In the quest to make faster computer chips and more efficient semiconductor lasers, scientists are exploring the relatively new field of "spintronics," in which the direction an electron spin is pointing is just as important as its charge.
Bringing spintronics down to the atomic level, researchers hope to create new "quantum computers" that encode information in different spin states -- up, down or a mixture of both -- instead of in binary digits (1 or 0). A major hurdle in reaching that goal is to develop new, non-invasive techniques to study electron spin states deep inside semiconductors.
Physicists at Yale and Lucent Technologies' Bell Labs have done just that with nuclear magnetic resonance (NMR) and have found that the spin states are surprisingly long-lived. Their results were reported in the July 31 issue of the journal Science and in the July 20 issue of Physical Review Letters.
"In a magnetic field, the energy of an electron depends upon whether its spin orientation is up or down," says Sean E. Barrett, assistant professor of physics and applied physics and leader of the research team. "Future quantum computers may make use of the fact that an electron can be placed simultaneously in both up and down spin states, a possibility outside the reach of classical computers based on binary digits."
The researchers were able to "heat" the electron spins using either laser or radio-frequency radiation. Then they used NMR to monitor the "cooling" time. While extremely fleeting by most standards -- in the neighborhood of 100 microseconds or 1/10,000th of a second -- the cooling time is at least 1,000 times longer than other electronic processes measured in semiconductors. In other words, the electrons stay surprisingly long in one spin state before "flipping back" in the other direction. The discovery may bode well for the feasibility of spin-based quantum bits, Barrett says.
"By using lasers and magnets at extremely low temperatures, we can study the spin physics of an electronic system. In such a system, the collective behavior from electron-electron interactions becomes apparent, like the organized motion of a flock of birds in flight," says Barrett, a former Bell Labs postdoctoral fellow. Other members of the research team were graduate students Nicholas N. Kuzma and Pankaj Khandelwal of Yale, and Loren N. Pfeiffer and Ken W. West of Bell Labs.
Possible applications of the new discovery are manifold. "Recently, interest in electronic spin polarization embedded in solid-state systems has grown with a view toward creating spin transistors and spin memory devices, and for making use of spin coherence in semiconductors for quantum computation," note James M. Kikkawa and David Awschalom of the University of California, Santa Barbara, in an accompanying "Perspectives" article in the journal Science.
The research team's demonstration that electron spins can be manipulated by radio frequency radiation "suggests the exciting possibility that resonance techniques conventionally targeted at nuclear spins may ultimately prevail in controlling these electronic spins as well," Kikkawa and Awschalom write.
Barrett and his colleagues studied electrons confined to a very thin layer of gallium arsenide (GaAs), sandwiched between thick layers of aluminum gallium arsenide (AlGaAs). Both semiconductor compounds frequently are used in high-speed electronic components and semiconductor lasers. The researchers cooled their samples to near absolute zero (0.3 degrees Kelvin), or about -459 degrees Fahrenheit, thereby restricting electron motion to two dimensions.
A strong magnetic field (12 Tesla, or 240,000 times the earth's magnetic field) was applied perpendicular to the semiconducting layers, placing the electron system into the mysterious domain of the fractional quantum Hall effect, a novel "quantum liquid" state first discovered by Bell Labs scientists Daniel C. Tsui, Horst L. Stormer and Arthur C. Gossard in 1982. Quantum effects are special laws of physics governing the strange ways electrons behave at very small scales -- behavior that is not seen at larger scales. For example, electrons act more like waves than particles in small electronic devices and can do unexpected things like tunnel through barriers.
By using NMR, a technology frequently used for medical imaging, the Yale physicists were able to probe deep into the semiconductor layers without heating the electrons, thus enabling them to study electron spin magnetization there for the first time. Circularly polarized laser light was used to boost the NMR signal 100-fold from the tiny GaAs layers -- a crucial step called optical pumping, Barrett says.
The discovery could advance scientific understanding not only in electronics but more broadly in nuclear physics, particle physics and condensed matter physics. Not only do the findings appear to challenge current understanding of the fractional quantum Hall effect, they also provide new details about an exotic "particle" called a Skyrmion, first theorized four decades ago. Skyrmions can be thought of as twists or kinks in a spin space, caused by having a different spin than exists in the underlying fields. "Whether or not these exotic particles are relevant to our new findings is a subject of great current interest," Barrett says.
Funding for this research was from the National Science Foundation
and Lucent Technologies.
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