June 3 , 2010
Editors: A video can be downloaded at http://dailynews.mcmaster.ca/images/fermion.avi
showing waves as they bounce off thorium atoms and change in size. A schematic image of the electrons can be of downloaded at http://dailynews.mcmaster.ca/images/fermion.jpg
The yellow and blue balls represent electron wave functions around uranium atoms while the silver balls with arrows representing their spin represent electrons moving through the crystal at a low temperature of 55K. Please credit: McMaster University
Scientists capture secret dance of electrons that causes them to change their form
The findings by scientists from McMaster University, Cornell University, the U.S. Department of Energy’s Brookhaven and Los Alamos National Laboratories, are published in the current issue of Nature.
“When electrons interact in materials unpredictable things can happen,” says Graeme Luke, professor in the Department of Physics & Astronomy at McMaster University. “Heavy fermion behavior—where electrons behave as if they weigh 1,000 times or more their actual mass—is one of the most fascinating examples of these phenomena. In this case, they were doing a kind of wave-like dance that changes the form and very nature of the electrons.”
Using properties in a crystal composed of uranium, ruthenium and silicon— synthesized by doctoral candidate Travis
Williams and staff member Jim Garrett in Luke’s group using the facilities of the Centre for Crystal Growth at
McMaster’s Brockhouse Institute for Materials Research — the effects of heavy fermions began to appear as the
material was cooled below 55 Kelvin (-218 °C). But an even more unusual electronic phase transition occurred
Using a technique developed specifically for their experiment known as spectroscopic imaging scanning tunneling
microscopy (SI-STM), the team was able to track the arrangement and interactions of electrons in the crystals, and
watch how they react at different temperatures and see what happens when they passed through the mysterious
“For 25 years we have known that there’s a phase transition occurring in this material but we’ve never been able to identify what kind of order was occurring. It wasn’t magnetic order or superconductivity,” says Luke. “We didn’t know if it was related to the way electrons were behaving in a group or whether it was the result of interactions between individual electrons and uranium atoms. The microscope, however, allowed us to actually see a change in the microscopic electron states.”
“Imagine flying over a body of water where standing waves are moving up and down, but not propagating toward
the shore,” said study leader Séamus Davis, a physicist at Brookhaven and the J.D. White Distinguished Professor of
Physical Sciences at Cornell University. “When you pass over high points, you can touch the water; over low points,
you can’t. This is similar to what our microscope does. It images how many electrons can jump to the tip of our probe at every point on the surface.”
Based on these wavelength and energy measurements, scientists can calculate the effective electron mass for specific electron bands.
The researchers are continuing to probe a variety of related compounds with this new approach to further their understanding of heavy fermion systems.
The study was funded by the Natural Sciences and Engineering Research Council of Canada, the Canadian Institute
for Advanced Research, and the U.S. Department of Energy’s Office of Science. At Brookhaven, this research was
supported as part of the Center for Emergent Superconductivity, an Energy Frontier Research Center funded by the
U.S. Department of Energy, Office of Science.
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