Physicists crack superconductor mystery by listening to noise

The superconductor puzzle

Cuprate superconductors discovered in the late nineteen eighties work at higher temperatures than earlier materials, but physicists still disagree on why after nearly forty years.

Missing electrons

Above the temperature where cuprates superconduct, some electrons vanish. Measurements find far fewer of them at the energies where they should be, a phenomenon called the pseudogap.

“The pseudogap is a phenomenon of missing electrons.”

Two competing theories

One theory says electrons freeze into patterns like charge density waves. The other says they form pairs called Cooper pairs before superconductivity begins, creating a waiting room of sorts.

The measurement challenge

Different instruments all see something related to the pseudogap, but their readings are never conclusive. Physicists needed a way to see electron pairs directly.

“It's always something kind of incoherent, hard to grasp.”

Listening to shot noise

The team measured shot noise, tiny fluctuations from individual charges in an electric current. Paired electrons produce more noise than single electrons, like heavier raindrops drumming louder.

“If you have two electrons coming always together, because they're associated with pairing, then it will be more noise.”

The scanning tunneling microscope

Researchers used a scanning tunneling microscope to pass current between a needle-sharp tip and a cuprate sample, listening to noise at specific energy levels and different surface locations.

Years of craftsmanship

Making the instrument sensitive enough to detect pairs took years of work by talented and persistent students. There was no single trick, only electronics craftsmanship.

“What it took is very talented and persistent students who worked for, I don't know, four years on this to get it working.”

Pairing at the pseudogap

The team examined BSCCO crystals cooled to a few degrees above absolute zero. Extra noise from pairing appeared right at the pseudogap energy, not only at superconductivity energies.

“The fact that there is more noise indicates that pairing is associated with this large gap.”

Magnetic field confirmation

Placing samples in magnetic fields up to six tesla suppressed pairing while leaving the missing electrons pattern intact. As the field grew, noise dropped back to single-electron levels, confirming pairs.

Pre-formed pairs, then coherence

In conventional superconductors, pair formation and phase coherence happen together. In cuprates, the study shows pairing occurs at higher temperatures, then phase coherence sets in at lower temperatures.

“So the picture that emerges is that you first have pre-formed pairs, and then only at lower temperatures, phase coherence sets in.”

The bottleneck shifts

The pairing energy measured is well above the superconducting temperature. This suggests the bottleneck is not getting electrons to pair, but getting pairs to move together coherently.

Hope and caution

The findings suggest cuprate electrons could theoretically pair at room temperature, but the team believes achieving room-temperature superconductivity will not be simple due to other coupled effects.

“If you have pairing at such high energies, that means that you might be able to push Tc higher.”