Unique SSRL–Stanford Collaboration Illuminates Rare Materials
Take two high-tech X-ray beam lines, a table-top apparatus, half a dozen researchers and a material made with one of the rarest elements on earth. Combine them in a collaborative venture to bring to light the best, first clear picture of the material's properties.
That is the heart of recent work done at the Geballe Laboratory for Advanced Materials at Stanford University and at beam lines 5-4 and 7-2 at the Stanford Synchrotron Radiation Laboratory (SSRL). Such a synergistic effort is an example of how to solve one puzzle by applying the power of multiple researchers and approaches.
Combining their resources and techniques, the researchers examined little-studied compounds made of tellurium—a semi-metal that is one of the rarest elements on earth—and one of the "rare earth" elements, which actually aren't so rare.
With three different techniques brought to bear on these rare-earth tri-telluride compounds, "We get a much more complete picture," said SSRL physicist Mike Toney. "All of the studies are complementary, related to understanding the same phenomenon."
The phenomenon is the formation of a "charge density wave," a kind of electronic instability where the electrons aren't where you'd expect them to be inside the material. The movement of electrons to those new locations also distorts the positions of atoms within the material, changing aspects of the material's behavior.
At the Geballe Lab, Nancy Ru and Ian Fisher used their table-top lab equipment to run current through the materials to measure the resistance to electron flow at different temperatures. These data provided the first evidence for the occurrence of electronic instabilities in these materials, and indicated the most interesting temperature regions for the two groups at SSRL to investigate.
At beamline 5-4, Rob Moore and Z.-X. Shen of SLAC's X-ray Laboratory for Advanced Materials (XLAM) shone intense UV-light on the material to eject electrons and then analyze their energy and momentum using a technique called Angle-Resolved Photoemission Spectroscopy (ARPES). This technique reveals the material's electronic structure—where the electrons are—and how it changes due to the existence of the charge density wave.
Over on beam line 7-2, Ru, coworker Kyungyun Shin, SSRL postdoctoral researcher Cathie Condron, Fisher and Toney sent X-rays through the material to get diffraction patterns at different temperatures. These patterns showed changes in the atomic structure that were directly correlated with changes in the electronic structure observed in the ARPES data.
The combined results enable a deeper understanding of the driving forces behind the changes in atomic and electronic structure in this and related materials.
"Understanding a simpler system like this might help us understand why materials behave this way," said Moore. "The ultimate goal is to understand the fundamental physics so we could design materials with properties we desire, such as superconductors that operate at room temperature."
Heather Rock Woods, SLAC Today, January 24, 2008
Above image: SSRL Researcher Rob Moore at beamline 5-4's Angle Resolved Photoemission Spectroscope (ARPES). (Click image for larger version. Photo credit Brad Plummer.)