Phase inhomogeneities in the charge-orbital-ordered manganite Nd_0.5Sr_0.5MnO₃ revealed through polaron dynamics

Ultrafast midinfrared spectroscopy is used to probe dynamics in the intermediate bandwidth manganite Nd_0.5Sr_0.5MnO₃. In the majority paramagnetic and ferromagnetic phases, the early time dynamics are consistent with the excitation and subsequent redressing of uncorrelated lattice polarons, with longer time dynamics related to spin-lattice thermalization. These polaron excitations reveal the intrinsically inhomogeneous nature of these phases. At lower temperatures we observe ultrafast melting of charge-orbital order, liberating quasiparticles that subsequently relax into bound polaronic states on a subpicosecond time scale. The temperaturedependent amplitude of the polaron excitations scales with the volume fraction of the CE phase. Thus, polaron dynamics, as measured using ultrafast spectroscopy, serve as a sensitive probe of phase inhomogeneity.