Theses

Ferroix group:

Cherenkov second harmonic microscopy characterization of multiaxial ferroelectrics

Throughout the last years, Cherenkov second harmonic microscopy (CSHG) has been successfully applied in our group for the three-dimensional position tracking of domain walls (DWs) in uniaxial ferroelectrics such as lithium niobate (LiNbO₃). Within this project, this approach shall be extended to multiaxial materials such as BaTiO₃ and KNbO₃. Thereby, we do not only aim at obtaining the DW position information, but moreover at a deeper understanding of the process of SHG at DWs by analyzing the intensity and polarization dependencies.
Research team: FERROIX
Contact: Prof. Lukas M. Eng
Type: BA, MA

Domain wall conductivity measurements of stochastic domain patterns in lithium niobate

The transport behavior of ferroelectric domain walls (DWs) in LiNbO₃ (LNO) single crystals has been extensively studied in our group for the case of single domains that were specifically written for this purpose. However, it is likewise possible to generate a stochastic multi-domain structure in such crystals by applying a homogeneous electric field to a large (~mm²) sample area. Here, we wish to quantify the variance in conductivity of these as-grown DWs by evaporating a microstructured electrode and measuring I-V-curves between the individual electrode fields.
Research team: FERROIX
Contact: Prof. Lukas Eng
Type: BA, MA

SKY group:

Skyrmions in lacunar spinels

Magnetic force microscopy is ideally suited to explore magnetic phase diagrams of non-collinear magnetic states in real space. Materials of the lacunar spinel family are multiferroic – i.e. they host at the same time ferroelectric and non-collinear magnetic states. We will use the lacunar spinel GaV₄S₈ (GVS) and explore the magnetic phase diagram at temeratures below 10 K. For this purpose, a special sample holder hosting a calibrated temperature sensor has to be build.
Research team: SKY
Contact: Dr. Peter Milde
Type: MA

Skyrmions in electric fields

Cu₂OSeO₃ is an insulating helimagnet hosting a variety of non-collinear spin textures such as skyrmions. In reciprocal space measurements as well as by magnetometry an influence of external electric field on the stability and extend of the skyrmion phase has been observed. We want to use our low-temperature magnetic force microscope to explore the magnetoelectric coupling in real space observing expected changes in the skyrmion texture. Being able to switch the skyrmion phase „on“ and „off“ by electric field also allows to study phase transitions including the topologically non-trivial unwinding of the skyrmion texture in detail and with high precision.
Research team: SKY
Contact: Dr. Peter Milde
Type: MA

Superheterodyne interferometer

In our low-temperature scanning force microscopy the motion of a cantilever holding the probe tip is measured via a homodyne interferometer. We work on an upgrade to a superheterodyne detection promising substantial improvements in stability and sensitivity. In this work the high frequency electronics for signal processing has to be designed and built. Hence, this project is ideally suited for students with background and interest in electronics design and handicraft work.
Research team: SKY
Contact: Dr. Peter Milde
Type: BA, MA

SNOM group:

High-resolution probing in the mid-IR to THz spectral regime

Scattering scanning near-field optical microscopy (SNOM) enables the nanoscopic optical examination of sample surfaces with a wavelength-independent resolution of few 10 nanometers. Particularly in the IR-to-THz regime, materials show a strong response to electro-magnetic radiation due to excitation of lattice vibrations, chemical bonds, and charge carrier oscillations. Probing such responses on the local scale enables unique scientific characterization of modern materials and nanostructured devices, which shall be studied in this work.
This thesis includes working with optics in the visible, infrared, and THz regime with unique light sources (state-of-the-art table top lasers and free-electron laser at HZDR) and understanding fundamental optical material response on the nanoscopic scale.
This work is particularly suitable to be followed by a PhD thesis. 
[Tags: IR-/THz-optics, lasers, free-electron laser, SNOM, scanning probe microscopy, nanoooptics]
Research team: SNOM
Contact: Susanne C. Kehr
Suitable for: MA/PhD

Nanoscopic infrared characterization of structured van-der-Waals-materials

The infrared optical response of matter is determined by fundamental material parameters such as lattice vibrations, chemical bonds, and charge carrier oscillations. Here, 2-dimensional (2D)- and van der Waals (vdW) materials characteristically show a strongly anisotropic response that even reaches hyperbolic behavior, which lead to intriguing effects such as unidirection propagation and high confinement of THz fields that are urgently wanted for nanotechnological applications. In this work, the roles of geometrical and excitation parameters shall be studied for a range of materials such as MoO₃, GeS, and hBN and utilized to tailor the materials’ response and to design optical devices for THz applications.
This thesis includes working with optics in the infrared to THz regime including near-field (high-resolution) probing along with various scanning probe techniques, sample preparation and modelling of material responses.
This work is particularly suitable to be followed by a PhD thesis.
[Tags: Nanooptics, nanotechnology, SNOM, AFM, Laser, IR-Optics]
Research team: SNOM
Contact: Susanne C. Kehr
Suitable for: MA/PhD

Nanometer-scale investigations at low temperatures

In many material systems, properties such as optical response, conductivity and/or work function change drastically at low temperatures. Combining different scanning probe techniques including atomic force microscopy (AFM), conductive-AFM (c-AFM), Kelvin-probe force microscopy (KPFM) and scattering scanning near-field optical microscopy (s-SNOM), this response can be measured on the local scale with a lateral resolution on the nanometer length scale, which enables the fundamental understanding of the material properties. With our setup, all of these measurements can be performed from room temperature down to T=10 K allowing for the local investigation of e.g. phase transitions in complex oxides. 
This thesis includes working with different scanning probe techniques at cryogenic temperatures, examination of various material system as well as analysis of the results.
This work is particularly suitable to be followed by a PhD thesis.
[Tags: Nanooptics, nanotechnology, SNOM, AFM, Laser, IR-Optics, low-temperatures]
Research team: SNOM
Contact: Susanne C. Kehr
Suitable for: MA/PhD

Infrared-spectroscopic examination of 2D-/van-der-Waals materials

In the infrared regime, materials show a characteristic response to electromagnetic radiation due to the excitation of e.g. lattice vibrations, chemical bonds, and charge carrier oscillations. By probing this response via different spectroscopic techniques, materials can be identified as well as regimes of interest, e.g. spectral locations of a materials Reststrahlenband and of intriguing hyperbolic behavior in van der Waals materials. Particularly by comparing the polarization-sensitive response of the complementary techniques of Raman spectroscopy and Fourier-Transform Infrared Spectroscopy (FTIR), these examinations give a full picture of the optical material properties of a sample.
This thesis includes working with optics in the visible and infrared regime, understanding fundamental optical material response, comparing different methods of spectroscopy and fitting them to different models of sample excitation.
This work is particularly suitable to be followed by a PhD thesis.
[Tags: IR, Fourier-Transform Infrared Spectroscopy (FTIR), Raman-Spectroscopy]
Research team: SNOM
Contact: Susanne C. Kehr
Suitable for: MA/PhD