Phase transitions and polarizations in epitaxial BaTiO3/SrTiO3 superlattices studied by second-harmonic generation

${\mathrm{BaTiO}}_3/{\mathrm{SrTiO}}_3$ superlattices are found to have a critical periodicity of 30 ${\mathrm{SrTiO}}_3$ unit cells, below which the room-temperature symmetry is $\text{mm2}$ orthorhombic. The orthorhombic-tetragonal phase-transition temperature is shifted upward from the bulk value $\text{(<3\hspace{0.17em}}\mathrm{K}$ and normally not observed at finite temperature) of the quantum paraelectric pure strontium titanate to temperatures above ambient (599 K for 30/30-unit-cell superlattices). Antidipole patterns appear in superlattices with unit cells below 10/10. A model is presented that accounts quantitatively for the change in second-harmonic generation intensity with superlattice periodicity from 4 unit cells to 50.