Revealing Hidden Order in Superconductors,
Revealing Hidden Order in High-Temperature Superconductors
Superconductivity, the complete loss of electrical resistance in some elements, intermetallic alloys, and compounds, occurs at temperatures near absolute zero. First observed in 1911 in mercury by Dutch physicist Heike Kamerlingh Onnes of Leiden University, the mechanism of superconductivity remained unexplained until 1957, when Illinois physicists John Bardeen, Leon Cooper, and J. Robert Schrieffer determined that electrons, normally repulsive, could form pairs and move in concert in superconducting materials below a certain critical temperature, Tc.
For more than a decade, scientists have been baffled by superconductivity in the copper oxides, which occurs at liquid-nitrogen temperatures and does not seem to behave according to standard BCS theory. A tantalizing goal, which would have enormous implications for electronics and power distribution, is to achieve superconductivity at room temperature. A large piece of the puzzle has been to understand how the coherent dance of electrons that gives rise to superconductivity changes when the material is heated.
In a paper appearing February 12 in Science, researchers at the University of Illinois show that when heated, the orderly superconducting dance of electrons is replaced, not by randomness as might be assumed, but by a distinct type of movement in which electrons organize into a checker-board pattern. Professor Ali Yazdani and his physics graduate students Michael Vershinin and Shashank Misra used a powerful new scanning tunneling microscope (STM) to map electron waves in cuprate superconductors at high temperatures. In the figure shown here, a Fourier analysis of electron waves, in which the four central peaks that are the unique signature of the unusual checkerboard electron ordering discovered at Illinois, is superimposed above an STM image of the distinctive checkerboard map of electrons moving in BiSrCaCuO.
These new experimental findings imply that the two types of electron organization-coherent motion and spatial organization-are in competition in the copper oxides, an idea that may break the logjam on the mystery of high-temperature superconductivity.
The organization of electronic states in spatially ordered patterns ("stripes"), and their role in the mechanism of high temperature superconductivity, has been anticipated theoretically by Professor Eduardo Fradkin, in collaboration with Steven Kivelson (UCLA) and with Vic Emery (deceased). (See Rev. Mod. Phys. 75, 1201 [2003]). However, the experimental discovery of such pattern formation was made possible by the ultrahigh vacuum, low-temperature STM, which was designed and constructed by Yazdani's group at Illinois.
Work was done in collaboration with colleagues at the Central Research Institute of Electric Power Industry in Japan. Further information is available from Professor Yazdani and graduate students Michael Vershinin and Shashank Misra.
This work was supported under NSF (DMR-98-75565 & DMR-03-1529632), DOE through the Frederick Seitz Materials Research Laboratory (DEFG-02-91ER4539), ONR (N000140110071), Willett Faculty Scholar Fund, and the Alfred P. Sloan Foundation. Conclusions presented are those of the authors and do not necessarily reflect those of the funding agencies.