A team of physicists from the US and China have found unexpected signs of a strange metallicity in a material in which electric charge is carried not by electrons, but by more “wave-like” objects called Cooper pairs.
Strange metals, also known as non-Fermi liquids, are a class of materials that do not obey the traditional laws of electrical engineering. Their behavior was first discovered about 30 years ago in cuprates. These copper oxide materials are best known as high-temperature superconductors—they conduct electricity with zero resistance at temperatures far above those of conventional superconductors.
But even at temperatures above the critical temperature for superconductivity, cuprates behave strangely compared to other metals. With an increase in their temperature, the resistance of cuprates increases strictly linearly.
In normal metals, the resistance increases only up to a certain limit, becoming constant at high temperatures in accordance with the so-called Fermi liquid theory.
Resistance occurs when electrons in a metal collide with the vibrating atomic structure of the metal, causing them to dissipate.
Fermi liquid theory sets the maximum rate at which electrons can scatter. But strange metals don’t follow the laws of a Fermi liquid, and no one knows exactly how they work.
What physicists do know is that the relationship between temperature and resistance in strange metals appears to be related to two fundamental constants of nature: the Boltzmann constant, which represents the energy created by random thermal motion, and the Planck constant, which is related to the energy of a photon. . .
“To try to understand what’s going on in these strange metals, people have taken mathematical approaches similar to those used to understand black holes,” said Dr. Valles, who led the study.
In 1952, Nobel laureate Leon Cooper discovered that in ordinary superconductors, electrons combine into Cooper pairs, which can slide through an atomic lattice without resistance.
Even though they are made up of two electrons, which are fermions, Cooper pairs can act like bosons.
“Fermionic and bosonic systems usually behave very differently. Unlike individual fermions, bosons are allowed to have the same quantum state, which means they can move collectively, like water molecules in the ripples of a wave,” Valles explained.
In 2019, researchers showed that paired Cooper bosons can exhibit metallic behavior, meaning they can conduct electricity with some resistance. This in itself was a surprising discovery, because elements of quantum theory suggested that this phenomenon was impossible.
For the new study, the scientists wanted to see if the Cooper pair bosonic metals are also strange metals. They used a cuprate material – yttrium-barium-copper oxide – with a pattern of tiny holes that induce the metallic state of the Cooper pair.
They then cooled the material just above its superconducting temperature to observe changes in its conductivity. They found, like fermionic strange metals, the metallic conductivity of the Cooper pair, which is linear with temperature.
“Our work shows that if you are going to model charge transport in strange metals, this model must be applied to both fermions and bosons — even though these types of particles obey fundamentally different rules,” the scientist said.