Epitaxial growth of polar KTaO$_{3}$ thin-films on polar perovskite substrates

The atomic polarity plays an important role in a wide range of physical phenomena at heterointerfaces. For example, the polar/non-polar nature of a LaAlO$_{3}$/SrTiO$_{3}$ system induces partial conducting electrons at the heterointerfaces to avoid diverging electrostatic potential, the so-called ``polar catastrophe,'' which results in intriguing two-dimensional transport and magnetic properties. In this presentation, we discuss another system in which the role of the polar interface is important: the KTaO$_{3}$/GdScO$_{3}$ (KTO/GSO) polar/polar system. At the KTO/GSO interface, there is a ``polar conflict'' heterointerface along the [001] direction, where the AO and BO$_{2}$ layers have reversed net charges so that there is a conflict between the chemical bonding and the electrostatic charges, i.e. K$^{1+}$O$^{2-}$(1-)/Sc$^{3+}$O$_{2}^{4-}$(1-) or Ta$^{5+}$O$_{2}^{4-}$(1$+)$/Gd$^{3+}$O$^{2-}$(1$+)$, which is unstable in the electrostatic point of view. We ask a fundamental question: ``How is the polar conflict resolved in the atomically flat heterointerfaces of such polar/polar systems?'' We have synthesized epitaxial KTO thin films on GSO substrates using pulsed laser deposition. From X-ray diffraction and high-resolution transmission electron microscopy, we have observed that the polar conflict is quite effectively avoided by forming only two non-polar mono-layers at the heterointerface, resulting in high-quality epitaxial thin films on top of the layers. Our result suggests a new way to create two-dimensional confined layers using the polar conflict of the heterointerfaces of two polar materials.