A concept from quantum information theory appears to explain at least some of the peculiar behaviour of so-called “strange” metals. The new approach, which was developed by physicists at Rice University in the US, attributes the unusually poor electrical conductivity of these metals to an increase in the quantum entanglement of their electrons. The team say the approach could advance our understanding of certain high-temperature superconductors and other correlated quantum structures.
While electrons can travel through ordinary metals such as gold or copper relatively freely, strange metals resist their flow. Intriguingly, some high-temperature superconductors have a strange metal phase as well as a superconducting one. This phenomenon that cannot be explained by conventional theories that treat electrons as independent particles, ignoring any interactions between them.
To unpick these and other puzzling behaviours, a team led by Qimiao Si turned to the concept of quantum Fisher information (QFI). This statistical tool is typically used to measure how correlations between electrons evolve under extreme conditions. In this case, the team focused on a theoretical model known as the Anderson/Kondo lattice that describes how magnetic moments are coupled to electron spins in a material.
Correlations become strongest when strange metallicity appears
These analyses revealed that electron-electron correlations become strongest at precisely the point at which strange metallicity appears in a material. “In other words, the electrons become maximally entangled at this quantum critical point,” Si explains. “Indeed, the peak signals a dramatic amplification of multipartite electron spin entanglement, leading to a complex web of quantum correlations between many electrons.”
What is striking, he adds, is that this surge of entanglement provides a new and positive characterization of why strange metals are so strange, while also revealing why conventional theory fails. “It’s not just that traditional theory falls short, it is that it overlooks this rich web of quantum correlations, which prevents the survival of individual electrons as the elementary objects in this metallic substance,” he explains.
To test their finding, the researchers, who report their work in Nature Communications, compared their predictions with neutron scattering data from real strange-metal materials. They found that the experimental data was a good match. “Our earlier studies had also led us to suspect that strange metals might host a deeply entangled electron fluid – one whose hidden quantum complexity had yet to be fully understood,” adds Si.
The implications of this work are far-reaching, he tells Physics World. “Strange metals may hold the key to unlocking the next generation of superconductors — materials poised to transform how we transmit energy and, perhaps one day, eliminate power loss from the electric grid altogether.”
The Rice researchers say they now plan to explore how QFI manifests itself in the charge of electrons as well as their spins. “Until now, our focus has only been on the QFI associated with electrons spins, but electrons also of course carry charge,” Si says.
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