Focus: Spin Currents–Free of Charge
Electrons not only carry electric charge, they also behave like spinning tops. And by exploiting a quirk of quantum mechanics, two teams of physicists have independently produced currents of spin without the currents of electricity that, until now, have always accompanied them. Published in the 4 April and 30 May issues of PRL, the results show how the weird quantum rules that govern the microscopic realm can leave their mark in the macroscopic world.
Physicists have been striving to develop technologies that would take advantage of currents of electrons all spinning in the same direction. Such “spintronic” technologies could prove far more powerful than ordinary electronics. But first, researchers must perfect schemes for creating spin-aligned currents inside semiconductors, the materials used to make microchips.
One way is to shine a laser on a semiconductor to give some of its electrons a boost. Inside the material, electrons move in “bands,” which vaguely resemble the lanes on a congested freeway. The lower-energy valence band is chock full of electrons that cannot move through one another, like cars stuck in a traffic jam. The higher-energy conduction band is normally empty, like the high-occupancy vehicle lane on the highway. The laser light lifts electrons from the valence band into the conduction band, where they travel freely when a voltage is applied to the semiconductor. And if the laser is polarized in just the right way, the flowing electrons will all spin in the same direction.
But two laser beams acting in concert can pull off an even niftier trick. They can produce a flow of spin without an applied voltage or any net flow of electricity, reports a team led by Martin Stevens and Arthur Smirl of the University of Iowa, in Iowa City, and another team led by Jens H?ner and Wolfgang Rühle of the University of Marburg, in Germany.
Both teams of researchers shined two overlapping laser beams straight down on to a sample of semiconductor. The photons in one beam packed exactly half as much energy as those in the other, so an electron could climb from the valence band into the conduction band by absorbing one photon from the high energy beam, or two photons from the low energy beam. That gave electrons two ways to jump from one band to the other. According to quantum mechanics, when a particle can get from one state to another in two ways, the two processes can cancel or reinforce each other, in a phenomenon known as quantum interference. Such interference leads to drastically different results than just adding the rates of the two independent processes.
The tag-team lasers affected electrons differently depending on whether the electrons spun clockwise (or “down”) or counter-clockwise (or “up”) when viewed from above the surface. If the lasers were polarized in perpendicular directions, then, through a complicated interaction, the quantum interference would amplify the flow of spin-up electrons in one direction along the surface and diminish it in the opposite direction. At the same time, it would amplify the flow of spin-down electrons so that it directly opposed the flow of the spin-up ones. Because equal numbers of electrons moved in opposite directions, no net electrical current flowed across the surface. However, because a spin-up electron moving to the right has the same effect as a spin-down electron moving to the left, the flows of spin reinforced each other, leading to a pure spin current. The two teams used different optical techniques to spot the spin-only currents, which flowed for only a few nanometers.
The experiments nicely confirm predictions made in the 1990s, says David Awschalom of the University of California, Santa Barbara. “The combination of these papers is really quite elegant,” he says. “The data is incontrovertible.” Spin-only currents might prove useful for controlling the interactions of light in technologies that shuttle photons instead of electrons, Awschalom says, but more immediately they underscore how tiny quantum effects can produce surprising macroscopic effects.
Adrian Cho is a freelance science writer in Grosse Pointe Woods, Michigan.