Generic approach for integrating ferroelectricity and superconductivity into a single two-dimensional monolayer

The coexistence of ferroelectricity and superconductivity is a forefront research topic in condensed matter physics, and achieving this coexistence in a single-phase material is particularly challenging, as the former is typically restricted to insulators while the latter demands metallicity. Here, we show that coexistence of robust ferroelectricity and superconductivity can be realized by properly hole doping a fertile ferroelectric insulator that possesses antibonding states as its highest valence bands. Using a prototypical two-dimensional ferroelectric insulating SnS monolayer as an example, we demonstrate that a moderate level of hole doping (\ensuremath{\sim}0.30 hole per formula unit) not only effectively converts the system into a metal, but counterintuitively also enhances its ferroelectricity rather than weakens or even completely destroys the ferroelectricity as commonly encountered. The underlying physical origin is traced to the shifting of the Fermi level into the occupied antibonding bands, which renders the system metallicity and simultaneously also strengthens the in-plane bonding nature associated with polar displacement. We further show that the resultant ferroelectric metal can harbor superconductivity with a pronounced superconducting transition temperature of \ensuremath{\sim}7 K, due to the emergence of strong Fermi-surface nesting upon hole doping and softening of the out-of-plane acoustic phonon mode that couples effectively with the itinerant electrons. These findings provide a generic approach for achieving ferroelectric superconductivity with appealing device potentials.