Domain Walls in LNT Mixed Crystals

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The proposal is part of the research group "Periodic low-dimensional defect structures in polar oxides", which is dedicated to the correlation of defect structure, electron and ion transport, and electromechanical properties using the model system lithium niobate-lithium tantalate (LiNb1-xTaxO3, LNT). The present proposal specifically focuses on domain walls in these polar LNT (mixed) crystals, that are present in every such ferroic system due to energy minimization. The thermodynamic equilibrium naturally depends on a manifold of parameters, including size, dimensionality, chemical composition, and others more. The specifically targeted chemical doping of the LNT systems (i.e with MgO, Hf, Fe) will vary this balance, and lead to a broad variety of domain and domain wall distributions that we will explore here. Notably, we expect to discover domain walls in these LNT systems with unprecedented and novel properties, as are tunable domain wall conductivities, variable optical refractive indices, or ferroelectric topologies. Firstly, we will explore the dielectric and electronic properties of domains and domain walls by applying macroscopic hysteresis measurements that allow deducing their integral responses, such as their coercive fields, the spontaneous polarization, and the leakage currents across the various LNT mixed crystals. A clear focus, however, lies on the domain wall electronic properties, in quantifying their AC and DC transport characteristics at both ultra-high and ultra-low temperatures. Hall-transport measurements in a variable magnetic field will allow to specify charge carrier type and densities, as well as their mobilities within a single domain wall. Complementary, we will apply a novel AC transport technique to these domain walls that provides direct access to the defect characteristics in these 2-dimensional transport channels through higher harmonic analysis. Secondly, a set of dedicated, non-invasive optical methods will complement the above electronic investigations of the LNT mixed crystals. Applying µ-Raman-spectroscopy, Second-Harmonic-Microscopy/Polarimetry and Fluorescence microscopy analysis to domains and domain walls, allows quantifying their chemical, structural and dielectric properties. Specifically at domain walls, we expect the local electric fields, and hence the local tensor properties to vary dramatically, which can be taken as a direct hint towards the presence of locally non-trivial topologies.