Materials Characterization

Magnetic fluctuations are good for superconductivity

Atomic resolution STEM images of different iron-based superconductors. In the inset of each image the expected crystal structure is indicated with FeAs and FeSe planes indicated by blue (Fe) and green (As or Se) dots, respectively. Colors were chosen to provide the maximum contrast for each image.

Atomic scale measurements of the strength of the magnetic fluctuations in a series of iron-based superconductors were made using high- resolution electron spectroscopy. Surprisingly, the superconducting transition temperature was higher when the magnitude of the fluctuating iron magnetic moment or “spin” was larger. This discovery provides experimental evidence that certain magnetic interactions can act as glue to form superconducting electron pairs. The study also demonstrated that electron energy loss spectroscopy in the aberration-corrected scanning transmission electron microscope can measure the local magnetic moment of Fe atoms in iron-based and similar classes of materials. This work establishes a method to map the dynamic magnetic moments atomic-column by atomic-column, which is not possible by any other technique. This unique approach provides key local information that leads to surprising insights into unusual material properties. 

Claudia Cantoni, Jonathan Mitchell, Andrew May, Michael McGuire, Juan-Carlos Idrobo, Tom Berlijn, Matthew Chisholm, Elbio Dagotto, Wu Zhou, Stephen Pennycook, Athena Sefat, and Brian Sales, “Orbital occupancy and charge doping in iron-based superconductors,” Advanced Materials (2014).   DOI: 10.1002/adma.201401518. 

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