Study Probes High-Temp Superconductivity

Daniel S. Burgess

The results of infrared spectroscopy experiments with a bismuth-containing copper oxide known as Bi-2212 suggest that high-temperature superconductivity in such materials originates from a magnetic effect, not from the action of phonons. The work, conducted by researchers at McMaster University in Hamilton, Ontario, Canada, and at Brookhaven National Laboratory in Upton, N.Y., gives further weight to the idea that the phenomenon in the copper oxides is fundamentally different from that in low-transition-temperature superconductors.

Below a superconductor's transition temperature, electrical resistance disappears, and direct current is carried unimpeded by coupled electrons called Cooper pairs. For low-temperature superconductors, this phonon-mediated coupling process is well understood in terms of the Bardeen-Cooper-Schrieffer theory.

Based on observations from inelastic neutron-scattering experiments and angle-resolved photo-emission spectroscopy, however, researchers had suspected that some other effect was at work in the high-temperature superconductors, such as magnetic coupling resulting from electron spin coordination. The new work reinforces this position by monitoring the behavior of a peak in the optical single-particle self-energy (the difference between the observed kinetic energy of electrons and that predicted by a simple collisional theory) of Bi-2212 at different temperatures and doping levels.

Using a Fourier transform infrared spectrometer from Bruker Optics Inc., the scientists collected reflectance data from samples of optimally doped Bi-2212 and three types of overdoped material. They noted that the amplitude in the peak in the self-energy weakened as the temperature and the doping level increased, and even disappeared in a superconducting overdoped sample with a transition temperature of 55 K.

Because the peak was not apparent in all cases of superconductivity, it cannot be necessary to the effect, explained Tom Timusk, professor emeritus in the department of physics and astronomy at McMaster. Moreover, he said, the peak cannot be the result of phonon interactions because it would be expected to be present at all temperatures and doping levels. The scientists concluded that it is caused by the interaction of the electrons with a magnetic resonance.

Timusk noted that the optical technique offers data similar to that from angle-resolved photoemission spectroscopy, but using a simpler, inexpensive and much more compact setup. Angle-resolved photoemission spectroscopy and neutron scattering can require synchrotron sources costing hundreds of millions of dollars and necessitating large support staffs. In contrast, this approach requires a commercial spectrometer.

The technique should be suitable for investigations into superconductivity in other copper oxides and promises to open the door to a better understanding of the effect in these novel materials.