Anomalous terahertz dielectric phase in charge-ordered La_1/3Sr_2/3FeO₃ thin film

Orthoferrites are the model systems for understanding the origin of complex valance-skipping charge ordering (CO) associated with antiferromagnetic spin ordering as a sequence of Fe⁺³Fe⁺³Fe⁺⁵Fe⁺³Fe⁺³Fe⁺⁵ … along the pseudocubic (111) direction. Here, we have investigated the low-energy dynamics of one such system La_1/3Sr_2/3FeO₃ along the (111) crystal orientation using terahertz (THz) time domain spectroscopy. The temperature dependent THz optical constants reveal a purely electronic semiconductor to a charge-ordered Mott insulator transition, which is contrary to the reported metal–insulator transition. Above the transition of ∼180 K, THz conductivity shows a thermally activated charge-carrier hopping type conduction with activation energy 92.7 meV (87.3 meV) when measured in cooling (heating). The dielectric constant also indicates that the charge ordering formation starts at 200 K, which exists down to 20 K. Further analysis shows a reduction in the effective number of carriers by a fraction of about six as temperature decreases from 300 K to the CO transition at 200 K; this depletion of carriers triggers the intersite interactions required for CO. These findings suggest that a high dielectric constant associated with charge order in the THz region could be useful for THz applications in communication.