  The origin of the effect of colossal magneto-resistance (CMR) remains still
unexplained. In this work we revisit the spin dynamics of the pseudo-cubic
La1-xSrxMnO3 along the Mn-O-Mn bond direction at four x doping values (x < 0.5)
and various temperatures and report a new lattice dynamics study at x0=0.2,
representative of the optimal doping for CMR. We propose an interpretation of
the spin dynamics in terms of orbital polarons. This picture is supported by
the observation of a discrete magnetic energy spectrum Enmag (q) characteristic
of the internal excitations of "orbital polarons" defined by Mn3+ neighbors
surrounding a Mn4+ center with a hole. Because of its hopping, the hole mixes
up dynamically all the possible orbital configurations of its surrounding Mn3+
sites with degenerate energies. The Enmag values indicate a lift of orbital
degeneracy by phonon excitations. The number n varies with the spatial
dimension D of the polaron and the q-range determines its size. At x=0.125 and
x=0.3 the spectrum reveals 2D polarons coupled by exchange and 3D "free"
polarons respectively, with sizes l=1.67a < 2a in all bond directions. At
x0=0.2, the spin and the lattice dynamics provide evidence for chains of
orbital polarons of size l=2a with a periodic distribution over ~ 3a and an
interaction energy ~ 3 meV. At T < Tc the charges propagate together with the
longitudinal acoustic phonons along the chains enhancing their ferromagnetic
character. The phase separation between metallic and ferromagnetic chains in a
non-metallic matrix may be crucial for CMR.
