Scientists understand quite well how temperature affects electrical
conductance in most everyday metals like copper or silver. But in recent
years, researchers have turned their attention to a class of materials that
do not seem to follow the traditional electrical rules. Understanding these
so-called "strange metals" could provide fundamental insights into 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 metal mix. In research published in the journal
Nature, the team found strange metal behavior in a material in which
electrical charge is carried not by electrons, but by more "wave-like"
entities called Cooper pairs.
While electrons belong to a class of particles called fermions, Cooper pairs
act as bosons, which follow very different rules from fermions. This is the
first time strange metal behavior has been seen in a bosonic system, and
researchers are hopeful that the discovery might 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, a professor of
physics at Brown and the study's corresponding author. "What this says is
that any theory to explain strange metal behavior can't be specific to
either type of particle. It needs to be more fundamental than that."
Strange metals
Strange metal behavior was first discovered around 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 (a particle of light).
"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's some very fundamental physics happening in
these materials."
Of bosons and fermions
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, now a Brown professor emeritus of physics, 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 behavior, meaning they can conduct electricity with some
amount of resistance. That in itself was a surprising finding, the
researchers say, 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.
The team 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 new discovery will give theorists something new to
chew on as they try to understand strange metal behavior.
"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."
Ultimately, a theory of strange metals could have massive implications.
Strange metal behavior could hold the key to understanding high-temperature
superconductivity, which has vast potential for things like lossless power
grids and quantum computers. And because strange metal behavior seems to be
related to fundamental constants of the universe, understanding their
behavior could shed light on basic truths of how the physical world works.
Reference:
Jie Xiong, Signatures of a strange metal in a bosonic system, Nature (2022).
DOI: 10.1038/s41586-021-04239-y.