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Probing the workings of heavy fermion compounds, researchers find that rather than hindering superconductivity, magnetism is an essential ingredient -- and if controlled, may be a key for future advances in the field.
A new imaging technology is giving scientists unprecedented views of the processes that affect the flow of electrons through materials.
By modifying a familiar tool in nanoscience - the Scanning Tunneling Microscope - a team at Cornell University's Laboratory for Atomic and Solid State Physics have been able to visualize what happens when they change the electronic structure of a "heavy fermion" compound made of uranium, ruthenium and silicon. What they learned sheds light on superconductivity - the movement of electrons without resistance -which typically occurs at extremely low temperatures and that researchers hope one day to achieve at something close to room temperature, which would revolutionize electronics.
The researchers found that, while at higher-temperatures magnetism is detrimental to superconductivity, at low temperatures in heavy fermion materials, magnetic atoms are a necessity. "We found that removing the magnetic atoms proved detrimental to the flow [of electrons]," said researcher Mohammad Hamidian. This is important, Hamidian explains, because "if we can resolve how superconductivity can co-exist with magnetism, then we have a whole new understanding of superconductivity, which could be applied toward creating high-temperature superconductors. In fact, magnetism at the atomic scale could become a new tuning parameter of how you can change the behavior of new superconducting materials that we make."
To make these findings, the researchers modified a scanning microscope that lets you pull or push electrons into a material. With the modification, the microscope could also measure how hard it was to push and pull - a development that Hamidian explains is also significant. "By doing this, we actually learn a lot about the material's electronic structure. Then by mapping that structure out over a wide area, we can start seeing variations in those electronic states, which come about for quantum-mechanical reasons. Our newest advance, crucial to this paper, was the ability to see at each atom the strength of the interactions that make the electrons 'heavy.'"
The Cornell experiment and its results are presented this week by the Proceedings of the National Academy of Sciences (See PNAS, available online). The research team included J.C. Séamus Davis, a member of the Kavli Institute at Cornell for Nanoscale Science and developer of the SI-STM technique. Working with synthesized samples created by Graeme Luke from McMaster University (Canada), the experiment was designed by Hamidian, a post-doctoral fellow in Davis' research group, along with Andrew R. Schmidt, a former student of Davis at Cornell and now a post-doctoral fellow in physics at UC Berkeley. This research was supported by the DOE's Office of Science, the Natural Sciences and Engineering Research Council of Canada, and the Canadian Institute for Advanced Research. Additional collaborators included Ines Firmo of Brookhaven National Laboratory and Cornell, and Andy Schmidt now at the University of California, Berkeley.
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