Quirky quantum materials could hold fundamental answers for quantum scientists

Metals are a class of materials that are fairly well-understood—we’ve used them in technology for centuries, from arrowheads to smartphones—but “strange metals” are a certain kind of metal that scientists don’t know how to explain. While it’s more often seen in a lab than in our everyday lives, figuring it out could provide new answers about quantum mechanics, superconductivity…and, oddly, black holes.

“You’d think, in the year 2023, we would understand metals,” said Peter Abbamonte, a physics professor at the University of Illinois Urbana-Champaign (UIUC) and the director of its Center for Quantum Sensing and Quantum Materials, a Department of Energy Frontier Research Center. “It’s not like they’re a new phase of material. But we can’t even begin to explain strange metals, and that’s very frustrating to a physicist.”

Abbamonte and other researchers at the Quantum Sensing and Quantum Materials Center have been studying strange metals, using qubits and other quantum systems as sensors to measure density fluctuations in the quirky materials. These metals have potential applications as high-temperature superconductors, but Abbamonte said he is drawn by something else: his own voracious curiosity for something he doesn’t understand.

“These metals pose really, really fundamental questions,” he said. “How can a thing in which an electron has no identity carry a current? How can such a phase of matter exist?”

The “strange” property of strange metals seems rather innocuous. Resistivity describes the ability of a material to oppose the flow of current; most metals hit a maximum resistivity when you raise the temperature high enough, but the resistivity of strange metals as they’re heated blows past where this maximum should be. While this may seem rather trivial, it’s almost impossible to explain using current theories. This implies that there is something extremely abnormal about these strange metals on a quantum level.

Resistivity in metals is caused by electrons bouncing off of one another—the more they bounce around, the slower they carry electricity through the metal. The reason why normal metals have a maximum to their resistivity is because eventually, the electrons simply can’t bounce anymore—they are packed together like passengers on a subway train at rush hour.

So what are the electrons doing in a strange metal when its resistivity keeps increasing past that limit? One theory is that they collectively become part of a single quantum state—as if the crowded train passengers could move through each other, becoming indistinguishable—that allows them to dissipate energy at a faster rate.

The quantum abnormality seen in these metals can be found in one other physical system that similarly defies traditional theories: the event horizon of a black hole. This means that this topic has garnered the attention of string theorists and cosmologists as well, who typically do not work in the realm of materials.

“It's a strange confluence of condensed matter physicists and string theorists,” Abbamonte said, adding that although these specialized fields of physics deal with extremely different systems, surprisingly, a small portion of the math is exactly the same.

He cited a meeting he attended in 2021 in the Netherlands where scientists from those two fields came together to discuss strange metals.

“It was fascinating because we basically spent two days just trying to understand what the other group was saying,” he said “What does this mean? What does that mean? It was like being thrown on a desert island with a bunch of people who speak a different language. It was extremely interesting.” Abbamonte hopes that the work being done at the Quantum Sensing and Quantum Materials Center may lend some insight into what happens with strange metals. Whatever it is, he said, this phenomenon can’t be explained by what we currently know, so the answer, when it’s found, will be completely new physics—for superconductors and black holes.