Mysteries in Science: Measuring spin currents
(Professor, Department of Physics)
When an electric current flows through materials, lights switch on, televisions turn on, and electric cars run. An electric current is the flow of electrons and the electric charge they carry. However, electrons have not only an electric charge but also a property called spin. Spin is a magnetic property that causes each electron to be like a tiny compass needle. Therefore, flowing electrons carry both electric charge and spin. Recently, the field of condensed matter physics is taking a closer look at the concept of spin current, and research on phenomena caused by spin currents instead of electric currents has become more widespread. Members of the Department of Physics, such as Associate Professor Masamitsu Hayashi, Professor Shuji Hasegawa, Associate Professor Hosho Katsura, and Professor Masao Ogata, are conducting research in this area. However, although we can measure an electric current using an ammeter, a device that can directly measure a spin current has yet to be developed. It’s quite frustrating, like an itch you can’t scratch.
As illustrated in Figure (a), according to quantum mechanics, an electron’s spin is like a compass needle that can only point in two directions: north (spin-up) or south (spin-down). (The figures below depict a spin magnetic moment.) A spin is a physical quantity with the same dimensions as the angular momentum, which is the momentum of rotation. Therefore, as shown in Figure (a), the spin-up state can be regarded as a state in which the electron is rotating clockwise and the spin-down state as the electron rotating counterclockwise.
As shown in Figure (b), when spin-up electrons flow to the right and the same number of spin-down electrons flow to the left, the electric currents cancel each other out and become zero; however, the spin currents are not actually zero. The flow of the spin-up electrons and spin-down electrons are time-reversed from each other. When considering that spin corresponds to the rotation direction of an electron, as exhibited in Figure (a), this time-reversal reverses not only the direction of flow but also the direction of rotation. Therefore, in materials that maintain time-reversal symmetry, the flow of spin-up electrons to the right are equivalent to the flow of spin-down electrons to the left. Hence, if we define the spin current as the flow of spin-up electrons, the spin current is flowing to the right in Figure (b).
In other words, in the state depicted in Figure (b), no Joule heat is generated because the electric current is zero, but the spin current flows nevertheless. Information can then be carried on the spin current and sent from left to right by changing the size of the spin current or creating an alternating spin current. This means that information can be transmitted without dissipating energy, so it’s no wonder researchers are excited about the possibility of creating the ultimate energy-saving devices.
However, one may wonder whether it’s actually possible to create a state such as Figure (b) in a material. In fact, a surprising discovery has revealed that a material called a topological insulator is automatically in the state illustrated in Figure (b). As shown in Figure (c), the directions of electrons’ flow on the surface of a topological insulator and the directions of their spins are locked at right angles, which is then nothing but the spin current flowing.
Until recently we were unaware of the concept of the spin current as well as a topological insulator, which has spin currents flowing on its surface. As we don’t have a way to directly measure a spin current yet, we await the invention of a “spin current meter” that can be used as easily as an ammeter.
Figures (a) to (c): Electron spin and spin current. These figures depict a spin magnetic moment. (Source: Hasegawa, Shuji. Toporojikaru busshitsu to wa nanika (What is a Topological Material?). Kodansha Bluebacks, 2021.)
― This article is from the "Mysteries in Science" series in The Rigakubu News ―
Translated by Kristina Awatsu, Office of Communication
― Office of Communication ―