(reprinted, with permission, from Currents, vol. 30, no. 21)
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.
"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."
"We would very much like for phonons not to be there, and 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 their study of 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 laser-like 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 in many previous ARPES studies of these materials. The ability to fine-tune the energy of the undulator's photons also made it possible for them to directly probe the electrons responsible for superconductivity. A comparison of the resulting energy spectra revealed a kink shared by all the samples at an energy matching that of an oxygen phonon.
"Until now there has been little direct evidence for electron-phonon coupling in the electron dynamics 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 indicates 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. In the meantime, however, they expect their finding to "stimulate more work" for the theorists.
In addition to Shen and Hussain, 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.
Members of the international collaboration that identified the "kink" include (inset) Scot Kellar, (seated) Pavel Bogdanov and Alessandra Lanzara, and (back row from left) Wantli Yang, Xingjian Zhou, Zahid Hussain, Zhi-Xun Shen, and Veronique Broet.