NSF-Europe: Atomic Defects and Their Dramatic Influence on Nanoscale Electrical, Elastic, and Optical Properties in Ferroelectrics

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The central goal of this proposal is to probe the static and dynamic interactions between domain walls and atomic defects structure, and their influence on the nanoscale electrical, elastic and optical properties of ferroelectrics. Ferroelectrics have built-in electrical polarization in their crystal structure that can be switched by an external field. In classic ferroelectrics lithium niobate, LiNbO3 and lithium tantalate, LiTaO3, the PIs have recently discovered dramatic order-of-magnitude changes in the macroscale properties (such as coercive fields, internal fields, domain structure, lattice strain and optical properties) with small amounts of non-stoichiometry in the crystals. The nanoscale local structure of a single ferroelectric domain wall also exhibits wide regions of strain, electric fields and optical birefringence (over micrometers) that are contrary to the theoretical expectations from an atomically sharp antiparallel wall (nanometer). Experimental evidence suggests point defect complexes as the main reason for this discrepancy. These discoveries open up a host of fundamental questions about the interaction between atomic defects and ferroelectric lattice from nano-to-macro scales. A focused multifaceted approach is used combining experimental and theoretical tools. This approach consists of experimentally probing defects using optical and magnetic spectroscopy of "designer" probe ions, combined with probing the local structure of domain walls such as strains, local electric fields, polarization gradient, and nanoscale optical properties using X-ray synchrotron imaging, and near-field optical and scanning probe microscopies. These experimental studies will be closely coupled with atomistic modeling of point defect complexes, domain walls, and their interactions using electronic-structure and atomic-level approaches. There are many applications of ferroelectric materials, such as micro-drills used in eye surgery, high speed optical modulators for a fast internet, underwater pressure sensors in submarines, and bar-code readers at grocery checkout counters. In these applications, the material is manipulated by adding small amounts of dopants that can dramatically alter the macroscopic properties. This largely empirical body of knowledge is exploited in industry today but lacks a fundamental grounding in precisely how these point defects function on a nanoscale and how these interactions scale up to influence macroscale properties. This work aims at a level of understanding that would enable science-based strategies to "design" materials with desired properties. This NSF project is co-funded by the Division of Materials Research (Ceramics) and the International Office (Western Europe) as a Cooperative Activity in Materials Research between the NSF and Europe (NSF 02-135). This project is being carried out in collaboration with the Applied Physics and Optical Communications groups (Profs. Sohler, Zrenner, and Noe) at the University of Paderborn, Germany and the Applied Optics Group (Prof. Buse) at the University of Bonn, Germany.