For about 40 years, the semiconductor industry has been able tocontinually shrink the electronic components on silicon chips, packingever more performance into computers. Now, fundamental physical limitsto current technology have the industry scouring the research world foran alternative. In a paper published in the Aug. 1 online edition ofPhysical Review Letters (PRL), Stanford University physicists present''orbitronics,'' an alternative to conventional electronics that couldsomeday allow engineers to skirt a daunting limit while still usingcheap, familiar silicon.
''The miniaturization of the present-day chips is limited by powerdissipation,'' says Shoucheng Zhang, a professor of physics, appliedphysics and, by courtesy, electrical engineering, who co-authored thePRL study. ''Up to 40 percent of the power in circuits is being lost inheat leakage,'' which he says will eventually make miniaturization aforbidding task.
In recent years, the search for an alternative to conventionalsemiconductors has resulted in the discovery of a nanotechnology called''spintronics,'' which uses a property of electrons called ''spin'' toproduce a novel kind of current that integrated circuits can process asinformation. Spin refers to how an electron rotates on its axis,similar to the rotation of the Earth. In 2003, Zhang and colleagues atthe University of Tokyo showed that producing and manipulating acurrent of aligned electron spins with an electric field would notinvolve any losses to heat-a technique they called spintronics.
Zhang now co-directs the IBM-Stanford Spintronic Science andApplications Center, along with Stanford electrical engineeringProfessor James Harris and IBM research fellow Stuart Parkin. Thecenter, established in 2004, is investigating many applications ofspintronics, including room-temperature superconductors and quantumcomputers.
Playing the angles
For all its potential, a drawback of spintronics is that itdoesn't work very well with lighter atoms, such as silicon, which themicroelectronics industry prefers. Enter Zhang's new research. In thePRL paper, he and graduate students B. Andrei Bernevig and Taylor L.Hughes show how, in theory, silicon could be used in a relatedtechnology they dubbed orbitronics. By using orbitronics, Zhang says,computer chip makers could get the benefits of spintronics withouthaving to abandon silicon.
Both orbitronics and spintronics involve a physical quantitycalled ''angular momentum,'' a property of any mass that moves around afixed position, be it a tetherball or an electron.
Like an electric current, which is the flow of negativelycharged electrons in a conventional integrated circuit, an orbitalcurrent would consist of a flow of electrons with their angular momentaaligned in an orbitronic circuit. ''If you push electrons forward withan electric field, then an orbital current will be generatedperpendicular to this electric current,'' Zhang says. ''It will notcarry charge, but will carry orbital angular momentum perpendicular tothe direction in which the electrons are moving.''
Therefore, he explains, with orbitronics, silicon would stillbe able to provide a useful current with no losses to heat at roomtemperature. Some alternative technologies require cold temperaturesthat are difficult and expensive to maintain, he adds.
From theory to application
The authors point out that orbitronics still has a long way togo to become an applied technology in the semiconductor industry.''This is so new,'' Zhang acknowledges. ''When something is firstdiscovered it is hard to say. There are many difficulties in thepractical world.''
Harris agrees, noting that spintronics will likely still takedecades to become a mature commercial technology. ''It's not going tohappen immediately, even if we are incredibly successful,'' he says.
But if orbitronics turns out to indeed be an economicallyfeasible technology to manufacture, it will be a boon to the industryto stick with silicon, Zhang says. ''There is a huge, huge investmentin processing silicon,'' he says. ''We don't want to switch overnightto a new material.''
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