Portland, Ore. -- Researchers believe they have unlocked the mystery to what makes high-temperature superconductors tick. According to a team from Oak Ridge National Laboratory and the University of Tennessee, the reason these materials superconduct at such high temperatures may be a magnetic resonance that causes their anti-ferromagnetic lattice to oscillate opposing-spin orientations in synchronization with the opposing-spin orientations of the so-called Cooper pairs passing through the superconductor's molecular lattice.
"This has been the most important problem in condensed-matter physics," said University of Tennessee professor Pengcheng Dai, who collaborated on the work with National Institute of Standards and Technology (NIST) physicist Jeffrey Lynn and UT doctoral candidate Stephen Wilson. The team worked at the NIST Center for Neutron Research in Gaithersburg, Md. "The magnetic resonance we observed is a spin excitation that is intimately related to superconductivity," Dai said. "It's when a layer of anti-ferromagnetic molecules begins switching their spins back and forth. This may be the glue that binds the Cooper pairs, since it begins just below the critical [superconducting] temperature and intensifies as superconduction progresses.
In a nutshell, "What we are reporting for the first time is that magnetic resonance is universal in both major classes of high-temperature superconductors," he said. "We have observed it in electron-doped superconductors and others had already observed it in hole-doped superconductors."
"If these researchers have found the key mechanism for high-temperature superconductors, the significance goes beyond being just a theoretical breakthrough," said Elie Track, a senior partner at Hypres Inc. (Elmsford, N.Y.), a specialist in fabricating superconducting microelectronics. "Because once we really understand how high-temperature superconductivity comes about, we can start predicting how new materials can be synthesized to raise the transition temperature even higher."
Magnetic-resonance excitation within an electron-doped high-temperature superconductor is believed to be the mechanism that generates Cooper electron pairs in high-critical-temperature superconductors. Recent experiments at the National Institute of Standards and Technology have confirmed the theory in the superconductor called praseodymium lanthanum cerium copper oxide. |
The Oak Ridge-UT team also reported a universal law governing all high-temperature materials--their magnetic-resonance energy is proportional to their superconductivity transition temperature. If the researchers are correct that magnetic resonance serves the same function as phonon lattice vibrations in low-temperature superconductors, then room-temperature superconductors could be on the horizon. "Our ultimate goal was to understand why these high-temperature materials become superconducting in the first place," said Dai. "If we can understand what makes it superconducting, then we can potentially design materials that have those desired features at even higher temperatures."
Since the discovery of a high-temperature superconductor (bismuth strontium calcium copper oxide, for which IBM received a Nobel Prize in 1987), researchers have been trying to understand why these materials superconduct at such a high temperature, in hopes of designing materials that superconduct at even higher ones. Earlier this year, IBM researchers confirmed that pairing in high-temperature superconductors has a distinct wave-function symmetry, indicating that the pairing mechanism must be different than in low-critical-temperature superconductors. But the mechanism responsible for condensing the Cooper pairs remained a mystery until now.
Low-temperature superconductors were discovered in 1911 by Dutch physicist Heike Kammerlingh Onnes. Since then, they have achieved unrivaled feats, such as levitating entire trains with billion-dollar superconducting magnets.
In 1972, University of Illinois researchers John Bardeen, Leon Cooper and Robert Schrieffer received the Nobel Prize for discovering the mechanism that causes superconduction. As a result, researchers understand that low-temperature superconducting does not involve magnetism; rather, it is electron-lattice interactions that cause a net attraction between electrons that normally repel one another. These bound electron pairs--called Cooper pairs--can flow in a loop in synchronization with the lattice forces, and consequently do not collide with any other atoms. Thus, they support current flow without any resistance.
Low-temperature superconductors are formed from a single element. For instance, lead atoms near absolute zero will bind with each other in a highly repetitive symmetrical pattern. But a copper-oxide-based high-temperature super- conductor has layers of copper and oxygen atoms bonded together in planes separated from one another by other elements such as yttrium, barium, lanthanum and cerium.
For low-temperature materials to superconduct, they must be cooled to a temperature of less than 5 Kelvin. The material IBM discovered that superconducted at 33 K was considered a high-temperature material. Since then, super- conducting materials have been reported at temperatures as high as 135 K, and today high-temperature superconductors are all operating above the boiling point of nitrogen: 77 K, or –195.79°C. Since liquid nitrogen can be used to cool high-temperature superconductors instead of the much more expensive liquid helium (boiling point 4.22 K, or –268.93°C), high-temperature superconductors promise to be more economical to use. However, theoretically the electron-lattice coupling that enabled low-temperature superconduction begins fading at about 23 K and completely ceases at about 50 K, which makes it impossible for high-temperature superconductors to use the same phonon mechanism as low-temperature superconductors.
"We needed another explanation for why high-temperature superconductors form Cooper pairs," said Dai.
To solve the puzzle, Dai's group studied the commonalities among the high-temperature superconductors, focusing on the fact that all high-temperature superconductors today have one thing in common: They employ copper oxide sheets that are ordered anti-ferromagnetically. "To us, this suggested that perhaps magnetism was the agent that binds together the Cooper pairs," said Dai.
The team used neutron probes to observe the magnetic resonance in an electron-doped high-temperature superconductor called praseodymium lanthanum cerium copper oxide. The results showed that the magnetic resonance previously observed by other researchers for hole-doped high-temperature superconductors was also present in electron-doped high-temperature superconduc- tors. This commonality led the researchers to postulate a universal law governing all high-temperature materials: that their magnetic-resonance energy is proportional to their superconductivity transition temperature, suggesting that magnetic resonance in high-temperature superconductors serves the same function as phonon-lattice vibrations in low-temperature superconductors.
The researchers are hoping that other labs worldwide will independently verify their results. In the meantime, they plan to characterize high-temperature superconductors further in hopes of understanding them even better. "We want to do doping-dependence studies," said Dai. "We want to apply a magnetic field to see if we can prevent superconductivity. We want to use different instruments to study their internal structure. We have a lot of follow-up research planned."