Interface control of emergent ferroic order in Ruddlesden-Popper Sr _n+1Ti _nO _3n+1

We discovered from first principles an unusual polar state in the low n Sr _n+1Ti _nO _3n+1 Ruddlesden-Popper (RP) layered perovskites in which ferroelectricity is nearly degenerate with antiferroelectricity, a relatively rare form of ferroic order. We show that epitaxial strain plays a key role in tuning the "perpendicular coherence length" of the ferroelectric mode, and does not induce ferroelectricity in these low-dimensional RP materials as is well known to occur in SrTiO ₃. These systems present an opportunity to manipulate the coherence length of a ferroic distortion in a controlled way, without disorder or a free surface.