Domain structure and dielectric properties of metal-ferroelectric superlattices with asymmetric interfaces

$\mathrm{Pb}\mathrm{Ti}{\mathrm{O}}₃/\mathrm{Sr}\mathrm{Ru}{\mathrm{O}}₃$ superlattices deposited on $\mathrm{Sr}\mathrm{Ti}{\mathrm{O}}₃$ substrates are studied using a combination of x-ray diffraction, piezoresponse force microscopy, scanning transmission electron microscopy, transport measurements, and impedance spectroscopy. The superlattices are found to have two inequivalent interfaces resulting from differences in the growth modes for $\mathrm{Pb}\mathrm{Ti}{\mathrm{O}}₃$ and $\mathrm{Sr}\mathrm{Ru}{\mathrm{O}}₃$. X-ray diffraction measurements show that, despite being sandwiched between metallic $\mathrm{Sr}\mathrm{Ru}{\mathrm{O}}₃$ layers, the ferroelectric layers possess dense nanoscale domains. The observed domain sizes are comparable to those found in ferroelectric-dielectric systems, and they are attributed to the depolarizing field caused by the finite screening length of the $\mathrm{Sr}\mathrm{Ru}{\mathrm{O}}₃\ensuremath{-}\mathrm{Pb}\mathrm{Ti}{\mathrm{O}}₃$ interface. The macroscopic capacitance of the ultrathin $\mathrm{Pb}\mathrm{Ti}{\mathrm{O}}₃$ layers was measured, and its temperature dependence was found to be consistent with permittivity enhancement due to domain wall motion.