The Material That Shouldn't Behave This Way

The Electrons That Shouldn't Work

Here is a puzzle from the latest issue of Nature Physics: in a particular iron-based superconductor, the electrons that scatter at the fastest rate allowed by quantum mechanics — the ones that should be too chaotic to form anything coherent — turn out to be the ones that carry the superconducting current.

Not some of them. Not a fraction of them. Mainly them.

The finding, from N.P. Armitage's group at Johns Hopkins published Thursday, comes from time-domain terahertz spectroscopy measurements of two FeTe1-xSex samples. The researchers found two conduction channels operating in parallel: one orderly, temperature-independent, doing what standard theory predicts; one sharp and governed by a scattering rate that scales linearly with temperature — right at the Planckian limit, the fastest electrons can scatter off each other while staying consistent with quantum mechanics. In a conventional metal, electron-electron scattering produces a quadratic temperature dependence. In this material, something pushes the rate right up against the fundamental speed limit.

Then the spectroscopy showed where the superconducting current comes from. "We show that it is mainly drawn from the channel that undergoes Planckian scattering," the authors write. The chaotic channel is not a limitation. It is the source.

The finding would be notable on its own. It becomes significant when you notice the pattern it joins.

Planckian linear resistivity — the same kind of anomalous electron scattering — has been observed in cuprate superconductors, in heavy fermion compounds, and in twisted bilayer graphene. These material families have nothing structural in common. They share only this: they all sit near a phase transition, some tipping point where one state of matter gives way to another. And near those transitions, electrons collectively scatter at the Planckian rate. Nobody knows why.

In cuprates, the linear resistivity was the original strange metal problem — a behavior that defied explanation and never stopped appearing no matter how carefully researchers looked. The iron chalcogenide result is the cleanest evidence yet that the strange metal behavior and the superconductivity are not awkward neighbors sharing the same material. They are related. The chaotic channel appears to be generative, not incidental.

"we show that it is mainly drawn from the channel that undergoes Planckian scattering." That is a direct quote from the paper. It is also, physicists will note, a sentence that should not be true under conventional Fermi liquid theory.

The FeTe0.55Se0.45 composition has a separate claim attached to it: prior work has identified it as a topological superconductor, with topological surface states and, potentially, Majorana bound states — the latter being of interest for fault-tolerant quantum computing. The current paper does not measure those states directly; the connection to quantum computing hardware is from prior literature. The paper's own contribution is the condensate attribution.

On the broader physics: a March 2025 paper by Gleis and colleagues proposed that in heavy fermions, the Planckian dissipation arises from a Kondo-breakdown quantum critical point — a moment where collective electron behavior flips between two competing states. The framework describes the dynamical response in those materials with precision. Whether it accounts for the iron chalcogenide result is not resolved by the current paper; the authors report what they measured, not what theory best explains it.

What the result does do is add a constraint. Any theory of high-temperature superconductivity will now need to account for why the superconducting order parameter preferentially sources from the highest-scattering channel, not the most coherent one. The field has been tuning chemistry, doping, pressure, and crystal structure across thousands of material systems — guided by frameworks that do not explain where the strange metal behavior comes from. If the Planckian channel is not a symptom but a cause, it becomes a design target.

The FeTe1-xSex result is specific to iron chalcogenides. The mystery it illuminates is not.