A study at Berkeley Lab's Advanced Light Source (ALS) has revealed that, contrary to what many scientists have argued, the physics behind the high-temperature superconductivity of copper oxides may be every bit as kinky as that behind their low-temperature metal counterparts.

Working with undulator light from ALS Beamline 10.0, an international collaboration led by Stanford University physicist Zhi-Xun Shen has identified a "kink" in the energy spectrum of low-energy electrons in three different families of copper oxide high-temperature superconductors. This spectral kink is the signature of an interaction or "coupling" between an electron and a phonon, a vibration in the ions that form the lattice of a superconductor's crystal. Electron-phonon coupling is behind the low-temperature superconductivity of metal alloys.

The team that found the "kink" in the spectra of high-temperature superconductors included (insert) Scot Kellar, (seated) Pavel Bogdanov and Alessandra Lanzara, and (back row from left) Wangli Yang, Xingjiang Zhou, Zahid Husssain, Zhi-Xun Shen, and Veronique Broet.

"We see in all of these copper oxide materials an abrupt change of electron velocity at 50-80 meV" — the kink in the spectrum — "which we cannot explain by any known process other than the coupling with phonons," says Shen. "This suggests that electron-phonon coupling strongly influences the electron dynamics in high-temperature superconductors and must therefore be included in any microscopic theory of superconductivity."

Superconductivity is a state in which a material loses all electrical resistance: once established, an electrical current will flow forever. Following the discovery in the 1950s of niobium-based metal alloys that become superconducting at temperatures around 18 degrees Kelvin (18 K), superconductivity has become a staple of high-tech magnet technology in scientific research. However, the cost and difficulty of chilling these metals with liquid helium limit their applications elsewhere.

The discovery in the 1980s of ceramic copper oxides that become superconducting at temperatures above the 77 K boiling point of liquid nitrogen — meaning they are much cheaper to chill than metals — brought forth the tantalizing prospect of room temperature superconductors and their promise of such prizes as dirt-cheap electrical power and magnetically levitated high-speed trains. For this promise ever to be realized, however, scientists need a better understanding of the physics behind the superconductivity of these materials.

The physics behind low-temperature superconductivity has been successfully explained by BCS theory, named for its Nobel-prize winning authors, John Bardeen, Leon Cooper, and Robert Schrieffer. BCS theory says that a superconductor loses electrical resistance through the formation of electron pairs, in which any motion on the part of one member of the pair instantly causes an equal countermotion in the other. This electron pairing cancels out any disruption in the flow of an electrical current caused by crystal impurities, a source of electrical resistance.

Electrons that would naturally repel one another because of their mutual negative charge are able to pair up because of the interaction of one of them with a phonon, which can be thought of as a sort of atomic sound wave. The first electron alters the phonon, and the second electron is affected by this alteration, like passing ships influencing each other's motion through their effect on the surrounding waves. Scientists believe electron pairing is also behind the superconductivity of high-temperature superconductors; however, the prevailing thought has been that this pairing involves different physics, such as fluctuations of the magnetic spins of atomic nuclei, rather than electron-phonon coupling.

	Relationships of electron energy and momentum help scientists better understand the physics behind high-temperature superconductivity. In this raw ARPES data, photoemission intensity appears as a function of electron binding energy and electron momentum.

"We would very much like for phonons not to be there. We have a completely new mechanism for high-temperature superconductivity that has nothing to do with phonons," Shen says. "However, our data suggest that the phonon is still a major player in high-temperature superconductivity."

The team led by Shen, which included Berkeley Lab physicist Zahid Hussain and researchers with Stanford University and the University of Tokyo, conducted its study of the three families of copper oxide superconductors at the ALS using a technique called ARPES (angle-resolved photoemission spectroscopy). With ARPES, light is flashed on a sample, causing electrons to be emitted through the photoelectric effect. Measuring the kinetic energy of emitted electrons and the angles at which they are ejected identifies their velocity and scattering rates. This in turn yields various energy spectra.

By lighting their samples with the laserlike beams of photons generated at ALS beamline 10.0.1's undulator magnet, Shen and his colleagues obtained an angular resolution in their ARPES measurements that was an order of magnitude better than any previous experiments. The ability to fine-tune the energy of the undulator's photons made it possible for them to zero in on the electrons that are directly responsible for superconductivity and to plot energy spectral curves. The spectral curves revealed a kink shared by all the samples at an energy matching that of an oxygen phonon.

In these energy spectra, three different families of high-temperature superconductors all show a common "kink" (arrow) at the energy that matches an oxygen phonon.

"Until now there has been little direct evidence for electron-phonon coupling in high-temperature superconductors," says Shen. "This was a hard job and we were blessed with both the performance of the ALS and the amount of beam time made available to us."

While the data gathered by Shen and his team indicate that phonons strongly influence the motion of electrons in high-temperature superconductors, it is still not clear to what extent BCS theory can explain the physics involved. Shen and his colleagues are already planning new experiments to help answer this question but, in the meantime, they expect their finding will "stimulate more work" for the theorists.

In addition to Shen and Hussain, other U.S. members of the discovery team were Stanford University's Alessandra Lanzara, Pavel Bogdanov, Xingjiang Zhou, Scot Kellar, and Donglai Feng, plus former ALS staffer Erdong Lu.

Additional information:

• More about Z.X. Shen and ARPES research group
• More about the Advanced Light Source