Outline of the research

In this research project, we theoretically investigate four different Dirac electron systems, where the bulk measurement is available. By the careful comparison with the experimental results, we study the universal nature of Dirac electron systems.
Especially, we aim at the following three points, which do not appear in graphene.

1. Clarification of the conditions for the emergence and stabilization of the Dirac electron systems.
2. Problems of the internal degrees of freedom of Dirac electron systems.
3. Effects of spin-orbit interaction, magnetic field, and correlation between spin and charge.

We aim at the extension and unification of the concepts of the Dirac electron systems through these studies.

Subject

“Dirac electrons in solids”

In 1928, P.A.M. Dirac, who is one of the founders of quantum mechanics, predicted the existence of the “Dirac electron”, in his theory of relativistic quantum mechanics. Recently, it is becoming evident that the Dirac electron is present in the familiar materials and exhibits various properties. We are exploring universal and exciting physics in these Dirac electron systems.

“Dirac electrons in bismuth” (Fuseya, Ogata, Fukuyama)

Bismuth has the largest spin-orbit interaction in the safe elemental solids. Because of this large spin-orbit interaction, bismuth shows various interesting properties. For example, we have found that the pure spin-current (or the spin-Hall insulator), which is non-dissipative, or the fully spin-polarized electric current are possible in bismuth. We are conducting research aiming to develop new transport phenomena by Dirac electrons in bismuth.

“Theoretical study of spintronics phenomena” (Tatara)

Various spintronics phenomena such as spin injection, spin pumping effect, inverse spin Hall effect, spin Seebeck effect and magneto-optical effects, are studied based on a microscopic theory. In these effects, a key role is played by the spin-orbit interaction.

“Study of Dirac electron in organic conductors” (Kobayashi, Suzumura)

The origin of the Dirac electron which has been found in organic conductor, α(BEDT-TTF)₂I₃ salt, is not yet clarified owing to the accidental degeneracy of the energy band in 4x4 matrix Hamiltonian. The purpose of the present study is to comprehend the property of the wave function, and the symmetry of the transfer energy which are relevant to the Dirac point. Further we are planning to understand more general case beyond the above material.

“Topological effects of Dirac fermions” (Morinari, Tohyama)

Dirac fermions are characterized by chirality. Massless Dirac fermions are either left-handed or right-handed. This chirality makes crucial difference in physical properties of solids. We investigate physical phenomena created by chirality and their topological characterization.