Superconductivity with onset temperature of 27 K in tetragonal CoSe monolayers on redox-modified SrTiO3(001)

High-temperature superconductivity has been successively realized in cuprates, iron pnictides/chalcogenides, and nickelates, whereas the cobalt-based high-temperature superconductors remain to be explored. We report the discovery of superconductivity with an onset temperature of 27 K in tetragonal CoSe monolayers synthesized on redox-engineered SrTiO3(001) substrates. Using molecular beam epitaxy, we prepared atomically thin CoSe films on FeOx overlayers that expand the in-plane lattice and reduce the surface work function relative to TiOx-terminated surfaces. In-situ scanning tunneling spectroscopy reveals a symmetric superconducting gap of 14 meV with well-defined coherence peaks. Ex-situ transport measurements confirm a sharp resistance drop with zero resistance at 11 K and a Meissner diamagnetic signal at 10 K. In contrast, CoSe monolayers on TiOx-terminated surfaces show no superconducting transition down to 4 K. These results demonstrate that interface engineering via metal oxide functionalization enables tunable lattice strain and charge doping, establishing CoSe as a new platform for exploring cobalt-based high-temperature superconductivity.