Doping dependent evolution of the polaron metal

Experimental and theoretical evidence has already suggested that the ferromagnetic metallic (FM) phase in colossal magnetoresistive manganites is not a conventional metal but rather a polaronic conductor. In the bilayer manganites La$_{2-2x}$Sr$_{1+2x}$Mn$_{2}$O$_{7}$ (LSMO), Angle Resolved Photoemission (ARPES) experiment revealed that the FM phase is a polaronic metal with a strong anisotropic character of the electronic excitations [1,2]. A small but well-defined quasiparticle (QP) with heavy mass along the [110] or ``nodal'' direction is found to account for the metallic properties and their temperature dependent evolution [2]. In this talk, we will discuss recent ARPES results on the x = 0.60 composition and contrast them to the x = 0.40 results. Recent work has shown that the region in proximity of x = 0.60 constitute the most metallic bilayer manganite with DC conductivity about one order of magnitude higher than that corresponding to the region 0.30 $<$ x $<$ 0.40. Much as in the x = 0.40 composition, for x = 0.60 along the nodal direction we observe a peak-dip-hump structure with QP of heavy effective mass. Quantitative differences in the electron-phonon coupling constant $\lambda $, the QP spectral weight and the hump energy are fully consistent with the doping evolution of the transport properties. [1] Nature \underline {438}, 474 (2005), [2] Phys. Rev. B \underline {76}, 233102 (2007).