Terahertz-infrared spectroscopy of overdoped manganites La1−xCaxMnO3

Abstract

Terahertz and infrared spectra of dielectric permittivity and optical conductivity of overdoped manganites La1−xCaxMnO3 (0.5≤x≤1) have been measured at frequencies ν from 4cm−1 to 700cm−1 and at temperatures T from 5K to 300K. 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<0.85) 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 (~ν2). 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. At low temperatures, for the range of dopings 0.5≤x<0.85, the conductivity spectra show signatures of variable-range hopping mechanisms of transport.