Daiju Tsuya

Quantum-dot nanodevices with carbon nanotubes

We review our recent work on quantum-dot devices with carbon nanotubes. We conclude that the single-wall carbon nanotube quantum dot is an artificial atom with two- or four-electron shell structures. Zeeman splitting of single particle levels was observed, which is advantageous for the spin based quantum computing device (spin qubit) because the single spin is generated by putting one electron in the shell. Single-electron devices such as single-electron inverter and single-electron exclusive-OR gates have been fabricated, and their performance has been demonstrated at liquid-helium temperature. Despite the expected room-temperature operation from the single-electron charging energy, the operation temperature of our devices was limited to ∼10K because of the low height of the tunnel barrier.

Carbon nanotube quantum dots fabricated on a GaAs∕AlGaAs two-dimensional electron gas substrate

Single-wall carbon nanotube (SWCNT) quantum dots have been fabricated on a GaAs∕AlGaAs two-dimensional electron gas (2DEG) substrate, and the single electron transport measurements have been carried out at 2.8 K and 22 mK with the 2DEG used as a gate. It was demonstrated that the gating by the 2DEG could be switched on and off by controlling a quantum point contact fabricated between the dot and the Ohmic contact to the 2DEG gate. The unique combination of the SWCNT dot and the 2DEG may open a door to realize flexible hybrid devices.

Quantum dots and their tunnel barrier in semiconducting single-wall carbon nanotubes with a p-type behavior

Electrical transport properties have been measured in an individual p-type semiconducting single-wall carbon nanotube at the temperature range from 1.5 K to 160 K. The quantum dot transport has been observed, which depends on the gate voltage range. In the small positive gate voltage range, the single quantum dot behavior was observed, while the multi-dot behavior was observed in the further positive gate voltage. In the sufficiently large positive gate voltage range, no current flowed. The barrier height in the single quantum dot regime was estimated to be ~10 meV by measuring the temperature dependence of the current–voltage curve at a Coulomb peak position.

Fabrication of a Single-Electron Inverter in Single-Wall Carbon Nanotubes

A single-electron inverter has been fabricated in single-wall carbon nanotubes (SWNTs) by connecting two single-electron transistors (SETs) in series. Each SET was fabricated in different SWNTs only by depositing metallic contacts on them. For one SET, SWNTs appeared to be single and semiconducting, while they appeared to form a bundle for the other SET. The inverter performance was obtained at 1.5 K with a full voltage swing and a gain of ~0.6, although the SETs were not fabricated from the simple individual metallic nanotubes.

One-dimensional shell-filling and single spin polarization in carbon nanotube quantum dots

Quantum dots have been fabricated in an individual single-wall carbon nanotube, and their single electron transport has been measured at low temperatures with magnetic field up to 5T. The Coulomb diamonds have shown two and four electron periodicities in different gate voltage ranges. The results can be understood by the two- or four-electron shell filling model. In magnetic fields, the Zeeman splitting of single particle levels has been observed. This means that the single spin polarization is realized when an odd number of electrons are in the dot. The experimental observation suggests an important step for realization of the spin qubit.

Magnetoresistance over 1000 % in CoFe/carbon nanotube/CoFe junctions

We have studied spin‐dependent transport through a single‐wall carbon nanotube, contacted by two CoFe electrodes with a gap of about 300 nm. At 1.8 K, the junction exhibits extraordinarily high magnetoresistance ratio over 1000 %.