Domain Walls in LNT Mixed Crystals

• Applicant: Professor Dr. Lukas M. Eng
• Subject Area: Synthesis and Properties of Functional Materials ; Experimental Condensed Matter Physics
• Term: since 2024
• Project identifier: Deutsche Forschungsgemeinschaft (DFG) - Project number 426703838

The nanoscopic picture of Lithium Niobate-Tantalate (LNT) solid solutions, i.e. crystalline mixtures of Lithium Niobate and Lithium Tantalate to form LNT, comes with a couple of surprises, hence demanding for a much deeper analysis of how the overall and macroscopically-determined LNT structure correlates to the local dielectric polarization, the local conductivity, and the apparent domain distribution. Defects and local chemical heterogeneity hence dramatically influence the local electric field distribution inside LNT single crystals, resulting in both a finite electronic and ionic conductivity. As a consequence, electrically poling LNT and thus inducing domains and domain walls (DWs) becomes very challenging. – The first goal (a) of this TP6 within the FOR5044 hence is to shed light onto this correlation at the 10-nanometer length scale, by applying a set of dedicated scanning-force-microscopy (SFM)-based tools to locally quantify the possible origins and behaviors of both the local conductivity and the dielectric function. Optical, electronic, phononic properties and their anisotropies will be recorded to provide correlated local information from one and the same spot. These local fingerprints then should converge and be able to equally interpret the macroscopic, integral findings. – Our second goal (b) focuses on applying both the above SFM-tools as well as macroscopic methods (Hall-transport, Second-Harmonic-Generation microscopy, Raman spectroscopy) to DW structures prepared in a three-fold set of LNT samples: (iii) Naturally-grown DWs in LNT; (ii) as-poled DWs into LNT-single-crystals, including the use of finger electrodes; and (iii) DWs induced into wafer-bonded LNT-systems; the latter technique, direct wafer-bonding, provides a completely novel approach of how to engineer charged DWs and interfaces into ferroelectrics, which becomes especially interesting when using purposely doped LNT-systems (Mg, Hf, Zn) or LNT-crystals with an altered Ta- or Li-concentration. Besides the local-scale SFM measurements, we will especially investigate the domain wall conductivity (DWC) by applying external stimuli while measuring Hall-transport in single DWs; these are: (i) photo-induced generation of electron-hole-pairs at the DW; (ii) applying mechanical (tensile, compressive) strain using a piezoelectric strain-cell; and (iii) recording the Hall-signatures at variable temperatures between ambient and 4 K. – Lastly, (c), by making use of the broad DWC tunability from above, we will propose three novel device concepts especially applicable with LNT: a DW-pn-diode, a DW-based transistor device, and a photovoltaic cell. All three applications will be thoroughly investigated and characterized with both our nanoscale and macroscopic tools mentioned in (a) and (b), and evaluated for their best performance with respect to the three-fold set of how DWs can be engineered into LNT samples.

• DFG Programme: Research Units
• Subproject of: FOR 5044: Periodic low-dimensional defect structures in polar oxides