Probing Nanoscale Ferroelectricity by Ultraviolet Raman Spectroscopy

We demonstrate that ultraviolet Raman spectroscopy is an effective technique to measure the transition temperature, Tc, accurately in ferroelectric ultrathin films and superlattices. We show that one-unit-cell thick BaTiO3 layers in BaTiO3/SrTiO3 superlattices are not only ferroelectric (Tc as high as 250 K), but also polarize the quantum paraelectric SrTiO3 layers adjacent to them. Tc was tuned by ~ 500 K by varying the thicknesses of BaTiO3 and SrTiO3 layers, revealing the essential roles of electrical and mechanical boundary conditions for nanoscale ferroelectricity