Researchers at the University of Bath in England have discovered, contrary to expectations, that in the field of “Valleytronics,” valley polarization can make electron spin polarization in silicon transistors easier. This discovery could have an impact on the development of silicon-based “spintronic” devices and quantum computing.
Despite this blog having covered the field dubbed “Valleytronics” with increasing frequency over the past year, it remains a fairly esoteric research area. A shorthand definition for valleytronics would be a movement away from exploiting the electrical charge of electrons as a means for storing information, and instead using the wave quantum number of an electron in a crystalline material to encode data. The “valley” of the portmanteau “valleytronics” refers to the shape of the graph you get when you plot the energy of electrons relative to their momentum: the resulting curve features two valleys.
Our coverage of the field has focused on efforts to achieve this effect with two-dimensional semiconductors such as graphene and tungsten diselenide. However, the UK researchers have focused their research on valleytronics in silicon.
The history of valleytronics in silicon is not one of achievement but more of an annoying curse. In silicon transistors, valleys cause electrons to lose speed. And in research for quantum-information-based devices, the valleys lead to decoherence, which can ruin the quantum state of so-called quantum computers.
In research published in the journal Nature Communications, the UK researchers looked at the behavior of electrons in the valleys of silicon-on-insulator quantum wells when exposed to a magnetic field. Conventional wisdom suggested that it would be more difficult to polarize the electrons after having polarized the valley, but the researchers discovered that the opposite was true.
Kei Takashina, lead author of the research said in a press release:
Our paper establishes the effect valley-polarization has on spin polarization in silicon transistors by using our unique capability to polarize valleys in the steady state. According to a simplistic way in which electrons are often thought about—that they move around independently of each other—it should become twice as difficult to polarize spins when valleys are polarized. In stark contrast, we find that at low enough electron density, it becomes easier to align their spin when valleys are frozen. This is a striking demonstration of how interactions between electrons lead to qualitatively new behavior.
Takashina believes that being able to control the valley polarization electrically will lead to new approaches to the development of either new silicon-based “spintronic” devices or interface complementary metal-oxide semiconductor (CMOS) technology with silicon-based quantum information processing.
Dexter Johnson is a contributing editor at IEEE Spectrum, with a focus on nanotechnology.