  The latest discovery of high temperature superconductivity near 80K in
La3Ni2O7 under high pressure has attracted much attention. Many proposals are
put forth to understand the origin of superconductivity.The determination of
electronic structures is a prerequisite to establish theories to understand
superconductivity in nickelates but is still lacking. Here we report our direct
measurement of the electronic structures of La3Ni2O7 by high-resolution angle
resolved photoemission spectroscopy. The Fermi surface and band structures of
La3Ni2O7 are observed and compared with the band structure calculations. Strong
electron correlations are revealed which are orbital- and momentum dependent. A
flat band is formed from the Ni-3dz2 orbitals around the zone corner which is
~50meV below the Fermi level and exhibits the strongest electron correlation.
In many theoretical proposals, this band is expected to play the dominant role
in generating superconductivity in La3Ni2O7. Our observations provide key
experimental information to understand the electronic structure and origin of
high temperature superconductivity in La3Ni2O7.
