  High-temperature superconductivity (HTSC) remains one of the most challenging
and fascinating mysteries in condensed matter physics. Recently,
superconductivity with transition temperature exceeding liquid-nitrogen
temperature is discovered in La3Ni2O7 at high pressure, which provides a new
platform to explore the unconventional HTSC. In this work, using
high-resolution angle-resolved photoemission spectroscopy and ab-initio
calculation, we systematically investigate the electronic structures of
La3Ni2O7 at ambient pressure. Our experiments are in nice agreement with
ab-initio calculations after considering an orbital-dependent band
renormalization effect. The strong electron correlation effect pushes a flat
band of d_(z^2 ) orbital component below the Fermi level (EF), which is
predicted to locate right at EF under high pressure. Moreover, the d_(x^2-y^2 )
band shows a pseudogap-like behavior with suppressed spectral weight and
diminished quasiparticle peak near EF. Our findings provide important insights
into the electronic structure of La3Ni2O7, which will shed light on the
understanding of the unconventional superconductivity in nickelates.
