Special CPM Seminar

Carbon nanotubes and semiconductor nanowires for quantum nanodevice applications

Koji Ishibashi

Advanced Devices Laboratory
RIKEN

In this talk, I review research activities in my group rather than talking about specific topics to exchange information between McGill and RIKEN. In my group, we are interested in quantum nanodevices such as quantum bits (qubits) and quantum THz detectors, as well as nanofabrication in a molecular scale. To make use of quantum effects, the smaller structures are required, so that we use carbon nanotubes and semiconductor nanowires that are self assembly formed with an extremely small diameter. We use single-wall carbon nanotubes (SWCNTs), Si/Ge nanowires and InAs naowires, depending on the type of quantum nanodevices. In this talk, we show main experimental results on these devices listed below. Part of the works is done in collaboration with NIMS and NTT basic research lab.

1) Towards spin qubit with SWCNT and Ge quantum dots
To realize the spin qubit, we have to begin with preparing a single spin in a quantum dot (QD). To do so, there are two methods. The simplest method is to prepare absolute one electron in the dot. The other method is to realize an unpaired electron in the uppermost quantum level with many electrons in the dot. The former method is conceptually simple, but is not always easy in practice. To realize the latter condition, there are requirements among energy scales with E (level spacing) much larger than electron-electron interaction energies. We demonstrate each case with the Ge QDs and SWCNT QDs where artificial atom behaviours are observed.

2) Towards Andreev qubit and study of transport physics with a large spin-orbit interaction in InAs nanowires
The InAs nanowires are known to easily make Ohmic contacts with metallic contacts. We have been studying basic properties of individual InAs nanowires with superconducting contacts (SNS), which could be a basic building block of the Andreev qubit. We will show basic transport properties of the SNS structures, which includes supercurrent modulated by gate voltage, its magnetic field and temperature dependence, and microwave effects. These results indicate the dirty Josephson junction behaviours.

3) Quantum response of the SWCNT quantum dots
One of the unique features of the SWCNT QDs is large energy scales, associated with the artificial atom (QD), which fall in a teraherz (THz) range. This fact made us to explore the quantum response of the SWCNT QDs to the THz wave. In fact, we have observed the THz photon assisted tunnelling in the Coulomb blockade oscillations with frequency-dependent satellite peaks.

4) SWCNT/Molecule heterustructures for molecular scale nanostructures
Another unique feature of the SWCNT would be a possible chemical modification of the nanotube ends and a surface. This makes it possible to fabricate chemically bonded SWCNT/molecule heterojunctions. As examples, we show chemically bonded individual SWCNT rings and SWCNT/molecule heterostructures to fabricate a QD. The structures are characterized by a scanning tunnelling microscope with simultaneous optical spectroscopy, such as Raman and photo-current spectroscopy and the electric field modulation spectroscopy.

References
[1] S. Moriyama, T. Fuse, M. Suzuki, Y. Aoyagi, K. Ishibashi, Phys. Rev. Lett. 94, 186806 (2005)
[2] T. Nishio, T. Kozakai, S. Amaha, M. Larsson, H. Nilsson, H. Q. Xu, G. Q. Zhang, K. Tateno, H. Takayanagi and K. Ishibashi, Nanotechnology, 44, 5701 (2011)
[3] Y. Kawano, T. Fuse, S. Toyokawa, T. Uchida, K. Ishibashi, J. Appl. Phys. 103, 034307 (2008)