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ABSTRACT: The electronic states of lateral many-electron quantum dots in high magnetic fields are analyzed in terms of energy and spin. In a regime with two Landau levels in the dot, several Coulomb-blockade peaks are measured. A zigzag pattern is found as it is known from the Fock-Darwin spectrum. However, only data from Landau level 0 show the typical spin-induced bimodality, whereas features from Landau level 1 cannot be explained with the Fock-Darwin picture. Instead, by including the interaction effects within spin-density-functional theory a good agreement between experiment and theory is obtained. The absence of bimodality on Landau level 1 is found to be due to strong spin polarization.
Physical Review Letters 07/2010; 105(4):046802. · 7.37 Impact Factor
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ABSTRACT: We present the full three dimensionality of an electrostatically calculated stability diagram for triple quantum dots. The stability diagram maps out the favored charge configuration of the system as a function of potential shifts due to gate voltages. For triple dots only a three dimensional visualization allows for the complete identification of all its components. Those are most notably the so called quadruple points where four electronic configurations are degenerate, and quantum cellular automata (QCA) processes. The exact positions of these features within the stability diagram are now revealed. Furthermore the influence on transport is studied by comparing the model with a two path triple quantum dot made with local anodic oxidation. The two path setup allows to study the influence of the dots arrangement. Comment: 11 pages, 5 figures
08/2009;
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M. C. Rogge
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ABSTRACT: We performed charge detection on a lateral triple quantum dot with starlike geometry. The setup allows us to interpret the results in terms of two double dots with one common dot. One double dot features weak tunnel coupling and can be understood with atomlike electronic states, the other one is strongly coupled forming moleculelike states. In nonlinear measurements we identified patterns that can be analyzed in terms of the symmetry of tunneling rates. Those patterns strongly depend on the strength of interdot tunnel coupling and are completely different for atomiclike or moleculelike coupled quantum dots allowing the noninvasive detection of molecular bonds.
Phys. Rev. B. 10/2008; 78(15).
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ABSTRACT: We performed charge detection on a lateral triple quantum dot with star-like geometry. The setup allows us to interpret the results in terms of two double dots with one common dot. One double dot features weak tunnel coupling and can be understood with atom-like electronic states, the other one is strongly coupled forming molecule-like states. In nonlinear measurements we identified patterns that can be analyzed in terms of the symmetry of tunneling rates. Those patterns strongly depend on the strength of interdot tunnel coupling and are completely different for atomic- or molecule-like coupled quantum dots allowing the non-invasive detection of molecular bonds. Comment: 4 pages, 4 figures
07/2008;
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J. Phys: Condens. Matter. 01/2008; 20:454204.
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ABSTRACT: We present an advanced lateral triple quantum dot made by local anodic oxidation. Three dots are coupled in a starlike geometry with one lead attached to each dot thus allowing for multiple path transport measurements with two dots per path. In addition charge detection is implemented using a quantum point contact. Both in charge measurements as well as in transport we observe clear signatures of states from each dot. Resonances of two dots can be established allowing for serial transport via the corresponding path. Quadruple points with all three dots in resonance are prepared for different electron numbers and analyzed concerning the interplay of the simultaneously measured transport along both paths.
08/2007;
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ABSTRACT: We study the magnetoconductance of lateral quantum dots in high magnetic fields. We use spin blockade and Kondo effect to investigate the spin configuration and the spin filling mechanism and find a dependence on the electron number of the quantum dot. © 2007 American Institute of Physics
AIP Conference Proceedings. 04/2007; 893(1):809-810.
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ABSTRACT: We demonstrate new results on electron redistribution on a single quantum dot caused by magnetic field. A quantum point contact is used to detect changes in the quantum dot charge. We are able to measure both the change of the quantum dot charge and also changes of the electron configuration at constant number of electrons on the quantum dot. These features are used to exploit the quantum dot in a high magnetic field where transport through the quantum dot displays the effects of Landau shells and spin blockade.
AIP Conf. Proc. 04/2007; 893(1):793.
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ABSTRACT: Experimental results for the sequential transport through a lateral quantum dot in a perpendicular magnetic field are compared with theory. Regular patterns of negative differential conductances are observed in the nonlinear regime. We attempt to reproduce theoretically these patterns in a simplified model which captures the essential features of the experimental system. Orbital and spin effects are treated in terms of the Fock–Darwin model. The transport properties are described by employing a master equation with tunable tunnelling and relaxation rates. We show that the essential physics underlying the experiment can be described within our approach if the timescales of the different transport channels are well separated.
New Journal of Physics 11/2006; 8(12):298. · 4.18 Impact Factor
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ABSTRACT: Single electron tunneling is studied in a many electron quantum dot in high magnetic fields. For such a system multiple transitions of the spin configuration are theoretically predicted. With a combination of spin blockade and Kondo effect we are able to detect five regions with different spin configurations. Transitions are induced with changing electron numbers.
Physical Review Letters 11/2006; 97(17):176801. · 7.37 Impact Factor
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ABSTRACT: We report on spin blockade observed in several different lateral quantum dot systems. We observe the typical features of alternating position and amplitude of Coulomb peaks as a function of a magnetic field perpendicular to the sample surface. In a nonlinear regime strong negative differential conductance is observed with a striking regularity. This is not a universal behaviour. Instead we propose a complex interplay of spin blockade, electron interaction and relaxation. At strong tunnel coupling to the leads we use spin blockade as a spectroscopy tool to reveal the spin configuration in a Kondo regime with a pronounced chessboard pattern. By comparing measurements of different samples we find that Kondo effect and spin blockade can be combined in two different ways depending on the spin configuration of the dot. Thus the combination of both effects can be used to detect spin polarized filling of dot orbitals instead of regular filling. (© 2006 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)
physica status solidi (b) 10/2006; 243(15):3864 - 3868. · 1.32 Impact Factor
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ABSTRACT: We investigate the Kondo effect and spin blockade observed in a many-electron quantum dot and study the magnetic field dependence. At lower fields, a pronounced Kondo effect is found, which is replaced by the spin blockade at higher fields. In an intermediate regime, both effects are visible. We make use of this combined effect to gain information about the internal spin configuration of our quantum dot. We find that the data cannot be explained assuming regular filling of electronic orbitals. Instead, spin polarized filling seems to be probable.
Physical Review Letters 03/2006; 96(4):046802. · 7.37 Impact Factor
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ABSTRACT: Results of combined experimental and theoretical studies of transport spectra of a quantum dot in a magnetic field are reported. In the linear regime, the observed oscillatory behavior of the conductance peaks with varying magnetic field is interpreted in terms of successive rearrangements in the electronic ground state configurations of Fock–Darwin energy levels. Spin-blockade related bimodal behavior is also observed in the peak intensities due to spin dependent tunneling. In the nonlinear regime, a regular pattern of negative differential conductance is detected. This is compared with different theoretical cases demonstrating a subtle interplay between spin dependent tunneling and source and drain resistance asymmetries.
01/2006; 1025(72).
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ABSTRACT: With non-invasive methods, we investigate ground and excited states of a lateral quantum dot. Charge detection via a quantum point contact is used to map the dot dynamics in a regime where the current through the dot is too low for transport measurements. In this way we investigate and compare the tunneling rates from the dot to source and drain. We find a symmetry line on which the tunneling rates to both leads are equal. In this situation ground states as well as excited states influence the mean charge of the dot. A detailed study in this regime reveals that the coupling symmetry depends on the number of states contributing to transport and on the spatial distribution of individual states.
09/2005;
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ABSTRACT: Experimental results for sequential transport through a lateral quantum dot in the regime of spin blockade induced by spin dependent tunneling are compared with theoretical results obtained by solving a master equation for independent electrons. Orbital and spin effects in electron tunneling in the presence of a perpendicular magnetic field are identified and discussed in terms of the Fock-Darwin spectrum with spin. In the nonlinear regime, a regular pattern of negative differential conductances is observed. Electrical asymmetries in tunnel rates and capacitances must be introduced in order to account for the experimental findings. Fast relaxation of the excited states in the quantum dot have to be assumed, in order to explain the absence of certain structures in the transport spectra.
08/2005;
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ABSTRACT: We demonstrate electron redistribution caused by magnetic field on a single quantum dot measured by means of a quantum point contact as non-invasive detector. Our device which is fabricated by local anodic oxidation allows to control independently the quantum point contact and all tunnelling barriers of the quantum dot. Thus we are able to measure both the change of the quantum dot charge and also changes of the electron configuration at constant number of electrons on the quantum dot. We use these features to exploit the quantum dot in a high magnetic field where transport through the quantum dot displays the effects of Landau shells and spin blockade. We confirm the internal rearrangement of electrons as function of the magnetic field for a fixed number of electrons on the quantum dot.
05/2005;
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ABSTRACT: We present transport measurements on a lateral double dot produced by combining local anodic oxidation and electron beam lithography. We investigate the tunability of our device and demonstrate, that we can switch between capacitive and tunnel coupling. In the regime of capacitive coupling we observe the phenomenon of spin blockade in a magnetic field and analyze the influence of capacitive interdot coupling on this effect.
11/2003;
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ABSTRACT: We have combined direct nanofabrication by local anodic oxidation with conventional electron-beam lithography to produce a parallel double quantum dot based on a GaAs/AlGaAs heterostructure. The combination of both nanolithography methods allows fabrication of robust in-plane gates and Cr/Au top-gate electrodes on the same device for optimal controllability. This is illustrated by the tunability of the interdot coupling in our device. We describe our fabrication and alignment scheme in detail and demonstrate the tunability in low-temperature transport measurements. © 2003 American Institute of Physics.
Applied Physics Letters 09/2003; · 3.84 Impact Factor
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ABSTRACT: In recent years several attempts have been made to fabricate coupled quantum dots as a crucial element of quantum computing devices. One important challenge is to achieve a reliable control of the interdot tunneling. For this purpose we have combined direct nanolithography by local anodic oxidation (LAO) with standard electron-beam lithography. LAO is used to produce parallel double quantum dots. Additional metallic split gates are responsible for the control of the interdot coupling. We describe our fabrication scheme and demonstrate the function in low-temperature transport measurements.
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ABSTRACT: We study the influence of asymmetric tunneling rates of a lateral quantum dot connected to source and drain leads. We use a quantum point contact (QPC) to detect charging events on the dot. Since the mean charge depends on the ratio of the tunneling rates the QPC can be used to detect symmetric configurations where both rates are equal. Thus, the transport properties of the dot can be investigated concerning this symmetry. We concentrate on the visibility of features that correspond to transitions with excited states. We interpret the results with the number of channels used for transport, the electron flow direction and the rate symmetry.
