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)
2023 Research Strategy Advisor (Deputy Executive Director), RIKEN (-present)


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


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


Mott-insulator to metal transition in artificial oxide quantum wells

It is widely known that many examples of unconventional superconductivity emerge when carriers are doped in two dimensional Mott insulators. For many years, efforts have been focused on artificially fabricating two-dimensional Mott insulators with thin film technology of transition metal oxides. However, there have been few examples that succeeded in achieving metallic states by carrier doping.

In this study, we have employed a gas source molecular beam epitaxy system with our original design. Confined single quantum well structures are constructed by sandwiching a correlated metal SrVO3 layer with two band insulator SrTiO3 layers. High quality SVO3 films exhibits clearer transition from metal to insulator at about 1-2 nm when the thickness is reduced. When electron carriers are doped in these quantum well Mott insulators, metallic state is successfully induced. Thus, the SrVO3 quantum wells are demonstrated as an ideal playground for studying physics of correlated electron systems.

Metal-insulator transition in SrVO3 two-dimensional artificial lattices


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


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.


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.


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.


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



Observation of shift current photovoltaic effect in an organic ferroelectric compound

Noncentrosymmetric crystals show spontaneous current by photoexcitation. The origin of this phenomenon is theoretically revealed to be the shift current driven by the Berry phase of the Bloch wave function. Recently, shift current attracts attention due to its topological origin as well as the large potential for applications in solar cells and photo-detectors. Here, we focus on an electronic ferroelectric, a material showing a spontaneous polarization dominantly originates from the displacement of electron clouds, as a candidate material showing large shift current. An organic charge transfer complex TTF-CA is one of the representative electronic ferroelectrics. In addition, this compound has a large response to visible and infrared light due to the narrow bandgap of 0.5 eV. We observed a sizable zero-bias photocurrent in TTF-CA below the ferroelectric transition temperature under simulated solar radiation. The current density is larger by several orders compared to other ferroelectric compounds. We also revealed that the zero-bias photocurrent propagates quite a long distance by means of local photoexcitation. This is a clear evidence for the non-local nature of shift current.

Schematic of shift current generation in an organic ferroelectric TTF-CA




Masashi Kawasaki

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

Masao Nakamura

Senior Research Scientist masao.nakamura[at]

Denis Maryenko

Senior Research Scientist maryenko[at]

Noriyuki Takahara

Junior Research Associate
Jobu Matsuno Visiting Scientist


  1. K. S. Takahashi, Y. Tokura, and M. Kawasaki

    Metal-insulator transitions in dimensionality controlled LaxSr1-xVO3 films

    APL Mater. 10, 111114 (2022)
  2. M. Nakamura, S. Inagaki, Y. Okamura, M. Ogino, Y. Takahashi, K. Adachi, D. Hashizume, Y. Tokura, and M. Kawasaki

    Band structures in orientation-controlled CuI thin films under epitaxial strain

    Phys. Rev. B 106, 125307 (2022)
  3. R. Nishino, T. C. Fujita, and M. Kawasaki

    Electric field control of anomalous Hall effect in CaIrO3/CaMnO3 heterostructure

    APL Mater. 10, 081104 (2022)
  4. M. Ohno, S. Minami, Y. Nakazawa, S. Sato, M. Kriener, R. Arita, M. Kawasaki, and M. Uchida

    Maximizing intrinsic anomalous Hall effect by controlling the Fermi level in simple Weyl semimetal films

    Phys. Rev. B 105, L201101 (2022)
  5. J. Falson, I. Sodemann, B. Skinner, D. Tabrea, Y. Kozuka, A. Tsukazaki, M. Kawasaki, K. von Klitzing, and J. H. Smet

    Competing correlated states around the zero-field Wigner crystallization transition of electrons in two dimensions

    Nat. Mater. 21, 311–316 (2022)