Spectral properties and infrared absorption in manganites

Abstract

Within a recently proposed variational approach it has been shown that, in $La_{1-x}A_xMnO_3$ perovskites with $0<x<0.5$, near the metal-insulator transition, the combined effect of the magnetic and electron-phonon interactions pushes the system toward a regime of two coexisting phases: a low electron density one made by itinerant large polarons forming ferromagnetic domains and a high electron density one made by localized small polarons giving rise to paramagnetic or antiferromagnetic domains depending on temperature. Employing the above-mentioned variational scheme, in this paper spectral and optical properties of manganites are derived for $x=0.3$ at different temperatures. It is found that the phase separation regime induces a robust pseudogap in the excitation spectrum of the system. Then the conductivity spectra are characterized by a transfer of spectral weight from high to low energies, as the temperature $T$ decreases. In the metallic ferromagnetic phase, at low $T$ two types of infrared absorption come out: a Drude term and a broad absorption band due respectively to the coherent and incoherent motion of large polarons. The obtained results turn out in good agreement with experiments.