Sep. 9, 1997 CHAMPAIGN, Ill. -- Recent experiments on yttrium-barium-copper-oxide (YBCO) superconductors have generated a clearer understanding of the peculiar behavior of this unconventional material. Most significant among the findings, by studying a characteristic called the zero-bias conductance peak, scientists at the University of Illinois -- working with scientists at Northwestern University -- have discovered the first example of a solid superconductor displaying broken time-reversal symmetry.
"The really hot news in the field of high-temperature superconductors is that the zero-bias conductance peak splits at low temperatures in the absence of an externally applied magnetic field," said Laura Greene, a U. of I. professor of physics who directed the research effort. "Not only does our experiment again prove that the dominant symmetry in YBCO superconductors is d-wave, it also shows that two different pairing mechanisms -- or order parameters -- can coexist in the same material, creating spontaneous currents that are a signature of broken time-reversal symmetry."
The surprising result offers proof of a new state of matter that has eluded researchers for years, Greene said. "This is the first case of a solid superconductor breaking both gauge symmetry and time-reversal symmetry. The only other material proven to break both symmetries is the unconventional superfluid helium-3, the discovery of which was awarded the 1996 Nobel Prize in physics."
To perform the experiment, Greene and her colleagues grew thin films of YBCO by off-axis magnetron sputter deposition. The researchers then used planar tunneling spectroscopy to measure the tunneling conductance across different junctions as a function of crystallographic orientation, temperature and externally applied magnetic field.
"Our team of experimentalists worked closely with Jim Sauls, a theorist at Northwestern," Greene said. "In fact, our two papers appeared together in the July 14 issue of Physical Review Letters. What we found was exactly what Sauls had predicted."
According to the experimental results, at 90 degrees Kelvin (the critical temperature for YBCO) the superconductor has d-wave symmetry, Greene said. "When cooled to about 7 degrees Kelvin, however, a second superconducting channel opens up which has s-wave symmetry. Because the two symmetries coexist, the differences between their phases spontaneously generate a current. The current creates a magnetic field, and that is what splits the zero-bias conductance peak."
The spontaneously generated current is also what breaks the time-reversal symmetry, Greene said. "Because the current is flowing in a certain direction, you can tell whether it's going forward or backward with respect to time."
The research team also comprised Mark Covington, Marco Aprili and Elvira Paraoanu at the U. of I., and Chad Mirkin, Feng Xu and Jun Zhu at Northwestern. The research was supported by the National Science Foundation through the Science and Technology Center for Superconductivity.
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