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, La0.5Srn+1−0.5TinO3n+1, is driven by intercalating an intrinsically insulating SrTiO3 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

Atomic layer deposition of environmentally benign SnTiOx as a potential ferroelectric material.

Inspired by the need to discover environmentally friendly, lead-free ferroelectric materials, here the authors report the atomic layer deposition of tin titanate (SnTiOx) aiming to obtain the theoretically predicted perovskite structure that possesses ferroelectricity. In order to establish the growth conditions and probe the film structure and ferroelectric behavior, the authors grew SnTiOxfilms on the commonly used Si(100) substrate. Thin films of SnTiOx have been successfully grown at a deposition temperature of 200 °C, with a Sn/Ti atomic layer deposition(ALD) cycle ratio of 2:3 and postdeposition heat treatments under different conditions. X-ray photoelectron spectroscopy revealed excellent composition tunability of ALD.X-ray diffraction spectra suggested anatase phase for all films annealed at 650 and 350 °C, with peak positions shifted toward lower 2-theta angles indicating enlarged unit cell volume. The film annealed in O2 at 350 °C exhibited piezoresponse amplitude and phase hysteresis loops, indicative of the existence of switchable polarization

Improper ferroelectricity in perovskite oxide artificial superlattices

Ferroelectric thin films and superlattices are currently the subject of intensive research(1,2) because of the interest they raise for technological applications and also because their properties are of fundamental scientific importance(3-5). Ferroelectric superlattices(6) allow the tuning of the ferroelectric properties while maintaining perfect crystal structure and a coherent strain, even throughout relatively thick samples. This tuning is achieved in practice by adjusting both the strain(7-10), to enhance the polarization, and the composition, to interpolate between the properties of the combined compounds(11-15). Here we show that superlattices with very short periods possess a new form of interface coupling, based on rotational distortions, which gives rise to 'improper' ferroelectricity. These observations suggest an approach, based on interface engineering, to produce artificial materials with unique properties. By considering ferroelectric/paraelectric PbTiO3/SrTiO3 multilayers, we first show from first principles that the groundstate of the system is not purely ferroelectric but also primarily involves antiferrodistortive rotations of the oxygen atoms in a way compatible with improper ferroelectricity. We then demonstrate experimentally that, in contrast to pure PbTiO3 and SrTiO3 compounds, the multilayer system indeed behaves like a prototypical improper ferroelectric and exhibits a very large dielectric constant of epsilon(r)approximate to 600, which is also fairly temperature-independent. This behaviour, of practical interest for technological applications(16), is distinct from that of normal ferroelectrics, for which the dielectric constant is typically large but strongly evolves around the phase transition temperature and also differs from that of previously known improper ferroelectrics that exhibit a temperature-independent but small dielectric constant only