10/25/94
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STANFORD -- The paramount mystery of how high-temperature superconductors carry electrical current without resistance may be revealed by a subtle difference in the way these materials reflect light when they switch between normal and superconducting states.
Writing in the Oct. 24 issue of the journal Physical Review Letters, a Stanford research team reports measuring such a difference and argues that it provides the most direct evidence yet obtained of the nature of the force or forces that bind the current-carrying electrons into pairs, a necessary condition for superconductivity.
The researchers are Stanford physics Professor William A. Little, chemistry Professor James P. Collman and postdoctoral fellow Matthew J. Holcomb.
"At a minimum, this observation puts a powerful new constraint on existing theories. A number of theories that are currently popular will have difficulty explaining it," Little maintains.
Since superconductivity was first discovered in 1911, an ultimate objective has been to develop a material that conducts electricity without resistance at room temperature. Such a material might provide dramatically more efficient ways to store and distribute electricity, make magnetically levitated vehicles practical, make electric motors more powerful and efficient, and allow the design of faster and more powerful computers, among a number of other possible uses.
From 1911 until 1986 the effect was restricted to materials cooled down to ultralow temperatures. They had "critical temperatures" - the highest temperatures at which they remain superconducting - limited to a few tens of degrees above absolute zero (-273 degrees Celsius). Then the discovery of the first "high-temperature" superconductors that broke this cryogenic barrier fostered a flurry of research that rapidly produced compounds with critical temperatures as high as -148 degrees Celsius, achievable with liquid nitrogen refrigeration. Since then, however, success pushing up the temperature at which superconductors operate has slowed markedly.
This slowdown has increased the importance of efforts to understand which features in these new materials confer the ability to superconduct. From the very first, discovery of these compounds came as a shock to theorists, who had generally believed that the effect was limited to within 40 degrees Celsius of absolute zero, and a number of conflicting theories have been advanced to explain this phenomenon.
Figuring out what is going on in these new materials is made more difficult by their complex structures. They are ceramics that contain microscopic sheets of copper and oxygen atoms separated by layers of other elements. By contrast, low-temperature superconductors tend to have simple, metallic structures.
In metals and other normal conductors, the electrons that carry current are continually colliding with atoms, with defects in the crystal structure and with each other. The net effect is electrical resistance and energy loss. In superconductors, however, the electrons avoid all this bouncing around. In low-temperature superconductors, they do so by forming pairs, which can then move without friction. The attractive force necessary to bind the electron pairs arises from a weak distortion, called a phonon, that takes place in the crystal lattice near the current-carrying electrons. However, this binding force is quickly overwhelmed by thermal vibrations at temperatures much above absolute zero.
The key to understanding these new materials is determining the nature of the force or forces that bind the electron pairs together strongly enough to sustain higher temperatures. One highly touted hypothesis envisions a magnetic-like interaction that arises from electron spin replacing phonons as the binding force. Another theory suggests that phonon-bound electron pairs gain additional strength by skipping from one copper-oxygen layer to another.
According to the scientists, their experimental results provide strong support for a less radical modification to the original BCS theory that explains low-temperature superconductivity, one in which electron excitations supplement phonon forces. Little proposed this mechanism over 30 years ago as part of an effort to design an organic superconductor.
If the Stanford scientists' model is correct, it implies that further increases in critical temperature should be possible. "Theoretically, you can alter the strength of the binding energy by altering the local field in the layers. Nobody knows how to do this in a sensible fashion, but there is no reason to believe that the effect is limited to -150 degrees Celsius. Another factor of two is not unlikely," Little says.
Little and his colleagues have attempted to determine the nature of the binding force in the new class of superconductors by a technique called thermal difference spectroscopy. The technique, developed by Holcomb, allowed them to measure temperature-induced changes in reflectivity as small as one part in 50,000. A special micro- miniature gas refrigeration system that Little invented allowed them to control the temperature of the material extremely precisely and with very little vibration. In this fashion they were able to measure the way in which one of the high-temperature superconductors (Tl2Ba2Ca2Cu3O10) reflects light when its temperature is 5 degrees above and then 5 degrees below the material's critical temperature.
The experiment is based on the argument that the electron pairs resonate, or ring, at an energy that corresponds to that of the binding force. As a result, the material will absorb light more strongly at this energy level and will reflect more photons at an adjacent level.
The resultant difference in reflectivity is extremely slight, within one part in 500. But the Stanford scientists report that they find such a change in the material's reflectivity while superconducting comparing to its reflectivity in the normal state at about 1.6 electron volts (an electron volt is the amount of energy required to move a single electron across a 1 volt potential).
According to the scientists, this reflectivity difference is caused by both phonon distortions in the lattice and an electron excitation created by non-conducting electrons jumping back and forth between oxygen and copper atoms. In these materials, the superconducting electrons move through the oxygen lattice. As a conduction electron passes an oxygen atom, one of the oxygen's non-conducting electrons temporarily jumps to a neighboring copper atom, leaving the oxygen briefly with an excess positive charge before it jumps back. This positive charge attracts the following electron, strengthening the bond between the pair. The energy required for one of these electrons to jump between the oxygen and copper atoms is 1.6 electron volts.
When the scientists plug this model into the BCS theory, as reformulated in 1960 by Gerasim M. Eliashberg from the Landau Institute in Moscow, to take into account stronger binding forces, they are able to derive reflectivity curves with all the features that they observed experimentally.
"We find remarkably good agreement between theory and experiment," the scientists write.
Little expects that their observations will be equally difficult for advocates of magnetic binding forces and of purely phonon mechanisms. Magnetic forces should cause a reflectivity shift opposite of that which they have observed, Little calculates, while phonon mechanisms should have no effect at all.
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