Improper ferroelectricity in perovskite oxide artificial superlattices

In the field of 'oxide electronics', artificially layered structures based on thin films of complex oxides are designed to obtain novel, technologically interesting electric or magnetic properties. Bousquet et al. studied artificial superlattices made from ferroelectric (PbTiO3) and paraelectric (SrTiO3) oxides. They have found a previously unknown type of atom rearrangement at the interfaces between thin films of these materials, leading to an unusual 'improper' ferroelectric effect. The system has a very large dielectric constant that, unlike conventional ferroelectricity, is fairly temperature independent. This is of considerable technological interest as a route to improved materials for high-permittivity dielectric layers in traditional microelectronics as well as for emerging needs in magneto-electric applications. Artificial superlattices made from a ferroelectric and a paraelectric oxide are studied, and a new kind of atom rearrangement is found to take place at the interfaces between thin films of these materials, leading to an unusual, 'improper' ferroelectric effect. As a result, the system has a very large dielectric constant that, in contrast to conventional ferroelectricity, is fairly temperature independent; this observation is of considerable interest for practical applications. Ferroelectric thin films and superlattices are currently the subject of intensive research1,2 because of the interest they raise for technological applications and also because their properties are of fundamental scientific importance3,4,5. Ferroelectric superlattices6 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 strain7,8,9,10, to enhance the polarization, and the composition, to interpolate between the properties of the combined compounds11,12,13,14,15. 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 ground-state 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 εr ≈ 600, which is also fairly temperature-independent. This behaviour, of practical interest for technological applications16, 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.