Signatures of ambient pressure superconductivity in thin film La₃Ni₂O₇

Recently, the bilayer nickelate La₃Ni₂O₇ has been discovered as a new superconductor with transition temperature T_c near 80 K under high pressure^1-3. Despite extensive theoretical and experimental work to understand the nature of its superconductivity^4-29, the requirement of extreme pressure restricts the use of many experimental probes and limits its application potential. Here we present signatures of superconductivity in La₃Ni₂O₇ thin films at ambient pressure, facilitated by the application of epitaxial compressive strain. The onset T_c varies roughly from 26 to 42 K, with higher T_c values correlating with smaller in-plane lattice constants. We observed the co-existence of other Ruddlesden-Popper phases within the films and dependence of transport behaviour with ozone annealing, suggesting that the observed low zero resistance T_c of around 2 K can be attributed to stacking defects, grain boundaries and oxygen stoichiometry. This finding initiates numerous opportunities to stabilize and study superconductivity in bilayer nickelates at ambient pressure, and to facilitate the broad understanding of the ever-growing number of high temperature and unconventional superconductors in the transition metal oxides.