Researchers at Chalmers University of Technology, Sweden, have discovered a striking new behavior of the “strange metal state” of high-temperature superconductors. The discovery is an important piece of the puzzle for understanding these materials. The findings were published in the prestigious journal Science.
Superconductivity, in which electric current is transported without losses, holds enormous potential for environmentally friendly technologies. For example, if it works at sufficiently high temperatures, it could enable the lossless transport of renewable energy over long distances. Studying this phenomenon is the goal of the research field of high-temperature superconductivity. The current record is ‑130 degrees Celsius, which may not seem like a high temperature, but it is a high temperature compared to standard superconductors, which only work at temperatures below ‑230 degrees Celsius. While standard superconductivity is well understood, various aspects of high-temperature superconductivity are still a mystery to be solved. The newly published research focuses on the least explored property — the so-called “strange metal” state that occurs at temperatures higher than those that enable superconductivity.
“This ‘strange metal’ state is aptly named. The materials really behave in a very unusual way, and it’s a mystery to researchers. Our work now offers a new understanding of the phenomenon. Through novel experiments, we have gained crucial new information about how the strange metal state works.”
- Floriana Lombardi, professor at the Laboratory for Quantum Physics
Presumably it is based on quantum entanglement
The strange metal state got its name because its behavior in conducting electricity is, at first glance, far too simple. In an ordinary metal, many different processes affect the 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 is a complicated function of temperature. In stark contrast, resistance in foreign metals is a linear function of temperature, meaning a straight line from the lowest temperatures achievable to the point where the material melts.
“Such simple behavior calls for a simple explanation based on a powerful principle, and for this type of quantum material, the principle is believed to be quantum entanglement,” says Ulf Gran, a professor in the Department of Subatomic, High Energy and Plasma Physics at Chalmers’ Faculty of Physics.
“Quantum entanglement is what Einstein called ‘spooky action at a distance’ and represents a way for electrons to interact with each other that has no equivalent in classical physics. To explain the counterintuitive properties of the strange metallic state, all particles must be entangled with each other, resulting in a soup of electrons in which individual particles are undetectable and which represents a radically new form of matter.”
Exploring the connection with charge density waves
The most important result of the work is that the authors have figured out what accounts for the strange metal state. Charge density waves (CDW) occur in high-temperature superconductors, which are waves of electric charge generated by patterns of electrons in the material lattice when the strange metal phase collapses. To investigate this relationship, nanoscale samples of the superconducting metal yttrium-barium-copper oxide were placed under voltage to suppress the charge density waves. This then led to the reappearance of the strange metal state. By stretching the metal, the researchers were able to extend the strange metal state into the region previously dominated by the CDW — making the “strange metal” even stranger.
“The highest temperatures for the superconducting transition were observed when the strange metal phase was more pronounced. Understanding this new phase of matter is therefore of utmost importance to be able to design new materials that are superconducting at even higher temperatures,” Floriana Lombardi explains.
The researchers’ work suggests a close connection between the appearance of charge density waves and the breaking of the strange metal state — a potentially crucial clue to understanding the latter phenomenon, which could represent one of the most impressive proofs of quantum mechanical principles at the macro scale.
