Chinese Physicists Find Nickelate Superconductor's Pairing Mechanism Differs From Cuprates

Researchers at Nanjing University, the University of Science and Technology of China, Hong Kong Polytechnic University and other Chinese institutes have investigated the electronic processes behind superconductivity -- the ability of a material to carry electric current with zero resistance -- in a nickel-oxide compound called La3Ni2O7 (lanthanum nickel oxide). The compound has shown superconductivity at temperatures up to about 80 Kelvin (minus 193 degrees Celsius) under very high pressure. Their findings, published in the journal Nature Physics, suggest superconductivity in specially engineered La3Ni2O7 films arises from an electron-pairing mechanism distinct from the one found in copper-based cuprate superconductors.

"The discovery of superconductivity in bilayer nickelates has sparked widespread interest, providing a novel platform to explore unconventional high-temperature superconductivity," co-senior author Professor Donglai Feng told Phys.org. Unlike cuprates, where a single electron orbital dominates, bilayer nickelates have two orbitals sitting close together in energy, raising questions about how their electrons pair up to become superconducting.

To answer this, the team first grew high-quality, strontium-doped La3Ni2O7 films using molecular-beam epitaxy at Nanjing University, then transported the atomically flat samples in a customized ultra-high-vacuum container to the Shanghai Synchrotron Radiation Facility. There, researchers used angle-resolved photoemission spectroscopy (ARPES) -- a technique that illuminates a sample and measures the energy and momentum of emitted electrons -- to directly probe the material's superconducting gap, a forbidden range of electron energies that opens when electrons form superconducting pairs.

The measurements provided clear, reproducible evidence of this gap forming across the superconducting transition, a direct spectroscopic observation that had not been achieved before for this material. "Surface quality is everything here, and it's very fragile," said Dr. Wenjie Sun, one of the researchers who synthesized the films, describing overnight shifts during the beamtime before the team succeeded.