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 epsilon(r) approximately 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.

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The evolution of tetragonality with thickness has been probed in epitaxial c-axis oriented PbTiO3 films with thicknesses ranging from 500 down to 24 A. High resolution x ray pointed out a systematic decrease of the c-axis lattice parameter with decreasing film thickness below 200 A. Using a first-principles model Hamiltonian approach, the decrease in tetragonality is related to a reduction of the polarization attributed to the presence of a residual unscreened depolarizing field. It is shown that films below 50 A display a significantly reduced polarization but still remain ferroelectric.

Artificial PbTiO3/SrTiO3 superlattices were constructed using off-axis RF magnetron sputtering. X-ray diffraction and piezoelectric atomic force microscopy were used to study the evolution of the ferroelectric polarization as the ratio of PbTiO3 to SrTiO3 was changed. For PbTiO3 layer thicknesses larger than the 3-unit cells SrTiO3 thickness used in the structure, the polarization is found to be reduced as the PbTiO3 thickness is decreased. This observation confirms the primary role of the depolarization field in the polarization reduction in thin films. For the samples with ratios of PbTiO3 to SrTiO3 of less than one a surprising recovery of ferroelectricity that cannot be explained by electrostatic considerations was observed.The construction of artificial ferroelectric oxide superlattices with fine periodicity presents exciting possibilities for the development of new materials with extraordinary properties and furthermore is an ideal probe for understanding the fundamental physics of ferroelectric materials.The most studied system at present is BaTiO 3 /SrTiO 3 [1,2,3,4,5,6,7,8] In BaTiO 3 /SrTiO 3 , first principles studies [5] suggest that both the SrTiO 3 and BaTiO 3 layers are polarized such that the polarization is approximately uniform throughout the superlattice. The driving force behind this is the large electrostatic energy penalty for a buildup of charge at the interface caused by discontinuous polarization in the normal direction. The electrostatic model proposed by Neaton and Rabe [5] to explain their first principles results for BaTiO 3 /SrTiO 3 superlattices is very similar to the electrostatic model applied to calculate the effect of the depolarization field in ultra-thin ferroelectric films with realistic electrodes [18,19,20]. Experimentally it was recently shown that the reduced polarization observed in monodomain thin PbTiO 3 can be explained by the presence of a depolarization field resulting from imperfect screening of the polarization [21]. Recent work also suggests that, under certain conditions, the electrostatic energy due to depolarization fields will drive the system to form domains as observed by Fong et al. [23] and Nagarajan et al. [24]. In this letter we use PbTiO 3 /SrTiO 3 superlattices to probe the effect of a reduced ferroelectric thickness in a dielectric environment. Our data show that the behaviour observed in PbTiO 3 thin films is reproduced for PbTiO 3 layers thicker than three unit cells. However, for thinner ferroelectric layers a surprising recovery of ferroelectricity that cannot be explained by electrostatic considerations was observed.The superlattices of PbTiO 3 /SrTiO 3 were prepared on conducting 0.5% Nb doped (001) SrTiO 3 substrates using off-axis RF magnetron sputtering with conditions similar to those used for growing high quality epitaxial c-axis PbTiO 3 thin films [21]. For all the samples discussed in this paper, the SrTiO 3 thickness was fixed at three unit cells (about 12Å). At room temperature the in-plane lattice parameters of tetragonal ferroelec...

The polarization of PbTiO3/SrTiO3 superlattices is experimentally tuned from 0-60 mu C/cm(-2) and the transition temperature from room temperature to 1000 K while maintaining a perfect crystal structure and low leakage currents (see figure). A simple model based on Landau theory is developed as a guide for the straightforward production of samples with ferroelectric properties designed for particular applications