Imagine trying to overhear a whisper between two people in a noisy hall. That's roughly the challenge physicists face when they want to understand how electrons pair up inside a superconductor. And if the "hall" is also squeezed by a giant press to pressures of hundreds of thousands of atmospheres, the task seems impossible.
The authors of the new study found an ingenious solution. They placed a tiny crystal in a high-pressure cell, small enough to fit in the palm of your hand, and brought a thin gold wire into contact with it. The contact point became a sort of "microphone": they ran a current through it and watched how the resistance changed with voltage. This method, called Andreev spectroscopy, directly measures the energy gap — the energy needed to break apart an electron pair. Previously, doing this under pressure was nearly impossible: contacts broke, samples shifted. Here, they used a clever mounting and wire-splitting technique that ensured a stable contact at pressures up to 15 GPa.
First, they tested the method on pure niobium — a classic superconductor. Everything matched: the gap size came out exactly as theory predicts. Then they moved on to more mysterious materials: the kagome metal CsCr3Sb5 and the bilayer nickelate La2PrNi2O7. In both cases, the spectra showed a characteristic peak at zero voltage — a sign that electron pairs may have nontrivial symmetry, meaning they behave differently than in conventional superconductors. The scientists tracked how this peak changed with temperature, magnetic field, and pressure, and obtained compelling evidence of unconventional superconductivity.
Thus, a bridge was built between two worlds: macroscopic measurements of electrical resistance and microscopic probes sensitive to the quantum nature of electron pairs. Now this bridge is open to dozens of other materials that become superconducting only under pressure. Perhaps that's where the key to room-temperature superconductivity lies.
Andreev reflection, on which the method is based, is named after physicist Alexander Andreev. It occurs at the interface between a superconductor and a normal metal: an electron approaching the interface doesn't just reflect; it "picks up" another electron to form a pair, while a hole goes back into the metal, as if a ball, hitting a wall, turned into its shadow and flew back.