Abstract
Lanthanum manganites with a massive concentration of La defects can be stabilized in the form of thin films, by exploiting the structural stress produced by a substrate such as SrTiO3. They undergo an insulator-to-metal transition (IMT) like those doped by divalent ions, which is here studied by measuring the optical conductivity of LaxMnO3-delta films with x = 0.66, 0.88, 0.98 and 1.10, and with delta similar or equal to 0, from the far infrared to the near ultraviolet, and between 20 and 300 K. The IMT is here a slow process which continues down to 100 K at least, more than 250 K below its onset at the Curie temperature T-c and at the T-IMT measured from the dc resistivity. The metallization is here monitored through the increase of the Drude term and a transfer of spectral weight from a 'hard' midinfrared band MIR-2 peaked between 3000 and 5000 cm(-1) at room temperature, to a 'soft' midinfrared band MIR-1 at similar to 1000 cm(-1) and to the Drude term. This evidence is in good agreement with a model of phase separation below Tc, where insulating and paramagnetic regions of small polarons coexist with conducting and ferromagnetic regions populated by large polarons and free carriers.