- Scientists in Canada claim to have reinvented the transistor,
the workhorse of electronic and information technology. Their 'spintronic'
transistor uses a new principle for controlling and switching electrical
current, based on a property of fundamental particles called spin1. Spin
makes every electron like a tiny magnet whose poles can point in one of
two directions: 'up' or 'down'.
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- The discovery follows hard on the heels of two recent
reports of transistor-like behaviour in single atoms and molecules2,3,
also controlled using spin. These developments raise hopes that spintronic
devices many times smaller than today's transistors could one day replace
electronics.
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- The new spintronic transistor is made from a 'quantum
dot' - a kind of electrical well that holds just a handful of electrons.
Electric fields emanating from tiny electrodes on a semiconductor surface
create the well. Pawel Hawrylak and colleagues at the Institute for Microstructural
Science in Ottawa built the transistor.
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- The quantum dot holds onto electrons just as the nucleus
of an atom does. It is, in other words, a kind of artificial atom, many
times bigger than a real one, to which the researchers add electrons one
by one. The spin of the electrons determines whether or not they can flow
in a current through the quantum dot.
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- In normal electronics, electrons' spins are irrelevant
- all that matters is how many electrons there are in the current. Transistors
and other microelectronic devices compute by switching between states that
let a current pass and others that block it.
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- Spintronic devices aim to block or pass a current depending
on whether the spins of its electrons are up or down. This entails creating
and maintaining currents in which all of the electrons have the same spin.
This is challenging, but researchers are beginning to achieve it.
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- Spintronic devices would be capable of feats that are
beyond the reach of conventional electronics. Data would not be wiped from
processors when the power fails. Better still, quantum computers, in which
aligned electron spins would process information using the rules of quantum
mechanics, would boost the capacity and power of information technology
many times over.
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- Researchers have created relatively simple spintronic
devices before: structures like diodes, for example. But what they really
need is a transistor, in which a voltage applied to one 'gate' terminal
controls the current that passes between two more.
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- Hawrylak and colleagues use a magnetic field to tune
their quantum dot so that the spins of electrons hopping onto or off it
must be aligned up or down. This is just the kind of gate mechanism that
a spintronic transistor demands - it turns the current through the dot
'on' or 'off', according to the electrons' spins.
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- This means that information can be stored, read out and
erased by manipulating the spins of the electrons in the well.
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- The single-atom and single-molecule transistors reported
previously2,3 also used a gate electrode to change atomic spin states and
hence electrical conductivity.
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- None of these devices can be incorporated into any practical
spintronic circuit yet, but they show what is possible - and just how small
such circuits might ultimately be.
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- References Ciorga, M. et al. Collapse of spin-singlet
phase in quantum dots. Physical Review Letters, 88, 256804, (2002). Park,
J. et al. Coulomb blockade and the Kondo effect in single-atom transistors.
Nature, 417, 722 - 725, (2002). Liang, W., Shores, M. P., Bockrath, M.,
Long, J. R & Park, H. Kondo resonance in a single-molecule transistor.
Nature, 417, 725 - 729, (2002).
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- http://www.nature.com/nsu/020701/020701-9.html
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