  It is highly desirable to search for promising two-dimensional (2D) monolayer
materials for deep insight of 2D materials and applications. We use
first-principles method to investigate tetragonal perovskite oxide monolayers
as 2D materials. We find four stable 2D monolayer materials from SrTiO$_3$,
LaAlO$_3$, KTaO$_3$, and BaFeO$_3$, denoting them as STO-ML, LAO-ML, KTO-ML,
and BFO-ML. Our further study shows that through overcoming dangling bonds the
first three monolayers are 2D wide-gap semiconducotors, and BFO-ML is a 2D
isotropic Heisenberg ferromagnetic metal. There is a large electrostatic
potential energy difference between the two sides, reflecting a large
out-of-plane dipole, in each of the monolayers. These make a series of 2D
monolayer materials, and should be useful in novel electronic devices
considering emerging phenomena in perovskite oxide heterostructures.
