Title: Tunable inversion symmetry in heterostructures of layered oxides
Abstract: Traditional approaches to create and control functional electronic materials have focused on new phases in previously unknown bulk minerals. More recently, interlayer physics has spawned interest in known materials in unexplored atomic scale geometries, especially in complex transition metal oxides (TMO), where heterostructures with targeted structures can be created on demand. In this talk, I show that although epitaxial strain routinely induces (enhances) electric polarizations, biaxial strain can also induce an unanticipated polar-to-nonpolar (P-NP) structural transition in (001) thin films of naturally layered An+1BnO3n+1 (n = 1– ∞) oxides. Density functional theory calculations and a complete phenomenological model for Ca3Ti2O7 are used to show that the origin of the P-NP transition originates from the interplay of trilinear-related lattice mode interactions active in the layered oxides, and those interactions are directly strain tunable. Moreover these acentric layered oxides exhibit a quasi-two dimensional phonon mode---an acoustic branch with quadratic dispersion, enabling unusual membrane effects such as tunable negative thermal expansion. This behavior is different from that occurring in the corresponding centrosymmetric polymorphs. Next, I show how to utilize structural coherency at the heterointerface to lift inversion in nominally centric metals, thereby creating a new family of polar metals in nickelate oxides. I conclude by emphasizing that broken inversion symmetric structures offer a plentiful playground for realizing new functionalities in thin films, including new multiferroics from polar metals.
Colloq flyer 3-29 Rondinelli