T. H. Oosterkamp

Microwave spectroscopy of a quantum-dot molecule

Quantum dots are small conductive regions in a semiconductor, containing a variable number of electrons (from one to a thousand) that occupy well-defined, discrete quantum states—for which reason they are often referred to as artificial atoms. Connecting them to current and voltage contacts allows the discrete energy spectra to be probed by charge-transport measurements. Two quantum dots can be connected to form an ‘artificial molecule’. Depending on the strength of the inter-dot coupling (which supports quantum-mechanical tunnelling of electrons between the dots), the two dots can form ‘ionic’ (refs 2–;6) or ‘covalent’ bonds. In the former case, the electrons are localized on individual dots, while in the latter, the electrons are delocalized over both dots. The covalent binding leads to bonding and antibonding states, whose energy difference is proportional to the degree of tunnelling. Here we report a transition from ionic bonding to covalent bonding in a quantum-dot ‘artificial molecule’ that is probed by microwave excitations. Our results demonstrate controllable quantum coherence in single-electron devices, an essential requirement for practical applications of quantum-dot circuitry.

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]

Electronic states in quantum dot atoms and molecules

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.

Several- and Many-Electron Artificial-Atoms at Filling Factors between 2 and 1.

We introduce new phenomena that can be studied in an artificial-atom vertical single electron transistor. As we move from the few-electron regime to the several-electron regime, and then the many-electron regime, features in the conductance peaks related to magnetic field induced spin polarization evolve. This allows us to probe the spin-flip region bounded by the last single-particle crossing at low field, and the eventual formation of a maximum density droplet at high field.

Photon Assisted Tunneling in Quantum Dots

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. The manipulation of quantum dots by a microwave field is an important ingredient for the possible application of quantum dots as solid state quantum bits, which forms a motivation for this review.

Singlet-triplet transitions in a few-electron quantum dot

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 (0–9 T) 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.

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.

Vertical quantum dots at high magnetic fields beyond the few-electron limit

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.

Elastic and inelastic single electron tunneling in coupled two dot system

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.

Coupling Characteristics Of Semiconductor Quantum Dots In Coulomb Blockade Regime

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.