T. H. Oosterkamp

Technische Universiteit Delft, Delft, South Holland, Netherlands

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Publications (21)166.7 Total impact

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    ABSTRACT: We review the electronic ground states in few-electron quantum dots studied by measuring Coulomb oscillations in the linear transport regime. The number of electrons in our vertical quantum dots are changed one-by-one from zero up to about a hundred by means of the gate voltage. We identify the quantum numbers of the states by measuring the magnetic field dependence. We find that transitions in the ground states originate from crossings between single-particle states and Hund’s rule. Most features can be explained with an extended constant interaction (CI) model. This model describes the electron-electron interactions by a charging energy plus an exchange energy which are both independent of magnetic field.
    10/2007: pages 139-152;
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    ABSTRACT: We have measured photon-assisted tunneling through a quantum dot with zero dimensional (0D) states. When the photon energy exceeds the separation between 0D-states we observed photon induced excited state resonances, as well as photon sideband resonances. We study the strength of these resonances as a function of the applied microwave field, and compare them to calculations.
    Physica Scripta Volume T. 01/2006;
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    ABSTRACT: We describe phenomena that can be studied in vertical quantum dot single electron transistors. Moving from the few-electron to the several-and many-electron regimes, features in the conductance peaks initially related to spin polarization evolve with magnetic ÿeld. This allows us to ÿrst probe the spin--ip region beyond the last single-particle crossing at low ÿeld, and then the formation and stability of the spin-polarized maximum density droplet at high ÿeld. According to a simple capacitance model, charge redistribution in the dot at higher magnetic ÿelds is accompanied by abrupt changes in the area of the droplet. ? 2000 Elsevier Science B.V. All rights reserved.
    Physica E Low-dimensional Systems and Nanostructures 01/2000; 6:358-363. · 1.86 Impact Factor
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    ABSTRACT: We describe phenomena that can be studied in vertical quantum dot single electron transistors. Moving from the few-electron to the several- and many-electron regimes, features in the conductance peaks initially related to spin polarization evolve with magnetic field. This allows us to first probe the spin-flip region beyond the last single-particle crossing at low field, and then the formation and stability of the spin-polarized maximum density droplet at high field. According to a simple capacitance model, charge redistribution in the dot at higher magnetic fields is accompanied by abrupt changes in the area of the droplet.
    Physica E Low-dimensional Systems and Nanostructures 01/2000; · 1.86 Impact Factor
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    ABSTRACT: We review experiments on single electron transport through single quantum dots in the presence of a microwave signal. In the case of a small dot with well-resolved discrete energy states, the applied high-frequency signal allows for inelastic tunnel events that involve the exchange of photons with the microwave field. These photon assisted tunneling (PAT) processes give rise to sideband resonances in addition to the main resonance. Photon absorption can also lead to tunneling via excited states instead of tunneling via the ground state of the quantum dot.
    05/1999;
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    ABSTRACT: Microwave experiments on an artificial two-level system formed by a double quantum dot, are discussed for different coupling and microwave power regimes. When the inter-dot coupling, T, is weak, an ionic-like bonding is observed. The current through the double dot is power dependent. In the strong coupling regime, a covalent-like bonding is formed and the energy separation between the symmetric and anti-symmetric eigenstates, ΔE∗, becomes power dependent as well. It is given by ΔE∗={ΔE}2+{2J0(eVAC/hf)T}2, where ΔE is the uncoupled energy splitting, J0 the zeroth-order Bessel function of the first kind, VAC the microwave amplitude, and f the frequency. We show that in the case of strong coupling and low microwave power (eVAC⪡hf,J0≈1), the observed energy separation is well described by ΔE∗={ΔE}2+{2T}2. For larger microwave powers (eVAC≳hf) it is shown that the energy separation is modified according to the Bessel function term.
    Physica B Condensed Matter 01/1999; 272(1):31-35. · 1.28 Impact Factor
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    ABSTRACT: We have measured electron transport through a vertical quantum dot containing a tunable number (between 0 and 40) of electrons. Over a region of the magnetic field the electrons are spin polarized and occupy successive angular momentum states. This is the maximum-density-droplet (MDD) state. The stability region where the MDD is the ground state decreases for increasing electron number. The instability of the MDD and other transitions in this high B region are accompanied by a redistribution of charge which abruptly changes the area of the electron droplet.
    Physical Review Letters 01/1999; · 7.73 Impact Factor
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    ABSTRACT: Elastic and inelastic tunneling between zero-dimensional states are studied for a laterally coupled two dot device and for a vertically coupled two dot device. The resonance current observed in both devices consists of a symmetric peak of elastic tunneling and an asymmetric broad peak of inelastic tunneling. The elastic peak width compares to the energy of tunnel coupling. The inelastic current is related to acoustic phon on emission from detailed study on the temperature dependence.
    Microelectronic Engineering - MICROELECTRON ENG. 01/1999; 47(1):101-105.
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    ABSTRACT: We report photon-assisted tunnelling (PAT) through a quantum dot with zero-dimensional (0D) states. PAT allows electrons to reach previously inaccessible energy states by absorbing or emitting photons from a microwave signal. We discuss a model based on a master equation for a quantum dot with 0D states and include PAT processes. Simulations are compared with measurements.
    Semiconductor Science and Technology 12/1998; 11(11S):1512. · 1.92 Impact Factor
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    ABSTRACT: We have measured spin-singlet-spin-triplet (ST) transitions in a vertical quantum dot containing up to four electrons. Current through the dot is measured as a function of gate voltage and magnetic field (0-9T) at both small and large source drain voltages. The ST transitions cannot be explained within the framework of single-particle states in combination with a constant Coulomb interaction. Taking into account exchange interaction and a magnetic field dependent direct Coulomb interaction is essential for describing the observed ST transitions.
    Physica B Condensed Matter 12/1998; 256(1-4):173-177. · 1.28 Impact Factor
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    ABSTRACT: We have measured electron transport through a vertical quantum dot containing a tunable number of electrons between 0 and 40. Over some region in magnetic field the electrons are spin polarized and occupy successive angular momentum states, i.e. the maximum density droplet (MDD) state. The stability region where the MDD state is the ground state, decreases for increasing electron number. The instability of the MDD is accompanied by a redistribution of charge which increases the area of the electron droplet.
    11/1998;
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    ABSTRACT: Quantum dots are small conductive regions in a semiconductor, containing a variable number of electrons (N=1 to 1000) that occupy well defined discrete quantum states. They are often referred to as artificial atoms with the unique property that they can be connected to current and voltage contacts. This allows one to use transport measurements to probe the discrete energy spectra. To continue the analogy with atoms, two quantum dots can be connected to form an 'artificial molecule'. Depending on the strength of the inter-dot coupling, the two dots can have an ionic binding (i.e. electrons are localized on the individual dots) or a covalent binding (i.e. electrons are delocalized over both dots). The covalent binding leads to a bonding and an anti-bonding state with an energy splitting proportional to the tunnel coupling. In the dc current response to microwave excitation we observe a transition from an ionic bonding to a covalent bonding, when we vary the inter-dot coupling strength. This demonstrates controllable quantum coherence in single electron devices.
    Nature 10/1998; · 38.60 Impact Factor
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    ABSTRACT: Not Available
    Microprocesses and Nanotechnology Conference, 1998 International; 08/1998
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    ABSTRACT: A double quantum dot device is a tunable two-level system for electronic energy states. A dc electron current directly measures the rates for elastic and inelastic transitions between the two levels. For inelastic transitions energy is exchanged with bosonic degrees of freedom in the environment. The inelastic transition rates are well described by the Einstein coefficients, relating absorption with stimulated and spontaneous emission. The most effectively coupled bosons in the specific environment of our semiconductor device are acoustic phonons. The experiments demonstrate the importance of vacuum fluctuations in the environment for little circuits of coherent quantum devices. Comment: 5 pages, 3 figures
    Science 08/1998; · 31.20 Impact Factor
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    SM Cronenwett, TH Oosterkamp, LP Kouwenhoven
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    ABSTRACT: A tunable Kondo effect has been realized in small quantum dots. A dot can be switched from a Kondo system to a non-Kondo system as the number of electrons on the dot is changed from odd to even. The Kondo temperature can be tuned by means of a gate voltage as a single-particle energy state nears the Fermi energy. Measurements of the temperature and magnetic field dependence of a Coulomb-blockaded dot show good agreement with predictions of both equilibrium and nonequilibrium Kondo effects.
    Science 08/1998; 281(5376):540-4. · 31.20 Impact Factor
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    ABSTRACT: We report measurements of the Kondo effect in the Coulomb blockade regime of a small lateral quantum dot. In seven consecutive Coulomb valleys we observe decreasing conductance in alternating valleys for increasing temperature from 50 mK to 300 mK. Even-odd peak spacings fluctuations are also observed for the same alternating valleys and indicate spin-degeneracy in the dot. The Kondo peak in the differential conductance of alternating valleys is studied in both parallel and perpendicular magnetic field from 0T to 9T (nu = 1 at 9T for perpendicular field).
    03/1998;
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    ABSTRACT: From accurate measurements of the energy states in a double quantum dot we deduce the change in magnetization due to single electron tunneling. As a function of magnetic field we observe crossings and anti-crossings in the energy spectrum. The change in magnetization exhibits wiggles as a function of magnetic field with maximum values of a few effective Bohr magnetons in GaAs. These wiggles are a measure of the chaotic motion of the discrete energy states versus magnetic field. Our results show good agreement with a numeric calculation but deviate significantly from semiclassical estimates. Comment: 6 pages, plus 3 figures
    Physical Review Letters 02/1998; · 7.73 Impact Factor
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    ABSTRACT: We study electronic states in disk-shaped semiconductor artificial atoms and molecules containing just a few electrons. The few-electron ground states in the artificial atom show atomic-like properties such as a shell structure and obey Hund’s rule. A magnetic field induces transitions in the ground states, which are identified as crossings between single particle states, singlet–triplet transitions and spin polarization. These properties are discussed in conjunction with exact calculation in which the effect of finite thickness of the disk is taken into account. An artificial molecule is made from vertically coupling two disk-shaped dots. When the two dots are quantum mechanically strongly coupled, the few-electron ground states are de-localized throughout the system and the electronic properties resemble those of a single artificial atom.
    Physica E Low-dimensional Systems and Nanostructures 01/1998; · 1.86 Impact Factor
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    ABSTRACT: Studies of the ground and excited states in semiconductor quantum dots containing 1 to 12 electrons showed that the quantum numbers of the states in the excitation spectra can be identified and compared with exact calculations. A magnetic field induces transitions between the ground and excited states. These transitions were analyzed in terms of crossings between single-particle states, singlet-triplet transitions, spin polarization, and Hund's rule. These impurity-free quantum dots allow "atomic physics" experiments to be performed in magnetic field regimes not accessible for atoms.
    Science 01/1998; 278(5344):1788-92. · 31.20 Impact Factor
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    ABSTRACT: We study atomic-like properties of artificial atoms by measuring Coulomb oscillations in vertical quantum dots containing a tunable number of electrons starting from zero. At zero magnetic field the energy needed to add electrons to a dot reveals a shell structure for a two-dimensional harmonic potential. As a function of magnetic field the current peaks shift in pairs, due to the filling of electrons into spin-degenerate single-particle states. When the magnetic field is sufficiently small, however, the pairing is modified, as predicted by Hund's rule, to favour the filling of parallel spins.
    Physica B Condensed Matter 01/1998; 249:191-196. · 1.28 Impact Factor