Research topics
I am interested in many-body physics in two-dimensional electron systems (2DESs). In particular, I investigate electron spin states in a quantum Hall system by resistively-detected nuclear magnetic resonance. Recently, I started research on plasmon transport in GaAs/AlGaAs 2DESs or graphene using time-resolved transport measurement.

  • Plasmon transport in graphene and GaAs/AlGaAs 2DESs

  • Quantum Hall effect

  • Bilayer quantum Hall state at nu=1

  • Bilayer quantum Hall state at nu=2

  • Bilayer quantum Hall state at nu=2/3

  • Control of nuclear spins in semiconductors

  • Other topics
    • Observation of Bogoliubov excitations in exciton-polariton condensates
      S. Utsunomiya, L. Tian, G. Roumpos, C. W. Lai, N. Kumada, T. Fujisawa, M. Kuwata-Gonokami, A. Loffler, S. Hofling, A. Forchel, and Y. Yamamoto
      Nature Physics 4, 700 (2008).
    Exciton-polaritons in a semiconductor microcavity, which are elementary excitations created by strong coupling between quantum-well excitons and microcavity photons, were proposed as a new BEC candidate in solid-state systems. In this letter, we report the first observation of interaction effects on the exciton-polariton condensate and the excitation spectra, which are in quantitative agreement with the Bogoliubov theory.

    • Boundary-mediated electron-electron interactions in quantum point contacts
      V. T. Renard, O. A. Tkachenko, V. A. Tkachenko, T. Ota, N. Kumada, J-C Portal, and Y. Hirayama
      Phys. Rev. Lett. 100, 186801 (2008).
    An unusual increase of the conductance with temperature is observed in clean quantum point contacts for conductances larger than 2e2/h. A model accounting for electron-electron interactions mediated by boundaries (scattering on Friedel oscillations) qualitatively describes the observation.

    • Coherent zero-state and pi-state in an exciton-polariton condensate array
      C. W. Lai, N. Y. Kim, S. Utsunomiya, G. Roumpos, H. Deng, M. D. Fraser, T. Byrnes, P. Recher, N. Kumada, T. Fujisawa, and Y. Yamamoto
      Nature 450, 529 (2007).
    we report the observation of spontaneous build-up of in-phase (zero-state) and antiphase (pai-state) 'superfluid' states in an array of exciton-polariton condensates connected by weak periodic potential barriers within a semiconductor microcavity. These inphase and antiphase states reflect the band structure of the one-dimensional polariton array and the dynamic characteristics of metastable exciton-polariton condensates.