Emergent multiferroicity and strain-driven metal-semiconductor transitions in LaMnO3/RMnO3 superlattices (R = Pr, Pm, Sm and Gd)

It is known that rare-earth manganites LnMnO3 with Ln = La to Gd are typical Mott insulators favoring the A-type antiferromagnetic (A-AFM) state. Certainly no ferroelectricity can be possible although the alternatively stacked LnO layers are both polar. Nevertheless, under the inspiration that one plus one is more than two, it is appreciated to add functionality by combining two components of this manganite series into a superlattice. In this work, we construct (001)-oriented LaMnO3/RMnO3 (R = Pr, Pm, Sm and Gd) superlattice and investigate the possible emergent ferroelectricity by means of the first-principles calculations. It is revealed that the lattice matching in these superlattices may generate lattice distortions to each component based on the scenario of hybrid improper ferroelectricity, resulting in spontaneous ferroelectric polarization, which is larger than the traditional type II LnʹMnO3 (Lnʹ radius is smaller than Gd) polarization. In the meantime, the A-AFM state remains the magnetic ground state of these superlattices. Furthermore, it is predicted that the externally imposed in-plane compressive strain can trigger the semiconductor to half-metal transitions in accompanying with the A-AFM to ferromagnetic (FM) transitions. The present work sheds light into the possibility to design multiferroic materials and functionality by tailoring artificial superlattices/heterostructures from those non-ferroelectric systems, and to design electronic device by utilizing the electronic transport properties under epitaxial strain.