Optical study of the insulator-to-metal transition in LaxMnO3 thin films

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−δ films with x = 0.66, 0.88, 0.98 and 1.10, and with δ ≃ 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 100K at least, more than 250K below its onset at the Curie temperature Tc and at the TIMT 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 ∼ 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 Q1 and ferromagnetic regions populated by large polarons and free carriers.