Ferroelectric superlattices

Studying the temperature evolution of UV Raman spectra was demonstrated to be an effective approach to determine the ferroelectric phase transition temperature in ferroelectric ultrathin films and superlattices, which is a critical but challenging step for understanding ferroelectricity in nanoscale systems. The T. determination from Raman data is based on the above mentioned fact that perovskite-type crystals have no first order Raman active modes in paraelectric phase. Therefore, Raman intensities of the ferroelectric superlattice or thin film phonons decrease as the temperature approaches Tc from below and disappear upon ti ansition into paraelectric phase. Above Tc, the spectra contain only the second-order features, as expected from the symmetry selection rules. This method was applied to study phase transitions in BaTiOs/SrTiOs superlattices. Figure 21.3 shows the temperature evolution of Raman spectra for two BaTiOs/SrTiOa superlattices. From the shapes and positions of the BaTiOs lines it follows that the BaTiOs layers remain in ferroelectric tetragonal... 

Dawber M, Stucki N, Lichtensteiger C, Gariglio S, Ghosez P, Triscone J-M (2007) Tailoring the properties of artificially layered ferroelectric superlattices. Adv Mater 19 4153... 

Lee HN, Christen HM, Chisholm ME, Rouleau CM, Lowndes DH (2005) Strong polarization enhancement in asymmetric three-component ferroelectric superlattices. Nature 433 395... 

Nakhmanson SM, Rabe KM, Vanderbilt D (2006) Predicting polarization enhancement in multicomponent ferroelectric superlattices. Phys Rev B 73 060101(R)... 

Pertsev NA, Janolin P-E, Kiat J-M, Uesu Y (2010) Enhancing permittivity of ferroelectric superlattices via composition tuning. Phys Rev B 81 144118... 

Figure 6.22 ferroelectric superlattice composed of m = 2 unit cells of SrTiO and n = 3 unit cells of BaTiO,... 

Although the properties of ferroelectric superlattices can be governed by domain structure, no systematic study of this effect has been performed. In other words, the physics of the domain structure formation at the FE phase transition temperature T=Tc in the FE multilayers remains poorly understood. Here we address the question of a phase transition temperature in a periodic superlattice structure consisting of alternate ferroelectric (FE) and paraelectric (PE) layers of nanometric thickness. To get rid of the effect of 90° ferroelastic domains we assume that FE layers have either natural or strain-induced c-oriented uniaxial symmetry. 

The supramolecular structure of block co-polymers allows the design of useful materials properties such as polarity leading to potential applications as second-order nonlinear optical materials, as well as piezo-, pyro-, and ferroelectricity. It is possible to prepare polar superlattices by mixing (blending) a 1 1 ratio of a polystyrene)-6-poly(butadiene)-6-poly-(tert-butyl methacrylate) triblock copolymer (SBT) and a poly (styrene)-Apoly (tert-butyl methacrylate) diblock copolymer (st). The result is a polar, lamellar material with a domain spacing of about 60 nm, Figure 14.10. 

The major trends in ferroelectric photonic and electronic devices are based on development of materials with nanoscale features. Piezoelectric, electrooptic, nonlinear optical properties of fe are largely determined by the arrangement of ferroelectric domains. A promising way is a modification of these basic properties by means of tailoring nanodomain and refractive index superlattices. 

Nanometer scale domain configurations in fe bulk crystals pave the way for a new class of photonic devices. As an example, preliminary calculations show that a uv laser (A = 300 nm) based on second harmonic generation in LiTaC>3 crystal requires a periodic nanodomain superlattice with domain widths of around 700 nm. In addition, the current domain gratings in ferroelectric crystals are suitable only for quasi-phase-matched nonlinear interactions in the forward direction, where the pump and generated beams propagate in the same direction. Sub-micron ferroelectric domain gratings are the basis for a new family of devices based on backward nonlinear quasi-phase-matched optical interactions in which the generated beam travels in a reverse or another non-collinear direction to the incident beam. Non-collinear... 

Ultraviolet Raman spectroscopy has emerged as a powerful technique for characterization of nanoscale materials, in particular, wide-bandgap semiconductors and dielectrics. The advantages of ultraviolet excitation for Raman measurements of ferroelectric thin films and heterostructures, such as reduced penetration depth and enhanced scattering intensity, are discussed. Recent results of application of ultraviolet Raman spectroscopy for studies of the lattice dynamics and phase transitions in nanoscale ferroelectric structures, such as superlattices based on BaTiOs, SrTiOs, and CaTiOs, as well as ultrathin films of BaTiOs and SrTi03 are reviewed. 

Periodic structures containing alternating layers of different ferroelectric and non-ferroelectric materials - superlattices (SLs) - have received increased attention. High-quality ferroelectric SLs with nearly atomically sharp interfaces in various perovskite systems have been synthesized and investigated [14, 24—27, 49-56], and also studied theoretically [44, 57-64]. Properties of such structures are not just a simple combination of the properties known for constituent bulk materials, as they are affected by both mechanical (lattice-mismatch-induced strain) and electrostatic boundary conditions at multiple interfaces located within close proximity to each other. Strain engineering is one of the most appealing ways to... 

UV Raman Spectroscopy of Nanoscale Ferroelectric Thin Films and Superlattices 589... 

Characterization of Ferroelectric Nanostructures BaTiOs/SrliOs Superlattices... 

Raman spectra as a function of temperature are shown in Fig. 21.6b for the C2B4S2 SL. Other superlattices exhibit similar temperature evolution of Raman spectra. These data were used to determine Tc using the same approach as described in the previous section, based on the fact that cubic centrosymmetric perovskite-type crystals have no first-order Raman active modes in the paraelectric phase. The temperature evolution of Raman spectra has indicated that all SLs remain in the tetragonal ferroelectric phase with out-of-plane polarization in the entire temperature range below T. The Tc determination is illustrated in Fig. 21.7 for three of the SLs studied SIBICI, S2B4C2, and S1B3C1. Again, the normalized intensities of the TO2 and TO4 phonon peaks (marked by arrows in Fig. 21.6b) were used. In the three-component SLs studied, a structural asymmetry is introduced by the presence of the three different layers, BaTiOs, SrTiOs, and CaTiOs, in each period. Therefore, the phonon peaks should not disappear from the spectra completely upon transition to the paraelectric phase at T. Raman intensity should rather drop to some small but non-zero value. However, this inversion symmetry breakdown appears to have a small effect in terms of atomic displacement patterns associated with phonons, and this residual above-Tc Raman intensity appears too small to be detected. Therefore, the observed temperature evolution of Raman intensities shows a behavior similar to that of symmetric two-component superlattices. 

Dawber M, Lichtensteiger C, Cantoni M, Veithen M, Ghosez P, Johnston K, Rabe KM, Triscone J-M (2005) Unusual behavior of the ferroelectric polarization in PbTi03/SrTi03 superlattices. Phys Rev Lett 95 177601... 

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