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A Fresh Spin In Quantum Physics:
The 'Spin Triplet' Supercurrent
Providence, RI, Feb 20:
Superconductivity occurs when electrical current moves without resistance, a
phenomenon that gave rise to particle accelerators, magnetic resonance imagining
machines and trains that float, friction-free, on their tracks.
Under quantum physics theory,
conventional superconductivity is not supposed to occur in ferromagnets. When
electrons pass through these crystalline materials, they realign in ways that
won’t allow resistance-free conductivity. While supercurrent through a
ferromagnet has been observed, it moved only an extremely short distance before
resistance kicked in.
But a team of scientists from Delft
University of Technology, Brown University and the University of Alabama has now
accomplished this physics feat, creating a “spin triplet” supercurrent through a
unique ferromagnet.
As explained in the current issue of
Nature, the team’s experimental system converts the spin, or rotation, of
pairs of electrons in such a way that suggests they exist in three quantum
states inside the new magnet. There’s the standard “spin up” and “spin down” – a
reference to an electron’s angular momentum – but also a middle state. Picture a
planet that was thought to rotate two ways: With its North Pole pointing up or
pointing down. But now it’s found that this planet can be made to rotate on its
side, with its North Pole pointing out in a 90-degree angle.
While such a “spin triplet”
conversion in a ferromagnet was predicted in theory, the team offers the first
experimental evidence for the phenomenon.
The team also showed that this
current travels a comparatively long distance. In previous experiments, current
passed through a ferromagnet sandwiched between superconductors spaced one
nanometer apart. Under the new system, the space between superconductors was 300
nanometers apart.
“It’s a beautiful thing,” said Gang
Xiao, a Brown professor of physics. “What we’ve done was considered almost
impossible. But physicists never take ‘no’ for an answer.”
Xiao spent eight years perfecting
the ferromagnet with Brown graduate students and colleagues from the University
of Alabama. The magnet is black, about the size of a postage stamp, and measures
only 1,000 atoms thick. To make it, chromium oxide was heated until it
vaporized. That vapor was transported onto a titanium oxide film, so that only a
single crystal layer coated the titanium material.
The magnet was sent to scientists at
Delft University of Technology in the Netherlands. A team there placed dozens of
tiny superconducting electrodes on top of the magnet then used an electron beam
to cut the electrodes, creating the 300-nanometer gap between them. Scientists
then tested the system to measure the flow of current.
Xiao hopes that the new ferromagnet
can help create technologies in the hot new field of “spintronics,” short for
spin-based electronics. While conventional electronics tap the charge of an
electron to conduct current, spintronic devices use the spin as well as the
charge. The promise: smaller, faster and cheaper computer memory storage and
processing.
Already, spintronic technology can
be found in computer hard drives. A magnetic version of a random access memory
device and a spin-based transistor are under development. So are “quantum
computers,” which can perform hyperfast calculations.
Xiao said the spin triplet current
created with the ferromagnet would allow for new control in spintronics
development.
“Once you understand this new
behavior of electrons, you can apply the knowledge in new ways to commercial
products,” he said. “The consequences can be significant.”
The Nederlandse Organisatie voor
Wetenschappelijk Onderzoek and the National Science Foundation funded the work.
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