Physicists Use Electrical Noise to Hunt Electron Pairs in Cuprates

Physicists have found a way to look directly for paired electrons inside a copper-based superconductor, a question the field has argued about for nearly 40 years, by recording the faint electrical noise of current flowing into the material. The study appears in the journal Science and was led by Milan Allan of Ludwig Maximilian University of Munich and Ingmar Swart of Utrecht University in the Netherlands, Ars Technica reported.

The target is the pseudogap, a puzzling state in which electrons seem to disappear. Explaining it could help physicists understand why these materials superconduct at temperatures much warmer than most superconductors need.

Superconductors that work with liquid nitrogen

A superconductor carries electric current with no resistance at all. Most only manage this a few degrees above absolute zero, which takes expensive cooling. In the late 1980s, scientists found that a family of copper-based compounds called cuprates superconducts at higher temperatures, ones that cheap liquid nitrogen can reach. The mechanism behind that behavior is still disputed.

The case of the missing electrons

Just above the temperature at which a cuprate becomes a superconductor, measurements find far fewer electrons than expected at certain energies. Allan told Ars Technica the pseudogap is “a phenomenon of missing electrons.” Many kinds of instruments detect the effect, he said, but nobody knows why it happens.

Over the decades, two main explanations have competed. In the first, the electrons get locked into an ordered pattern, such as a charge density wave (a repeating ripple in how electric charge is spread through the material), and superconductivity develops separately. In the second, the electrons go missing because they have already joined up into Cooper pairs, the two-electron units that carry current in a superconductor.

Under that second idea, pairs would form as the material cools but could not yet move together in step. The pseudogap would then be a kind of waiting room, and pairing would be a precondition for superconductivity in these materials.

Why earlier measurements fell short

Separating the two explanations has been very hard. Allan said many different experimental signals end up grouped under the pseudogap label, and each instrument sees something, yet the readings have never settled the matter. He described them to Ars Technica as incoherent and difficult to pin down.

What the field lacked was a measurement that could reveal electron pairs directly. The Munich and Utrecht team built its approach around the tiny electrical crackle, or noise, that appears as current flows into a cuprate, and used that signal as a window onto pairs.

What it means for the superconductor debate

The two camps make different predictions. If Cooper pairs already exist in the pseudogap, then pairing comes first and superconductivity follows once the pairs start moving together. If the missing electrons are instead frozen into patterns, the pseudogap is a separate effect, and superconductivity arises by another route. A method that can detect pairs directly gives physicists a way to test those ideas against each other after decades of indirect and inconclusive signals.

Cuprates are of practical interest because they superconduct at temperatures that liquid nitrogen can reach, which is far cheaper than the cooling most superconductors require. Understanding why they work is a step physicists have not managed since the materials were discovered.

What remains open

The result comes from one study, published in Science, and the researchers describe it as providing some answers about the pseudogap. Physicists have backed competing explanations for decades, and results from a single technique on a single material will sit alongside the many earlier measurements that pointed in different directions.

Source: Ars Technica