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Study of A-site substituted quantum paraelectric strontium titanate crystals by terahertz time-domain spectroscopic ellipsometry

The dielectric properties of the terahertz frequency range were studied in A-site substituted quantum paraelectric strontium titanate SrTiO3 (STO) crystals with the perovskite structure. The complex dielectric constants were determined accurately by the terahertz time-domain spectroscopic ellipsometry. The soft optic mode of STO has T1u symmetry in a cubic phase. The hardening of a soft mode frequency of pure STO at 2.7 THz was observed with increasing the La content of the A-site of the perovskite structure up to 3.8 THz of a La 5.0 at% doped STO crystal. However, the softening of a soft mode frequency was observed in a Ca doped STO crystal reflecting the enhancement of ferroelectric instability.