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SUNDAY SCIENCE: A STRANGE FASCINATION  
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Zack Savitsky
May 22, 2025
Science
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_ Studies of exotic materials called “strange metals” point to a
whole new way to understand electricity _

Two tiny fingers of a strange metal (above, at right) are ready for
their close-up, in which a beam of neutrons will probe the behavior of
the sample’s electrons., DMYTRO INOSOV/DRESDEN UNIVERSITY OF
TECHNOLOGY

 

Something strange is afoot in Silke Bühler-Paschen’s lab at the
Vienna University of Technology. The walls of the room are plastered
in copper foil to keep out electromagnetic waves. A blue refrigerator
dangles through a hole in the ceiling, suspended from robotic shock
absorbers that precisely counteract the slightest vibrations,
including from subway cars passing deep underground. Condensation
drips down the fridge into a Minion-themed kiddie pool. Inside, a
hair-thin sample of an exotic material is cooled to thousandths of a
degree above absolute zero. What happens within this material, and the
way it conducts electricity, is one of the biggest mysteries in
condensed matter physics.

The electrons begin their journey through Paschen’s lab from an
ordinary wall outlet. According to the standard theory of electricity,
they migrate individually or in small clusters through the wires
leading to the refrigerator. But once the electrons reach the
sample—a compound of ytterbium, rhodium, and silicon—this simple
picture breaks down. The sample belongs to a class of materials that
physicists call “strange metals.” For 4 decades, they’ve puzzled
over the fact that in these compounds, the standard theory of
electricity just doesn’t work.

Recent experiments in Paschen’s lab and others suggest that in
strange metals, electrons lose their individuality. “They magically
disappear,” she says. Instead, electric charge appears somehow to
pass through the metal as a diffuse amorphous blob—like water
without individual H2O molecules. Researchers are still debating the
microscopic details of this bizarre picture. But it’s already clear
that the stakes are higher than just understanding a dozen or so
oddball materials. “It’s really a mysterious state with big
consequences,” Paschen says.

The hallmark of strange metals is electrical resistivity that climbs
higher than that of ordinary metals when they are warmed from low
temperatures. They also lose their resistivity altogether, becoming
superconductors, at lower temperatures—though above those of
conventional superconductors. Some researchers believe this
high-temperature superconductivity is simply the flip side of strange
metallicity—that they’re two manifestations of the same underlying
phenomenon. If so, then the path to room-temperature superconductors,
a long-sought goal that could revolutionize technologies from power
grids to transportation, may lead through an understanding of strange
metals. “You have to get strange metals right before you can get
superconductivity right,” says Philip Phillips, a physicist at the
University of Illinois Urbana-Champaign (UIUC). “That’s the heart
of it all.”

But the implications go beyond just building better superconductors. A
theory that explains strange metals may force a fundamental rethinking
of how electricity works in all materials. It might subsume the
standard theory the way general relativity, with its curved spacetime,
subsumed Isaac Newton’s theory of gravity—and prove just as
unsettling. Strange metals are forcing physicists to ask whether the
very idea of an electron, or any particle for that matter, is an
oversimplification of what’s really going on. “The violation of
the standard theory of solids in these strange metals is so
dramatic—it’s in your face,” says Qimiao Si, a physicist at Rice
University who collaborates with Paschen. “There’s no question
there’s new physics.”

That prospect has energized researchers. “The strange metal problem
is the hardest problem we have in condensed matter physics,” says
Peter Armitage of Johns Hopkins University. “Since when do
physicists run away from hard problems?”

RISE OF THE QUASIPARTICLE

The theory of electricity in solid materials has been overhauled
before. For a few decades after electrons were discovered at the end
of the 19th century, physicists treated them as independent particles
that passed through the lattice of atoms in a conductor like pinballs,
scattering off the atoms. Researchers knew reality was more
complicated—that electrons also repel one another because of their
negative charge—but calculating the effects of zillions of such
interactions was unfeasible.

In 1956, Russian physicist Lev Landau found a shortcut: He could
account for at least some of these electron interactions by treating a
clump of electrons as a single heavier particle called a
“quasiparticle.” It wasn’t a physical particle—it was a state
of excitation shared by many particles, like a wave of sports fans
standing up in a stadium. But mathematically, quasiparticles act like
conventional particles coursing through a metal, scattering off atoms
and one another. That scattering generates electrical resistivity, and
quasiparticles made it possible to calculate resistivity more
accurately. Landau’s model, called Fermi liquid theory, remains the
canonical understanding of how electricity flows through solid
materials.

The model captures the properties of metals on the periodic table
remarkably well. That is, apart from superconductivity. In 1911, when
physicists at Leiden University used liquid helium to cool solid
mercury below 4°C above absolute zero, its resistivity suddenly
dropped to zero. Landau’s model alone couldn’t explain this
behavior, but just 1 year after he proposed his theory, physicists
came up with a workaround.

Called BCS, the theory holds that at such low temperatures, electrons
trigger vibrations in the superconductor’s atomic lattice that
effectively glue pairs of electrons together, in spite of their
electrostatic repulsion. These Cooper pairs (named for one of the
authors of BCS theory) then settle into their lowest energy state. In
that state they can’t scatter off the lattice, because that would
require them to lose even more energy. So they flow through the
material resistance-free.

This picture worked fine for decades. But it was challenged again in
1987 when physicists at IBM in Switzerland found that certain
copper-based compounds, or cuprates, could superconduct about 30°C
warmer than liquid helium—too hot for the BCS mechanism to operate.
This time, there was no quick fix. To this day, scientists haven’t
identified the extrastrong “glue” that could be binding electrons
together at such high temperatures. As they have continued to discover
materials that superconduct at higher and higher temperatures, the
mystery has grown more acute.

Silke Bühler-Paschen holds a strange metal made of cerium, palladium,
and silicon. She found evidence that its electrons are
quantum-mechanically entangled. Photo: Matthias Heisler/vienna
University of Technology

Yet a clue lurked in the original experiments at IBM. When the
scientists heated their cuprate sample above the critical temperature,
they noticed another peculiar feature: The resistivity rose in a
straight line instead of along an exponential curve, as it does in all
other known metals. Fermi liquid theory could not explain this odd
behavior. Furthermore, in some materials, the resistivity continued to
rise steadily instead of flattening out as the temperature climbed
further (making them particularly bad conductors). To explain such
high resistance in Landau’s framework would require electron
quasiparticles to be scattering at distances shorter than there are
things to scatter off, in the empty spaces of the atomic lattice.

Gradually, physicists began to realize the unusual resistivity of
these “strange metals,” as these materials came to be called, was
just as important a puzzle as their high-temperature
superconductivity. “It has the hallmark of a really good physics
problem: It’s very simple, and it seems to require a big conceptual
change,” says Peter Abbamonte, a physicist at UIUC.

In 2004, Dutch physicist Jan Zaanen noted something else about
resistivity in strange metals. The slope of a material’s resistivity
is a measure of how fast it dissipates an electrical current as heat.
In normal metals, the electron scattering rates depend on microscopic
details of the material. But in strange metals the dissipation—hence
the resistivity—always seems to rise at the fastest possible rate.
Moreover, that rate is proportional to Planck’s constant, a key
value in quantum mechanics that determines how precisely certain
properties of particles can possibly be measured. “Planckian
dissipation,” as Zaanen dubbed it, implied that the behavior of
electrons in strange metals must reflect blurry quantum effects that
aren’t accounted for by Landau’s quasiparticles.

By 2019, Planckian dissipation had turned out to be a general property
of cuprates. That year Zaanen drafted a 40-page manifesto challenging
his colleagues to consider linear resistivity as the “expression of
a new, truly fundamental physics” that required them to kill the
quasiparticle model.

Theorists are now trying to do just that. In a review
in _Science_ in 2022, a team led by Phillips concluded that in
strange metals, at least, “electrons are no longer the primary
charge carriers. When the particle picture breaks down, no local
entity carries the current.” The question is what does.

THE SILENT PLACE

For a long time many physicists resisted Zaanen’s call to arms.
Phillips was not one of them. “Quasiparticles were always a crutch,
and we need to throw away the crutches,” Phillips says. Lately a
flurry of new experimental techniques has begun to highlight all the
strangeness that Landau’s simplified picture had overlooked.

On sabbatical at Rice in 2016, Paschen teamed up with Si and his
colleague Doug Natelson to concoct a way to, in effect, listen to the
current inside strange metals. Just as recording the pitter-patter of
rain on your roof can tell you about the size and frequency of
raindrops, measuring the fluctuations in current along a strange metal
wire can tell you about the nature of whatever is carrying the
current. Rather than picking up the inconsistent crackling of passing
electrons or quasiparticles, the noise the researchers heard was dead
consistent. Electricity seemed to be flowing through the wire as a
homogeneous soup.

The results, reported in 2023, dealt the strongest blow yet to the
notion of quasiparticles. Paschen says the experiment totally reframed
her mental image of what’s going on inside a strange metal. In her
presentations, she used to represent it as a chaotic tornado of messy
electron interactions. Now, she thinks “it’s actually something
very controlled. It’s the silent place.”

Other researchers have found ways to probe the properties of strange
metals more directly. Armitage shines far-infrared light at samples
and finds no evidence for quasiparticles. Stephen Hayden, a physicist
at the University of Bristol, shoots a beam of neutrons at them and
finds waves of magnetism that slow down as the sample cools, a hint
that the material is undergoing a transition to a new state. Abbamonte
uses an electron gun to explore variations in the density of electrons
in the material, and finds instead a uniform spread of charge.
“There’s no measurement you can do with the system that tells you
how many electrons are in it,” Abbamonte says. “They really just
behave in a very bizarre way.”

All three of these recent scattering experiments also suggest
different properties of electrons in these strange metals are “scale
invariant.” Measure, for instance, fluctuations in the charge
density as a function of temperature, and it will follow the same
general curve at a narrow temperature range as at a wide one.
Observing the physical phenomena inside a strange metal is like
zooming in on a snowflake: Things look the same at all scales.

Meanwhile, physicists are seeing hints of the same strange electron
behavior in materials very different from the original cuprate
compounds, from rotated sheets of graphene to starshaped lattices of
nickel and indium. The challenge to the existing picture of
electricity seems to extend far beyond a few outlier metals.

In 2023, Zaanen fell into a coma after treatment for his esophageal
cancer— a consequence of a lifelong devotion to cigarettes, one that
transcended mere addiction. (“Nicotine fueled his mathematics,”
Abbamonte says.) When he woke up in the hospital, his family told him
they were already planning his memorial service. He asked whether he
could go. That summer, Zaanen threw his own funeral in Leiden, and
physicists flew in from around the world to join the party. He died a
few months later.

“All of us feel an urgency to get this problem solved, and it’s in
no small way because he died,” says Phillips, who considered Zaanen
among his closest friends. “We owe it to Jan to solve this
problem.”

REWIRING ELECTRICITY

Landau’s theory is so ingrained that physicists aren’t sure how to
talk about electricity without it. “The words we use very often
presuppose that we have these electronic quasiparticles,” Armitage
says. “It may be that a different lens is needed, but what that is
we don’t know.”

Zaanen’s manifesto left some hints. He proposed that at the center
of strange metal behavior is “entanglement,” the quantum
phenomenon that links properties of particles, allowing them to act
almost as a single object, even when they’re too far apart to
communicate. “We are dealing with a completely new form of matter
controlled by a system where literally everything is entangled with
everything,” he wrote.

Maximally entangled electrons would form a diffuse “liquid” that
would have the lowest possible viscosity, according to Zaanen. Imagine
dropping a rock into a soup—the thinner the broth, the faster the
ripples will dissipate. Because the quantum soup has minimal
viscosity, it will dissipate energy at the fastest possible rate,
explaining why the resistivity of a strange metal rises so fast as
it’s warmed. He stopped short of explaining how this entangled
electron soup might form. But three theorists are now putting forth
distinct attempts.

The first comes from physicist Subir Sachdev of Harvard University.
Three decades ago, he helped develop a mathematical model of how
random interactions of electrons lead them to become entangled. In
a _Science_ paper in 2023, Sachdev used an updated model to argue
that in strange metals, quasiparticles scatter off magnetic waves and
imperfections in the atomic lattice and break apart, leaving a highly
entangled soup of electrons.

Hayden says Sachdev’s theory “seems to have a good stab at
explaining what we see” in his recent neutronscattering experiments,
particularly the behavior of the magnetic waves. Paschen finds the
theory’s mathematical and explanatory power “very appealing,”
but she doesn’t buy the importance of randomness and lattice
imperfections in driving entanglement. Many strange metal samples,
including her own, are exquisitely pure and regular, she says.

Last year, Paschen found a way to actually probe the entanglement
inside a material by following an approach proposed by Peter Zoller of
the University of Innsbruck in 2016. With Si and Fakher Assaad at the
University of Würzburg, she used the technique to estimate the
minimum amount of entanglement present in a strange metal made of
cerium, palladium, and silicon. Posted as a preprint in March, their
result—at least nine entangled electrons—sounds tiny, but it’s
an extremely conservative estimate, Paschen says, and also the
strongest entanglement ever documented in any solid-state system.

The experimental confirmation of multiple-particle entanglement led Si
to a second theory of strange metals, published in _Nature
Communications_ in March. Si invokes interactions between two sets of
electrons in the metal: the conduction electrons that can move freely,
and the inner electrons that are locked to metal atoms. In normal
metals, the conduction electrons and the spins of inner electrons link
to form quasiparticles. But in the strange-metal state, the inner
electrons are so strongly entangled among themselves—as evidenced by
Paschen’s experiments—that they’re restrained from talking with
the outer electrons. This leads to a breakdown of the quasiparticles,
leaving a maximally entangled blob of conduction electrons that passes
through the lattice only with difficulty, Si says—hence the high
resistivity. But if the material is cooled below its critical
temperature, the agitated electron soup will reorganize into a
superconducting state.

Silke Bühler-Paschen’s lab in Vienna has copper-clad walls to
shield it from electromagnetic noise that could drown subtle signals
from strange metals. Photo: Duy Ha Nguyen/vienna University of
Technology.

Paschen plans to explore the transition Si has proposed by using her
big blue fridge to run her noise experiments at lower and lower
temperatures. In a sparkly white clean room, her team is busy spraying
material samples onto thin films. Once they cut these samples into
wires and load them in the chamber, Paschen will listen to the noise
in hopes of hearing a change of entanglement in the material as it
transitions from strange metal to superconductor.

Si says the ability to measure entanglement is a breakthrough that’s
already yielding new insights. “I think there’s potentially a
floodgate that’s being opened up,” he says. Phillips agrees.
“It’s a really, really big deal.” In a preprint in March, he
used a similar approach to calculate the level of entanglement
observed in his colleague Abbamonte’s electron-scattering
experiment. He concluded it was significantly larger than in normal
metals. Furthermore, Phillips believes Abbamonte’s experiment is
evidence that whatever carries charge in strange metals doesn’t have
a well-defined mass or energy. This belief underlies his third
explanation for strange metal behavior.

Over the past decade, Phillips has argued that current in strange
metals is carried by something radically different from electrons,
even entangled ones. He favors what Harvard physicist Howard Georgi
called “unparticles”—a mind-bending, still hypothetical form of
matter. Unlike all known particles, which have a well-defined mass at
rest, an unparticle could take on any possible mass, depending on how
it’s measured. Phillips says a soup of variable-mass unparticles is
the only way he can make sense of all the bewildering experimental
data on strange metals. But he’s not sure yet how that soup would
give rise to strange metals’ hallmark—their linear resistivity.

Phillips acknowledges that his idea is further out than Sachdev’s or
Si’s. “I seem to be fundamentally incapable of doing anything that
is well accepted,” he says. As a Black physicist, he explains,
he’s grown accustomed to the experience of not fitting in. “It’s
easier to go against the grain when you are not part of the grain,”
Phillips says.

Inside this machine, Silke Bühler-Paschen’s team sprays an
ultrathin strange metal film onto a substrate to be used in her
experiments. Photo: Z. Savitsky/science

Yet his distaste for particles does fit a trend that extends outside
the strange metal community, says Meigan Aronson of the University of
British Columbia, who studies how new kinds of matter emerge near
quantum phase transitions. “We learned about electrons in school, so
we’re all very attached to them,” Aronson says. But over the past
few decades, breakthroughs in the field have repeatedly shown how the
collective behavior of electrons can explain phenomena that individual
electrons can’t. Like Phillips, Aronson suspects that “maybe
electrons are not the fundamental particle in condensed matter
physics.”

Zaanen would have approved. “Twentieth-century physics revolved
around the particle idea,” he wrote in his manifesto. But when it
comes to strange metals, “it cannot be emphasized enough how
misleading the very idea of a particle is.”

THE GLUE IS SOUP

In April, at a workshop at the Max Planck Institute for the Physics of
Complex Systems in Dresden, Germany, a handful of strange metal
enthusiasts assembled to compare their new ideas. Although the
theorists retain their healthy rivalry, “We’re all going in the
same direction,” Si says. He, Sachdev, and Phillips all agree that
current flows through strange metals as a diffuse quantum soup lacking
any localized electron quasiparticles. They just each have different
ideas about the exact ingredients of that soup.

” feel we’re pretty close to understanding strange metals,”
Sachdev says. “The ideas are not so far from each other—we’re
talking about the same elephant from different points of view.”

The theorists are also united in their conviction that the new
insights from strange metals are the key to making better and higher
temperature superconductors. For decades, investigators have focused
on figuring out the “glue” that might bind electrons into Cooper
pairs at higher temperatures. But if charge actually travels as some
entangled quantum blob, that effort may be misguided. “You don’t
need glue if the whole thing is a soup,” Si says.

The way to find revolutionary, ambient-condition superconductors, Si
says, is to look at materials that are particularly bad conductors at
higher temperatures. “It’s mind-boggling, but that’s the strange
metal paradigm,” he says. Si thinks the high resistivity of strange
metals indicates electrons in this state are “frustrated,” and
when cooled enough they naturally reorder into a more comfortable
superconducting state. His goal is to lift that transition to room
temperature: “You make these particles as unhappy as possible so
that they can reorganize into something that we actually care about
for certain purposes, such as saving the world.”

For his part, Phillips thinks unparticles could be the path not just
to room-temperature superconductors, but to rethinking electricity
without the electron. At the Dresden workshop, he filled a blackboard
with math describing how to replace quasiparticle descriptions with
unparticle ones. “You have to go and show in your theory how the
quasiparticle is killed,” he says. “If you kill the quasiparticle,
you have a new starting point, and that’s what we’ve all been
trying to find.”

An extracurricular passion far removed from physics fortifies Phillips
for this creative destruction. At 3:30 p.m. on the dot, he excused
himself from the workshop: It was time for opera rehearsal. In a
windowless office kitchen with cherry trees blooming outside the
windows, Phillips unleashed his powerful bass. Tea mugs trembled as he
recited a song, in its original German, from Mozart’s _The
Abduction from the Seraglio_. One line stood out: “I study day and
night, I rack my brains, and I won’t rest until I see you killed.”

Quasiparticles are on notice.

_Zack Savitsky is a journalist specializing in the physical
sciences._

_SCIENCE has been at the center of important scientific discovery
since its founding in 1880—with seed money from Thomas Edison.
Today, Science continues to publish the very best in research across
the sciences, with articles that consistently rank among the most
cited in the world. In the last half century
alone, Science published:_

* _The entire human genome for the first time_
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Advances._

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May 21, 2025

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