Terahertz-infrared spectroscopy of overdoped manganites La1-xCaxMnO3

Terahertz and infrared spectra of dielectric permittivity and optical conductivity of overdoped manganites La(1 - x)Ca(x)NnO(3) (0.5 <= x <= 1) have been measured at frequencies v from 4 cm(-1) to 700 cm(-1) and at temperatures T from 5 K to 300 K. The samples were prepared in the form of bulk polycrystals (ceramics) and epitaxial films (free-standing and on MgO substrates). Strongly asymmetric absorption bands have been found in the compounds which are in charge-ordered state (0.5 < x < 085) and have commensurate calcium doping. The bands are located close to the position of lowest energy van Hove singularity of the Folded Brillouin zone. The bands are associated with a boson peak, i.e., with the excess of lattice acoustical vibrational states over the standard Debye contribution (similar to v(2)). In addition, Raman scattering measurements on ceramic samples have been conducted, which confirmed our assignment of the bands' origin. We firmly identify contribution to the low energy spectra due to quasi-free charge carriers. In the paramagnetic phase all samples show a Drude-like response corresponding to the band type of charge transport. AL low temperatures, for the range of &pings 0.5 <= x < 0.85, the conductivity spectra show signatures of variable-range hopping mechanisms of transport. (C) 2014 Elsevier B.V. All rights reserved