Electron transport and coherence in semiconductor quantum dots and rings

A number of experiments on electron transport and coherence in semiconductor vertical and lateral quantum dots and semiconductor rings is described. Quantum dots are often referred to as "artificial atoms", because of their similarities with real atoms. Examples of such atom-like properties that have been studied, are spin-singlet-triplet transitions and the Kondo effect. A strong Kondo effect is observed where Coulomb blockade is overcome completely and the conductance reaches the unitary-limit value at 2e2/h. It is shown that phase-coherent transport through a Kondo quantum dot is possible, by measuring electron interference in an Aharonov-Bohm ring with the dot embedded in one of its arms.Where single quantum dots are regarded as "artificial atoms", two quantum dots can be coupled to form an "artificial molecule". Motivated by their relevance for realizing solid-state quantum bits, electron transport experiments on two lateral quantum dots coupled in series are reviewed. Finally, an electro-magnetic Aharonov-Bohm effect in a 2D electron gas ring is studied. A new method is developed to measure the non-equilibrium electron dephasing time with a focus on the role of electron-electron interactions.Applied Science

Fabrication and low-temperature transport properties of selectively grown dual-gated single-electron transistors

We report on the fabrication of a dual-gated single-electron transistor (SET) based on a quantum dot (QD) formed by selective area growth of metalorganic vapor-phase epitaxy, and its low-temperature transport properties. We observe clear Coulomb oscillations in a SET fabricated in combination with direct growth of nanostructures and lithographically defined metal gates. The magnetic field dependence of the Coulomb oscillations as well as the Coulomb diamonds suggest strong carrier confinement in our QD.Kavli Institute of NanoscienceApplied Science

Two-stage kondo effect in a quantum dot at a high magnetic field

We report a strong Kondo effect (Kondo temperature ~ 4K) at high magnetic field in a selective area growth semiconductor quantum dot. The Kondo effect is ascribed to a singlet-triplet transition in the ground state of the dot. At the transition, the low-temperature conductance approaches the unitary limit. Away from the transition, for low bias voltages and temperatures, the conductance is sharply reduced. The observed behavior is compared to predictions for a two-stage Kondo effect in quantum dots coupled to single-channel leads.Comment: 4 pages, 5 figure