Terahertz spectroscopy of low-energy excitations in charge-ordered La0.25Ca0.75MnO3

The low-energy electrodynamic response of a series of La0.25Ca0.75MnO3 samples of different morphology is systematically studied in the frequency range from 4 to 700 cm(-1) (0.1-20 THz, quantum energies 0.4-90 meV) with particular attention focused at frequencies below 40 cm(-1). In the antiferromagnetically ordered phase below 140 K, a resonancelike absorption band appears in the spectra around 20-40 cm(-1) that is assigned to acoustical phonons which acquire optical activity due to a fourfold superstructure appearance in the crystallographic a direction and correspondent Brillouin-zone folding. Nanosize effects in charge and magnetic subsystems are investigated by measuring the spectra of samples with grain diameters reduced from several micrometers down to 40 nm. A strong relaxation in the lowest-frequency spectra is observed above 100 K in all samples, indicating that the charge-order transition shows characteristics typical for order-disorder transitions.