T. Fuse

Four-electron shell structures and an interacting two-electron system in carbon-nanotube quantum dots.

Low-temperature transport measurements have been carried out on single-wall carbon-nanotube quantum dots in a weakly coupled regime in magnetic fields. Four-electron shell filling was observed, and the magnetic field evolution of each Coulomb peak was investigated. Excitation spectroscopy measurements have revealed Zeeman splitting of single particle states for one electron in the shell, and demonstrated singlet and triplet states with direct observation of the exchange splitting at zero-magnetic field for two electrons in the shell, the simplest example of Hunds rule.

Terahertz photon-assisted tunneling in carbon nanotube quantum dots

The authors have studied the transport properties of carbon nanotube quantum dots under terahertz (THz) wave irradiation. The experimental data have shown that the satellite currents are generated with the THz irradiation, and that the peak position of the satellite currents varies linearly with the THz photon energy. These results provide experimental evidence for photon-assisted tunneling in the THz region. The present observation provides the interesting possibility of developing a highly sensitive and frequency-tunable THz detector capable of high-temperature operation.

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.

Quantum response of carbon nanotube quantum dots to terahertz wave irradiation

Single-electron transport measurements have been carried out at 1.5 K on single-wall carbon nanotube (SWCNT) quantum dots (QDs) under terahertz (THz) irradiation with a frequency of 2.5 THz. The most important finding in this study was new side-peaks that appeared only under the THz irradiation. From the detailed analysis with energy scales associated with the QDs, we conclude that the side-peaks originate from the photon-assisted tunnelling (PAT) of an electron in the QD to the drain electrode. The present THz-PAT is the first observation in QDs because of the larger energy scales in a SWCNT QD, compared with those of standard semiconductor QDs.

Coulomb peak shifts under terahertz-wave irradiation in carbon nanotube single-electron transistors

The authors have studied the effect of terahertz irradiation on single-electron transistors (SETs) based on single-wall carbon nanotubes, and have observed that the radiation generates Coulomb peak shifts. Time-resolved measurements of the terahertz response have revealed that the peak-shift signal has very long time constants, measured in seconds, and that the time trace of the signal after the terahertz irradiation is turned off deviates from a single-exponential curve. These experimental results suggest that the terahertz irradiation causes a charging process in trap states in the close vicinity of the SET, leading to a change in its effective gate voltage.

Excitation spectroscopy of two-electron shell structures in carbon nanotube quantum dots in magnetic fields

Single-quantum dots have been fabricated in an individual single-wall carbon nanotube, and the single-electron-transport measurements have been carried out at 40 mK. Coulomb diamonds showed an alternate change of the diamond size, an even-odd effect, indicating the two-electron shell filling. An excitation spectroscopy in which a Coulomb oscillation peak was measured in magnetic fields with a large source-drain bias voltage revealed the Zeeman splitting of single-particle states. The results suggest an important step for preparation of the spin two-level system.

Effect of the large current flow on the low-temperature transport properties in a bundle of single-walled carbon nanotubes

A large current flowing process has been used at 1.5 K to modify transport properties of quantum dots in a bundle of single-walled carbon nanotubes. After the processes were applied to the samples a few times, Coulomb oscillations started to be observed that did not appear before the process. The number of Coulomb peaks decreased as the process was repeated. The experimental observations could be explained by the simple model that the nanotube bundle was composed of many single and multiquantum dots in parallel, and some of them were broken by the current flowing process.

Highly sensitive and frequency-tunable THz detector using carbon nanotube quantum dots

We have developed a new type of THz detector using carbon nanotube quantum dots. We found that THz illumination generates new-side currents, whose peak position linearly depends on the THz-photon energy. This observation leads to highly sensitive and frequency-tunable detection of THz photons.

Single electron transport of carbon nanotube quantum dots under THz laser irradiation

Single‐electron transport properties have been measured at 1.5K on single‐wall carbon nanotube (SWNT) quantum‐dots (QDs) under terahertz (THz) laser irradiation with a frequency of 2.5THz. The interesting finding in this study was new side‐peaks in Coulomb oscillations, which appeared only under the THz irradiation. From the detailed analysis with energy scales associated with the QD, we conclude that the side‐peaks originate from the THz‐photon assisted tunneling (PAT) of an electron in the QD to the drain electrode.

Selecting single quantum dots from a bundle of single-wall carbon nanotubes using the large current flow process

Abstract We demonstrate selecting a Coulomb peak which originates from a single quantum dot, from a bundle of many single-wall carbon nanotubes. The method uses the previously reported current flowing process, by which the number of nanotubes can be reduced for the transport. By adjusting the gate voltage in an appropriate range, the single peak belonging to the single quantum dot has been selected. The effect of the high frequency application on the peak has been investigated, and it is shown that the basic response can be explained by the adiabatic response of the single dot to the high frequency signal.