Quantum Oscillations in an Insulator: Metal-like Behavior Detected Deep Inside YbB12
Imagine walking into a perfectly soundproofed room—double walls, acoustic foam, an airtight door—designed so no sound can penetrate. You close the door and expect silence.
But you hear singing.
Not leaking through the door, not coming through the walls, but emanating from the room itself—from the floor, the ceiling, and the interior of every inch of "soundproofing material."
In October 2025, University of Michigan physicist Lu Li and his international team experienced the physics version of this moment. They discovered quantum oscillations inside a material that should be "completely insulating"—a phenomenon that for decades was thought to belong exclusively to metals. Even more striking: the oscillations do not come from the material's surface, but from deep within its bulk.
The research, published in *Physical Review Letters*, was described by peers as "really bizarre and exciting."
The Electrons' Spring Dance
To understand how absurd this discovery is, we first need to discuss what quantum oscillations are.
In metals, electrons are free. They move about like pedestrians wandering a city, unbound. When you apply a magnetic field to a metal, these free electrons orbit around the field lines—like a ball swung on a string. Physicists call this "cyclotron motion."
If you slowly increase the magnetic field, the electrons' orbital radii shrink; each time they shrink to a particular size, they "resonate" with the material's quantum energy levels. It's like pushing a swing: when your pushing frequency matches the swing's natural frequency, it swings highest. In experimental data, this resonance appears as periodic oscillations in quantities like electrical resistivity and magnetic susceptibility—hence "quantum oscillations."
Li favors a more intuitive metaphor: electrons act like springs. Change the magnetic field, and the spring's vibration frequency changes accordingly. Recording these frequency shifts lets you read out information about the electronic world inside the material.
Quantum oscillations are a powerful tool for studying metal electronic structures. Since the de Haas–van Alphen effect was discovered in 1930, physicists have used this method to "listen" to the song of electrons inside metals, inferring Fermi surface shapes, effective electron masses, and other key information.
The crucial prerequisite: you need free electrons.
Insulators: The Electron Prison
Insulators are a completely different world.
In an insulator, electrons are firmly locked into atomic orbitals—like a city under total curfew, everyone confined at home, unable to go out. No free electrons means no cyclotron motion, and no quantum oscillations. This is basic condensed-matter physics, written into every textbook.
But starting in the 2010s, that conventional wisdom began to waver.
Scientists observed quantum oscillation signals in several unusual insulators. It was like hearing singing in a soundproof room—it shouldn't happen, but it did.
One explanation quickly gained traction: perhaps it was a surface effect. There is a class of materials called "topological insulators," whose bulk is insulating but whose surfaces host an extremely thin conductive layer—like a safe with a solid steel body plated with gold. If the quantum oscillations came only from that surface "gold plating," everything made sense.
That explanation preserved the textbook while retaining promising applications—the surface conductivity of topological insulators is exactly what quantum computing and novel electronics have dreamed of. Everyone breathed a sigh of relief.
But Li's team says: No—the song is not coming from the surface. It's coming from inside the room.
35 Tesla: Hunting Truth in Extremes
Determining whether the oscillations originate from the surface or the bulk requires extremely strong magnetic fields.
Ordinary laboratory magnets produce roughly 10–15 tesla (hospital MRIs about 1 tesla). Not enough. Li's team went to the National High Magnetic Field Laboratory (National MagLab), home to the strongest artificial magnetic fields in the world.
They placed YbB₁₂—ytterbium boride, a "Kondo insulator"—into the magnet and ramped the field up to 35 tesla.
What does 35 tesla mean? About 35 times an MRI magnet—enough to levitate a frog (in 2000, Andre Geim levitated a frog at 16 tesla; he later won the Nobel Prize for graphene).
Under these extreme conditions, the team measured YbB₁₂'s heat capacity—its ability to absorb heat. Quantum oscillations leave periodic "fingerprints" in heat capacity. If the oscillations came from the surface, the fingerprint would be extremely faint (surface atoms are a tiny fraction of the total); if from the bulk, the fingerprint would be much stronger.
The result was clear: an oscillation frequency of 700 tesla, with a signal strength entirely consistent with a bulk origin.
"For years, scientists have asked a fundamental question: where do the carriers in this exotic insulator come from—bulk or surface? Are they intrinsic or extrinsic?" said researcher Chen Guanwen (Guanwen Chen). "We're excited to provide clear evidence—it's bulk and it's intrinsic."
Kondo Insulators: A Self-Contradictory Existence
YbB₁₂ is no ordinary insulator. It belongs to the "Kondo insulators," a name honoring Japanese physicist Jun Kondo.
The story of Kondo insulators is strange in itself. These materials conduct at high temperatures, but as temperature drops, their resistivity rises until they become insulators—counterintuitive, since metals usually conduct better when cold.
The reason is a peculiar quantum effect: at low temperatures, the material's "itinerant electrons" and "localized electrons" become strongly quantum-entangled, like two strangers tied together, neither able to move. This entanglement "locks up" the originally free electrons, turning the metal into an insulator.
In YbB₁₂, ytterbium atoms provide localized f-electrons, while the boron framework provides itinerant electrons. At low temperatures, the two "perish together," losing their freedom.
But the quantum oscillations tell us: under extreme magnetic fields, these "locked-up" electrons seem to come back to life. The entire material behaves like a metal, even though it is clearly an insulator.
A New Duality: Conductor and Insulator at Once
Li places this discovery in a larger framework he calls a "new duality."
A century ago, physicists discovered the "old duality": light is both wave and particle; the electron is both particle and wave. Wave-particle duality overturned classical physics, gave birth to quantum mechanics, and ultimately produced lasers, semiconductors, MRI—nearly every modern technology we rely on.
Li believes we may be witnessing the birth of another duality: a material that is simultaneously a conductor and an insulator.
"We've actually shown that that naive picture—good conductivity at the surface that could be used for electronic devices—is completely wrong," Li said. "It's the whole compound behaving like a metal, even though it's an insulator."
There is a catch: this "crazy metallicity" appears only under extreme magnetic fields of 35 tesla. In everyday life, YbB₁₂ remains an honest insulator.
"I wish I knew what to do with it, but at this stage we have no idea," Li admitted. "What we have right now is experimental evidence of an extraordinary phenomenon. We've documented it, and hopefully someday we'll figure out how to use it."
The Unknown Carriers
Perhaps the most fascinating part: we still don't know what is oscillating.
In metals, the carriers of quantum oscillations are free electrons—that's clear. But in YbB₁₂, electrons are locked up by the Kondo effect; there should be no free carriers. So what is executing cyclotron orbits?
"It's exciting to confirm that the oscillations are bulk and intrinsic," said graduate student Zhu Yuan (Yuan Zhu). "But we don't yet know which neutral particle is responsible for this observation. We hope our finding will stimulate more experimental and theoretical work."
"Neutral particle"—that phrasing is cautious and intriguing. If the carrier is charged, it should be some kind of electron; but if neutral, it could be an entirely new quasiparticle, something we don't yet understand.
Throughout the history of physics, every "signal that shouldn't exist" eventually pointed to new physics. In 1964, Gell-Mann predicted quarks; in 1974, Ting and Richter discovered the J/ψ particle; in 2012, the Higgs boson was finally captured. Each time, an "impossible signal" came first, and a new understanding of the world followed.
Could the quantum oscillations in YbB₁₂ be the next such signal?
Curiosity Before Application
Li is honest: he doesn't know what this discovery is good for.
In an era demanding that research be "useful," such honesty is almost a luxury. But the history of science tells us the deepest breakthroughs often come from "useless," curiosity-driven research.
When quantum mechanics was born, no one knew what it could do. The Schrödinger equation, Heisenberg's uncertainty principle—these looked like pure intellectual games. But decades later they became transistors, lasers, MRI, smartphones—the foundations of modern civilization.
The Kondo effect itself followed the same path. When Jun Kondo proposed his theory in 1964, it was only to explain a tiny low-temperature anomaly in metal resistivity—a "useless" little problem. But the Kondo effect later became a cornerstone for understanding heavy-fermion materials, high-temperature superconductivity, and quantum critical phenomena.
Perhaps the quantum oscillations in YbB₁₂ will be the same. Today it's just a "really bizarre" phenomenon, a puzzle that makes physicists scratch their heads. But maybe fifty years from now, someone will build technologies we cannot imagine today on the basis of this "signal that shouldn't exist."
Or maybe not. But that's the charm of basic science—you never know where curiosity will take humanity, but you must first let it set out.
---
*References:*
- *Quantum Oscillations in the Heat Capacity of Kondo Insulator YbB₁₂, Physical Review Letters (2025), DOI: 10.1103/ms3x-pjsk*
- *University of Michigan News: "Really bizarre and exciting: The quantum oscillations are coming from inside"*
- *ScienceDaily: "Really bizarre quantum discovery defies the rules of physics"*