Research projects

Tailored domain structures in LiNb_(1-x)Ta_xO₃ solid solutions (DFG - FOR 5044)

Project TP05 within the DFG Research Unit FOR 5044 - Periodic low-dimensional defect structures in polar oxides
Project management: Dr. Michael Rüsing
Research Teams: FERROIX and iSPEX
Funding: DFG
Period: 10/2020-09/2024
This project aims at the fabrication of tailored ferroelectric domain structures in the model system lithium niobate tantalate via electric field poling. Structured ferroelectric domains are the fundamental building block to realize various applications in ferroelectric materials, such as in nonlinear and quantum optics, ferroelectric surface structuring, piezotronics, or electronics. The reliable and reproducible fabrication of homogeneous ferroelectric domains structures requires a profound understanding of the underlying physical mechanism related to the poling process over the complete range of stoichiometric compositions in the mixed crystals. In the context of the mixed crystals defects and local variations in the Nb/Ta stoichiometry create new challenges for realizing homogeneous domain structures. At the same time the mixed crystals promise novel possibilities to control and structure ferroelectric domains and domain walls via control of stoichiometry.
The fabrication of structured domain grids is realized with electric field poling at room temperature. In combination with standard lithography or local UV illumination this enables the realization of two-dimensional ferroelectric domains in tailored, arbitrary patterns. To achieve a complete picture of the poling process in the model system, investigations are performed on the polar (c-surface), as well as non-polar surfaces (a- and b-surfaces). To analyze the poling process and the influence of various parameters, complementary methods will be used to investigate the fabricated domain structures, such as nonlinear microscopy, piezo-response force microscopy, polarization microscopy and μ-Raman spectroscopy. This allows for the analysis of material constants directly related to the poling process, such as coercive fields or domain propagation speeds, as well as for the analysis of the influences of defects or local stoichiometry on the formation of ferroelectric domains and domain walls. The fabricated domain structures and their domain walls, which can be understood as low dimensional extended defects, will be used to investigate the interaction of domain structures with other material parameters, such as ion and electronic transport, the electromechanical properties, thermal stability or polarons, which are investigated within the research group.

Domain Walls in LNT Mixed Crystals (DFG - FOR 5044)

Project TP06 within the DFG Research Unit FOR 5044 - Periodic low-dimensional defect structures in polar oxides
Project management: Prof. Dr. Lukas M. Eng
Research Teams: FERROIX
Funding: DFG
Period: 10/2020-09/2024
The present project 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.

Local-Scale Fingerprinting of 2D Hybrid Materials (DFG - CRC 1415)

Scientific project B06 of the Collaborative Research Center CRC 1415 - Chemistry of Synthetic Two-Dimensional Materials
Project management: Prof. Lukas M. Eng
Research Team:  iSPEX
Funding: DFG
Period: 07/2020-06/2024
The project will focus on the in-situ vibrational and electronic characterization of 2DMs down to the 1-nm length scale. When applying scanning probe methods, i.e. Kelvin probe force microscopy (KPFM) and coherent anti-Stokes Raman scattering (nano-CARS) at the nanometre-length scale, it will be able to quantify the local electric field environment and local bond strengths, that then allows differentiation between defects and dopants from intact stacked 2DMs by their physico-chemical fingerprints. Such methods allow specifying the surface reactivity and chemical functionality at different edge and basal-plane sites. A time-resolved variant of KPFM constituting a novel approach for investigating electronic transport in 2DMs will be developed.

TiNaII - Time-resolved Nanoscopy in the deep THz regime - part II: Development of ultrafast pump-probe near-field methods for THz-driven processes (BMBF)

Project management: Prof. Lukas M. Eng
Research team:  SNOM
Funded by: BMBF
Period: 07/2019-06/2022
Goal of this project is the implementation of optical near-field microscopy for the THz-frequency regime (abbreviated: T-SNOM) from 10 THz to 100 GHz. This T-SNOM will enable the examination of transient dynamic processes with a time-resolution within the sub-30-femto-second regime and with a spatial resolution of view 10 nanometers only.

NanOMapII -  Nanoscopic Optical Material Probing at FELBE - phase II: Implementation of a New Use-Friendly Near-Field Microscope (BMBF)

Project management: Dr. Susanne C. Kehr
Research team: SNOM
Funding: BMBF
Period: 07/2010-06/2022
Near-field microscopy (SNOM) allows for the spectroscopic surface mapping of solids with a wavelength-independent resolution of about 30 nm. Particularly in the IR- to THz-regime this enables us to study nanoscopic structures on a scale way below the examination wavelength, a dimension that is usually inaccessible due to the wave-nature of light. Here, we utilize two unique near-field microscopes at the free-electron-laser source FELBE for optical examinations of semiconductors and complex oxides on a spatial scale of about 30 nm, within a wavelength regime of 4 to 250 μm, and at temperatures from 4 to 300 K.

Skyrmions in confined spaces: A local-scale SPM analysis (DFG - SPP 2137)

Project within the DFG Priority Program SPP 2137: Skyrmionics: Topological Spin Phenomena in Real-Space for Applications
Project management: Prof. Lukas M. Eng
Research team: SKY
Funding: DFG
Period: 2018-2021
In this project, we apply our dedicated scanning-probe (SPM) methods (i.e. Magnetic Force Microscopy (MFM), Piezoelectric Force Microscopy (PFM), and Kelvin Probe Force Microscopy (KPFM), etc.) to the fundamental analysis of skyrmion (SKY)-hosting thin-films and nanostructures. Additionally, these SPM methods will also be used to either locally or globally induce SKY manipulation in these thin films, both by electrical, magnetic, and mechanical means. We hence profit here from our vast SPM experience having investigated SKYs and skyrmion lattices (SkLs) at the surface of the B20, the Cu2OSeO3, and GaV4S8-(GVS)-family-type bulk materials. The GVS systems are very favorable, since being multiferroic semiconductors, which is in favor for both, exploring the mutual interplay of ferroelectric and magnetic textures (SkLs) in thin-films including the contribution of bulk and interfacial Dzyaloshinskii-Moriya-interaction (DMI), as well as inducing changes in the magnetic SKY texture solely by electrical means. A third way of field stimuli (beyond electrical and magnetic) that is explored here is mechanical stress, as exerted onto the thin film either through epitaxial misfit, or when being mounted onto a piezoelectric substrate carrier for linear contraction / expansion. These manipulations will all be performed in-situ, hence providing additional degrees of freedom when recording the extended phase diagrams of such films. The central focus in this project lies on thin-film samples of the GVS-family that will be prepared with reduced dimensions, i.e. samples confined to 2-dimensional (2D), 1D, and 0D nanostructures. This is realized by applying 4 different preparation routes, i.e. two top-down and two bottom-up strategies: thinned-down bulk thin films will be realized either through Focused-Ion-Beam -Milling or Chemical-Mechanical-Polishing with the goal to reach ~50-nm-thick, free-standing thin films. The bottom-up approaches include the GVS-sample-growth by Pulsed Laser Deposition and using single grains of polycrystalline GVS as 0D nanostructures. A specialty introduced here are wedge-like sample-cuts, that allow to in-situ investigate the different competing interactions by SPM at buried interfaces. In addition, we focus our SPM research also on a second class of thin-film SKY materials, i.e. SrIrO3 / SrRuO3 multilayers. In contrast to the GVS-type samples, interfacial DMI between individual (mono)layers stands in our focus here. Again, the full analysis by the SPM methods as well as the local manipulation of SKYs will be realized. -- Note that investigating these two prominent and novel sample systems, i.e. the GVS-type and oxide thin-films, will uniquely be possible through applying our elaborated SPM methods; no other (real or reciprocal-space) technique is known to date that might deliver as much insight into these nanoscale systems. Hence, our methodologies might equally be applied to other prospective materials within this SPP.

TOPELEC - The Topology of Conductive Ferroelectric Domain Walls (DFG)

Project management: Prof. Lukas M. Eng
Research team: FERROIX
Funding: DFG
Period: 2018-2021
Domain walls (DWs) in ferroelectrics have become a topic of major interest over the last 10 years because of their exceptional dielectric, optical, magnetic, electronic and mechanical properties. The DWs represent nanometric interfaces that extend across the full bulk system and display an ultra-high electronic conductivity, reaching several 10 µA for a single DW in bulk single crystalline LiNbO3 (LNO). These remarkable properties propel ferroelectric DWs as one of the most promising functional nanostructure for modern-type and reconfigurable applications in nanoelectronic devices. According to recent studies, ferroelectric DWs could contain novel topological structures in their dielectric polarization that are much more complex than the Ising-type configuration, which is the traditionally expected DW type in uniaxial ferroelectrics. The local sym¬metry breaking at the DWs is particularly important as it can promote exotic polar topological structures, similar to those observed in magnetic systems. Exploring the detailed ferroic structure of ferroelectric DWs is a prerequisite for the understanding and control of DW properties. The goal of this joint research project is to elucidate the local symmetry and topology at such DW regions and to investigate and quantify their interrelated physical and optical properties when being rendered highly conductive.The two teams allied within this joint German-French project have shown that DWs can be elegantly tuned for transporting high electronic currents along the two-dimensional DW. In LNO, the free charge carrier density within such a wall can be steered by simply varying the DW’s inclination with respect to the polar axes. We then expect this DW to convert from its pure Ising-type configuration into a Bloch- or Néel-type state, depending on both the material under investigation, a possible sample doping, or an electrical bias field applied across the crystal. In addition, we have developed sophisticated local probe and nonlinear optical techniques that are able to quantify and three-dimensionally map the presence of such non-Ising configurations. We accordingly intend to engineer chiral DWs in the LNO single crystals family, both with and without Mg doping, and monitor their behavior in real time and real space using, for instance, second-harmonic generation polarimetry. This project is expected to deliver groundbreaking insight on the origin and build-up of such non-Ising, often chiral polarization structures at DWs, as is necessary for the profound understanding and tuning of future optoelectronic nano-devices based on ferroelectric DWs.

Scanning probe microscopy: Magnetic structures and topological states (DFG  - CRC 1143)

Subproject C05 of CRC 1143: Correlated Magnetism: From Frustration To Topology 
Project management: Prof. Lukas M. Eng
Research team: SKY
Funded by: DFG
Period: 01/2015-12/2022
This project investigates the electronic and magnetic real-space structure of frustrated materials down to the atomic length scale using scanning probe techniques. Scanning-tunneling microscopy and spectroscopy (STM/STS) will be used for extracting local electronic densities of states and quasiparticle interference patterns. Magnetic force microscopy (MFM) and Kelvin-probe force microscopy (KPFM) will enable us to study magnetic textures and magnetoelectric effects. Novel quantitative methods at low temperature will be employed, in particular for MFM. Materials at focus are quantum spin liquid candidates, skyrmion systems, and Weyl semimetals.

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Finished projects