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A Fresh Spin in Quantum Physics: The 'Spin Triplet' Supercurrent
February 16, 2006PROVIDENCE, R.I. - 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.
Brown University
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.
Brown University
Discovery by UC Riverside physicists could enable development of faster computers
Physicists at UC Riverside have made an accidental discovery in the lab that has potential to change how information in computers can be transported or stored. Dependent on the "spin" of electrons, a property electrons possess that makes them behave like tiny magnets, the discovery could help in the development of spin-based semiconductor technology such as ultrahigh-speed computers.
Physicists exploit ultra-cold gases to measure ultra-small magnetic fields
Capturing the coldest atoms in the universe within the confines of a laser beam, University of California, Berkeley, physicists have made a device that can map magnetic fields more precisely than ever before.
Nature press release for 19 July issue
[412352] LIFELINES: TRIPLE CHECK (pp352-355; N&V) There are currently two well established main checkpoints in cell division. In this week’s Nature, researchers report a new type of checkpoint in yeast. This finding could lead to a better understanding of how cells form different tissues. For cell division to work, each daughter cell must get one complete set of chromosomes. To achieve this, the chromosomes attach to a framework of proteins called the spindle, which aligns them and pulls them to opposite poles of the cell. The newly discovered checkpoint ensures that the spindle forms in the correct position by monitoring another network of filaments, made of the protein actin, that c
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