06/20/2002 07:00:00

Researchers make progress in understanding the basics of high-temperature superconductivity

High-temperature superconductors have long been the darlings of materials science because they can transfer electrical current with no resistance or heat loss. Already demonstrated in technologies such as magnetic sensors, magnetic resonance imaging (MRI), and microwave filters in cellular-phone base stations, superconductors are potentially one of the greatest technological triumphs of the modern world if they could just be made to operate more reliably at higher temperatures. But getting there will probably require a much better understanding of the basic principles of superconductivity at the microscopic level.

Now, physicists at the California Institute of Technology have made progress in understanding at a microscopic level how and why high-temperature superconductivity can occur. In a new study appearing in the June 3 issue of Physical Review Letters, Caltech physics professor Nai-Chang Yeh and her colleagues report on the results of an atomic-scale microprobe revealing that the only common features among many families of high-temperature superconductors are paired electrons moving in tandem in a background of alternately aligned quantum magnets. The paper eliminates many other possibilities that have been suggested for explaining the phenomenon.

Yeh and her collaborators from Caltech, the Jet Propulsion Laboratory, and Pohang University of Science and Technology in Korea report on their findings on "strongly correlated s-wave superconductivity" in the simplest form of ceramic superconductors, which are based on copper oxides, or cuprates. The paper differentiates the behavior of the two basic types of high-temperature superconductors that have been studied since the mid-1980s—the "electron doped" type that contains added electrons in its lattice-work, and the "hole-doped" type that has open slots for electrons.

The cuprate materials were discovered to be superconductors in the 1980s, thereby instantaneously raising the temperature at which superconductivity could be demonstrated in the lab. This allowed researchers to produce devices that could be cooled to superconductivity with commonly available liquid nitrogen, which is used in a huge variety of industrial processes throughout the world. Before the high-temperature superconducting materials were discovered, experts could achieve superconductivity only by cooling the materials with liquid helium, which is much more expensive and difficult to make.

The arrival of high-temperature superconductivity heralded speculation on novel applications and machines, including virtually frictionless high-speed magnetically levitated trains, as well as power transmission at a fraction of the current cost. Indeed the progress of the 1980s led to demonstrations of technologies such as magnetic sensors, microwave filters, and small-scale electronic circuits that could potentially increase the speed of computers by many thousands of times.

A certain amount of progress has been made since the high-temperature superconductors were discovered, and researchers remain optimistic that even the current generation may be adequate for such futuristic devices as extremely high-speed computers, provided that other technological hurdles can be overcome. But a primary roadblock to rapid progress has been and continues to be a limited understanding of precisely how high-temperature superconductivity works at the microscopic level.

A better fundamental understanding would allow researchers better to determine which materials to use in applications, which manufacturing procedures to employ, and possibly how to design new cuprates with higher superconducting transition temperatures. This is important because researchers would have a better idea of the molecular architecture most essential to the desired properties.

In this sense, Yeh and her colleagues' new paper is a step toward a more fundamental understanding of the phenomenon. "The bottom line is that we can eliminate a lot of things people thought were essential for high-temperature superconductivity," she says. "I feel that we have narrowed down the possibilities for the mechanism."

More specifically, the type of cuprates investigated by the Caltech team has the simplest form among all cuprate superconductors, with a structure consisting of periodic stacks of one copper oxide layer followed by one layer of metal atoms. This structure differs from all other cuprates in that multiple layers of complex components between consecutive copper oxide layers are absent, and the latter are known to be the building blocks of high-temperature superconductivity.

This unique structure appears to have a profound effect on the superconducting properties of the cuprate, resulting in a more three-dimensional "s-wave pairing symmetry" for the tandem motion of electrons in the simplest cuprate, in contrast to the more two-dimensional "d-wave pairing symmetry" in most other cuprates. This finding eliminates the commonly accepted notion that d-wave pairing may be essential to the occurrence of high-temperature superconductivity.

Another new finding is the absence of the "pseudogap phenomenon," the existence of which would imply that electrons or holes could begin to form pairs at relatively high temperatures, although these pairs could not move in tandem until the temperature fell below the superconducting transition temperature. The pseudogap phenomenon is quite common in many cuprates, and physicists have long speculated that its existence may be of fundamental importance. The absence of pseudogap, as found in the simplest form of cuprates, can now effectively rule out theories for high-temperature superconductivity based on the pseudogap phenomenon.

Contact: Robert Tindol (626) 395-3631