Signatures of superconductivity near 80 K in a nickelate under high pressure.

High-transition-temperature (high-T c ) superconductivity in cuprates has been discovered for more than three decades, but the underlying mechanism remains a mystery 1-4 . Cuprates are the only unconventional superconducting family that host bulk superconductivity with T c s above the liquid nitrogen boiling temperature at 77 K. Here we observe signatures of superconductivity in single crystals of La 3 Ni 2 O 7 with a maximum T c of 80 K at pressures between [Formula: see text] GPa from high-pressure resistance and mutual inductive magnetic susceptibility measurements. The superconducting phase under high pressure exhibits an orthorhombic structure of Fmmm space group with the 3[Formula: see text] and 3[Formula: see text] orbitals of Ni cations strongly mixing with oxygen 2p orbitals. Our density functional theory calculations suggest the superconductivity emerges coincidently with the metallization of the σ-bonding bands under the Fermi level, consisting of the 3[Formula: see text] orbitals with the apical oxygens connecting Ni-O bilayers. Thus, our discoveries not only reveal important clues for the high-T c superconductivity in this Ruddlesden-Popper double-layered perovskite nickelates but also provide a new family of compounds to investigate the high-T c superconductivity mechanism.