A team of scientists studying the parent compound of a cuprate (copper-oxide) superconductor has discovered a link between two different statesor phasesof the material. They have written a mathematical theory to describe that relationship. This work will help scientists predict the material's behavior under varying conditions. It also may help to explain how it is transformed below a critical temperature into a superconductor that can carry current with minimal loss.
Ultimately, the researchers hope to design copper-oxide materials with practical properties. Examples include superconductors that operate at sufficiently warm temperatures to allow more widespread use in energy-distribution applications. For any material, though, this requires a theoretical understanding of how it works under different conditions. Copper-oxide superconductors have many other states that can compete with superconductivity.
To begin to understand these different phases, the team used a technique called spectroscopic image-scanning tunneling microscopy. It allowed them to visualize the electrons in each phase at the atomic level. this technique was developed by J.C. Samus Davis, Director of the Center for Emergent Superconductivity at Brookhaven National Laboratory and the J.D. White Distinguished Professor of Physical Sciences at Cornell University.
They found that one state has a periodic modulation of the electronic structuresimilar to a wave with periodic peaks and valleysthat imparts a "stripe" pattern over the material's crystalline structure (see figure). The other state has variations within every unit cell of the same crystal (i.e., variations in a property of each individual electron). Davis' technique aids the detection of "topological defects"swirling vortex-like distortions in the striped component of the electronic structure. Those distortions provide a link from one ordered phase to the other.
The topological defects are similar to those observed in liquid-crystal states. This realization led the group's theoretical physicists to devise a theory that draws on experience with those materials. (Those physicists are Eun-Ah Kim of Cornell, Michael Lawler of Binghamton University, Subir Sachdev of Harvard University, and Jan Zaanen of Leiden University.) This new theory explains the coexistence of the two cuprate states. It also predicts their interplay at the atomic scale.
The theory will foretell how the material behaves in the real world and how that behavior varies as a function of material-specific conditions, such as crystal symmetry. Eventually, the researchers hope to gain information that relates to the mechanism of high-temperature superconductivity.
This research was supported by the DOE Office of Science (through the Center for Emergent Superconductivity, an Energy Frontier Research Center), the National Science Foundation, the Japanese Ministry of Science and Education, the Japan Society for the Promotion of Science, and the Netherlands Organization for Scientific Research.