Electrical Noise Reveals How Cuprate Superconductors Begin to Pair

A faint electrical crackle has given physicists a new way to hear what happens inside a cuprate superconductor. By tracking tiny fluctuations in current, they found evidence that electron pairing begins at the same energy as the mysterious pseudogap.

The result brings new focus to one of the biggest unanswered questions in superconductivity: do electrons pair before cuprates begin carrying electricity without resistance? A study in Science, led by Milan Allan of Ludwig Maximilian University of Munich and Ingmar Swart of Utrecht University in the Netherlands, points toward an answer.

A Missing-Electron Mystery

Superconductors carry electricity without resistance, but most only work a few degrees above absolute zero. In the late 1980s, copper-based materials called cuprates changed that picture when researchers found they could superconduct at higher temperatures reachable with affordable liquid nitrogen.

Nearly 40 years later, physicists still do not agree on why cuprates superconduct at those higher temperatures. The central puzzle involves the pseudogap, a phenomenon that appears above the temperature at which cuprates become superconducting.

“The pseudogap is a phenomenon of missing electrons,” Allan said. Measurements find far fewer electrons at energy levels where they should be, but the reason remains unclear. “Everybody knows that from different probes, you see that there’s these vanishing electrons, but nobody knows why,” Allan said.

Physicists have developed two main explanations. One says the electrons become trapped in a structure, freezing into patterns such as charge density waves while superconductivity develops separately. The other links the missing electrons to superconductivity itself: they disappear from those energy levels because they form pairs.

Those pairs are called Cooper pairs, and they carry current in a superconductor. If Cooper pairs appear as the material cools, before the material becomes superconducting, they would be a precondition for superconductivity.

Listening for Cooper Pairs

Settling the question proved difficult because many experiments detected related signals without delivering a conclusive answer. “Lots of different experimental signatures are kind of lumped inside this,” Allan said. “It’s always something kind of incoherent, hard to grasp.”

The team needed a measurement that could reveal pairing through the behavior of individual charges. They turned to shot noise: tiny electrical fluctuations caused by the arrival of individual charges that form the stream of an electric current.

The researchers used a scanning tunneling microscope, which passed current between a needle-sharp tip and a cuprate sample. The instrument allowed them to listen to noise at specific energy levels and at different locations across the surface.

In conventional superconductors, electron pairing creates a clean doubling of the noise. Cuprates make that signal harder to detect because single electrons continue flowing alongside the pairs. Making the instrument sensitive enough to separate the pairing signal took four years of work.

“What it took is very talented and persistent students who worked for, I don’t know, four years on this to get it working,” Allan said. “There was not a single trick. It was electronics craftsmanship.”

The team examined crystals of a cuprate known as BSCCO and cooled them to a few degrees above absolute zero. By adjusting the voltage between the microscope tip and the sample, the researchers selected the energy of the electrons they were listening to, then compared the noise with the energies where electrons went missing.

The Noise Rises at the Pseudogap

If frozen patterns caused the pseudogap by themselves, the researchers reasoned, extra noise from pairing should appear only at energy levels linked to superconductivity. Instead, the pairing signal emerged at the pseudogap energy.

The pseudogap energy changes from one spot to another on the surface, yet the same relationship held throughout the sample. Wherever the team listened, extra noise began rising at the local pseudogap energy. The pattern also appeared in samples with different chemical compositions.

Allan compared the effect to raindrops. “If you imagine that you have raindrops and then all of a sudden you make the drops twice as heavy, there’ll be more noise,” he said. “With the electrons it’s similar. If you have two electrons coming always together, because they’re associated with pairing, then it will be more noise.”

Ruling out other sources of noise, including atoms shifting on the surface, took a lot of effort. The decisive test placed the samples in magnetic fields of up to six tesla. Those fields suppress pairing while leaving the overall pattern of missing electrons largely intact.

As the magnetic field grew, the noise dropped back toward the single-electron level. That response tied the extra noise to pairing and showed that the missing-electron pattern can remain even when the pairing signal weakens.

“The fact that there is more noise indicates that pairing is associated with this large gap,” Allan said. The finding supports the explanation that electrons form pairs at the pseudogap energy, placing pairing before the material becomes superconducting.

“Noise is super hard to interpret, and that was basically the thing that we had the most sleepless nights about,” Allan said. Now, the electrical crackle has supplied a new way to study the cuprate puzzle, connecting the pseudogap to Cooper pairs through a signal that changes under magnetic fields.

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