Control of nonlinear self-localisation phenomena of charge carriers with strong coupling in LiNb1-xTaxO3 (LNT) solid solutions

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

The present proposal describes a subproject for the continuation of the research group 5044 'Periodic low-dimensional defect structures in polar oxides', which is dedicated to the correlation of defect structure, electron and ion transport as well as electromechanical properties using the model system lithium niobate-lithium tantalate (LiNb1-xTaxO3, LNT). The main focus is on the interaction of incoherent transport phenomena of strongly coupled charge carriers with (i) the intrinsic defect structure of the solid solution system including the electronic structure, (ii) extended defect structures and their dielectric/electronic properties, in particular with domain walls, interfaces & mechanical strain in thin films and (iii) compensating coherent transport properties and ionic diffusion. The project thus aims to address the question of the microscopic origin of polaron-based, volume photovoltaic net current densities in polar oxide material systems and their control via intrinsic and extended defect structures. Based on the findings of the first funding phase, LNT represents an excellent model system for this, as it exhibits novel polaronic (mixed) states in a multi-component defect landscape, enables the control of the self-localisation of charge carriers, the incoherent transport and the concentration of polaronic states by composition and opens up experimental access to the polaron-based, volume photovoltaic net current density through optically induced refractive index inhomogeneities. From a methodological point of view, an extension of nonlinear optical spectroscopy methods is required, which allows a correlation of the relaxation properties of transient absorption and refractive index with pulse-induced, (incoherent) photoelectric and (coherent) electric/ionic properties (TP2, TP3, TP7). Specifically, (a) the time range of time-resolved spectroscopy across scales is to be extended, (b) the method of time-resolved multi-wave mixing based on pulse-induced index changes is to be applied and correlated with (c) photoelectric studies under repeated fs-pulse train illumination. The experimental results on LNT crystals of different composition and doping (SP1), with/without pretreatment (SP3), on interfaces (SP5) and layered systems (SP9) are to be reproduced, taking into account the nanoscopic domain structure (SP6), ionic transport (SP2) and atomistic model calculations (SP8) with numerical modelling of the transport and recombination kinetics of self-localised charge carriers. The subproject is expected to develop a generalised, defect-structure related model to describe the volume photovoltaic effect on the basis of self-localised charge carriers in mechanically strained, polar oxide systems.

• DFG Programme: Research Units
• Subproject of: FOR 5044: Periodic low-dimensional defect structures in polar oxides
• International Connection: Italy
• Cooperation Partner: Professor Marco Bazzan