Feel the Force: Magnetic probe finds lone electron

When a patient undergoes magnetic resonance imaging (MRI), the scanner makes tissues visible by tapping into a magnetlike property of many trillions of protons in the patient’s body. Known as spin, this quantum-mechanical trait also shows up in electrons, atomic nuclei, and many other elementary particles. Now, scientists have devised an ultrasensitive MRI technique and used it to detect the spin of a single electron.

QUIVERING BLOODHOUND. Changes of this tiny silicon cantilever’s vibration frequency betray a single electron’s presence. The electron’s spin influences the minuscule permanent magnet (inset, blue) in the cantilever’s tip. B.W. Chui/IBM; (inset) H.J. Mamin/IBM

Daniel Rugar and his colleagues at IBM Almaden Research Center in San Jose, Calif., report their feat in the July 15 Nature.

“This signal achievement will dramatically alter the horizons for high-resolution imaging,” comments P. Chris Hammel of Ohio State University in Columbus in the same journal issue.

Near-term uses for the technique could include atom-scale microscopy of impurities and defects within electronic materials. The method could also prove useful in two up-and-coming fields. One is spintronics, a new type of circuitry that manipulates electron spins instead of—or in addition to—the electron charges exploited by today’s circuitry. The other is the discipline known as quantum information processing, in which spins may serve as quantum bits that can represent more than one number simultaneously.

Rugar has been pursuing what scientists call magnetic-resonance force microscopy since 1992, when John A. Sidles of the University of Washington in Seattle proposed that MRI might be pushed to make measurements on the nuclear scale (SN: 3/7/92, p. 150). Sidles’ idea was to scan individual molecules and pinpoint the type and three-dimensional location of each of their atoms by sensing the spin of their nuclei.

Such precise MRI, Sidles notes, could be a boon to molecular biology by providing a way to determine the structures of proteins as they function in living cells without first extracting and crystallizing them, as today’s techniques require. Toward that end, the new IBM work is “a major breakthrough,” he says.

He, Rugar, and other researchers in the field have built minute slivers, or cantilevers, of silicon or other materials that bend or vibrate like tiny diving boards in response to extremely small forces. They’ve used those cantilevers with magnetic coils and other components resembling those in ordinary MRI scanners.

To detect the spin of a single electron, Rugar and several colleagues prepared a piece of quartz with a smattering of so-called dangling bonds. At these sites, electrons lack companion electrons with opposite magnetism, which often has a canceling effect. The scientists also glued a tiny magnet to the tip of their cantilever, chilled the device to nearly absolute zero, and then set the cantilever vibrating.

Whenever the cantilever was moved over the quartz, the spins of individual electrons influenced the magnet, and the cantilever changed vibration frequency slightly.

Other researchers have already detected a single electron’s spin, for instance, by observing a shift in an atom’s optical property. Hammel expects the new approach to be more versatile. He says that, with improvements, it might even detect the far weaker spins of individual atomic nuclei in a molecule. The magnetic strength of a nucleus is only about one six-hundredth that of an electron’s spin.

It will take a thousandfold improvement in the sensitivity of the single-electron-detection technique to achieve MRI of single nuclei. Rugar notes that, in a dozen years, he and other scientists have already boosted their probes’ sensitivities by a factor of 10 million.

“Even though it has taken us a while, we’ve come a long way,” he says.

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