In the left panel, the copper-oxide planes of YBCO are presented in the strange metal state, where the strong interaction between electrons, the 'quantum entanglement', is illustrated as lightning. In the right panel, the same planes are presented when the CDWs appear. Here, the symmetry of the system is reduced by the appearance of these local modulations of the conducting electrons, which cause the suppression of the strange metal phase. Image: Chalmers University of Technology/Yen Strandqvist.Researchers from Chalmers University of Technology in Sweden have uncovered a striking new behavior of the ‘strange metal’ state in high-temperature superconductors. This discovery, reported in a paper in Science, represents an important piece of the puzzle for understanding these materials.
Superconductivity, where an electric current is transported without any losses, holds enormous potential for green technologies. For example, if it could be made to work at high enough temperatures, it could enable the lossless transport of renewable energy over great distances. Investigating this phenomenon is the aim of the research field of high-temperature superconductivity.
The current record for high-temperature superconductivity stands at −130°C. Although this might not seem like a high temperature, it is when compared to standard superconductors, which only work below −230°C.
While standard superconductivity is well understood, several aspects of high-temperature superconductivity are still a puzzle to be solved. This newly published research focuses on the least understood property – the so called ‘strange metal’ state, which appears at temperatures higher than those that allow for superconductivity.
“This ‘strange metal’ state is aptly named,” says Floriana Lombardi, professor in the Quantum Device Physics Laboratory at the Department of Microtechnology and Nanoscience at Chalmers. “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.”
The strange metal state got its name because its behavior when conducting electricity is, on the face of it, far too simple. In an ordinary metal, lots of different processes affect electrical resistance – 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 – meaning a straight line from the lowest attainable temperatures up to where the material melts.
“Such a simple behavior 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,” says Ulf Gran, professor in the Division of Subatomic, High-Energy and Plasma Physics at the Department of Physics at Chalmers.
“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.”
The key finding of the paper is that the researchers discovered what kills the strange metal state. In high-temperature superconductors, charge density waves (CDW), which are ripples of electric charge generated by patterns of electrons in the material lattice, occur when the strange metal phase breaks down.
To explore this connection, the researchers put nanoscale samples of the superconducting metal yttrium barium copper oxide (YBCO) under strain to suppress the charge density waves. This then led to the re-emergence of the strange metal state. By straining the metal, the researchers were thus able to expand the strange metal state into the region previously dominated by CDWs – making the ‘strange metal’ even stranger.
“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,” explains Lombardi.
The researchers’ work indicates a close connection between the emergence of CDWs and the breaking of the strange metal state. This is a potentially vital clue to understanding the latter phenomenon, and might represent one of the most striking pieces of evidence of quantum mechanical principles at the macro scale. The results also suggest a promising new avenue of research, using strain control to manipulate quantum materials.
This story is adapted from material from Chalmers University of Technology, with editorial changes made by Materials Today. The views expressed in this article do not necessarily represent those of Elsevier. Link to original source.