Dimensionality-driven insulator-metal transition in A-site excess non-stoichiometric perovskites.

Coaxing correlated materials to the proximity of the insulator-metal transition region, where electronic wavefunctions transform from localized to itinerant, is currently the subject of intensive research because of the hopes it raises for technological applications and also for its fundamental scientific significance. In general, this tuning is achieved by either chemical doping to introduce charge carriers, or external stimuli to lower the ratio of Coulomb repulsion to bandwidth. In this study, we combine experiment and theory to show that the transition from well-localized insulating states to metallicity in a Ruddlesden-Popper series, La(0.5)Sr(n+1-0.5)Ti(n)O(3n+1), is driven by intercalating an intrinsically insulating SrTiO(3) unit, in structural terms, by dimensionality n. This unconventional strategy, which can be understood upon a complex interplay between electron-phonon coupling and electron correlations, opens up a new avenue to obtain metallicity or even superconductivity in oxide superlattices that are normally expected to be insulators.