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 LaxMnO$_{3-delta }$ films with x = 0.66, 0.88, 0.98 and 1.10, and with $delta simeq 0$, 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 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 and ferromagnetic regions populated by large polarons and free carriers.

Optical study of the insulator-to-metal transition in LaxMnO3thin films

GALDI, ALICE;MARITATO, Luigi;
2014

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 LaxMnO$_{3-delta }$ films with x = 0.66, 0.88, 0.98 and 1.10, and with $delta simeq 0$, 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 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 and ferromagnetic regions populated by large polarons and free carriers.
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Utilizza questo identificativo per citare o creare un link a questo documento: http://hdl.handle.net/11386/4479858
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