Metal-insulator transition in (Pr, Y)0.7Ca0.3CoO3 in the far-infrared spectral region

Bulk ceramics and thick nanogranular films of mixed-valence cobaltite (Pr1-yYy)(0.7)Ca0.3CoO3 were studied using time-domain terahertz transmission spectroscopy and infrared reflection spectroscopy. Thanks to the high probing frequency, the overall increase in the conductivity during the insulator to metal transition was observed both in the ceramics and in the nanogranular films. The terahertz conductivity spectrum shows that the transition is not a simple cross-over between two states. As the charges become delocalized during the insulator to metal transition, a broad resonant mode corresponding to the eigenfrequency of the binding potential appears in the terahertz conductivity spectra in the ceramics, and it further softens as the metallic state prevails. This is interpreted as a consequence of the complexity of charge motion in a temperature-dependent energy landscape, changing dramatically from the tightly bound charges on atomic distance in the insulating phase to almost free charges in the metallic phase. This behavior is not directly observed in the nanogranular films as their nature introduces spectral signatures of an additional localization due to grain boundaries which obscure the pronounced effects observed in the bulk. Nevertheless fine spectral signatures accompanying the phase transition are still resolved. The insulator to metal transition is also accompanied by a weak softening of infrared-active phonon modes, which is related to the expansion of lattice parameters upon phase transition.