Letter

Nature 452, 732-736 (10 April 2008) | doi:10.1038/nature06817; Received 7 August 2007; Accepted 30 January 2008

Improper ferroelectricity in perovskite oxide artificial superlattices

Eric Bousquet1,3, Matthew Dawber2,3,4, Nicolas Stucki2, Céline Lichtensteiger2, Patrick Hermet1, Stefano Gariglio2, Jean-Marc Triscone2 & Philippe Ghosez1

  1. Physique Théorique des Matériaux, Université de Liège, Allée du 6 Août 17 (B5), 4000 Sart Tilman, Belgium
  2. DPMC, University of Geneva, 24 Quai E.-Ansermet 1211, Geneva 4, Switzerland
  3. These authors contributed equally to this work.
  4. Present address: Department of Physics and Astronomy, Stony Brook University, Stony Brook, New York 11794-3800, USA.

Correspondence to: Matthew Dawber2,3,4Philippe Ghosez1 Correspondence and requests for materials should be addressed to P.G. (Email: philippe.ghosez@ulg.ac.be) or M.D. (Email: matthew.dawber@stonybrook.edu).

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 epsilon r approximately 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.