Scientists can make high-temperature superconductors, but they don't have a good theory for how they work. Understanding high-temperature superconductors will have significant impact on the modern condensed matter theory, and may someday allow scientists to design room-temperature superconductors. SLAC Photon Science and Stanford Professor Z.-X. Shen and colleagues, working at SSRL's Beam Line 5-4, recently made observations that will help shape the theory. Their results are published in the Nov. 1 issue of Nature.

Certain materials become superconductors—that is, losing all resistance to electric current—when they become colder than their transition temperature. So far, the warmest transition temperature recorded is minus 164 degrees Fahrenheit. Below the transition temperature, the electrons pair up. This pairing reduces the energy of the electrons. The strength of pairing is characterized as a superconducting gap in the single particle excitation spectrum, which is found below the transition temperature in both conventional and high-temperature superconductors.

In high-temperature superconductors, scientists also observe a gap above the transition temperature, called the pseudogap. So far it has been unclear whether the two gaps are unrelated, or if the pseudogap is a precursor to the superconducting gap. Using angle-resolved photoemission spectroscopy to measure the energy gap at different temperatures and momenta, Shen and colleagues found that the pseudogap and the superconducting gap coexist and exhibit different temperature dependence. Therefore, the two gaps seem to have different origins. This should provide an important step toward unveiling the mystery of the pseudogap phenomena.

W. S. Lee, I. M. Vishik, K. Tanaka, D. H. Lu, T. Sasagawa, N. Nagaosa, T. P. Devereaux, Z. Hussain and Z.-X. Shen, Nature 450, 81 (2007)

To learn more about this research see the full scientific highlight at:
http://www- ssrl.slac.stanford.edu/research/highlights_archive/high-tc_07.html