What it is
Bilayer nickelates of the Ruddlesden-Popper family are an emerging platform for high-temperature superconductivity, but the structure of their superconducting gap is under debate. The authors used atomically resolved scanning tunnelling microscopy and spectroscopy on superconducting (La,Pr)3Ni2O7 films 1.5 unit cells thick, grown on SrLaAlO4. When the films were moved rapidly between instruments in cryogenic ultrahigh vacuum, they kept an ordered surface reconstruction and gave reproducible U-shaped spectra with two gap scales of about 14 and 20 meV and an extended, nearly zero-conductance bottom. Films that spent longer at room temperature in vacuum during transfer gave V-shaped spectra instead, even though they kept the surface reconstruction and a superconducting transition onset in transport above 40 K. Wide-energy spectra and oxygen-control measurements indicate that oxygen loss possibly introduces density-wave-related spectral weight into the local spectra.
Why it matters
The shape of a superconducting gap is a clue to how electrons pair, and here the measured shape depended on how the films were moved between instruments. The authors link the difference to possible oxygen loss, read the U-shaped spectra as atomic-scale evidence for a fully opened superconducting tunnelling gap in oxygen-sufficient films, and establish oxygen-controlled transfer as a key condition for resolving pairing-related spectra. In their preprint, transfers shorter than 5 minutes consistently gave U-shaped spectra and transfers longer than 10 minutes only V-shaped ones, and they write that a V-shaped gap should not be taken, by itself, as evidence for d-wave superconductivity.
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Filed underPhysics of Superconductivity and Magnetism, Electronic and Structural Properties of Oxides, Magnetic and transport properties of perovskites and related materials