Associate Professor, Department of Physics, Tokyo Institute of Technology
The new field of electronics using the spin degree of freedom is called spintronics. The fundamental question in spintronics is how to operate spins and induce spin current in solids. Because the spin carries magnetism, a naïve way to produce a spin current is to use a ferromagnet.
In my research, I propose another way of producing a spin current in solids, but without a magnetic field. I address our theoretical proposal for intrinsic spin Hall effect (SHE), where an external electric field induces a transverse spin current in semiconductors and metals as shown in Fig. 1.
Fig.1) Schematic figure of the spin Hall effect.
This is caused by the “Berry phase in momentum space,” based on the wave nature of electrons. Since our theoretical proposal, this effect has been observed in various semiconductors and metals. The most prominent among them is platinum, where the SHE survives even at room temperature. This prominent SHE in platinum can be understood from its band structure. This effect enables us to inject or detect spin current in metals and semiconductors with a simple setup and has already been used as an experimental tool for characterizing spin-transport properties of various materials. Because the wave nature of electrons is necessary for the SHE, one can expect a similar effect for other types of waves such as light (electromagnetic wave). We find that the SHE of light appears where the refractive index varies spatially. In the case of interface between different media, this SHE appears as the Imbert shift, i.e., the transverse shift at the interface (Fig. 2).
Another topic of recent interest, which arose from the SHE, is quantum spin Hall (QSH) systems (topological insulators). In QSH systems, the bulk is gapped and insulating, while the edge or surface states are gapless and carry spin current. We proposed that bismuth ultrathin film is a good candidate for the QSH system. This prediction has opened the way for the experimental discovery of Bi1-xSbx as a three-dimensional topological insulator.
Fig.2) Imbert shift: an example for the spin Hall effect of light