Bilayer manganites: polarons in the midst of a metallic breakdown

The exact nature of the low temperature electronic phase of the manganite materials family, and hence the origin of their colossal magnetoresistive (CMR) transition is still a flagship issue in emergent correlated matter research. By combining new photoemission and tunneling data, we show that in the bilayer ($N$=2) manganite La$_{\mathrm{2-2x}}$Sr$_{\mathrm{1+2x}}$Mn$_2$O$_7$ the lattice/spin/orbital polaronic degrees of freedom win out, all across the CMR region of the phase diagram. This means that the generic ground state is that of a system in which strong interactions result in vanishing coherent quasi--particle spectral weight at the Fermi level for all locations in $k$--space. The incoherence of the charge carriers offers a unifying explanation for the anomalous charge-carrier dynamics seen in transport, optics and electron spectroscopic data. The stacking number $N$ is the key factor for true metallic behavior, as an intergrowth-driven breakdown of the polaronic domination to give a robust metal possessing a traditional Fermi surface is seen in the bilayer system.