Heavy fermion compound is a classical type of correlated materials, encompassing
unconventional superconductivity, strange metal, quantum criticality, magnetic order,
heavy electronic states, correlated topological states, etc, in which 4f electrons play a critical
role. With the advancement of high-resolution ARPES measurements and MBE thin film
growth techniques, direct observation of the band dispersion and spectral weight of 4f electrons
in momentum/energy space has become possible, providing spectroscopic insight for understanding
correlated electronic states and novel quantum phenomena. In this review paper, we
summarized the electronic studies of several typical heavy fermion compounds and thin films,
including Ce-115 families, CuCu2Si2, CeRh6Ge4 and Ce films, etc. The experimental results
provide direct evidence to understand the temperature evolution of heavy electronic states,
energy/moment-dependent Kondo hybridization, the interplay of heavy electronic state with
superconductivity, the competition between Kondo effect with other quantum states and the
dimensionality tuning of 4f electrons.