Boson peak in manganites

Abstract

After the phase transition occurs in manganites La1-xCaxMnO 3 (x ≥ 0.5) into a dielectric phase with charge order, right below the corresponding temperature of TCO = 240 K a series of absorption lines appear in the terahertz spectral range, only in case of commensurate x values. We show that the lines originate from acoustic phonons that become optically active by folding of the Brillouin zone. At even lower temperatures, a strongly asymmetric extra absorption band develops at frequencies around 20 cm-1-30 cm-1 corresponding to the position of the lowest-energy van Hove singularity in the reduced Brillouin zone. The band is assigned to the Boson Peak, i.e. to the excess of lattice vibrational states over the standard Debye contribution. The folded phonons and the boson peak do not show up for incommensurate calcium. © Published under licence by IOP Publishing Ltd.