Research Director: Dr. Seigo Tarucha
(Professor, Graduate School of Science, The University of Tokyo / Research Professor, Physics Science Laboratory, NTT Basic Research Laboratories)
Research Term: 1999-2004

The Tarucha Mesoscopic Correlation Field project aimed to make very well-defined semiconductor systems using state-of the-art technology, and then to test the related QM. This included such items as artificial atoms and molecules, where the electrons are well defined due to the correlation and quantum mechanical confinement.

Research result

Electron transport in low-dimensional systems:

Spin effects in artificial systems:

Spin effects in single quantum dots (artificial atoms) were studied. As a result, it was possible to control the spin in semiconductor quantum dots. Further, enhanced Kondo effects for a singlet-triplet degeneracy and a doublet-doublet degeneracy were found. Progress was also made regarding the unitary limit of the Kondo effect, spin-related nonequilibrium transport, and an extremely long time of spin-flip relaxation.

Pauli exclusion principle:

The Pauli exclusion effect was demonstrated by observing current rectification in a weakly coupled two-dot system. This can be a unique device sensitive to spin orientations (up or down). A classical theory for implementing a single-electron-tunneling current rectifier was newly developed. Quantum wires: The transport properties of quantum wires were investigated. Among the results achieved were a Fermi liquid and Tomonaga-Luttinger liquid effect in a quantum wire with a Bragg reflector, and a Coulomb drag effect in a coupled quantum wire.

Multi-particle correlations:

Electronic states: The electronic states in single quantum dots and quantum dot molecules were studied. Interesting results were found concerning ground state transitions associated with the isospin states and magic-number states in a magnetic as well as spin-polarized and electronic states in strongly coupled molecules.

Self-assembled single and double-dot systems:

The electronic properties of self-assembled single and double-dot systems were investigated.

Multi-particle correlating systems:

Among the important results were: a hot electron effect in a spin valve transistor consisting of a ferromagnetic metal/semiconductor system, an extraordinary ferromagnetic effect in a small dilute magnetic semiconductor, and LO phonon emission in a double quantum dot system.

Electron and nuclear spin coupling:

A quantum dot was made of GaAs, but with nuclear spins. Coupling took place between the electronic spin and the nuclear spin. A method was devised to turn on (activate) and off this coupling effect. Conservation of spin was found to take place between these systems. The electron spin has a short lifetime of a microsecond due to interactions with the surrounding space, while the nuclear spin lifetime is on the order of minutes. The nuclear spin becomes polarized, causing an extra magnetic field, as in MRI. Also, a special type of memory was obtained that could transfer information from the electron spin to the nuclear, and keep it for a much longer time, like 10 minutes.

Quantum transport and quantum information processing:

Quantum computation:

A laterally coupled two vertical dot system was fabricated to investigate molecular-like electronic properties and also the fundamental physics for implementing spin-type quantum computation. Inter-dot tunnel coupling between various types of orbitals and transitions between weak and strong tunnel coupling was found.

Carbon nanotube:

Carbon nanotubes were investigated, and were found to be very interesting not only for applications, but also fundamental physics. Though there are many theories about the electronic states and band structure, they have not been well studied experimentally.