Newly discovered type of 'strange metal' could lead to deep insights


Date:
January 12, 2022
Source:
Brown University
Summary:
A new discovery could help scientists to understand 'strange
metals,' a class of materials that are related to high-temperature
superconductors and share fundamental quantum attributes with
black holes.



FULL STORY ========================================================================== 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.

========================================================================== Story Source: Materials provided by Brown_University. Note: Content may
be edited for style and length.


========================================================================== Related Multimedia:
* Researchers_have_discovered_'strange_metal'_behavior ========================================================================== Journal Reference:
1. Chao Yang, Haiwen Liu, Yi Liu, Jiandong Wang, Dong Qiu, Sishuang
Wang,
Yang Wang, Qianmei He, Xiuli Li, Peng Li, Yue Tang, Jian Wang,
X. C. Xie, James M. Valles, Jie Xiong, Yanrong Li. Signatures of
a strange metal in a bosonic system. Nature, 2022; 601 (7892):
205 DOI: 10.1038/s41586-021- 04239-y ==========================================================================

Link to news story: https://www.sciencedaily.com/releases/2022/01/220112121457.htm
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