Strained enabled Ferroelectricity in CaTiO$_{3}$ Thin Films Probed by Nonlinear Optics and Scanning Probe Microscopy

Calcium titanate, CaTiO$_{3}$ is not a ferroelectric in its bulk form. However, first principles calculations predict that biaxially tensile strained CaTiO$_{3}$ thin films should become ferroelectric.~ Here, we indeed confirm that strained CaTiO$_{3}$ films become ferroelectric with a Curie temperature of $\sim $125K. Optical second harmonic generation (SHG) measurements, polarization studies, and in-situ electric-field measurements for a number of films with different strain values will be presented: CaTiO$_{3}$/DyScO$_{3}$(110), CaTiO$_{3}$/SrTiO$_{3 }$(100),$_{ }$CaTiO$_{3}$/GdScO$_{3}$/NdGaO$_{3}$(110), CaTiO$_{3}$/LaSrAlO$_{3}$(001) as well as for a single crystal CaTiO$_{3}$. From these studies, we conclude that strained CaTiO$_{3}$ films are ferroelectric with a point group symmetry of \textit{mm2}, and show reversible domain switching characteristics under an electric field. We also present results of variable temperature piezoelectric force microscopy for imaging the polar domains in the ferroelectric phase. These results suggest that strain is a valuable tool for inducing polar, long range ferroelectric order in even non-polar ceramic materials such as CaTiO$_{3}$.