Electronic States Spectroscopy Research Team

Principal Investigator

PI Name Kyoko Ishizaka
Degree Ph.D.
Title Team Leader
Brief Resume
2004Ph.D., Engineering, University of Tokyo
2004Research Associate, Institute for Solid State Physics, University of Tokyo
2010Associate Professor, Department of Applied Physics, School of Engineering, University of Tokyo
2014Associate Professor, Quantum-Phase Electronics Center, School of Engineering, University of Tokyo
2016Team Leader, Electronic States Spectroscopy Research Team, Strong Correlation Physics Division, RIKEN Center for Emergent Matter Science (-present)
2018Professor, Quantum-Phase Electronics Center, School of Engineering, University of Tokyo (-present)

Outline

We investigate the electronic states of materials showing a variety of physical properties, functions, and quantum phenomena. By utilizing spin- and angle-resolved photoemission spectroscopy, which can probe the energy, momentum, and spin of electrons, we investigate new materials, topological quantum states, and many-body effects in strongly correlated systems. We are also developing the pulsed-laser based ultrafast transmission electron microscopy, aiming for probing the dynamical states of nanoscale spin/lattice/charge textures, materials, and devices.

Research Fields

Physics, Materials Science

Keywords

Ultrafast time-resolved TEM
Photoelectron Spectroscopy
Strongly Correlated Electron System
Superconductivity
Topological Materials

Results

Ultrafast nematic electron excitation in superconductor FeSe

The electronic nematic phase is an unconventional state of matter that spontaneously breaks the rotational symmetry of electrons. In iron-pnictides/chalcogenides and cuprates, the nematic ordering and fluctuations have been suggested to have as-yet-unconfirmed roles in superconductivity. In this study, we used femtosecond optical pulse to perturb the electronic nematic order in FeSe. Through time-, energy-, momentum- and orbital-resolved photoelectron spectroscopy, we detected the ultrafast dynamics of electronic nematicity. In the strong-excitation regime, through the observation of Fermi surface anisotropy, we found a quick disappearance of the nematicity followed by a heavily-damped oscillation. This short-life nematicity oscillation is seemingly related to the imbalance of Fe 3dxz and dyz orbitals, and shows a critical behavior as a function of pump fluence. Our real-time observations reveal the nature of the electronic nematic excitation instantly decoupled from the underlying lattice.

Time- angle-resolved photoelectron spectroscopy (upper left), schematic transient Fermi surface (lower left), and transient Fermi momentum obtained by real-time measurement (right).

 

Observation of topological electronic state emerging at the surface of a superconductor

There has been increasing interest on topological materials with non-trivial surface edge states (topological surface states). The topological surface states possess the massless Dirac electrons with their spins polarized perpendicular to their momenta, which give us chances to generate novel electromagnetic functions than cannot be found in bulks. The development of new topological materials including insulators, metals, and superconductors is being extensively examined. Here we utilized spin- and angle-resolved photoemission spectroscopy to investigate the surface and bulk states of a superconductor PdBi2. We found the spin-polarized surface bands emerging at the surface, which was proved to have the topologically non-trivial character by the Z2 analysis based on the first-principle calculations. Possible novel phenomena associated with the topological surface states and the superconductivity will be investigated in the future.

The band structures (left, center) and the spin polarization (right) obtained by the spin- and angle-resolved photoemission spectroscopy.

Members

Kyoko Ishizaka

Team Leader kyoko.ishizaka[at]riken.jp R

Takahiro Shimojima

Research Scientist

Asuka Nakamura

Special Postdoctoral Researcher

Publications

  1. A. Nakamura, T. Shimojima, Y. Chiashi, M. Kamitani, H. Sakai, S. Ishiwata, H. Li, and K. Ishizaka

    Nanoscale Imaging of Unusual Photoacoustic Waves in Thin Flake VTe2

    Nano Lett. 20, 4932 (2020)
  2. N. Mitsuishi, Y. Sugita, M. S. Bahramy, M. Kamitani, T. Sonobe, M. Sakano, T. Shimojima, H. Takahashi, H. Sakai, K. Horiba, H. Kumigashira, K. Taguchi, K. Miyamoto, T. Okuda, S. Ishiwata, Y. Motome, and K. Ishizaka

    Switching of band inversion and topological surface states by charge density wave

    Nat. Commun. 11, 2466 (2020)
  3. T. Shimojima, Y. Suzuki, A. Nakamura, N. Mitsuishi, S. Kasahara, T. Shibauchi, Y. Matsuda, Y. Ishida, S. Shin, and K. Ishizaka

    Ultrafast nematic-orbital excitation in FeSe

    Nat. Commun. 10, 1946 (2019)
  4. A. Nakamura, T. Shimojima, M. Matsuura, Y. Chiashi, M. Kamitani, H. Sakai, S. Ishiwata, H. Li, A. Oshiyama, and K. Ishizaka

    Evaluation of photo-induced shear strain in monoclinic Vte2 by ultrafast electron diffraction

    Appl. Phys. Express 11, 092601 (2018)

Articles

お問い合わせ

2-1 Hirosawa, Wako, Saitama 351-0198 Japan

E-mail:
kyoko.ishizaka[at]riken.jp

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