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A material’s multiple personalities

Thu, 09/12/2013 - 7:57am

In order to understand how complex materials merge at the boundary, scientists look at cross-sections of an oxide superlattices. In this picture, peaks correspond to layers of cuprate superconductor and valleys to metallic manganites (bottom region). The power of scanning tunneling microscopy allows researchers to gain insight into both the material's topography as well as its electronic properties.Just like people, materials can sometimes exhibit “multiple personalities.” This kind of unusual behavior in a certain class of materials has compelled researchers at the U.S. Dept. of Energy (DOE)’s Argonne National Laboratory to take a closer look at the precise mechanisms that govern the relationships between superconductivity and magnetism.

Previous measurements of magnetic and electronic properties in these superconducing oxide materials relied on aggregate or “bulk” measurements of a large area. By using advanced scanning tunneling microscopy at the laboratory’s Center for Nanoscale Materials, Argonne physicist John Freeland and materials scientist Nathan Guisinger were able to develop a clearer picture of the physical and chemical behavior of boundary regions within the material.

According to Guisinger, the most important regions of study in oxide superconductors are the boundaries or interfaces.

Because these materials do not have natural cleavage planes that provide scientists an easy way of looking directly at the interfaces between two dissimilar oxides, the Argonne team needed a way of precisely probing tiny features along the edges of the materials.

“The surface often looks like broken glass—it’s jagged and rough,” Freeland said.  “But if you look at it closely enough, you can find plateaus in between the peaks and valleys, and some of those plateaus are level enough for us to extract the kind of information we need.”

The properties of electrons in regions near material boundaries in these structures are of special interest to physicists because they are not well understood.

“There are a lot of different ways these layers can interact,” Freeland said.  “Knowing where atoms are doesn’t necessarily tell you where the material superconducts.”

When one layer is superconducting and the other is magnetic, the Argonne researchers attempted to figure out how these two dissimilar phases met at their boundary. This required sampling the behavior of electrons in the region using especially sensitive scanning tunneling microscopy.

The planned upgrade to Argonne's Advanced Photon Source that will take place by the end of the decade has the potential to improve the data collection process. According to Freeland, current x-ray spectroscopy provides chemical information about the sample but does not give a highly resolved picture of its spatial or physical configuration, while electron microscopy reveals the second kind of information but not the former. 

Using the newly developed technique of coupling x-rays with scanning probes, the upgrade to the APS would allow researchers to combine the power of the two techniques in a single process, dramatically reducing the time and cost required to analyze the behavior of these materials.

The funding for the research comes from a laboratory-directed research and development grant. A paper based on the study can be found in Nature Communications.

Source: Argonne National Laboratory

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