Slow Spin Zone | Illumination

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Slow Spin Zone

Slow Spin Zone

Just over five years ago, Giovanni Vignale, the Millsap Professor of Physics at MU, and his student Irene D'Amico, now an assistant professor at Britain's University of York, announced what they considered to be an important insight into how electrons behave in semiconductors. It wasn't very well received.

Their paper, first published in the August 2000 edition of the journal Physical Review, theorized that "spin currents," circular patterns created by swarms of speeding electrons, would likely be slowed by resistance as they made their way through a semiconductor -- an effect of electrons colliding into one another. Vignale and D'Amico dubbed the phenomenon "spin Coulomb drag," for Charles Augustin de Coulomb, the 18th century French physicist who laid the groundwork for the modern understanding of friction, magnetic attraction and the distribution of electrical charges.

Predicting that spinning electrons would move more slowly than "charge currents," the more direct form of semiconductor power, was something of a maverick move. Scientists had long assumed that, since the same random motion of electrons generated both spin and charge currents, each would push through a semiconductor at the same rate of speed. Vignale and D'Amico thought otherwise, but without experimental data to prove it their paper generated little excitement and much doubt. And although the researchers encouraged experimentalists to test their theory, interest languished.

Research into electron spin, however, did not. It soon got its own trendy moniker, "spintronics," and rapidly became all the rage among forward-thinking aficionados of the ultra-high-tech, particularly engineers interested in building next-generation computers. While the specifics of making this happen are complicated, the principle behind the buzz is not: Because the movement of electrons spinning their way through a semiconductor can be classified as either "up" or "down," engineers can encode movement data in the 1s and 0s of a binary system, just as in today's charge-based chips. But unlike charge-based chips, those utilizing spin -- magnetic random access memory [MRAM] chips for example -- do not depend on active current to store data. They can thus store information in less space, retrieve it more quickly and use less power than charge-based chips. If the world is ever to have quantum computing, scientists say, spin will likely make it happen.

Among the most prominent of the spin proponents is Joe Orenstein, a physicist who holds a joint appointment with the Materials Sciences Division of the Lawrence Berkeley National Laboratory and the University of California-Berkeley physics department. Orenstein is a leader in developing experimental techniques for observing electron spin.

One of the most important is called "transient grating spectroscopy," a variant of a well-established method of using lasers to triangulate electron flow. When, in the fall of last year, Orenstein applied the technique to the movement of spin current in semiconductors, he observed something strange. The spin current wasn't moving as predicted.

"When talking about the motion of charge current, you can think of the electrons as acting like a swarm of bees moving in one direction," Orenstein told Lynn Yarris a writer with the web-based Science@Berkeley Lab. "Within that swarm, individual bees might be colliding, but momentum is conserved with each collision so that the total motion of the swarm is unaffected. When talking about the motion of spin current, the electrons act more like a swarm of honeybees and a swarm of bumble bees trying to move through one another. As the bees in these two populations collide, there is an exchange of momentum that slows the relative motion of each. Eventually both swarms may move in a single direction, but the overall effect has been a drag on their collective motion."

The effect Orenstein was describing was, of course, "spin Coulomb drag," though he didn't know it yet. Eventually, with the help of a review of the relevant literature, Orenstein figured out that he had confirmed the earlier MU theory.

Vignale received the welcome news at a conference in Los Angeles, where both he and Orenstein were presenting papers. There's a presentation you really shouldn't miss, Vignale recalls one of his colleagues telling him. "It was a complete surprise. They didn't know about our work and we didn't know about them," says Vignale. "Generally, a theorist gives an idea to an experimentalist and they are aware of each other from the beginning."

For now the experimentalists are still mulling over the effect of spin Coulumb drag, wondering whether it will complicate spin-related applications.

Orenstein sees the effect as a mixed bag. "Spin Coulomb drag definitely makes it harder to flow a pure spin current," he said last winter. "But if you have a technology that calls for separate packets of up and down spin, the drag effect means those separate packets will have a longer shelf-life, and that's a good thing. You could say the impact of spin Coulomb drag depends on what kind of spin you want to put on it."

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Published by the Office of Research.