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Osada Group

Research Associate

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Osada group aims to search, elucidate, and control novel electronic states, quantum transport phenomena, and topological phenomena in low-dimensional/atomic layer materials, topological materials, and artificial nanostructures, by experiments on quantum transport phenomena under high magnetic field, low temperature, and high pressure environments. Main experimental tools include device fabrication of atomic layers, their complex stacks, and artificial nanostructures using advanced microfabrication/evaluation equipment, precision measurement of double-axial magnetic field angle dependence, high magnetic field measurement with 40T-class miniature pulse magnet. Recently, we have focused on quantum transport in twisted bilayer graphene and black phosphorus ultrathin films, topological electronic states and transport phenomena in organic conductors such as α-(BEDT-TTF)2I3, and magnetic field-induced electronic phase transition in ultrathin graphite.

(a) Band dispersion of the τ-type organic conductor with a finite width and a gap due to the spin-orbit coupling. There appear the spin-polarized helical edge states along edges, so that the system is a topological insulator. (b) Energy levels of the τ-type organic conductor with a finite spin-orbit coupling under magnetic fields (Hofstadter butterfly). Although no Zeeman effect is considered, we can see additional spin splitting with the orbital origin. The energy gap at zero field corresponds to the gap of each spin subband with the Chern number of ±1.
Application example of the method for studying the anisotropy of the band structure by the interlayer magnetoresistance. (a) In-plane magnetic field orientation dependence of interlayer magnetoresistance of an layered organic conductor α-(BEDT-TTF)2I3. Measured from weak charge-ordered state to two-dimensional Dirac semimetal state under different pressure. A local minimum of resistance occurs in the magnetic field orientation perpendicular to the principal axis of the tilted Dirac cone. (b) Arrangement of experiments. (c) Schematic diagram of the band structure of α-(BEDT-TTF)2I3 with a tilted Dirac cone and a van Hove singularity (saddle point). The anisotropy of Dirac cone where the Fermi level is located can be measured at low temperatures, and the anisotropy of the van Hove singularity with high density of states at high temperatures due to thermally excited carriers.

Research Subjects

  1. Topological properties of an organic Dirac semimetal
  2. Electronic structure and quantum transport in atomic layers and their van der Waals stacks
  3. Quantum transport phenomena in layered topological materials
  4. Interlayer coherence and angle-dependent magnetotransport in layered conductors
  5. Magnetic-field-induced electronic phase transitions in thin-film graphite