Publications (22)178.42 Total impact
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ABSTRACT: We review the electronic ground states in fewelectron quantum dots studied by measuring Coulomb oscillations in the linear transport regime. The number of electrons in our vertical quantum dots are changed onebyone 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 singleparticle states and Hund’s rule. Most features can be explained with an extended constant interaction (CI) model. This model describes the electronelectron interactions by a charging energy plus an exchange energy which are both independent of magnetic field.Advances in Solid State Physics 38, 10/2007: pages 139152; 
Article: Photon induced resonances in the current through a quantum dot with zerodimensional states
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ABSTRACT: We have measured photonassisted tunneling through a quantum dot with zero dimensional (0D) states. When the photon energy exceeds the separation between 0Dstates 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.01/2006; 1997(T69). DOI:10.1088/00318949/1997/T69/014  [Show abstract] [Hide abstract]
ABSTRACT: We describe phenomena that can be studied in vertical quantum dot single electron transistors. Moving from the fewelectron to the severaland manyelectron regimes, features in the conductance peaks initially related to spin polarization evolve with magnetic ÿeld. This allows us to ÿrst probe the spinip region beyond the last singleparticle crossing at low ÿeld, and then the formation and stability of the spinpolarized 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 Lowdimensional Systems and Nanostructures 02/2000; 6(14):358363. DOI:10.1016/S13869477(99)001691 · 2.00 Impact Factor  [Show abstract] [Hide abstract]
ABSTRACT: Elastic and inelastic tunneling between zerodimensional 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 06/1999; 47(1):101105. DOI:10.1016/S01679317(99)001628 · 1.20 Impact Factor  [Show abstract] [Hide abstract]
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 wellresolved discrete energy states, the applied highfrequency 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.  [Show abstract] [Hide abstract]
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 maximumdensitydroplet (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 04/1999; 82(14). DOI:10.1103/PhysRevLett.82.2931 · 7.51 Impact Factor  [Show abstract] [Hide abstract]
ABSTRACT: We introduce new phenomena that can be studied in an artificialatom vertical single electron transistor. As we move from the fewelectron regime to the severalelectron regime, and then the manyelectron regime, features in the conductance peaks related to magnetic field induced spin polarization evolve. This allows us to probe the spinflip region bounded by the last singleparticle crossing at low field, and the eventual formation of a maximum density droplet at high field.Japanese Journal of Applied Physics 01/1999; 38(1):372375. DOI:10.1143/JJAP.38.372 · 1.13 Impact Factor  [Show abstract] [Hide abstract]
ABSTRACT: Microwave experiments on an artificial twolevel system formed by a double quantum dot, are discussed for different coupling and microwave power regimes. When the interdot coupling, T, is weak, an ioniclike bonding is observed. The current through the double dot is power dependent. In the strong coupling regime, a covalentlike bonding is formed and the energy separation between the symmetric and antisymmetric 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 zerothorder 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):3135. · 1.32 Impact Factor  [Show abstract] [Hide abstract]
ABSTRACT: We report photonassisted tunnelling (PAT) through a quantum dot with zerodimensional (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. DOI:10.1088/02681242/11/11S/010 · 2.19 Impact Factor  [Show abstract] [Hide abstract]
ABSTRACT: We have measured spinsingletspintriplet (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 (09T) at both small and large source drain voltages. The ST transitions cannot be explained within the framework of singleparticle 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(14):173177. DOI:10.1016/S09214526(98)005432 · 1.32 Impact Factor  [Show abstract] [Hide abstract]
ABSTRACT: Coupled quantum dots in series have been realized in two dimensional electron gas (2DEG) in GaAs/AlGaAs heterostructures using the surface gate, and transport measurements have been performed at the dilution refrigerator temperature. The electrical transport was found to be determined by the Coulomb blockade and the resonant tunneling. The energy width of the resonant peak can be smaller than the thermal energy when two dots are sufficiently isolated. The charge stability diagram, by changing two gates attached to each dot, was measured for weak and strong coupling conditions. The width of the current resonant peak changed significantly as the coupling was increased, while the capacitive coupling did not change so much. The experimental result was analyzed using a simple capacitance model.Japanese Journal of Applied Physics 12/1998; 37(12B):7161. DOI:10.1143/JJAP.37.7161 · 1.13 Impact Factor  [Show abstract] [Hide abstract]
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.  [Show abstract] [Hide abstract]
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 interdot 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 antibonding 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 interdot coupling strength. This demonstrates controllable quantum coherence in single electron devices.Nature 10/1998; 395(6705). DOI:10.1038/27617 · 41.46 Impact Factor  [Show abstract] [Hide abstract]
ABSTRACT: We study electronic states in diskshaped semiconductor artificial atoms and molecules containing just a few electrons. The fewelectron ground states in the artificial atom show atomiclike 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 diskshaped dots. When the two dots are quantum mechanically strongly coupled, the fewelectron ground states are delocalized throughout the system and the electronic properties resemble those of a single artificial atom.Physica E Lowdimensional Systems and Nanostructures 10/1998; 3(13):112120. DOI:10.1016/S13869477(98)002252 · 2.00 Impact Factor  [Show abstract] [Hide abstract]
ABSTRACT: Not AvailableMicroprocesses and Nanotechnology Conference, 1998 International; 08/1998  [Show abstract] [Hide abstract]
ABSTRACT: A double quantum dot device is a tunable twolevel 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 figuresScience 08/1998; 282(5390). DOI:10.1126/science.282.5390.932 · 33.61 Impact Factor  [Show abstract] [Hide abstract]
ABSTRACT: A tunable Kondo effect has been realized in small quantum dots. A dot can be switched from a Kondo system to a nonKondo 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 singleparticle energy state nears the Fermi energy. Measurements of the temperature and magnetic field dependence of a Coulombblockaded dot show good agreement with predictions of both equilibrium and nonequilibrium Kondo effects.Science 08/1998; 281(5376):5404. DOI:10.1126/science.281.5376.540 · 33.61 Impact Factor  [Show abstract] [Hide abstract]
ABSTRACT: We study atomiclike 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 twodimensional harmonic potential. As a function of magnetic field the current peaks shift in pairs, due to the filling of electrons into spindegenerate singleparticle 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 06/1998; 249:191196. DOI:10.1016/S09214526(98)000969 · 1.32 Impact Factor  [Show abstract] [Hide abstract]
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. Evenodd peak spacings fluctuations are also observed for the same alternating valleys and indicate spindegeneracy 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).  [Show abstract] [Hide abstract]
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 anticrossings 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 figuresPhysical Review Letters 02/1998; 80(22). DOI:10.1103/PhysRevLett.80.4951 · 7.51 Impact Factor
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3k  Citations  
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19972007

Technische Universiteit Delft
 Applied Geophysics and Petrophysics
Delft, South Holland, Netherlands
