  The atomic-scale synthesis of artificial oxide heterostructures offers new
opportunities to create novel states that do not occur in nature. The main
challenge related to synthesizing these structures is obtaining atomically
sharp interfaces with designed termination sequences. Here, we demonstrate that
the oxygen pressure (PO2) during growth plays an important role in controlling
the interfacial terminations of SrRuO3/BaTiO3/SrRuO3 (SRO/BTO/SRO)
ferroelectric capacitors. The SRO/BTO/SRO heterostructures were grown by the
pulsed laser deposition (PLD) method. The top SRO/BTO interface grown at high
PO2 (around 150 mTorr) usually exhibited a mixture of RuO2-BaO and SrO-TiO2
terminations. By reducing PO2, we obtained atomically sharp SRO/BTO top
interfaces with uniform SrO-TiO2 termination. Using capacitor devices with
symmetric and uniform interfacial termination, we were able to demonstrate for
the first time that the ferroelectric (FE) critical thickness can reach the
theoretical limit of 3.5 unit cells (u.c.).
