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Electron Spin Plays New Semiconductor Tricks

spin microscope



COURTESY OF DAVID AWSCHALOM
One of the simplest devices imaginable--a strip of electrified semiconductor--continues to befuddle physicists. The flow of charge is easy enough to understand. The mystery lies in the behavior of spin, or the angular momentum associated with an electron. Researchers studying currents in a room-temperature semiconductor wire have now observed a pair of odd spin effects thought extremely unlikely to occur in that material or at such a high temperature.

An electron's spin can point in any direction but is often most useful when polarized in particular directions, such as up or down. Polarizing spin usually requires a magnetic field, but researchers imagine adding a whole new dimension to electronics by learning how to control spin using electric fields instead. Microchips might then convert rapidly varying spins into shifting polarized light for faster communications inside the chip or secure cryptography over a network. Toward that goal, researchers have demonstrated that electrons coursing through a semiconductor made of gallium arsenide are spontaneously polarized in the plane of the wire, if chilled below 50 kelvins (-223 degrees Celsius). Several materials also cause flowing electrons to rapidly segregate themselves such that spin-up electrons creep along one side of the wire and spin-down electrons take the other, in what is called the spin Hall effect. Researchers thought both these behaviors depended on strong electric fields in the materials and would be drowned out at warmer temperatures.

Planning to perform a control experiment in a semiconductor with a naturally weak electric field, experimental physicist David Awschalom of the University of California, Santa Barbara, and his co-workers sent current through a zinc selenide strip and turned on a special scanning microscope capable of imaging and distinguishing spin states in real time. Unexpectedly, they witnessed both spin polarization and the spin Hall effect all the way from five to 295 kelvins (22 degrees Celsius), they report in the September 22 Physical Review Letters. "Why it occurs is a very big mystery," Awschalom says. "It means there's some other piece of physics underlying this."

Solid-state theorist Bertrand Halperin of Harvard University says it's still not clear whether existing explanations are wrong or the experiment is so sensitive it is detecting a subtle consequence of them. "These semiconductor systems look much simpler than high-temperature superconductors or other exotic materials, and yet we find that we don't understand them very well," he observes. That may be a hindrance in realizing applications, but for basic physics it's a field day.

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