In November 2025, at the National High Magnetic Field Laboratory in Tallahassee, Florida, a water-cooled resistive magnet hummed away, producing a 35-tesla magnetic field—roughly 35 times stronger than the field inside a hospital MRI machine, and one of the strongest continuous fields on Earth. Inside the magnet's chamber sat a tiny crystal, less than a centimeter across: ytterbium boride (YbB₁₂).
By the textbook definition, this is an insulator. It should not conduct electricity, should not carry heat, and above all should not show quantum oscillations.
Yet physicist Lu Li of the University of Michigan and his team were watching clear oscillation waveforms appear on their instrument screens—and the waves were coming not from the crystal's surface, but from its interior.
"I wish I could say this has some great application," Li said, "but my work keeps pushing that dream further away. What we found, though, is genuinely bizarre and exciting."
A Metal's Spring, an Insulator's Silence
To grasp how strange this is, you first need to understand what a "quantum oscillation" is.
Picture the electrons in a metal. They are not bound politely to atoms; like a crowd of wandering children, they spread through the entire lattice. When you apply a strong magnetic field, these electrons begin circular motion—because a magnetic field bends the paths of charged particles. Circular motion means oscillation, with a frequency set by the field strength.
Li likes an analogy: the electron is a spring, and the magnetic field is a hand plucking it. The stronger the field, the faster the spring vibrates. This "spring vibration" shows up in experimental data as periodic fluctuations in quantities like resistance, magnetic susceptibility, and heat capacity as the field strength varies. These are quantum oscillations—specifically the de Haas–van Alphen effect or the Shubnikov–de Haas effect—classic phenomena discovered in the 1930s.
The key point: quantum oscillations require freely moving charged particles. An insulator has no free electrons—all its electrons are locked into atomic orbitals or chemical bonds, unable to move. So an insulator should not exhibit quantum oscillations, just as a desert should not have tides.
But over the past decade or so, physicists have observed quantum oscillations in several insulators. The first reaction was: that must be a surface effect. Topological insulators are exactly that—conducting surfaces, insulating interiors. If the oscillations came only from that thin conducting skin, the story still held together: surfaces are metallic, metals oscillate, case closed.
The question Li's team wanted to answer was precisely this: surface, or bulk?
Judgment Under the Strongest Magnetic Field
Answering this required pushing the experimental conditions to the extreme. The 35-tesla field was not decoration—it was the instrument of judgment.
At such field strengths, surface and bulk effects show different "fingerprints." Specifically, the heat-capacity-versus-field curves for surface carriers and bulk carriers differ in oscillation frequency and phase. Li's team measured the heat capacity of YbB₁₂—a quantity that genuinely reflects the "whole material," unlike electrical conductivity, which is surface-sensitive.
The result was unambiguous: the oscillations come from the bulk, not the surface.
"We naively thought there was a good conducting layer on the surface that could be used for electronic devices—that picture is completely wrong," Li said. "The whole compound behaves as if it were a metal, even though it's an insulator."
In other words, every part of this crystal oscillates; every part plays the role of a "metal"—but when you stop interrogating it with a magnetic field, it dutifully goes back to being an insulator.
A New Duality
Li calls this phenomenon a "new duality."
The name is ambitious. A century ago, quantum mechanics was born with the "old duality"—wave-particle duality—which completely rewrote humanity's understanding of matter. Light is both wave and particle; the electron is both particle and wave. Not "sometimes a wave, sometimes a particle," but "both at once, depending on what you ask."
Li believes conductor–insulator may be the same kind of relationship. Certain materials, under certain conditions, are both conductor and insulator. Not a "division of labor" with a conducting surface and insulating interior, but the whole material holding both identities simultaneously.
The power of the analogy is this: when wave-particle duality was first discovered, it had no "application" either. It was just a headache-inducing paradox. Only later came semiconductors, lasers, solar cells, electron microscopes—nearly all of twentieth-century technology was built on that paradigm shift.
The "new duality" also has no application now. A 35-tesla field can only be produced at national laboratories; it is light-years away from everyday electronics. But as Li put it: "We now have experimental evidence of this remarkable phenomenon. We've recorded it, and hopefully at some point we'll figure out how to use it."
So What Exactly Is That "Thing"?
There is an even deeper mystery here.
In a metal, the carriers of quantum oscillations are electrons—negatively charged, massive, spin-1/2 fermions. But YbB₁₂ is an insulator; its electrons are locked down. So what is oscillating?
The team admits in their paper: they don't know.
"We don't yet know what neutral particle is responsible for this observation," said team member Yuan Zhu. "We hope this finding will stimulate more experimental and theoretical work."
"Neutral particle"—that is the key clue. If the oscillation carrier is neutral, it is not an electron. It might be some kind of exciton—a bound state of an electron and a hole, electrically neutral overall. Or it might be an even more exotic quasiparticle, perhaps a neutral fermion. In condensed-matter physics, a quasiparticle's "identity" can be very free—as long as the quantum numbers match, collective excitations can behave like almost anything.
It is a bit like the dark matter problem in astrophysics: we see the gravitational effects but don't know what particle causes them. In YbB₁₂, we see oscillations but don't know what quasiparticle causes them. We know it is electrically neutral, that it lives in the bulk, and that it only shows itself at 35 tesla—but its "true form" remains a mystery.
Tides in the Desert
Let me return to the metaphor.
If tides appear in a desert, there are a few possibilities: first, there is a sea beneath the desert (the surface-effect explanation—now ruled out); second, the desert itself becomes a sea under certain conditions (the "new duality" explanation); third, something we don't yet recognize is making the sand undulate like water (the "neutral particle" explanation).
The third possibility is the most exciting. It means our fundamental understanding of "what can happen inside an insulator" is incomplete. The textbook says an insulator's electrons are locked down, so there are no free carriers—but this "lockdown" may only be a low-energy approximation. Under extreme conditions (a 35-tesla field, temperatures near absolute zero), new collective modes may be excited—modes that simply don't appear under ordinary conditions.
This resembles a phenomenon in AI research: emergent capabilities. Large language models at small scale don't show certain abilities (say, multi-step reasoning or code generation), but past a threshold those abilities appear abruptly. Predict a large model's behavior from a small model's, and you will completely miss these capabilities—because they are not linear extrapolations of small-model behavior.
YbB₁₂ is a textbook insulator at low fields, but at 35 tesla it "emerges" metal-like oscillation behavior. Predict high-field behavior from low-field data, and you miss the phenomenon entirely.
Extreme conditions are physics' version of "scale." Just as parameter scale in AI research, timescales in evolution, or energy scales in cosmology—many phenomena only reveal themselves under extremes, and those are often the deepest ones.
A Discovery Without Application
Let me close with Li's own words: "I wish I could say this has some great application, but my work keeps pushing that dream further away."
It sounds self-deprecating, but it is science's most honest posture. Wave-particle duality had no application when discovered. General relativity's only "application" at first was explaining Mercury's tiny orbital anomaly. No one thought of CRISPR when the DNA double helix was found.
Discoveries that truly change the world often start without applications—because they change our basic understanding of what the world is, not our checklist of what we can do. Applications come later, built by engineers and inventors on the new paradigm. But bridges must be built on real ground, and the scientist's job is to discover that the ground is stranger, deeper, and less well-behaved than we assumed.
At 35 tesla, an insulator admitted it is also a metal. We don't yet know what that means. But at least we now know: the wall between conductor and insulator is not as thick as the textbook draws it.
Perhaps the next generation of textbooks will change "conductor or insulator" to "conductor and insulator"—just as today's textbooks already changed "wave or particle" to "wave and particle."
A one-word difference—but it is the difference between entire worldviews.
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References
- University of Michigan News: 'Really bizarre and exciting': The quantum oscillations are coming from inside (2025-11)
- Physical Review Letters: Quantum Oscillations in the Heat Capacity of Kondo Insulator YbB₁₂ (DOI: 10.1103/ms3x-pjsk)
- National High Magnetic Field Laboratory (35 Tesla magnet facility)