Origin of low-energy excitations in charge-ordered manganites

The low-energy excitations in the charge-ordered phase of polycrystalline La0.25Ca0.75MnO3 are explored by frequency-domain terahertz spectroscopy. In the frequency range from 4 cm-1 to 700 cm-1 (energies from 0.4 meV to 90 meV) and at temperatures down to 5 K, we do not detect any feature that can be associated with the collective response of the spatially modulated charge continuum. In the antiferromagnetically ordered phase, broad absorption bands appear in the conductivity and permittivity spectra around 30 cm-1 and 100 cm-1 which are assigned to former acoustic phonons optically activated due to a fourfold superstructure in the crystal lattice. Our results indicate that characteristic energies of collective excitations of the charge-ordered phase in La0.25Ca0.75MnO3, if any, lie below 1 meV. At our lowest frequencies of only few wave numbers a strong relaxation is observed above 100 K connected to the formation of the charge-ordered state.