COLUMBUS, OhioExperiments in doping molecule-thin films of ceramic with the rare-earth element cerium suggest that low-temperature superconducting states can exist at the higher temperatures of the copper-oxide-based superconductors. A research group at Ohio State University here found that as the doping concentration increases, the films switch from the "d-wave" superconducting state of high-critical-temperature superconductors to the "s-wave" state found in metals such as lead and aluminum when they are cooled to near absolute zero.
Lead researcher Thomas Lemberger said that s-wave-style superconductivity was observed at temperatures as high as 90 Kelvin ( - 298°F). "That might sound very cold, but it's much less expensive to achieve than 10 K [ - 442°F, - 263°C], the operating temperature of conventional metallic superconductors," said Lemberger, an Ohio State professor of physics. He collaborated with Tine Greibe and Michio Naito, materials scientists at the NTT Basic Research Laboratories in Japan, and with Ohio State postdoctorate researcher Mun-Seog Kim and doctoral candidate John Skinta.
Both types of superconductors are beginning to play important roles in technology. Microwave carriers are using low-temperature superconducting filters to extend the reach of remote communications stations, while high-temperature superconductors are going into power cables, motors and generators, and temporary energy-storage units that smooth out fluctuations in electric-power grids. More-visionary designs are looking at the use of superconductors in microelectronic systems such as a superconducting antennas that would shrink cell phones to the size of a watch.
"[When] buckyballs, those soccer-ball-shaped carbon molecules, were doped with potassium in 1991 at Bell Labs, it was discovered that they exhibit the kind of superconductivity we are looking for: s-wave superconductivity at 40 K," Lemberger said. "However, we hope our observation of s-wave superconductivity in cuprates could lead to high-temperature operation as high as 90 [K]."
A crystal structure of lanthanum, copper and oxide, called a cuprate ceramic, forms the basis for most high-temperature superconductors. Cuprates became interesting in 1986 when researchers at IBM Corp.'s Zurich Research Laboratories discovered that doping a lanthanum/copper/oxygen material a type known as a Mott insulator with a few strontium atoms changes it from an insulator to a high-temperature superconductor.
These materials seemed odd because they behave opposite to a normal band insulator, in which electron movement is prevented by the Pauli exclusion principle. When each cell in the lattice is occupied by two electrons, the exclusion principle averts any movement. In a Mott insulator, researchers found that when a single electron occupied each cell, electron movement was prevented by electron-electron repulsion. When the material is doped with rare-earth atoms such as yttrium, barium or strontium, donor electrons increase the cell occupancy to two electrons that coexist with their spin states in opposite directions. Unlike a normal insulator, the pairs are able to move through the lattice.
The resulting population strongly resembles the electron pairing of low-temperature superconductors, which leads to the resistanceless flow of electric charge. And indeed, it was discovered that these doped Mott insulators became superconducting when cooled.
However, high-temperature superconducting electron dynamics turn out to be much more complex than the low-temperature version.
In low-temperature superconductors, which have a much simpler metallic-lattice structure, vibrations in the crystal lattice (called phonons) overcome the electron's natural repulsion to its neighbor, allowing the pairs to move through the lattice without any resistance. The theory has been confirmed over many decades of work with low-temperature superconductors.
Moreover, the electron pairs, called Cooper pairs, form a large-scale system known as a Fermi liquid. The orientation of the spin pairs is symmetric, a phenomenon called s-wave superconductivity.
Unfortunately, the higher-critical-temperature superconductors exhibit a less desirable, and less well-understood, d-wave anti-symmetry. That means the superconducting behavior is much more complicated, making it difficult to develop engineering applications.
The breakthrough announced by Lemberger and his colleagues is that s-wave superconductivity can be obtained from cuprates by doping them with cerium. The discovery holds out the hope of simpler behavior that could lead to practical applications.
"This discovery had already started a lively discussion among researchers as to the mechanisms that make high-temperature superconduction work in cuprates," said Lemberger. "Hopefully my group or others will be able to use this work as a springboard to identifying much higher-temperature cuprates that exhibit s-wave superconductivity." The current record for high-critical-temperature superconductivity is 135 K.
Lemberger's team created thin films of cuprates with differing amounts of cerium doping, and characterized the results by measuring how deeply a magnetic field could penetrate each film. At low levels of doping, the material continued to exhibit d-wave superconductivity, but by "overdoping" the material with cerium, as Lemberger termed it, the resulting overabundance of free electrons switched the magnetic-field measurements from d-wave to s-wave.
"It may be that cuprates can go either way, that they can be either d-wave or s-wave depending on which behavior you suppress by doping," Lemberger said. Similar behavior has been found in doped magnesium-boride systems, which exhibited s-wave superconductivity at the "high" temperature of 30 K. In this case, overdoping suppressed the s-wave superconductivity, but no d-wave behavior was found.
If Lemberger is correct, and cuprates can be developed with s-wave superconduction at higher temperatures than metals, then composite superconductors could replace metals as cheaper, more efficient alternatives. Lemberger's group was financed by the National Science Foundation.