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

$BaTiO3/SrTiO3$ superlattices are found to have a critical periodicity of 30 $SrTiO3$ unit cells, below which the room-temperature symmetry is $mm2$ orthorhombic. The orthorhombic-tetragonal phase-transition temperature is shifted upward from the bulk value $(<3 K$ 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.