  Valley lines on total-energy surfaces for the zone-center distortions of
free-standing and in-plane strained SrTiO3 are investigated with a newly
developed first-principles structure optimization technique [Jpn. J. Appl.
Phys. vol.43 (2004), p.6785]. The results of numerical calculations confirmed
that the ferroelectricity is induced, and the Curie temperature is increased,
by applying biaxial compressive or tensile strains. Along the distortion,
strong nonlinear coupling between the soft- and hard-modes is demonstrated.
