New developments in artificially layered ferroelectric oxide superlattices
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Introduction The ability to control the growth of thin films at the atomic scale afforded by modern physical deposition techniques has enabled the possibility of making new types of materials by alternating the thin-film growth of two or more materials on top of each other and repeating the process a certain number of times.1 When the layers are on the order of a few unit cells in thickness, these structures are classed as superlattices, which refers to the fact that the predominant periodicity in the structure is the artificially produced one. Three motivations to produce these artificial materials are: (1) The possibility to modulate the properties of the two parent compounds by controlling the layer thickness ratio between the two building blocks, (2) to produce a material that has an enhancement of one or more of the initial properties of the parent compounds, and (3) to induce new properties that are completely absent in the parent compounds. These possibilities have been explored for ferroelectricity (the property where a material develops a spontaneous electrical polarization, the direction of which can be changed by application of an electrical field) mostly in superlattices made from two building blocks, also called bi-color superlattices (see Figure 1; the term “color” refers to a distinct parent material
within the artificial structure), which are the focus of this article. While, based on principles of electrostatics and mechanical strain, we can expect a modulation of the initial properties of the two bulk parent compounds for large layer thicknesses, present research in superlattices is especially motivated by the appearance of exotic interface effects that would induce new phenomena not present otherwise. In the field of ferroelectric perovskites (FE), several bi-color systems have been studied: FE/dielectric, FE/FE, FE/metal, and FE/incipient-FE (incipient ferroelectrics are materials that are close to being ferroelectric, but the onset of the ferroelectric order is suppressed by an external parameter). Interestingly, these systems can be roughly understood through simple electrostatic models that consider two dielectrics or a dielectric and a metal coupled by electrostatics and strain. However, these models are valid only for large thicknesses of the layers, while for small layer thicknesses, the interface effects can dominate the physics of the superlattices, and new models, explicitly taking into account these new effects, have to be built in order to understand the unexpected observed phenomena. In this article, we discuss interfaces both with and without electronic screening. The first class of interface contains technologically relevant ultrathin ferroelectric capacitors and the more recently studied
Matthew Dawber, Department of Physics and Astronomy, Stony Brook University, NY; [email protected] Eric Bousquet, University of Liège, Department of Physics, Belgium; [email protected] DOI: 10.1557/mrs.2013.263
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MRS BULLETIN • VOLUME 38 • DECEMBER 2013 • www.mrs.org/bulletin
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