Nicolas Stucki

Improper ferroelectricity in perovskite oxide artificial superlattices

Ferroelectric thin films and superlattices are currently the subject of intensive research because of the interest they raise for technological applications and also because their properties are of fundamental scientific importance. Ferroelectric superlattices 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, to enhance the polarization, and the composition, to interpolate between the properties of the combined compounds. 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 applications, 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.

Strain relaxation and critical temperature in epitaxial ferroelectric Pb(Zr0.20Ti0.80)O3 thin films

Strain relaxation and the ferroelectric critical temperature were investigated in a series of epitaxial Pb(Zr0.20Ti0.80)O3 thin films of different thicknesses grown on metallic 0.5% Nb-doped SrTiO3 substrates. Detailed x-ray diffraction studies reveal that strain relaxation progressively occurs via misfit dislocations as the film thickness is increased from fully coherent films (for films below 150A) to essentially relaxed films (for thicknesses above typically 800A). It is found that this change in the strain state does not modify the ferroelectric critical temperature which is found for all the samples to be around 680°C, a value much higher than the bulk.

Diodes with breakdown voltages enhanced by the metal-insulator transition of LaAlO3–SrTiO3 interfaces

Using the metal-insulator transition that takes place as a function of carrier density at the LaAlO3–SrTiO3 interface, oxide diodes have been fabricated with room-temperature breakdown voltages of up to 200 V. With applied voltage, the capacitance of the diodes changes by a factor of 150. The diodes are robust and operate at temperatures up to 270 C.

New phenomena at the interfaces of very thin ferroelectric oxides

We present a brief review of the role of interfacial physics in ferroelectric oxides, with an emphasis on the importance of boundary conditions that determine the properties of very thin ferroelectric films and superlattices. As well as discussing the screening problem, and the role of strain and electrostatics in ferroelectrics, we highlight some of the possibilities in fine period superlattices where the high density of interfaces can lead to new and potentially useful phenomena.

Ferroelectric Domains in PbTiO3/SrTiO3 Superlattices

Polydomain superlattices composed of ferroelectric PbTiO3 and paraelectric SrTiO3 were investigated by means of temperature-dependent X-ray diffraction and electrical measurements. The ferroelectric nanodomains were found to have an almost isotropic distribution of domain wall alignments and to be highly responsive to applied electric fields, giving rise to large enhancements in the overall dielectric response of the structure. Electrical measurements revealed a dramatic dependence of the dielectric permittivity, tunability, and polarization-voltage characteristics on the superlattice period. For short-period superlattices, double hysteresis loops were observed.