Discovery of Hidden Order in High-Tc Superconductors
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 in the journal Science (Science, Vol 303, Issue 5666, 1995-1998, 26 March 2004) researchers at 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 checkerboard pattern. The 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.
One of the most important and controversial problems here is the identification of the nature and mechanism of the anomalous pseudogap regime. Although the boundaries of this regime have been fully mapped out by various spectroscopies, the microscopic origin of this "phase" remains a mystery, as does its relation to the underlying d-wave superconducting phase. Our work uses large scale, atomic resolution STM to search for electronic structure in underdoped cuprates in the normal and superconducting states. We find incommensurate checkerboard structure in the pseudogap regime, indicating the presence of an organized state that competes with superconductivity in the material Bi2Sr2CaCu2O8. The origin of the structure is not known, but a likely candidate is an incommensurate spin density wave previously seen in neutron scattering experiments.
Understanding the properties of the cuprates is considered to be crucial to developing a microscopic model for high temperature superconductivity. The results presented here place strict limitations on various ordering scenarios being put forward to describe the complex phase diagram of these materials. These insights provide suggestions as to how these remarkable properties can be enhanced and provide new potential for impacting a diverse range of technologies, ones ranging from power transmission to the fabrication of the most sensitive detectors available for application in human medicine.
Senior FS-MRL PI: A. Yazdani