Thermodynamic Stability of Ferroelectric Oxide Surface in Pulsed Laser Deposition: Novel Way to Atomic-scale Interface Control.*

We demonstrated the selective fabrication of BaO-RuO$_{\mathrm{2}}$ and TiO$_{\mathrm{2}}$-SrO atomic sequence at the interface of epitaxial SrRuO$_{\mathrm{3}}$/BaTiO$_{\mathrm{3}}$/SrRuO$_{\mathrm{3}}$ (SRO/BTO/SRO) heterostructure using pulsed laser deposition (PLD). The growth conditions for stabilizing each interface structure are strongly related to the phase diagram of BTO surface structure expected from thermodynamic equilibria. The approach allows us precise control of desired sequence of atomic layers with abrupt interfaces. By simply adjusting the oxygen partial pressure, we obtained an abrupt BTO top interface with a TiO$_{\mathrm{2}}$ termination. The interface-controlled BTO exhibits a robust ferroelectricity down to a thickness of 3.5 unit cells (\textasciitilde 1.4 nm)--- a theoretical limit that has to date remained elusive in experiment. Despite PLD has been considered to have non-equilibrium nature, these results highlight that thermodynamic considerations are useful to identify the correct stoichiometric surface/interface terminations.