  High-transition-temperature (high-T_c) superconductivity in cuprates has been
discovered for more than three decades, but the underlying mechanism remains a
mystery. Cuprates are the only unconventional superconducting family that host
bulk superconductivity with T_cs above the liquid nitrogen boiling temperature
at 77 Kelvin. Here we report an observation of superconductivity in single
crystals of La3Ni2O7 with a maximum T_c of 80 Kelvin at pressures between
14.0-43.5 gigapascals using 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
3d_(x^2-y^2 ) and 3d_(z^2 ) orbitals of Ni cations strongly interacting with
oxygen 2p orbitals. Our density functional theory calculations suggest the
superconductivity emerges coincidently with the metallization of the
{\sigma}-bonding bands under the Fermi level, consisting of the 3d_(z^2 )
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.
