N.C. van der Vaart

Single electron charging effects in semiconductor quantum dots

We have studied charging effects in a lateral split-gate quantum dot defined by metal gates in the two dimensional electron gas (2 DEG) of a GaAs/AlGaAs heterostructure. The gate structure allows an independent control of the conductances of the two tunnel barriers separating the quantum dot from the two 2 DEG leads, and enables us to vary the number of electrons that are localized in the dot. We have measured Coulomb oscillations in the conductance and the Coulomb staircase in current-voltage characteristics and studied their dependence on the conductances of the tunnel barriers. We show experimentally that at zero magnetic field charging effects start to affect the transport properties when both barrier conductances are smaller than the first quantized conductance value of a point contact at 2e2/h. The experiments are described by a simple model in terms of electrochemical potentials, which includes both the discreteness of the electron charge and the quantum energy states due to confinement.

Changes in the magnetization of a double quantum dot

From accurate measurements of the energy states in a double quantum dot, we deduce the change in magnetization due to single electron tunneling. We observe crossings and anticrossings in the energy spectrum as a function of magnetic field. 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. [S0031-9007(98)06132-8]

Charging effects in quantum dots at high magnetic fields

Abstract We have studied charging effects in quantum dots, defined by gates in the two-dimensional electron gas of an AlGaAs-GaAs heterostructure. We have investigated Coulomb oscillations at zero magnetic field, and in the integer and fractional QHE regimes. Coulomb oscillations are found to appear only when both barrier conductances G are below the first well resolved plateau: at zero field G e 2 / h , in the IQHE regime G e 2 / h , and in the FQHE regime G 1 3 e 2 / h . The period of the oscillations is shown to be field independent. We furthermore studied the interplay between charging effects and discrete energy states in the dot. In the IQHE, we found a regular pattern in the amplitude of the Coulomb oscillations that corresponds to the number of Landau levels in the dot. In a smaller dot containing only 25 electrons, the excitation spectrum of the dot is investigated at finite source-drain voltages.

Photon-assisted tunnelling through a quantum dot

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.

Quantized current in a quantum dot turnstile

We have performed RF experiments on a lateral quantum dot defined in the two dimensional electron gas (2DEG) of a GaAs/AlGaAs heterostructure. The small capacitance of the quantum dot gives rise to single-electron charging effects, which we employed to realize a quantum dot turnstile device. By modulating the tunnel barriers between the quantum dot and the 2DEG leads with two phase-shifted RF signals, we pass an integer number of electrons through the quantum dot per RF cycle. This is demonstrated by the observation of quantized current plateaus at multiples ofef in current-voltage characteristics, wheref is the frequency of the RF signals. When an asymmetry is induced by applying unequal RF voltages, our quantum dot turnstile operates as a single-electron pump producing a quantized current at zero bias voltage.

High-frequency transport through mesoscopic structures

We have measured DC transport through a GaAs/AlGaAs quantum dot in the presence of a microwave signal of frequency f. We find features related to the photon energy hf whose positions in gate voltage are independent of the microwave power but vary linearly with frequency. The measurements demonstrate photon-assisted tunneling in the mesoscopic regime.

Microwave-assisted transport through a quantum dot

We present results on microwave-assisted transport through quantum dots. First, the important energy/frequency scales are discussed. Then, measurements of the current versus gate voltage characteristics in the presence of microwaves are presented. At finite source-drain bias, microwave-induced features are observed, and at zero source-drain bias, an oscillating photocurrent is observed. A model of photon-assisted transport is discussed that can account for the experimental observations.

Charging effects in quantum dots in a magnetic field

Abstract We have studied charging effects in a lateral split-gate quantum dot defined by metal gates in the two dimensional electron gas of a AlGaAs-GaAs heterostructure. Conductance oscillations as a function of magnetic field occur in the integer quantum Hall effect regime. We have observed a superposition of oscillations with two different periods. The larger period is associated with the removal of one electron from the dot. The smaller period is related to the transfer of an electron from the second to the first Landau level (LL). The measurements are interpreted in terms of a simple model which takes into account the charging energy between the LLs in the dot.

Photon induced resonances in the current through a quantum dot with zero-dimensional states

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.

Photon-Assisted Tunneling Through a Quantum Dot: Theory and Experiment

We have studied the effect of microwaves on Coulomb blockade devices, both theoretically and experimentally. Our model extends the Tien-Gordon theory for photon-assisted tunneling to encompass the correlated tunneling of electrons through small capacitance double-junction devices. The main theoretical results for a double-junction system are (1) the prediction of sharp current jumps reflecting the discrete photon energy hf in current-gate voltage characteristics and (2) the prediction of a zero-bias current whose sign changes when an electron is added to the island between the junctions. Preliminary measurements are presented on split-gate quantum dots with 19 GHz microwaves applied to one of the control gates. The observed photoresponse is in good agreement with the predictions of the model.