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

Although high-transition-temperature (high-T_c) superconductivity in cuprates has been known for more than three decades, the underlying mechanism remains unknown^1-4. Cuprates are the only unconventional superconductors that exhibit bulk superconductivity with T_c above the liquid-nitrogen boiling temperature of 77 K. Here we observe that high-pressure resistance and mutual inductive magnetic susceptibility measurements showed signatures of superconductivity in single crystals of La₃Ni₂O₇ with maximum T_c of 80 K at pressures between 14.0 GPa and 43.5 GPa. The superconducting phase under high pressure has an orthorhombic structure of Fmmm space group with the 3 d_x²−y² and 3 d_z² orbitals of Ni cations strongly mixing with oxygen 2p orbitals. Our density functional theory calculations indicate that the superconductivity emerges coincidently with the metallization of the σ-bonding bands under the Fermi level, consisting of the 3 d_z² orbitals with the apical oxygen ions connecting the Ni-O bilayers. Thus, our discoveries provide not only important clues for the high-T_c superconductivity in this Ruddlesden-Popper double-layered perovskite nickelates but also a previously unknown family of compounds to investigate the high-T_c superconductivity mechanism.