Smarter electronic circuitry will, in the future, store
information on the spin of an electron—up or down—rather than on the number of
electrons stored, thereby saving energy, generating less heat and operating at
higher speeds. Now University of Cincinnati
researchers claim to have found spintronics' "holy grail"—electrical
control of spin.
Today electronic devices store information—logical 1s and 0s—by
charging up a storage component, such as a capacitor or a floating gate.
Electronic charge accumulates in the storage device, raising its voltage until
it rises from a 0 to a 1 (in a 5-volt system a 1 is encoded as 2.7 volts or
above, and anything below 2.4 volts is a 0). Instead of storing electrical
charge to encode a logical 1, as opposed to 0 (no charge), future spintronics
devices will instead encode 1s and 0s as either the up or down spin of
individual electrons.
Electrons have always had the intrinsic property of spin—either
up of down—but as an effect of quantum mechanics it has been largely ignored.
For instance, when charge is used to encode 1s and 0s, like today, the spin of
the electrons stored is random. But by separating electrons by spin, and by
controlling the spin state of individual electrons, data can be encoded as
streams of electrons in the same spin state, rather than requiring thousands of
electrons to accumulate as charge.
Unfortunately, controlling the spin of electrons is not
easy. Since an electron's spin is associated with its magnetic moment, early
attempts used ferromagnetic materials to filter electrons by spin state. Once
separated, electrons in a known spin state can be metered out to encode
information.
The holy grain of spintronics, however, is to take any
electron as an input and change its spin to up or down under electrical
control. Once achieved, such all-electric spintronics will enable the whole
semiconductor industry to transfer to this new paradigm.
The researchers, led by professors Philippe Debray and Marc
Cahay, made their first steps toward that goal by harnessing quantum
confinement to flip an electron's spin orientation into the up state. First
they fabricated in an indium-arsenide semiconductor a short quantum wire—called
a quantum point contact—that constricted electrons as they pass through. Then
by tuning the voltage potential of gates adjacent to the wire, the researchers
were able to make one end of the wire more constricted than the other. Spin
polarization was subsequently triggered by coupling to the spin-orbits of the
electrons passing through the wire, an effect that was enhanced by Coulomb
electron–electron interactions.
Their current prototype only outputs electrons in the
"up" spin state, but for the future, the researchers plan to build
spin-polarizers that can emit electrons in either the up or down spin state.
Also, they plan to experiment with reproducing the circuit in gallium arsenide,
a more mainstream material than the more temperamental indium arsenide.
