Newly discovered type of ‘strange metal’ could lead to deep insights

Scientists understand very well how temperature affects electrical conductance in most common metals like copper or silver. But in recent years, researchers have turned to a class of materials that don’t seem to follow traditional electrical rules. Understanding these so-called “strange metals” could provide fundamental information about the quantum world and potentially help scientists understand strange phenomena like high-temperature superconductivity.

Now, a research team co-led by a Brown University physicist has added a new discovery to the strange mix of metals. In research published in the journal Nature, the team discovered strange metallic behavior in a material in which electrical charge is not carried by electrons, but by more “wavelike” entities called Cooper pairs.

While electrons belong to a class of particles called fermions, Cooper pairs act like bosons, which follow very different rules from fermions. It’s the first time strange metallic behavior has been observed in a bosonic system, and the researchers hope the discovery could be helpful in finding an explanation for how strange metals work, something that has eluded scientists for decades.

“We have these two fundamentally different types of particles whose behaviors converge around a mystery,” said Jim Valles, professor of physics at Brown and corresponding author of the study. “What this says is that any theory explaining the strange behavior of metals cannot be specific to any type of particle. It must be more fundamental than that.”

strange metals

The strange behavior of metals was first discovered about 30 years ago in a class of materials called cuprates. These copper oxide materials are best known for being high temperature superconductors, which means that they conduct electricity with zero resistance at temperatures much higher than those of normal superconductors. But even at temperatures above the critical temperature for superconductivity, cuprates act strangely compared to other metals.

When their temperature increases, the resistance of cuprates increases in a strictly linear fashion. In normal metals, the resistance increases only so far, becoming constant at high temperature in accordance with what is known as Fermi’s liquid theory. Resistance occurs when electrons flowing through a metal collide with the vibrating atomic structure of the metal, causing them to scatter. Fermi’s liquid theory defines a maximum rate at which electron scattering can occur. But strange metals don’t follow Fermi’s liquid rules, and no one knows how they work. What scientists do know is that the temperature-resistance relationship in strange metals appears to be related to two fundamental constants of nature: Boltzmann’s constant, which represents the energy produced by random thermal motion, and the constant of Planck, which relates to the energy of a photon. (a particle of light).

“To try to understand what’s going on in these strange metals, people have applied mathematical approaches similar to those used to understand black holes,” Valles said. “So there’s very fundamental physics going on in these materials.”

Of bosons and fermions

In recent years, Valles and his colleagues have studied electrical activity in which the charge carriers are not electrons. In 1952, Nobel laureate Leon Cooper, now Professor Emeritus of Brown Physics, discovered that in normal superconductors (not the high-temperature type later discovered), electrons combine to form pairs of Cooper, who can glide through an atomic network without resistance. . Although they are made up of two electrons, which are fermions, Cooper pairs can act like bosons.

“Fermi and boson systems generally behave very differently,” Valles said. “Unlike individual fermions, bosons are allowed to share the same quantum state, meaning they can move collectively like water molecules in the ripples of a wave.”

In 2019, Valles and his colleagues showed that Cooper bosons can produce metallic behavior, meaning they can conduct electricity with some resistance. That in itself was a surprising finding, the researchers say, because elements of quantum theory suggested the phenomenon shouldn’t be possible. For this latest research, the team wanted to see if the bosonic metals of the Cooper pair were also strange metals.

The team used a cuprate material called barium yttrium copper oxide to pattern tiny holes that induce the metallic state of the Cooper pair. The team cooled the material to just above its superconducting temperature to observe changes in its conductance. They found, like the fermionic strange metals, a Cooper pair metallic conductance that is linear with temperature.

The researchers say this new discovery will give theorists something new to chew on as they try to understand the strange behavior of metals.

“It’s been a challenge for theorists to find an explanation for what we see in strange metals,” Valles said. “Our work shows that if you want to model charge transport in strange metals, that model must apply to both fermions and bosons, even though these types of particles follow fundamentally different rules.”

Ultimately, a strange metal theory could have massive implications. Weird behavior of metals could hold the key to understanding high-temperature superconductivity, which has vast potential for things like lossless power grids and quantum computers. And because the strange behavior of metals seems to be tied to the fundamental constants of the universe, understanding their behavior could shed light on fundamental truths about how the physical world works.

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