Scanning tunneling microscope image of a 48-nanometer square sample of an iron-based superconductor finds a\n array of dumbbell-shaped “impurities” in the electronic structure, like the sample in the inset, confirming a theoretical prediction. The electronic impurities are associated with cobalt atoms (red cross in the inset). Credit: Davis Lab

Close look at iron-based superconductor advances theory

February 21, 2013 by Bill Steele

(—Cornell researchers have resolved a longstanding theoretical debate about the electronic structure of iron-based superconductors by directly observing it at the atomic scale. The work is reported in the Feb. 24 online edition of the journal Nature Physics.

A team led by J. C. Séamus Davis, the James Gilbert White Distinguished Professor in the Physical Sciences and director of the Center for Emergent Superconductivity at Brookhaven National Laboratory, studied a compound of iron, calcium and arsenic that is “doped” by replacing a few of the iron atoms with cobalt atoms. With around 8 percent doping the material becomes a superconductor. Somehow the cobalt atoms change the environment within this material in a way that allows some electrons to join into “Cooper pairs” that then move without resistance.

Surprisingly, however, when the material is “underdoped” with not quite enough cobalt to create superconductivity, an ordinary electric current moves easily along only one axis of the crystal—call it “lengthwise”—but encounters high resistance moving crosswise. This effect increases with the amount of cobalt doping, and may offer a clue to how superconductivity works.

Scanning tunneling microscope (STM) images of the electronic structure of the material in this underdoped state reveal an array of tiny, elongated groups of electrons in an unusual energy state, aligned with the long axis of the crystal, that act as barriers to electrons moving crosswise. It’s like having a lot of long, narrow islands in the ocean, all lined up the same way: Boats moving parallel to the islands can slide through between them, but boats going across are blocked and have to detour. The researchers call these structures “impurities” because they differ from the rest of the electronic structure.

“There has been an assumption that the cobalt dopant atoms are neutral,” Davis said. “That assumption is wrong. We showed in this project that the dopant atoms do other very surprising things.”

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