Abstract
The low-energy electrodynamic response of a series of samples of different morphology is systematically studied in the frequency range from 4 to (0.1–20 THz, quantum energies 0.4–90 meV) with particular attention focused at frequencies below . In the antiferromagnetically ordered phase below 140 K, a resonancelike absorption band appears in the spectra around that is assigned to acoustical phonons which acquire optical activity due to a fourfold superstructure appearance in the crystallographic 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.
2 More- Received 20 January 2010
DOI:https://doi.org/10.1103/PhysRevB.81.125132
©2010 American Physical Society