Far-infrared spectroscopy of a SrTiO3thin film

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Abstract
Far-infrared transmittance of a SrTiO3 thin film deposited on a sapphire substrate by laser ablation was measured in the frequency range 25–200 cm–1 at temperatures from 5K up to 300 K. An interesting difference between the temperature behaviour of the polar soft mode in the thin film and the one in bulk material was observed. Below the structural transition near 105 K the mode stops softening and its damping increases on further cooling. The values of the total low-frequency permittivity in the film are substantially lower than those in bulk SrTiO3. The suggested explanation is based on the fact that the film contains relevant internal stress.