"To solve the problem, we need to understand the many phases of these materials," said Kazuhiro Fujita, a physicist in the Condensed Matter Physics and Materials Science Department of the U.S. Department of Energy's Brookhaven National Laboratory. In a new study just published in Physical Review X, Fujita and his colleagues sought to find an explanation for an oddity observed in a phase that coexists with the superconducting phase of a copper-oxide superconductor.
The anomaly was a mysterious disappearance of vibrational energy from the atoms that make up the material's crystal lattice. "X-rays show that the atoms vibrate in particular ways," Fujita said. But as the material is cooled, the x-ray studies showed, one mode of the vibrations stops.
"Our study explored the relationship between the lattice structure and the electronic structure of this material to see if we could understand what was going on," Fujita said.
The Brookhaven team used a tool called a spectroscopic imaging scanning tunneling microscope (SI-STM). By scanning the surface of the layered material with trillionths-of-a-meter precision, they could map the atoms and measure the distances between them-while simultaneously measuring the electric charge at each atomic-scale location.
The measurements were sensitive enough to pick up the average positions of the atoms when they were vibrating-and showed how those positions shifted and became locked in place when the vibrations stopped. They also showed that the anomalous vibrational disappearance was directly linked to the emergence of a "charge density wave"-a modular distribution of charge density in the material.
The electrons that make up the charge density wave are localized, meaning in fixed positions-and separate from the more mobile electrons that eventually carry the current in the superconducting phase, Fujita explained. These localized electrons form a repeating pattern of higher and lower densities that can be visualized as ladders laying side-by-side (see diagram). It's the appearance of this pattern that distorts the normal vibrations of the atoms and shifts their positions along the direction of the "rungs."
"As the temperature goes down and the charge density wave (CDW) emerges, the vibrational energy goes down," Fujita said. "By measuring both charge distribution and atomic structure simultaneously, you can see how the emergence of the CDW locks the atoms in place."
"This result implies that, as the atoms vibrate, the charge density wave interacts with the lattice and quenches the lattice. It stops the vibrations and distorts the lattice," Fujita said.
So that's one more clue about how two of the characteristics of one phase of a superconducting material couple together. But there's still a lot to uncover about these promising materials, Fujita said.
"There are many variables. Electrons and the lattice are just two. We have to consider all of these and how they interact with each other to truly understand these materials," he said.
This work was supported by the DOE Office of Science (BES).
wPrecision atomic and electronic scanning
The spectroscopic imaging scanning tunneling microscope (SI-STM) used in this study achieves its extreme precision by being completely isolated from its surroundings. It's situated in a cube of concrete that "floats" on vibration-cushioning springs anchored to the ground separately from the foundation of the Interdisciplinary Science Building on Brookhaven's campus. An electromagnetically isolating Faraday cage, sound-insulating foam, and three layers of doors provide complete protection from any external vibrations.
"If there is any external vibration, that is going to kill the experiment," Fujita said. "We need vibration isolation to perform the experiment correctly."
When making measurements, a needle hovers over the sample at a distance of about one angstrom-one ten-billionth of a meter, or about the diameter of an atom-but not touching the surface. Applying varying voltages allows electrons to tunnel (or jump) from the sample to the tip, creating a current. The strength of the current at each location maps out the material's electron density while simultaneous spectroscopic imaging captures the sample's topographical features-including atomic positions and variations caused by impurities and imperfections.
Research Report:"Periodic Atomic Displacements and Visualization of the Electron-Lattice Interaction in the Cuprate"
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