Institute of Scientific and Industrial Research, Osaka University

   Oiwa Laboratory


Spin manipulation and spin coherence using self-assembled nanostructures

 The spin systems of electrons, holes in semiconductors can be regarded as an ideal two-level system. The rotation operation and the spin coherence are extensively studied toward the applications to quantum computing. Because of their inherent well-defined quantum structures and wide range of material choices, self-assembled quantum dots (QDs) and nanowires (NWs) would enable us to study the novel phenomena in solid states, for instance, the spin manipulation and spin coherence and the interplay between superconductivity and Kondo effect, which are representative many-body effects in solid states.

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Figure1.SEM image of an InAs self-assembled QD with nano-gap electrodes
Control of spin-orbit interaction and g-factor in QDs

 Because InAs self-assembled QDs have strong spin-orbit interaction and large g-factor, the characteristic features related to spin appears predominantly. Especially, it is an important feature to manipulate the spins without controlling the external magnetic field whereas the spin-orbit interaction is one of the prime mechanisms of the spin relaxation and decoherence. Until now, we have elucidated the origin of the spin-orbit interaction in the InAs self-assembled QDs [1] and have successfully demonstrated the electrical control the spin-orbit interaction [2]. Thus this allows us to design the novel devices that work by optimizing the spin controllability and spin relaxation. Moreover, we have shown the electrical control of g-factors, which governs the spin states, and have demonstrated the possibility of the spin manipulation via an electrical g-factor (g-tensor) modulation [3,4].
We are going to investigate the nanostructures consisting of Si and Ge. While all the atoms, Ga, In and As in III-V group materials have nuclear spins, resulting in a fluctuating nuclear spin field as a major source of spin decoherence via a hyperfine interaction, the natural abundance of nuclear spins in Si and Ge is about 5 % and the influence of the nuclear spins is suppressed. Thus, these materials are suitable candidates for realizing long spin coherence time needed for the quantum computing and quantum memories. Therefore, we study the spin physics and develop the spin qubits using the SiGe self-assembled QDs.

Junction with superconductors

 The self-assembled QDs and nanowires have an advantage for making junctions with various materials. For example, superconductor transistors have been realized using superconducting materials as electrodes [5,6]. Such junctions between superconductors and QDs are very fascinate systems where the attractive interaction of Cooper pairs in the superconductor competes to the repulsive Coulomb interaction in the QDs. We have realized the supercurrent in QD Josephson junctions [6,7] and the tunneling spectroscopy of Andreev bound state due to Andreev reflections in superconductor/QD/normal junctions [8,9]. Kondo effect is also an interesting spin many-body effect in solid states. This is attributed to the coupling between the localized spin in a QD and the electron spins in the leads and has been already observed [10]. Furthermore, it has been shown that the magnitude of the interaction can be electrically controlled and the interplay between Kondo effect and superconductivity has been elucidated [7,11].

Exploring topological superconductivity

 Recently, the new phase in solid states called as topological insulators has been comprehensively studied in the systems with strong spin-orbit interaction. One of the most interesting phenomena of the topological insulator is an emergence of Majorana fermions. Majorana particles have been originally proposed in particle physics. Though the neutrinos are the candidate of Majorana particles it has not been yet discovered. There is the theoretical proposals that the topological superconducting state can appear in systems with strong spin-orbit interaction coupled to s-wave superconductor under applied external magnetic field. Although the experimental results suggesting Majorana bound states in a one-dimensional system subsequently the existence of Majorana fermions are not verified satisfactory. The non-Abelian statistics would provide a root to realize topologically robust quantum computing. We are studying the topological superconductivity and Majorana fermions using narrow gap semiconductor nanostructures with strong spin-orbit interaction.

References
  1. "Large Anisotropy of the Spin-Orbit Interaction in a Single InAs Self-Assembled Quantum Dot", S. Takahashi, R. S. Deacon, K. Yoshida, A. Oiwa, K. Shibata, K. Hirakawa, Y. Tokura, S. Tarucha, Phys. Rev. Lett. 104, 246801 (2010).
  2. "Electrically tuned spin-orbit interaction in an InAs self-assembled quantum dot", Y. Kanai, R.S. Deacon, S. Takahashi, A. Oiwa, K. Yoshida, K. Shibata, K. Hirakawa, Y. Tokura, S. Tarucha, Nature Nanotechnology 6, 511 (2011). .
  3. "Electrically tunable three-dimensional g-factor anisotropy in single InAs self-assembled quantum dots", S. Takahashi, R. S. Deacon, A. Oiwa, K. Shibata, K. Hirakawa, and S. Tarucha, Phys. Rev. B 86, 161302(R) (2013).
  4. "Electrically tuned g tensor in an InAs self-assembled quantum dot", R. S. Deacon, Y. Kanai, S. Takahashi, A. Oiwa, K. Yoshida, K. Shibata, K. Hirakawa, Y. Tokura, and S. Tarucha, Phys. Rev. B 84, 041302 (2011).
  5. "Lateral electron tunneling through single self-assembled InAs quantum dots coupled to superconducting nanogap electrodes", K. Shibata, C. Buizert, A. Oiwa, K. Hirakawa, and S. Tarucha, Appl. Phys. Lett. 91, 112102 (2007).
  6. "Control of supercurrent in a self-assembled InAs quantum dot Josephson junction by electrical tuning of level overlaps", Y. Kanai, R.S. Deacon, A. Oiwa, K. Yoshida, K. Shibata, K. Hirakawa and S. Tarucha, Appl. Phys. Lett. 100, 202109 (2012).
  7. "Electrical control of Kondo effect and superconducting transport in a side-gated InAs quantum dot Josephson junction", Y. Kanai, R. S. Deacon, A. Oiwa, K. Yoshida, K. Shibata, K. Hirakawa, S. Tarucha, Phys. Rev. B 82, 054512 (2010).
  8. "Kondo-enhanced Andreev transport in single self-assembled InAs quantum dots contacted with normal and superconducting leads", R. S. Deacon, Y. Tanaka, A. Oiwa, R. Sakano, K. Yoshida, K. Shibata, K. Hirakawa, and S. Tarucha, Phys. Rev. B 81, 121308(R) (2010).
  9. "Tunneling Spectroscopy of Andreev Energy Levels in a Quantum Dot Coupled to a Superconductor", R. S. Deacon, Y. Tanaka, A. Oiwa, R. Sakano, K. Yoshida, K. Shibata, K. Hirakawa, and S. Tarucha, Phys. Rev. Lett. 104, 076805 (2010).
  10. "The spin-half Kondo effect in a single self-assembled InAs quantum dot at zero and a high magnetic field", Y. Igarashi, M. Jung, M. Yamamoto, A. Oiwa, T. Machida, K. Hirakawa, and S. Tarucha, Phys. Rev. B Rapid Commun. 76, 081303 R (2007).
  11. "Kondo universality for a quantum dot coupled to superconducting leads", C. Buizert, A. Oiwa, K. Shibata, K. Hirakawa, and S. Tarucha, Phys. Rev. Lett. 99, 136806 (2007).