Researchers used angle-resolved photoemission spectroscopy (ARPES) to study superconductivity in a new Ruddlesden–Popper nickelate heterostructure, La2PrNi2O7 grown on NdAlO3. They report a clear superconducting state that opens at the transition temperature and, importantly, shows no pseudogap — a partial suppression of electronic states that complicates measurements in other materials.

Because there is no pseudogap, the team could directly measure the superconducting order parameter. ARPES revealed a gap in the electronic spectrum that opens at the superconducting transition temperature and well-defined coherence peaks, which are fingerprints of a true superconducting condensate. From these spectroscopic features they also extracted the electronic specific heat and observed a jump at the transition, giving thermodynamic evidence that the films undergo a true phase transition into the superconducting state.

The electronic structure mapped by ARPES shows a multi-pocket Fermi surface made of three pockets labeled α, β and γ. This multi-orbital character is important because different pockets can host different kinds of electronic behavior. By changing epitaxial strain in the films, the researchers found the γ pocket appears in all samples they studied, whether the film was superconducting or not.

From the gap measurements the authors conclude the superconducting gap is nodeless in these films. In plain terms, the energy gap does not go to zero at any point on the measured Fermi surface. Together with the thermodynamic jump in electronic specific heat, these observations provide missing evidence that nickelate superconductivity in this family is a true bulk phase with a particular order-parameter symmetry.

Important caveats apply. These results come from a specific Ruddlesden–Popper nickelate heterostructure and from surface-sensitive ARPES measurements on thin films. The work establishes thermodynamic and symmetry facts for these samples, but it does not by itself identify the microscopic pairing mechanism or guarantee the same behavior in all nickelates. The role of strain and the presence of the γ pocket suggest the physics is sensitive to details of the film and substrate, so further studies will be needed to see how general these findings are.