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
    • Intrinsic gap and exciton condensation in the nu_T=1 bilayer system
      P. Giudici, K. Muraki, N. Kumada, and T. Fujisawa
      Phys. Rev. Lett. 104, 056802 (2010).
    Our result clarify the origin of the quasiparticle excitation in the nu_T=1 bilayer QH state demonstrating to be a pure pseudospin excitation. The intrinsic transition from the QH to compressible states can be explained in terms of a continuous transformation from a exciton condensate towards two weakly-interacting composite Fermions system. Furthermore, a relation between the order of the phase transition and the spin polarization of the compressible state can be inferred.

    • Spin-dependent phase diagram of the nu_T=1 bilayer electron system
      P. Giudici, K. Muraki, N. Kumada, Y. Hirayama, and T. Fujisawa
      Phys. Rev. Lett. 100, 106803 (2008).
    We show that the spin degree of freedom plays a decisive role in the phase diagram of the nu_T=1 bilayer electron system using an in-plane field in the regime of negligible tunneling. A new phase diagram as a function of d/l_B and the Zeeman energy is established.

    • Pseudospin soliton in the nu=1 bilayer quantum Hall state
      A. Fukuda, D. Terasawa, M. Morino, K. Iwata, S. Kozumi, N. Kumada, Y. Hirayama, Z. F. Ezawa, and A. Sawada
      Phys. Rev. Lett. 100, 016801 (2008).
    We investigate pseudospin soliton lattice in the bilayer quantum Hall state at nu=1. Rxx is anisotropic for the angle between the in-plane magnetic field and the current, indicating that the soliton lattice aligned parallel to the in-plane field is formed.

    • Spin degree of freedom in the nu=1 bilayer electron system investigated by nuclear spin relaxation
      N. Kumada, K. Muraki, K. Hashimoto, and Y. Hirayama
      Phys. Rev. Lett. 94, 096802 (2005).
    The nuclear-spin-relaxation rate has been measured in a bilayer electron system at and around nu=1. The results demonstrate that, as opposed to common assumption, the electron spin degree of freedom is not completely frozen either in the QH or compressible states.
    >>further information

  • Bilayer quantum Hall state at nu=2

  • Bilayer quantum Hall state at nu=2/3

  • Control of nuclear spins in semiconductors

  • Other topics