When it comes to high-temperature superconductors, ‘high’ is a relative term. In the field of superconductivity, ‘high temperature’ means anything that can still be superconductive at more than 30K, or a balmy -405°F.

The first high-temperature superconductor was discovered in 1986, in ceramic compounds of copper and oxygen known as cuprates. These materials could reach superconductivity at around 35K, or -396.67°F. In the following decades, this temperature limit increased, and to date researchers have achieved superconductivity in cuprates at temperatures of up to 135K.

It's important progress, to be sure, but room-temperature superconductivity, which requires operation at 300K, is still a long way off, if not impossible. One of the biggest obstacles is that researchers still don't completely understand the underlying mechanisms responsible for cuprate superconductivity and why there is such variability in the superconducting transition temperature among cuprate compounds.

Now, researchers at the Harvard John A. Paulson School of Engineering and Applied Sciences (SEAS) may have the answer. The researchers, led by Xin Li, assistant professor of materials science at SEAS, found that the strength of a particular chemical bond in cuprate compounds influences the temperature at which the material achieves superconductivity. They report their finding in a paper in Physical Review Letters.

"This could be a new start for designing materials with high-temperature superconductivity," said Li. "Our research sheds lights on a key component of the complicated phenomena in cuprates and points us in a new and exciting direction for materials design."

All cuprates have the same structural building blocks – layered planes of copper peroxide (CuO2) with an out-of-plane oxygen ion, known as the apical oxygen. This oxygen ion sits above each copper atom in the CuO2 plane, like a buoy on the surface of water. The key difference between cuprate compounds comes from what other element is attached to the oxygen buoy. This element is known as the apical cation and can include lanthanum, bismuth, copper or mercury.

The temperature at which a cuprate becomes superconducting changes depending on which element is used, but no one really knows why. By comparing simulation and experiments, Li and his team were able to demonstrate that the key is the bond between the apical cation and the apical oxygen – the stronger the chemical bond, the higher the temperature at which the material becomes superconducting.

"This could be a new start for designing materials with high-temperature superconductivity. Our research sheds lights on a key component of the complicated phenomena in cuprates and points us in a new and exciting direction for materials design."Xin Li, Harvard SEAS

But why does this bond raise superconducting temperatures?

Superconductors are often described as electron superhighways, or super carpool lanes, in which paired electrons are cars and the superconducting material is the special, frictionless road along which the cars move. However, electrons don't really move across a high-temperature superconductor like a car on a road. Instead, they hop. This hopping process is made a lot easier when the crystal lattice on which the electrons are moving oscillates in a particular way.

A strong chemical bond between the apical anion and apical cation increases the oscillation of both the lattice and the induced electric current.

Imagine a kite tied to a buoy and many such kite-buoy units lined up. If the bond between a kite and a buoy is strong, the kite can pull the buoy up and down, causing ripples and splashes in the water. These ripples are akin to the lattice oscillation and the splashes represent the electrons that get pushed out of the CuO2 plane. The ripples and splashes are not chaotic, rather they cooperatively follow certain rules that tell the buoys how to oscillate in the best way to help the electrons hop easily along the material.

"We demonstrated that this structural unit – the copper oxygen layer, the apical anion and the apical cation – is a fundamental building block that can couple dynamically to control the superconductive properties of the material," said Li. "This opens up an entirely new avenue to explore the superconductive properties of materials."

Next, the researchers aim to explore how this novel effect impacts our understanding of the mysterious phase diagram in high-temperature superconductors, including the pairing mechanism in these superconductors.

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