Correlation between Soft Mode Frequency and Carrier Concentration in Doped Strontium Titanate Crystals

The complex dielectric constants of the ferroelectric soft optic mode of doped SrTiO3 (STO) crystals with T1u symmetry were determined accurately by the terahertz time-domain spectroscopic ellipsometry. In La doped STO crystals at the A-site of perovskite structure, the hardening of soft mode frequency was observed as the La content increases. The similar behavior was also observed in Nb doped STO crystals at the B-site. These results suggest that the increase of carrier concentration introduced by heterovalent doping suppresses the ferroelectric instability.