Electron pairs can take the heat

Electrons zipping through a thin layer of strontium titanate interact and form pairs at higher temperatures than expected, researchers report in the May 14 Nature. The study is the first definitive evidence of coupled electrons in a solid material too warm for superconductivity, a state in which paired electrons move with no resistance. The research could help scientists better understand how superconductivity emerges and how to get materials to conduct electricity without resistance at or near room temperature.

Electrons tend to avoid each other, repelled by their negative charge. But within a select group of materials exposed to extremely low temperatures, electrons overcome their standoffishness and pair up. Two electrons mutually attracted to positively charged ions in the material lattice can couple to form a Cooper pair, which is crucial for superconductivity. Robert Schrieffer, who shared the 1972 Nobel Prize in physics for devising a theory of superconductivity, compared Cooper pairs to couples in a ballroom that all coordinate their dance steps, ensuring that nobody trips over each other. The combination of paired electrons and synchronized movement ensures that electric current can flow resistance-free.

Jeremy Levy, a condensed matter physicist at the University of Pittsburgh, and colleagues were working with strontium titanate, a compound that becomes superconducting when cooled to about 0.3 kelvins (just above –273° Celsius). But the researchers noticed that the material behaved strangely even when it was a bit too balmy for superconductivity.

Levy and his team used a transistor capable of detecting the passage of individual electrons to probe the strontium titanate under different conditions. At temperatures up to 0.9 kelvins and in the presence of a magnetic field, electrons entered a section of the transistor in pairs rather than individually. Unlike Cooper pairs, these coupled electrons did not coordinate their movements, Levy says. “It’s like swing dancing,” he says. “People hold hands, but different pairs are doing different things.” That means the detected electron pairs can bump into each other or into impurities in the solid, which dissipates energy and prevents the resistance-free flow seen in the superconducting state. Levy says these electron pairs resemble tightly bound molecules, whereas the partners in Cooper pairs are more spread apart.

The research could help physicists explore how electrons pair up and the role that pairing plays in superconductivity. In particular, the study may provide insight into the physical processes driving high-temperature superconductors, a class of materials including one that remains superconducting at up to 164 kelvins. Some scientists have proposed that electrons in these special materials pair up at temperatures too high for superconductivity, but the experimental evidence is controversial, says Ohio State University condensed matter physicist Mohit Randeria. He says it’s important to find other materials like strontium titanate to explore how materials transition to a superconducting state.