PORTLAND, Ore. Just last month, an Oak Ridge National Laboratory team offered evidence that magnetic resonance was a more likely candidate than phonons as the mechanism behind high-temperature superconducting.
But now a research team at Cornell University (Ithaca, N.Y.) has countered that careful characterization at the atomic scale reveals that the mechanism causing high-temperature superconducting may be phonons after all.
Since the discovery of high-temperature superconducting materials, no one has convincingly explained why they work. Low-temperature superconducting is caused by a boson modea phononthat interacts with electrons. But electron-phonon interaction in high-temperature superconductors has been elusive to observe, giving rise to the magnetic-resonance hypothesis.
"We were looking for the magnetic glue that others have proposed as an explanation, but what we found is that you can't ignore the electron-phonon interactions," said Cornell physics professor J.C. Séamus Davis. "We did not prove that phonons cause electron pairing, but you cannot ignore their presence."
If the mechanism that enables high-temperature superconducting can be quantified, then designers worldwide could craft materials that eventually would enable room-temperature superconductivity.
Davis and his team went back to the seminal material that IBM Corp. scientists originally brewed up in 1986, and for which they received the Nobel Prize for high-temperature superconductivity in 1987: bismuth strontium calcium copper oxide. Using a scanning-tunneling microscope to characterize the fine structure of its tunneling spectra during superconduction, the group identified telltale nonlinearities"kinks"in the spectra that they say indicate phonons and electron pairing.
Changing the doping had no effect, indicating magnetism was not involved. But changing the atomic weight of one isotope for another (oxygen 18 to 16) resulted in the expected 6 percent reduction in average-mode energy, indicating that electron-phonon interaction indeed may cause high-temperature as well as low-temperature superconducting. "We can't say whether the phonons were the cause or the effect, because the nature of our measurements is a snapshot," said Davis. "We can't see the sequence of events, but the phonons are definitely involved with the pairing."
Phonons are vibrations in the crystalline lattice of a material that interact with electrons by enabling pairs to overcome their natural repulsion to enter a lower-energy state called a Cooper pair. After pairing, they weave through the lattice without any of the usual atomic collisions that cause resistance. "We have many more experiments to perform to try to pin this down," said Davis.
Working with Davis was Cornell researcher Jinho Lee and professor Kazuhiro Fujita, visiting from Tokyo University, along with colleagues at Tokyo University, AIST Labs in Tsukuba, Japan, and Los Alamos National Laboratory.