  In order to reveal many-body interactions in the three-dimensional (3D)
perovskite manganite, we have performed an $in$ $situ$ angle-resolved
photoemission spectroscopy (ARPES) on La$_{0.6}$Sr$_{0.4}$MnO$_3$ (LSMO) and
investigated the behaviors of quasiparticles. We observe quasiparticle peaks
around the Fermi momentum, both in the electron and the hole bands, and clear
kinks throughout the hole Fermi surface in the ARPES band dispersion. The
isotropic behavior sharply contrasts to the strong anisotropic quasiparticle
excitation observed in layered manganites. These results suggest that polaronic
quasiparticles by coupling of the electrons with Jahn-Teller phonons play an
important role in the physical properties of the ferromagnetic metallic phase
in 3D manganite LSMO.
