Charge density wave linked to distortions in superconductor

Scientists are trying to unravel the key characteristics of superconductivity in order to ascertain what makes some materials carry current with no resistance. Harnessing superconductivity could lead to perfectly efficient powerlines, ultrafast computers and a range of energy-saving advances. Understanding these materials when they aren’t superconducting is a key part of the quest to unlock that potential. Kazuhiro Fujita, a physicist in the US Department of Energy’s Brookhaven National Laboratory, said that in order to solve the problem, the researchers needed to understand the many phases of these materials.

In a study published in Physical Review X, Fujita and his colleagues sought 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. According to Fujita, X-rays show that the atoms vibrate in particular ways, but as the materials 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,” Fujita said.

The researchers used a spectroscopic imaging scanning tunnelling microscope (SI-STM) to scan the surface of the layered material with trillionths-of-a-metre precision. The researchers then mapped the atoms and measured 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 positons 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 making up the charge density wave were localised (in fixed positions) and separated from the more mobile electrons that eventually carry the current in the superconducting phase. These localised electrons form a repeating pattern of higher and lower densities that can be visualised as ladders lying side-by-side. It’s the appearance of this pattern that distorts the normal vibrations of the atoms and sifts 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. 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.

While the researchers have determined how two of the characteristics of one phase of a superconducting material couple together, Fujita said that there is still more to uncover about these materials. “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,” Fujita said.

The spectroscopic imaging scanning tunnelling microscope used in this study achieves its 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.

“If there is any external vibration, that is going to kill the experiment. We need vibration isolation to perform the experiment correctly,” Fujita said.

While making measurements, a needle hovers over the sample at a distance of about one angstrom 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.

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