This image shows the positions of atoms (blue spheres) that make up the crystal lattice of a copper-oxide superconductor, superimposed on a map of electronic charge distribution (yellow is high charge density, dark spots are low) in charge-ordered states. Normally, the atoms can vibrate side-to-side (shadows represent average locations when vibrating). But when cooled to the point where the ladder-like charge density wave appears, the atomic positions shift along the ‘rungs’ and the vibrations cease, locking the atoms in place. Image: Brookhaven National Laboratory.
This image shows the positions of atoms (blue spheres) that make up the crystal lattice of a copper-oxide superconductor, superimposed on a map of electronic charge distribution (yellow is high charge density, dark spots are low) in charge-ordered states. Normally, the atoms can vibrate side-to-side (shadows represent average locations when vibrating). But when cooled to the point where the ladder-like charge density wave appears, the atomic positions shift along the ‘rungs’ and the vibrations cease, locking the atoms in place. Image: Brookhaven National Laboratory.

What allows some materials to carry current with no resistance? Harnessing this property, known as superconductivity, could lead to perfectly efficient power lines, ultrafast computers and a range of energy-saving advances. Understanding these materials when they aren’t superconducting is, however, a key part of unlocking that potential.

“To solve the problem, we need to understand the many phases of these materials,” said Kazuhiro Fujita, a physicist in the Condensed Matter Physics & Materials Science Department of the US Department of Energy’s Brookhaven National Laboratory. In a new study, 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. They report their findings in a paper in Physical Review X.

This oddity was the mysterious disappearance of vibrational energy from the atoms that make up the superconductor’s crystal lattice. “X-rays show that the atoms vibrate in particular ways,” Fujita said. But they also showed that as the material is cooled, 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.”

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, the team 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. 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.”

This provides 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.”

This story is adapted from material from Brookhaven National Laboratory, with editorial changes made by Materials Today. The views expressed in this article do not necessarily represent those of Elsevier. Link to original source.