ISSP - The institute for Solid State Physics

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Tokunaga Group
Associate Professor

Research Associate

Research Associate

Project Research Associate

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Magnetic fields have been extensively used in broad research fields of solid state physics because they can directly tune the spins, orbitals and phases of electrons in materials. We study various kinds of phase transitions in high magnetic fields with using non-destructive pulse magnets and developing/improving various experimental techniques; e.g. magnetization, magnetoresistance, electric polarization, polarizing optical microscopy, and so on. As one of our recent projects, we focus on the electronic states in the ultra-quantum limit state. Since charge carriers are confined in the smallest cyclotron orbit, Coulomb interaction dominates over the kinetic energy. Therefore, we can realize strongly correlated electron systems in the quantum limit states. In particular, we have been focusing on the semimetals having equal number of electrons and holes, and found a novel field-induced phase in graphite, complete valley polarization in bismuth, and anomalous quantum transport properties in black phosphorus under multiple extreme conditions. We are also studying multiferroic materials through high precision experiments in pulsed-fields. In BiFeO3, which is perhaps the most extensively studied multiferroic material, we found bipolar resistive memory effect, magnetic control of ferroelastic strain, and novel multiferroic phase at around room temperature. In addition to these in-house studies, we accept about 40 joint research projects per year and study various localized/itinerant magnets and topological materials in high magnetic fields.

Longitudinal magnetostriction of bismuth measured by the capacitance method in pulsed high magnetic field (blue line). Quantitative coincidence with theoretical calculation (broken line) suggests emergence of field-induced valley polarization predicted in this model. This result provides us of thermodynamic evidence for the complete valley polarization, namely one of the three Fermi pockets dries up, at around 40 T as illustrated in the inset.
(left) Magnetization curves of single crystals of BiFeO3. Anomalies of magnetization curves shown in the shaded area suggest emergence of a novel multiferroics phase (IM phase) in this field-temperature region. (right) Schematic illustration of the spin arrangement in the IM phase.

Research Subjects

  1. Field-induced transitions in multiferroic materials
  2. Electronic phase transitions in the quantum limit state
  3. High-speed polarizing microscope imaging in pulsed-high magnetic fields
  4. High-field study of topological materials