Scientists discover new behaviour in high temperature superconductors

Researchers from Chalmers University of Technology (CUT), Sweden, have uncovered a surprising new behaviour regarding the ‘strange metal’ state of high temperature superconductors.

What is superconductivity?

Superconductivity is the scientific process where an electric current is transported by high temperature superconductors without any losses in electrical resistance, and is a phenomenon that holds great potential regarding green technologies.

How would this work for renewable energy?

It is theorised that if this could work at high enough temperatures, it could potentially allow for a lossless transport of renewable energy over great distances. Investigating this possibility is the aim of CUT’s research into high temperature superconductors.

The highest temperature reached is −130 °C, which is substantially higher than the standard superconductors which only function below−230 °C. While standard superconductivity is well understood within the scientific field, there are several aspects of high temperature superconductors that are still perplexing researchers.

The research conducted by the CUT research team concentrates on the ‘strange metal state,’ which is the least understood property due to the way it appears at temperatures higher than those that allow for superconductivity.

What is the ‘strange metal state?’

This ‘strange metal state’ is aptly named. The materials really behave in a very unusual way, and it is something of a mystery among researchers. Our work now offers a new understanding of the phenomenon. Through novel experiments, we have learned crucial new information about how the strange metal state works,’ explained Floriana Lombardi, Professor at the Quantum Device Physics Laboratory at the Department of Microtechnology and Nanoscience at CUT.

The ‘strange metal state’ was named as such due to its behaviour when conducting electricity is ‘simple’. In an ordinary metal, various processes affect the electrical resistance. For example, electrons can collide with the atomic lattice, with impurities, or with themselves, and each process has a different temperature dependence.

This means that the resulting total resistance becomes a complicated function of the temperature. In sharp contrast, the resistance for strange metals is a linear function of temperature, which means there is a straight line from the lowest attainable temperatures up to where the material melts.

Ulf Gran, Professor at the Division of Subatomic High-Energy and Plasma Physics at the Department of Physics at CUT commented: “Such a simple behaviour begs for a simple explanation based on a powerful principle, and for this type of quantum materials the principle is believed to be quantum entanglement.

Quantum entanglement is what Einstein called ‘spooky action at a distance’ and represents a way for electrons to interact which has no counterpart in classical physics. To explain the counterintuitive properties of the strange metal state, all particles need to be entangled with each other, leading to a soup of electrons in which individual particles cannot be discerned, and which constitutes a radically novel form of matter.”

A connection noted with charge density waves

Scientists discovered that in high temperature superconductors, charge density waves (CDW) killed the ‘strange metal state.’ CDW are ripples of electric charge that are generated by patterns of electrons in the material lattice and occurs when the ‘strange metal’ phase breaks down.

To explore this connection, nanoscale samples of the superconducting metal yttrium barium copper oxide were put under strain to suppress the charge density waves. This led to the re-emergence of the ‘strange metal state.’ By straining the metal, the researchers were able to this state into the region previously dominated by CDW, thus making the ‘strange metal’ even more confounding.

The results achieved by CUT’s research team indicates a close connection between the emergence of charge density waves and the breaking of the strange metal state. This is a potentially vital clue to understand the latter phenomenon, which might represent one of the most striking pieces of evidence of quantum mechanical principles at the macro scale. The results further suggest a promising new avenue of research, using strain control to manipulate quantum materials.

“The highest temperatures for the superconducting transition have been observed when the strange metal phase is more pronounced. Understanding this new phase of matter is therefore of utmost importance for being able to construct new materials that exhibit superconductivity at even higher temperatures,” concluded Floriana Lombardi.

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