The ‘strange metal’ state of high-temperature superconductors has shown a stunning new behavior, according to researchers at the Chalmers University of Technology in Sweden. The result was reported in the journal Science, and it constitutes an essential piece of the jigsaw in understanding these materials.
Superconductivity, in which an electric current is carried without loss, has huge implications for green technology. It may, for example, allow for the lossless transfer of renewable energy across long distances if it could be engineered to function at high enough temperatures. The goal of high-temperature superconductivity research is to learn more about this phenomenon. The current record is 130 degrees Celsius, which may not appear to be a high temperature, but it is when compared to ordinary superconductors, which can only perform at temperatures below 230 degrees Celsius.
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.Floriana Lombardi
While ordinary superconductivity is widely understood, certain characteristics of superconductivity at high temperatures remain a mystery. The recently published research focuses on the least known feature of superconductivity, the so-called’strange metal’ state, which appears at temperatures greater than those required for superconductivity.
“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,” says Floriana Lombardi, Professor at the Quantum Device Physics Laboratory at the Department of Microtechnology and Nanoscience at Chalmers.
Believed to be based on quantum entanglement
On the surface, the odd metal state’s behavior when carrying electricity appears to be way too simple. Many distinct processes impact the electrical resistance of an ordinary metal. Electrons might collide with the atomic lattice, with impurities, or with each other, and each process has a different temperature dependency.
As a result, the overall resistance produced is a complex function of temperature. Strange metals, on the other hand, have a linear resistance that follows a straight line from the lowest possible temperatures 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 at 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 unusual metal state’s paradoxical qualities, all particles must be entangled with one another, resulting in a soup of electrons in which individual particles cannot be distinguished, resulting in a completely unique form of matter.”
Exploring the connection with charge density waves
The authors found what kills the unusual metal state, which is the paper’s main finding. When the weird metal phase breaks down in high-temperature superconductors, charge density waves (CDW) develop, which are electric charge ripples caused by patterns of electrons in the material lattice.
To investigate this link, researchers strained tiny samples of the superconducting metal yttrium barium copper oxide to suppress charge density waves. The weird metal condition reappeared as a result of this. The researchers were able to expand the weird metal state into the previously dominated by CDW area by stressing the metal, making the strange metal’ increasingly 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 Floriana Lombardi.
The findings show a link between the creation of charge density waves and the breaking of the odd mental state, which might be a crucial insight to understanding the latter event and one of the most stunning proofs of quantum mechanical principles at the macro scale. The findings also point to a possible new research direction: manipulating quantum materials via strain control.