A Chinese team has used ARPES to map an 18 meV nodeless gap and a 70 meV boson coupling kink in ambient pressure bilayer nickelate films, narrowing the candidate pairing symmetry without naming the pairing glue.
For three decades, the high-temperature superconductivity mystery has had a stubborn shape. Physicists could grow cuprate films and measure them cleanly, then argue forever about what glued the pairs. The nickelate cousin of that family, by contrast, has been the opposite problem. Crystals could be coaxed into superconducting states only under tens of thousands of atmospheres of pressure, and that pressure blanket also blocked the spectroscopic instruments that could have identified the pairing mechanism. A new measurement from a Chinese collaboration, published in May in Science and posted as arXiv:2502.17831, sidesteps that obstacle with a thin-film stabilization trick. The result is the cleanest spectroscopic picture yet of a superconducting bilayer nickelate, and it arrives with the pairing mechanism still genuinely unsolved.
The headline numbers are two. Angle-resolved photoemission spectroscopy (ARPES) on epitaxial (La,Pr,Sm)₃Ni₂O₇ films grown on SrLaAlO₄ substrates shows a finite superconducting gap of roughly 18 meV with a pronounced coherence peak along the Brillouin-zone diagonal, and a sharp band renormalization, the so-called kink, at 70 meV below the Fermi level. The gap persists across the entire Brillouin zone of the underlying Fermi surfaces, which is the signature of a nodeless gap. In copper-oxide high-Tc materials, a nodeless gap of this shape is what the s±-wave pairing symmetry would predict: sign-changing order parameter, no zeros on the Fermi surface, robust against impurity scattering.
According to SciTechDaily's coverage of the paper, the nodeless result constrains symmetry rather than settles it. The authors describe the data as consistent with s± pairing; they do not claim proof. That distinction matters. ARPES is sensitive to gap magnitude and nodal structure. It is not a direct order-parameter phase measurement. A reader looking for a definitive answer on whether nickelates and cuprates share a pairing symmetry will not find one in this paper.
The 70 meV kink is the second, and arguably more interesting, signature. Abrupt band renormalizations of that kind are fingerprints of electrons coupling to a boson. In conventional superconductors, that boson is a phonon. In iron pnictides, the same kind of kink has been tied to spin fluctuations. In underdoped cuprates, the boson remains contested. The 70 meV energy scale in the bilayer nickelate data, as reported on the arXiv preprint, establishes that an electron-boson coupling exists at a specific energy. It does not name the boson.
The experimental setup is the part of the story that turns a one-off measurement into a platform. The films were grown at the Southern University of Science and Technology (SUSTech) in Shenzhen by Qikun Xue and Zhuoyu Chen's group, then transferred under ultra-high-vacuum, liquid-nitrogen-cooled conditions to Junfeng He's group at the University of Science and Technology of China (USTC) in Hefei for ARPES. The quench-and-transfer step, which prevents the films from losing oxygen during the trip, is what made the measurement possible. People's Daily Online's English coverage of the collaboration describes it as a deliberate division of labor between growth and spectroscopy. The same kind of UHV sample shuttle is now standard in cuprate ARPES work, and reproducing it for nickelates is itself a step.
Two caveats belong in any honest version of this story. First, the doping recipe. (La,Pr,Sm)₃Ni₂O₇ is not bulk La₃Ni₂O₇. Samarium and praseodymium are added in small amounts to stabilize the bilayer Ruddlesden-Popper phase in thin-film form at ambient pressure. The superconducting state on display is real, but it lives inside a deliberately engineered film, not a free-standing crystal. Bulk La₃Ni₂O₇ still requires high pressure to superconduct. Second, ARPES is a surface-sensitive probe. The 18 meV gap is measured on the topmost layers of the film. A reader who wants to know whether the gap is uniform through the bulk of the film will have to wait for a bulk-sensitive measurement, such as muon spin rotation or penetration-depth experiments, to confirm.
What the paper actually delivers is a constraint-narrowing result. The pairing symmetry in bilayer nickelates is now consistent with s± rather than with a d-wave or nodal gap. The pairing glue in bilayer nickelates is real, has an energy of 70 meV, and is open for identification. A schematic of the measurement geometry, provided with the EurekAlert release, shows the Brillouin-zone cuts along which the gap was mapped. With a reproducible ambient-pressure sample and a working UHV transfer protocol, the community can now design the next experiment: polarized ARPES to look for gap anisotropy that s± would predict, resonant inelastic X-ray scattering to look for spin fluctuations the 70 meV boson could be, and tunneling spectroscopy to cross-check the gap magnitude.
The nickelate story is no longer a question of whether the system can be built. It is now a question of which boson the electrons are riding.