirect transition remained polarized several tens of nanoseconds after the pumping laser pulse, demonstrating directly a very long relaxation time of exciton spin. The observed spin memory effect exceeds the radiative lifetime of the indirect excitons.Comment: 4 pages, 2 figure
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