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Understanding the mysteries of high-temperature superconductors

The electronic states in a high-temperature superconducting material are strongest along the diagonal momentum direction. (Image by Donghui Lu)
Click here for a high resolution photograph.

High-temperature superconductors (HTSCs) operate in mysterious ways, but scientists are starting to understand their peculiarities by using a state-of-the-art spectroscopy system at SSRL.

One of the biggest mysteries is how a material that starts as an insulator—which does not conduct electricity—can become a high-temperature superconductor after being doped with electric carriers. Researchers Kyle Shen and Donghui Lu (both ESRD), working in Zhi-xun Shen’s group at SSRL and Stanford, looked at the evolution from insulator to superconductor by studying an HTSC material at different doping concentrations, including ones that are insulating. The team used angle-resolved photoemission spectroscopy (ARPES), a method of probing the electronic states in solids.

Their results, published in Science magazine on February 11, contribute to creating a fundamental understanding of the perplexing physics of HTSCs.

“The materials were discovered almost 20 years ago, but they are very complicated and not well understood,” Lu said. “We’d like to have a microscopic theory that tells us why they can be superconducting at a temperature much higher than conventional superconductors, and thereby how to improve the materials.”

HTSCs have huge potential for industry because they conduct electrical current without heat loss, yet need to be cooled only to liquid nitrogen temperatures (77 Kelvin) rather than the liquid helium temperatures (4 Kelvin) needed for conventional superconductors. While still chilly, that ‘high’ temperature is much less expensive to reach. HTSCs are used in niche applications, as the materials are currently too brittle for widespread use.

Below the superconducting transition temperature, electrons pair up and travel free of resistance. The pairs in conventional superconductors join up through well-understood interactions. In HTSCs, however, it is unclear what mechanism causes the electrons to pair up.

One clue came from another group that used scanning tunneling microscopy (STM) to look at where the electrons were distributed across the two-dimensional sheets that make up the HTSC material. They discovered an interesting checkerboard pattern, indicating an unusual charge ordering (the repeating pattern or arrangement of electrons).

The ARPES data added to the unexpected picture. It revealed electronic states that were much stronger along the nodal momentum direction (diagonal to the checkerboard squares) than along the anti-nodal (straight) direction. The anti-nodal direction is the one in which superconductivity is the strongest and where charge ordering manifests.

“The fact that there are fewer electronic states along the anti-nodal direction is surprising,” Lu said. “We thought they would be stronger to give rise to the checkerboard pattern observed by STM.” These results show that the difference in momentum direction is important to electronic structure, and therefore put strong constraints on proposed models trying to explain how HTSCs work.

For more information, see: http://www.sciencemag.org/content/vol307/issue5711/index.shtml



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