Study Results Provide New Insights into High-Temperature Superconductivity

Researchers at EPFL have studied strange quantum events taking place in an innovative superconducting material, paving the way to understand high-temperature superconductivity better.

Marco Grioni ©2014 Alain Herzog/EPFL

The material they have studied is cuprates, which hold potential to exhibit superconductivity at elevated temperatures (-120˚C). The superconductive characteristic of cuprates holds promise for electricity at a lower cost and with no loss of energy.

Researchers around the world are working on to gain insights into the physics of cuprates, with the attention of developing room-temperature superconductors. The research group headed by Marco Grioni at EPFL revealed the mechanism that governs superconductivity of cuprates using an advanced technique.

Traditional superconductors conduct electricity without electrical resistance at temperatures in the vicinity of absolute zero. The electrons of the material couple to form a quantum effect called “Cooper pairs” under these conditions.

Cooper pairs are capable of flowing with no electrical resistance. The formation of Cooper pairs typically occurs due the generation of an attractive force between electrons of the superconductor caused by the vibration of its atoms.

However, in some superconducting materials, their atoms push them together, thus denying the formation of Cooper pairs. Cuprates are such superconductors and these copper-based materials behave like magnets and electrical insulators under normal temperatures.

They are recognized for their superconductivity at elevated temperatures (just over -123.15°C or 150K) when compared to other materials, making them an ideal material to realize everyday superconductivity. However, earlier studies have proposed that superconductivity of cuprates is different from other materials. Hence, it is necessary to understand superconductivity of cuprates.

The EPFL team explored the superconductivity of cuprates using an advanced spectroscopic technique known as Resonant Inelastic X-ray Scattering, which is an analytical technique to determine a material’s electronic structure.

With the high-resolution Resonant Inelastic X-ray Scattering technique, the researchers monitored the status of the electrons of a cuprate sample when it converted into a superconductor.

“Normally, superconductors hate magnetism,” says Grioni. “Either you have a good magnet or a good superconductor, but not both. Cuprates are very different and have really surprised everyone, because they are normally insulators and magnets, but they become superconducting when a few extra electrons are added by gently tweaking its chemical composition.”

The existence of an electron property called ‘spin’ is the key ingredient of magnetism. Spins are capable of interacting with one another, creating spin waves all along the material. Spin waves are generated in disturbed magnetic materials and travel in ripples across their volume. These spin waves provide clues about the magnetic interaction and structure.

Cuprates retain their magnetic properties even after they become superconductors. “Something of the magnet remains in the superconductor, and could play a major role in the appearance of superconductivity ” says Grioni. “The new results give us a better idea of how the spins interact in these fascinating materials.”

The study results provide new insights into superconductivity in cuprates, and hold potential to understand other high-temperature superconductors better. The new findings about the role played by spin interactions in the superconductivity of cuprates help in the realization of high-temperature superconductors.

This study is a partnership of EPFL’s Institute of Condensed Matter with Swiss Light Source, Ruhr-Universität Bochum, IFW Dresden, the University of British Columbia, the National University of Science and Technology of the Russian Federation, and the University of Geneva.

The study results have been reported in the journal, Nature Communications.

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