‘Charge density waves’ are related to atomic distortion in superconducting candidates

(Nanowerk News) What makes some materials carry current without resistance? Scientists are trying to unravel complex characteristics. Harnessing this property, known as superconductivity, can lead to highly efficient power lines, ultra-fast computers and a series of energy-efficient advancements. Understanding these materials when they are not superconductors is an important part of the quest to unlock that potential.

“To solve this problem, we need to understand the many phases of this material,” said Kazuhiro Fujita, a physicist in the Condensed Matter Physics & Materials Science Department at the US Department of Energy’s Brookhaven National Laboratory. In a new study just published in X Physical Review (“Periodic Atomic Movement and Visualization of Electron-Lattice Interactions in Cuprate”), Fujita and his colleagues sought explanations for the oddities observed in the coexistent phase with the superconducting phase of copper-oxide superconductors.

The anomaly is the mysterious loss of vibrational energy of the atoms that make up the material’s crystal lattice. “X-rays show that atoms vibrate in a certain way,” said Fujita. But when the material is cooled, x-ray studies show, one mode of vibration stops (Natural Physics, “Spectroscopic fingerprints of charge sequence melting driven by quantum fluctuations in cuprate”). This image shows the positions of the atoms (blue spheres) that make up the copper-oxide superconducting crystal lattice, superimposed on a map of the electronic charge distribution (yellow is high charge density, black spots are low) in a charge-ordered state. Typically, atoms can vibrate from side to side (shadows represent the average location as they vibrate). But when cooled to the point where a ladder-like charge density wave appears, the position of the atoms shifts along the “rungs” and the vibration stops, locking the atoms in place. Understanding the state of this charge order can help scientists unlock other interactions that trigger superconductivity at lower temperatures. (Image: Brookhaven National Laboratory)

“Our study explores the relationship between the lattice structure and the electronic structure of these materials to see if we can understand what is going on,” said Fujita.

The Brookhaven team used a tool called a tunneling microscope scanning imaging spectroscopy (SI-STM). By scanning the surface of layered materials with a precision of a millionth of a meter, they can map atoms and measure the distances between them—while simultaneously measuring the electric charge at each atomic-scale location.

The measurements are sensitive enough to pick up the average positions of the atoms when they vibrate—and show how those positions shift and become locked in place when the vibrations stop. They also show that the loss of anomalous vibrations is directly related to the appearance of “charge density waves”—a modular distribution of charge densities in materials.

The electrons that make up the charge density wave are localized, meaning in a fixed position — and separated from the more moving electrons that ultimately carry the current in the superconducting phase, explains Fujita. These localized electrons form a repeating pattern of higher and lower densities which can be visualized as a ladder laid side by side (see diagram). It is the appearance of this pattern that distorts the normal vibrations of the atoms and shifts their position along the “rungs” direction.

“As the temperature drops and a charge density wave (CDW) appears, the energy of the vibrations drops,” says Fujita. “By measuring the charge distribution and atomic structure together, you can see how the appearance of CDW locks the atoms in place.”

“These results imply that, when the atoms vibrate, the charge density waves interact with the lattice and quench the lattice. It stops the vibrations and distorts the grid,” said Fujita.

So that’s one more clue as to how the two characteristics of a single phase of a superconducting material pair together. But much remains to be uncovered about this promising ingredient, says Fujita.

“There are lots of variables. Electrons and lattice are just two. We have to consider all of these and how they interact with each other to really understand this material,” he said.

Atomic scanning and precision electronics

The spectroscopic imaging scanning tunneling microscope (SI-STM) used in this study achieves its extreme precision by being completely isolated from its environment. It lies in a concrete cube that “floats” on a vibration-bearing spring anchored to the ground separately from the foundation of the Interdisciplinary Science Building on the Brookhaven campus. Electromagnetically isolated Faraday enclosure, sound insulation foam and three layers of doors provide complete protection from any external vibrations.

“If there were any external vibrations, it would shut down the experiment,” said Fujita. “We needed vibration isolation to do the experiment properly.”

When taking a measurement, the needle hovers over the sample at a distance of about one angstrom — a ten-billionth of a meter, or about the diameter of an atom — but does not touch the surface. Applying a varying voltage allows electrons to tunnel (or jump) from the sample to the other end, creating a current. Current strength at each location maps the electron density of the material while simultaneous spectroscopic imaging captures the topographical features of the sample—including atomic positions and variations caused by impurities and imperfections.