Strong Correlation Interface Research Group

Principal Investigator

PI Name Masashi Kawasaki
Degree D.Eng.
Title Group Director
Brief Resume
1989 D.Eng., University of Tokyo
1989 Postdoctoral Fellow, Japan Society for the Promotion of Science
1989 Postdoctoral Fellow, T. J. Watson Research Center, IBM, USA
1991 Research Associate, Tokyo Institute of Technology
1997 Associate Professor, Tokyo Institute of Technology
2001 Professor, Tohoku University
2007 Team Leader, Functional Superstructure Team, RIKEN
2010 Team Leader, Strong-Correlation Interfacial Device Research Team, RIKEN
2011 Professor, University of Tokyo
2013 Deputy Director, RIKEN Center for Emergent Matter Science (CEMS) (-present)
2013 Group Director, Strong Correlation Interface Research Group, Strong Correlation Physics Division, RIKEN CEMS (-present)

Outline

Thin films and interfaces of topological materials are the playground of our research. Chiral spin textures in real space and magnetic monopoles in momentum space are the sources of non-trivial Hall effect. Photo-excited polar crystals generate unconventional photocurrent. Not classical mechanics but quantum mechanics is needed to understand those examples. We will design and demonstrate possible devices that utilize expectedly dissipationless electron flow exemplified as above. The device physics study will open a new avenue towards topological electronics that manage flow of information and energy carried by such topological current.

Research Fields

Physics, Engineering, Chemistry, Materials Science

Keywords

Topological electronics
Thin films and interfaces
Topological materials
Unconventional photovoltaic effect
Unconventional Hall effect

Results

Excitonic properties of high-quality iodide thin films grown by molecular beam epitaxy

 Iodide semiconductors are quite promising materials for such optical devices as solar cells and light-emitting devices because of their strong optical absorption and high exciton stability. Moreover, iodides start to attract much attention as ideal platforms for exploring emergent quantum phenomena due to recently discovered topological magnetic structures and quantum transport. However, very few studies have been reported on the epitaxial films of iodides. We aim at establishing the growth method of high-quality iodide thin films employing molecular beam epitaxy (MBE), and exploring novel quantum physics and functionalities emerging at the interface of iodide heterostructures.

To begin with, we grew cuprous iodide (CuI) films on InAs substrates with excellent lattice matching, yielding in single-crystalline films having outstandingly high lattice coherence and atomically flat surfaces. The films exhibit extremely sharp free exciton emission at low temperatures that has never been observed in bulk single-crystals. This is a major step toward the development of iodide electronics.

Schematic of free exciton emission from epitaxially-grown CuI films

Schematic of free exciton emission from epitaxially-grown CuI films

 

Quantum transport phenomena of spin polarized electrons in high mobility magnetic semiconductor EuTiO3

Magnetic semiconductors are a vital component in future spintronics. Conventional magnetic semiconductors contain a large amount of magnetic impurities as the dopant, resulting in the low mobility. Therefore, it is difficult to observe interference effects of spin polarized electrons such as Shubnikov- de Haas (SdH) oscillation. Here, we have developed a gas source molecular beam epitaxy system, yielding in high crystalline quality films of La doped EuTiO3. The maximum mobility reaches 3,200 cm2V-1s-1 at 2 K, showing clear quantum oscillations in magnetoresistance. Using first-principles calculations, we show that the observed SdH oscillations originate genuinely from Ti 3d-t2g states which are fully spin-polarized due to their energetical proximity to the in-gap Eu 4f bands. This suggests that EuTiO3 film is an ideal magnetic semiconductor, offering a fertile field to explore quantum phenomena.

Figure

Schematic of an interference effect of spin polarized electrons

 

Generation and nonlocal spreading of shift current under local photoexcitation

Noncentrosymmetric materials exhibit a generation of photocurrent without an external electric field, called the bulk photovoltaic effect. In this study, we have revealed the mechanism of the photocurrent generation and spreading under local photoexcitation by potentiometric measurements for a prototypical ferroelectric semiconductor SbSI.

We shined a focused laser light on a SbSI single crystal, and measured the voltage inside and outside of photoirradiated region simultaneously. The results indicate that the photocurrent emerging in photoirradiated region is a less dissipative current driven by the Berry phase of electron’s wavefunction, termed shift current, whereas current spread outside as a dissipative current driven by the internal field. On the basis of result, we determined the equivalent circuit model to simulate the bulk photovoltaic effect in various devise structures. We have also succeeded in fabricating thin films of SbSI with aligned polarization axis by molecular beam epitaxy.

Figure

Schematic of photocurrent generation under local photoexcitation in ferroelectric semiconductor SbSI

 

Control of conduction electrons by magnetic monopoles in a magnetic semiconductor

Magnetic semiconductor is one of the candidates for novel spintronics devices with low power consumption because both magnetic and transport properties can be controlled. Because the Hall effect of magnetic semiconductors can be electrically tuned, magnetic semiconductors have attracted considerable interest from the view point of the application.

 Band crossing called as Weyl nodes which generate magnetic monopoles in momentum space is known to be one of the origin of anomalous Hall effect (AHE). We discovered a new phenomenon of AHE in a high electron mobility oxide magnetic semiconductor, EuTiO3. During the magnetization process of the spin moment on Europium (Eu) under applied magnetic field, it was found that the AHE is not proportional to the magnetization as usual magnets. We revealed that changing the energy position of Weyl nodes that create magnetic monopoles in momentum space by tiny change of Zeeman splitting in the magnetization process dramatically affect the trajectory of electron conductions.

Figure

Schematic of the trajectory of electron conduction modified by the monopole (red ball) in momentum space

 

 

Members

Masashi Kawasaki

Group Director m.kawasaki[at]riken.jp R
Kei Takahashi Senior Research Scientist kei.takahashi[at]riken.jp R

Masao Nakamura

Senior Research Scientist masao.nakamura[at]riken.jp R

Denis Maryenko

Senior Research Scientist maryenko[at]riken.jp
Jobu Matsuno Visiting Scientist R

Publications

  1. M. Uchida, S. Sato, H. Ishizuka, R. Kurihara, T. Nakajima, Y. Nakazawa, M. Ohno, M. Kriener, A. Miyake, K. Ohishi, T. Morikawa, M. S. Bahramy, T.-h. Arima, M. Tokunaga, N. Nagaosa, and M. Kawasaki

    Above-ordering-temperature large anomalous Hall effect in a triangular-lattice magnetic semiconductor

    Sci. Adv. 7, eabl5381 (2021)
  2. T. Yasunami, M. Nakamura, S. Inagaki, S. Toyoda, N. Ogawa, Y. Tokura, and M. Kawasaki

    Molecular beam epitaxy of two-dimensional semiconductor BiI3 films exhibiting sharp exciton absorption

    Appl. Phys. Lett. 119, 243101 (2021)
  3. D. Maryenko, M. Kawamura, A. Ernst, V. K. Dugaev, E. Y. Sherman, M. Kriener, M. S. Bahramy, Y. Kozuka, and M. Kawasaki

    Interplay of spin-orbit coupling and Coulomb interaction in ZnO-based electron system

    Nat. Commun. 12, (2021)
  4. S. Nishihaya, M. Uchida, Y. Nakazawa, M. Kriener, Y. Taguchi, and M. Kawasaki

    Intrinsic coupling between spatially-separated surface Fermi-arcs in Weyl orbit quantum Hall states

    Nat. Commun. 12, (2021)
  5. S. Inagaki, M. Nakamura, Y. Okamura, M. Ogino, Y. Takahashi, L. C. Peng, X. Z. Yu, Y. Tokura, and M. Kawasaki

    Heteroepitaxial growth of wide bandgap cuprous iodide films exhibiting clear free-exciton emission

    Appl. Phys. Lett. 118, 012103 (2021)

Articles

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E-mail:
m.kawasaki[at]riken.jp

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