Physicists Find 'Smoking Gun' for How Electron-Doped Cuprates Become Superconductors

An international team led from the University of Bristol says it has identified the mechanism that lets electrons pair up and carry current with no loss in one family of copper-oxide high-temperature superconductors, a question that has resisted answers for decades.

Superconductors power high-resolution medical scanners, move electricity efficiently and underpin ultrasensitive measuring devices. In conventional, lower-temperature superconductors, the pairing is mediated by vibrations of the ions in the crystal lattice. For years scientists have searched for a different mechanism in copper-oxide materials, likely involving something that fluctuates at higher frequencies than the ions. Electrons were the prime candidate because their spin and charge both fluctuate, but evidence tying the strength of the electron-fluctuation interaction to the critical temperature had been missing.

The researchers from Bristol, the Netherlands, China and the United States built a special thin film of lanthanum cerium copper oxide, depositing it layer by atomic layer on a substrate to create a smooth gradient of doping. They patterned electrical contacts along the film with lithography so they could measure resistance across finely spaced doping values while keeping disorder constant. Experiments at the European Magnetic Field Laboratory in the Netherlands then tracked how resistance changed with temperature and magnetic field strength.

"Employing some analytical tricks on the data obtained in high magnetic fields, we were then able to identify the dominant source of resistance in this material as scattering off the fluctuating spins," said lead experimentalist Dr. Caitlin Duffy, a postdoctoral researcher at the French National Center for Scientific Research. Analysis then confirmed that superconductivity in this material arises from the interaction between conduction electrons and spin fluctuations. The work was published in Nature Physics under the title "Evidence for spin-fluctuation-mediated superconductivity in electron-doped cuprates."

Co-lead author Nigel Hussey, a Bristol physics professor, called it "the smoking gun" that solves part of the equation. He was careful about the limits, though. Superconductivity in the parent copper oxide can be induced two ways: by adding electrons, as in this study with cerium replacing lanthanum, or by removing electrons to create "holes." The highest critical temperatures are found only on the hole-doped side.

That means simply strengthening the coupling to spin fluctuations is unlikely to raise the temperature in hole-doped materials. Comparing the new results with earlier high-field studies of hole-doped cuprates, Hussey said, shows there must be a second, missing ingredient. "Only then can theorists develop the full story for high-temperature superconductivity," he said, adding that whoever finds it will be a strong candidate for a future Nobel Prize in Physics.