Ubiquitous stripe phase and enhanced electron pairing in the interfacial high- Tc superconductor FeSe/BaTiO3

Monolayer FeSe grown on ${\mathrm{SrTiO}}₃$ provides a playground for high-${T}_c$ superconductivity. Authors of a recent study have shown the importance of stripy instability for superconductivity enhancement in FeSe/${\mathrm{SrTiO}}₃$. However, it is still under debate whether such a stripe phase is ubiquitously bound to the interfacial superconductivity. Here, we report on molecular beam epitaxy growth and a low-temperature scanning tunneling microscopy study of FeSe films on $\mathrm{BaTi}{\mathrm{O}}₃(001)$. We find that the stripe phase is more prominent in $\mathrm{s}/\mathrm{BaTi}{\mathrm{O}}₃$. Long-range stripes exist in bilayer and triple-layer FeSe films at 4 K and even persist up to 77 K in the case of bilayer FeSe, with enlarged superconducting gaps upon alkali-metal deposition. The results point out an intimate correlation between the interfacial superconductivity and the stripe phase.