Hot questions in superconductivity.

Hot questions in superconductivity 

Last month, researchers announcedthey had made a material that becomes completely superconducting at 94|K (-290|F). By losing all electrical resistance 17|K above the boiling point of the inexpensive coolant liquid nitrogen, it promises to make a host of technological dreams come true (SN: 2/21/87, p.116).

Now, in the March 2 PHYSICAL REVIEWLETTERS, the composition of the new material has been revealed by Paul C.W. Chu at the University of Houston, MawKuen Wu at the University of Alabama in Huntsville and their colleagues. It contains yttrium (Y), barium (Ba), copper (Cu) and oxygen (O), with the composition (Y0.6Ba0.4)2CuO4. Previous superconducting temperature records were set with lanthanum (La)-barium or strontium-copper oxides, with a typical composition of (La0.9Ba0.1)2CuO4.

These new data, together withprovocative but sketchy information on the crystal structure of the material revealed to SCIENCE NEWS this week by scientists at the Carnegie Institution of Washington (D.C.), raise a suite of scientific questions.

Chu and Wu were guided to the yttriummaterial by examining the behavior of the lanthanum compounds; they found that the relative sizes of atomic elements are important criteria in superconducting. But in spite of their success at navigating past the 77|K liquid nitrogen barrier, the hunt for high-temperature superconductors still involves a good measure of alchemy. And while scientists have a sound theory of superconductivity, they have yet to agree on what makes the yttrium and lanthanum compounds tick.

The basic theory of superconductivity,worked out 30 years ago, states that electrons in a crystal can communicate with one another by forming what are known as Cooper pairs. The conductivity of a crystal is enhanced because with Cooper pairs, the electrons scatter off the crystal lattice in a coherent, rather than random, way. The problem has been to explain the mechanism that couples normally repulsive electrons together.

The conventional mechanism holdsthat electrons interact through crystal vibrations called phonons. Marvin L. Cohen at the University of California at Berkeley thinks the electron-phonon interaction may still be a viable mechanism for the recent high-temperature superconductive materials. (Cohen and his colleagues announced March 2 that they had reached a superconductivity onset temperature of 100|K with a compound of the same composition as Chu's.)

But Chu and theorist Philip W. Andersonat Princeton (N.J.) University believe that the electron-phonon interaction cannot explain superconductivity at temperatures higher than about 40|K. Chu and Wu are intrigued by the fact that the yttrium material contains two different phases--two compounds of different composition and structure. A green phase is not itself superconducting, according to Chu. So the researchers believe that either the other (black) phase is responsible, or the superconductivity mechanism is taking place at the interface between the two phases, an idea first developed about 10 years ago.

However, Anderson says he would besurprised if the existence of two phases has anything to do with superconductivity in these kinds of materials. Instead he proposes in the March 6 SCIENCE another mechanism, in which the electrons in the yttrium and lanthanum oxides --and in other materials that are not quite able to become completely ordered magnetically--are coupled through magnetic and electronic interactions.

In interviews this week, Carnegie researcherswho were asked to analyze the material said it contains two novel crystal structures. "The new structure will really set theoreticians to thinking,' says Robert M. Hazen, "because [one aspect] lends itself to some very unusual electronic properties.' They plan to reveal the exact nature of the structures in a paper submitted to PHYSICAL REVIEW LETTERS.