A team of American and Chinese researchers have found strange metal behaviour in a material in which electrical charge is carried not by electrons, but by wave-like entities called Cooper pairs.
“Strange metals” are a class of materials that are related to high-temperature superconductors and share fundamental quantum attributes with black holes. These materials do not seem to follow the traditional electrical rules and, thus, have caught scientists’ attention in recent years as they could provide fundamental insights into the quantum world, and potentially help to understand strange phenomena like high-temperature superconductivity.
The recent discovery related to strange metals was published in the journal Nature. In the article, the researchers explain that while electrons belong to a class of particles called fermions, Cooper pairs act as bosons, which follow very different rules from fermions.
“We have these two fundamentally different types of particles whose behaviours converge around a mystery,” said Jim Valles, the study’s corresponding author. “What this says is that any theory to explain strange metal behaviour can’t be specific to either type of particle. It needs to be more fundamental than that.”
Valles explained that strange metal behaviour was first discovered 30 years ago in a class of materials called cuprates. These copper-oxide materials are most famous for being high-temperature superconductors, meaning they conduct electricity with zero resistance at temperatures far above that of normal superconductors. But even at temperatures above the critical temperature for superconductivity, cuprates act strangely compared to other metals.
As their temperature increases, cuprates’ resistance increases in a strictly linear fashion. In normal metals, the resistance increases only so far, becoming constant at high temperatures in accord with what’s known as Fermi liquid theory. Resistance arises when electrons flowing in a metal bang into the metal’s vibrating atomic structure, causing them to scatter. Fermi-liquid theory sets a maximum rate at which electron scattering can occur.
But strange metals don’t follow the Fermi-liquid rules, and no one is sure 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 Planck’s constant, which relates to the energy of a photon.
“To try to understand what’s happening in these strange metals, people have applied mathematical approaches similar to those used to understand black holes,” Valles said. “So there are some very fundamental physics happening in these materials.”
In recent years, Valles and his colleagues have been studying electrical activity in which the charge carriers are not electrons. In 1952, Nobel Laureate Leon Cooper discovered that in normal superconductors (not the high-temperature kind discovered later), electrons team up to form Cooper pairs, which can glide through an atomic lattice with no resistance. Despite being formed by two electrons, which are fermions, Cooper pairs can act as bosons.
“Fermion and boson systems usually behave very differently,” Valles said. “Unlike individual fermions, bosons are allowed to share the same quantum state, which means they can move collectively like water molecules in the ripples of a wave.”
In 2019, Valles and his colleagues showed that Cooper pair bosons can produce metallic behaviour, meaning they can conduct electricity with some amount of resistance. That in itself was a surprising finding because elements of quantum theory suggested that the phenomenon shouldn’t be possible. For this latest research, the team wanted to see if bosonic Cooper-pair metals were also strange metals.
They then used a cuprate material called yttrium barium copper oxide, patterned with tiny holes that induce the Cooper-pair metallic state. The team cooled the material down to just above its superconducting temperature to observe changes in its conductance. They found, like fermionic strange metals, a Cooper-pair metal conductance that is linear with temperature.
The researchers say this discovery will give theorists something new to chew on as they try to understand strange metal behaviour.
“It’s been a challenge for theoreticians to come up with an explanation for what we see in strange metals,” Valles said. “Our work shows that if you’re going 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.”
In Valles’ view, strange metal behaviour could hold the key to understanding high-temperature superconductivity, which has vast potential for things like lossless power grids and quantum computers.