Guiding electrons opens doors for 'new physics,' enables new devices
The wild spins of electrons in a semiconductor can be tamed by guiding their collective motions into a synchronized helix, new research shows. The study, published April 2 in Nature, uncovers new principles of physics and holds promise for the development of new information-carrying gadgets.
“The experiment is a fundamental discovery — a discovery with a device potential,” comments Jaroslav Fabian, of the University of Regensburg in Germany.
Electrons behave like spinning tops, complete with angular momentum. Because of this property, an electron winds up with a small magnetic field, as if the electron has a little bar magnet inside, says study coauthor Jake Koralek of Lawrence Berkeley National Laboratory in Calif. While the charge of the electron is always negative, the electron spin can be either up or down, depending on the orientation of the spin axis. “This [spin] is an untapped property of the electron,” Koralek says. Understanding and manipulating electron spins is the goal of a new research area dubbed spintronics.
The major challenge of making spins serve as useful data carriers is reliability. As electrons whiz through matter, the spins occasionally flip. A group of electrons with spins all up may quickly decay to have spins that are equally up and down — meaning they’d drop any information they were carrying.
“The environment is typically destructive for the spins,” says Fabian, who authored an accompanying commentary in the same issue of Nature. “Can the spins shield themselves against it? That is what the paper is about.”
To keep spins stable, Koralek and his team created a special semiconducting material that coaxed the individual spins in such a way that their orientations create a coordinated helix, which has never been observed before. The researchers set the wavelength of the helix by using two lasers to set alternating spin states — some up, some down — in a process called spin-grating.
The researchers found that incorporating two fine-tuned factors into their calculations led to stability and increased the life span of the helical spins 100-fold. These two factors — called the Rashba and the Dresselhaus — lead to “a new type of spin conservation,” says Koralek. “We can move spins around quickly, and still know exactly what each spin is.”
The researchers found that electrons in the new system had stable spins for up to a nanosecond or so, meaning the forces that make spins flip have “been made irrelevant,” for a relatively long period of time, says Koralek.
This new type of stability is relatively safe from temperature effects, Koralek says. Although stronger at very cold temperatures, the stable helix persists at room temperature. This may make the technology feasible for use in everyday devices in the future.
“It’s exciting because of this very unusual fundamental system,” says Koralek. “This opens up doors for a lot of new physics.”
Koralek, J.D., et al. 2009. Emergence of the persistent spin helix in semiconductor quantum wells. Nature 458(April 2):610-613. doi:10.1038/nature07871