Cooperative dynamics in doped manganite films: Phonon anomalies in the ferromagnetic state

Abstract

We present optical measurements of phononic excitations in La 2/3 Ca 1/3 MnO 3 (LCMO) and La 2/3 Sr 1/3 MnO 3 (LSMO) thin films covering the full temperature range from the metallic ferromagnetic to the insulating paramagnetic phase. All eight phonons expected for the R3c symmetry in LSMO and 17 out of the expected 25 phonons for the Pnma symmetry in LCMO have been determined. Close to the ferromagnetic-toparamagnetic transition both compounds reveal an anomalous behavior but with different characteristics. Anomalies in the phononic spectra are a manifestation of the coupling of lattice degrees of freedom (DOF) to electronic DOF. Specifically, the low-frequency external group proves to be an indicator for lattice modifications induced by electronic correlations. The enhanced electron-phonon coupling in LCMO is responsible for Fano-like interference effects of distinct phonon modes with electronic continuum excitations: we observe asymmetric phonon line shapes, mode splitting and spectral weight transfer between modes.

Novel aspects of charge and lattice dynamics in the hole-doped manganite La0.67Sr0.33MnO3

“…The reasons that other phonons cannot be clearly resolved are following: lower signal from the film whose thickness is much less than infrared wavelengths, the presence of strong phonon features of LAO substrate and the screening effect of mobile charges in the metallic phase. The oscillator parameters of the 340 cm −1 infrared phonon mentioned above agrees with previous reported results [36,38,39]. The inclusion of two additional Lorentzian oscillators was required in order to improve the fits to the data and these oscillators likely correspond to infrared-active phonons.…”

Infrared Reflectivity Spectra of Manganite Thin Films Grown on Different Substrates

“…The spectral resolution of phononic excitations in the films is superior to that in bulk LCMO samples and this is the reason we can resolve the stretching mode doublets in the Pnma symmetry. This has been already observed by Hartinger et al [13] in the IR spectra of LCMO thin films comparing to polycrystalline spectra [16]; only three broad modes have been detected in the polycrystalline sample while the degeneracy has been removed in thin films.…”

“…Two strong bands appear at the ∼500-700 cm −1 region where the JT F 1u stretching mode is expected. The splitting of the F 1u stretching mode to two A u and E u modes has been also observed by others and has been attributed to the JT distortion, which induces different Mn-O lengths [13,15]. The spectral resolution of phononic excitations in the films is superior to that in bulk LCMO samples and this is the reason we can resolve the stretching mode doublets in the Pnma symmetry.…”

“…Approximately, the phonon spectra of manganites can be separated into external (∼185 cm −1 ), bending (∼350 cm −1 ), and stretching (∼550 cm −1 ) modes with respect to cubic (Pm3m) symmetry [13]. Depending on the ion size and doping concentration, these triply degenerate modes split into pairs of non-degenerate (A) and doubly degenerate (E) modes, and, moreover, they become broader and overlap [14].…”

“…Our results indicate that the different substrates induce changes in the microscopic phases and in some cases in the structure of the thin films due to the different kind of strains. Although we did not obtain the optical conductivity from the Kramers-Kronig (KK) analysis due to the strong contribution by the substrate (substrate correction will be done in future work), thin films proved to be superior for the analysis of the phononic [13] and MIR spectra in the metallic manganites [19].…”