Abstract. A brief introduction to the ABINIT software package is given. Available under a free software license, it allows to compute directly a large set of properties useful for solid state studies, including structural and elastic properties, prediction of phase (meta)stability or instability, specific heat and free energy, spectroscopic and vibrational properties. These are described, and corresponding applications are presented. The emphasis is also laid on its ease of use and extensive documentation, allowing newcomers to quickly step in.

The nonlinear response of infinite periodic solids to homogenous electric fields and collective atomic displacements is discussed in the framework of density functional perturbation theory. The approach is based on the 2n + 1 theorem applied to an electric-field-dependent energy functional. We report the expressions for the calculation of the nonlinear optical susceptibilities, Raman scattering efficiencies, and electro-optic ͑EO͒ coefficients. Different formulations of third-order energy derivatives are examined and their convergence with respect to the k-point sampling is discussed. We apply our method to a few simple cases and compare our results to those obtained with distinct techniques. Finally, we discuss the effect of a scissors correction on the EO coefficients and nonlinear optical susceptibilities.

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...

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Nonlinear optical susceptibilities and nonresonant Raman scattering spectra of the ferroelectric phase of lithium niobate (LiNbO3) are computed using a first-principles approach based on density functional theory and taking advantage of a recent implementation based on the nonlinear response formalism and the 2n+1 theorem. Infrared reflectivity spectra of the ferroelectric phase of LiNbO3 are also calculated. New assignments are proposed for the E-modes, clarifying a longstanding debate in the literature. In addition, it is shown that knowledge of the nonlinear optical susceptibility tensor of LiNbO3 does not significantly alter the profile of its Raman spectra in a configuration where the longitudinal optic modes are involved.