@article{64864,
  abstract     = {{Probing novel properties, arising from twisted interfaces, has traditionally relied on the stacking of exfoliated two-dimensional materials and the spontaneous formation of van der Waals bonds. So far, investigations involving intimate covalent or ionic bonds have not been a focus. Yet, we show here that an established technique, involving thermocompressional wafer bonding, works well for creating twisted non-van der Waals interfaces. We have successfully bonded z-cut lithium niobate single crystals to create ferroelectric oxide interfaces with strong polar discontinuities and have mapped the associated emergent interfacial conductivity. In some instances, a dramatic change in microstructure occurs, involving local dipolar switching. A twist-induced collapse in the capability of the system to effec8tively screen interfacial bound charge is implied. Importantly, this only occurs around specific moiré twist angles with sparse coincident lattices and associated short-range aperiodicity. In quasicrystals, aperiodicity is known to induce pseudo-bandgaps and we suspect a similar phenomenon here.}},
  author       = {{Rogers, Andrew and Holsgrove, Kristina and Schäfer, Nils A. and Koppitz, Boris and McCluskey, Conor J. and Yedama, Shivani and Lynch, Ronan and Sloan, Keelan and Porter, Barry and Sykes, Adam and Catalan Daniels, Alex and Silva, Romualdo S. and Bruno, Flavio Y. and Seddon, Sam D. and Lu, Haidong and Rüsing, Michael and Fink, Christa and Fahler-Muenzer, Philipp and Fearn, Sarah and Heutz, Sandrine E. M. and Hadjimichael, Marios and Ramasse, Quentin M. and Alexe, Marin and Kumar, Amit and McQuaid, Raymond G. P. and Gruverman, Alexei and Sanna, Simone and Eng, Lukas M. and Gregg, J. Marty}},
  issn         = {{2041-1723}},
  journal      = {{Nature Communications}},
  number       = {{1}},
  publisher    = {{Springer Science and Business Media LLC}},
  title        = {{{Polar discontinuities, emergent conductivity, and critical twist-angle-dependent behaviour at wafer-bonded ferroelectric interfaces}}},
  doi          = {{10.1038/s41467-026-68553-7}},
  volume       = {{17}},
  year         = {{2026}},
}

@article{59276,
  abstract     = {{Stress plays a crucial role in thin films and layered systems, and thus significantly influences the material's electrical, mechanical and (nonlinear) optical responses. Despite lithium niobate's wide applicability as a nonlinear optical material, the impact of mechanical stress on its nonlinear optical properties is not well characterized. In this work, we systematically study both experimentally and theoretically, the nonlinear optical responses of thin film lithium niobate (TFLN) single crystals. Compressive and tensile stress is applied in our experiment using a piezodriven strain cell. We then record the second-harmonic-generated (SHG) response in back-reflection geometry, and compare these results to theoretical modeling using density functional theory (DFT). Both methods consistently reveal that uniaxial stress induces changes of the nonlinear optical susceptibility of certain tensor elements on the order of up to 1 pm/(V GPa). The exact value depends on the tensor element that is addressed in our SHG analysis, on the crystal orientation, and also whether using compressive or tensile stresses. Furthermore, a lowering of the crystal symmetry when applying stress along the <a:math xmlns:a="http://www.w3.org/1998/Math/MathML"><a:mi>x</a:mi></a:math> or <b:math xmlns:b="http://www.w3.org/1998/Math/MathML"><b:mi>y</b:mi></b:math> crystallographic axes is observed by the appearance of new nonlinear optical tensor elements within the strained crystals.}},
  author       = {{Pionteck, Mike N. and Roeper, Matthias and Koppitz, Boris and Seddon, Samuel D. and Rüsing, Michael and Padberg, Laura and Eigner, Christof and Silberhorn, Christine and Sanna, Simone and Eng, Lukas M.}},
  issn         = {{2469-9950}},
  journal      = {{Physical Review B}},
  number       = {{6}},
  publisher    = {{American Physical Society (APS)}},
  title        = {{{Second-order nonlinear piezo-optic properties of single crystal lithium niobate thin films}}},
  doi          = {{10.1103/physrevb.111.064109}},
  volume       = {{111}},
  year         = {{2025}},
}

@article{61338,
  abstract     = {{Conductive ferroelectric domain walls (DWs) represent a promising topical system for the development of nanoelectronic components and device sensors to be operational at elevated temperatures. DWs show very different properties as compared to their hosting bulk crystal, in particular with respect to the high local electrical conductivity. The objective of this work is to demonstrate DW conductivity up to temperatures as high as 400 °C which extends previous studies significantly. Experimental investigation of the DW conductivity of charged, inclined DWs is performed using 5 mol % MgO-doped lithium niobate single crystals. Current–voltage (  ) curves are determined by DC electrometer measurements and impedance spectroscopy and found to be identical. Moreover, impedance spectroscopy enables to recognize artifacts such as damaged electrodes. Temperature dependent measurements over repeated heating cycles reveal two distinct thermal activation energies for a given DW, with the higher of the activation energies only measured at higher temperatures. Depending on the specific sample, the higher activation energy is found above 160 °C to 230 °C. This suggests, in turn, that more than one type of defect/polaron is involved, and that the dominant transport mechanism changes with increasing temperature. First principles atomistic modeling suggests that the conductivity of inclined domain walls cannot be solely explained by the formation of a 2D carrier gas and must be supported by hopping processes. This holds true even at temperatures as high as 400 °C. Our investigations underline the potential to extend DW current based nanoelectronic and sensor applications even into the so-far unexplored temperature range up to 400 °C.}},
  author       = {{Wulfmeier, Hendrik and Yakhnevych, Uliana and Boekhoff, Cornelius and Diima, Allan and Kunzner, Marlo and Verhoff, Leonard M. and Paul, Jonas and Ratzenberger, Julius and Beyreuther, Elke and Gössel, Joshua and Kiseleva, Iuliia and Rüsing, Michael and Sanna, Simone and Eng, Lukas M. and Fritze, Holger}},
  issn         = {{0167-2738}},
  journal      = {{Solid State Ionics}},
  publisher    = {{Elsevier BV}},
  title        = {{{Demonstration of domain wall current in MgO-doped lithium niobate single crystals up to 400°C}}},
  doi          = {{10.1016/j.ssi.2025.116949}},
  volume       = {{429}},
  year         = {{2025}},
}

@article{61337,
  abstract     = {{<jats:p>Lithium niobate–tantalate mixed (LNT) crystals promise improved performance and new applications for optical, piezomechanical, or electrical devices when compared to the end composition compounds lithium niobate and lithium tantalate. The macroscopic properties of ferroelectrics highly depend on the structure of the underlying ferroelectric domains, which within mixed crystals can interact with the local changes in chemical compositions. In this work, we demonstrate how ferroelectric domain walls can unambiguously be identified and distinguished from local changes in composition by correlating piezoresponse force microscopy with second harmonic generation microscopy, using the Cherenkov contrast, reference crystal contrast, and negative phase mismatching contrast. We demonstrate how measuring the associated intensity change when approaching negative phase mismatching can be used to deduce the local tantalum concentration fast and over a large sample area. Based on these results, we study the natural domain structures that appear from Czochralski-grown, multi-domain LNT solid solution crystals. The developed results and methods serve as the central foundation to poling these mixed crystal systems and are key for their integration and applications.</jats:p>}},
  author       = {{Koppitz, Boris and Saxena, Tanya and Bernhardt, Felix and Ganschow, Steffen and Sanna, Simone and Rüsing, Michael and Eng, Lukas M.}},
  issn         = {{0021-8979}},
  journal      = {{Journal of Applied Physics}},
  number       = {{3}},
  publisher    = {{AIP Publishing}},
  title        = {{{Second harmonic generation contrasts of ferroelectric domain structures and composition in lithium niobate–tantalate mixed crystals}}},
  doi          = {{10.1063/5.0276183}},
  volume       = {{138}},
  year         = {{2025}},
}

@article{62749,
  abstract     = {{Coherent Raman scattering techniques as coherent anti-Stokes Raman scattering (CARS), offer significant advantages in terms of pixel dwell times and speed as compared to spontaneous Raman scattering for investigations of crystalline materials. However, the spectral information in CARS is often hampered by the presence of a nonresonant contribution to the scattering process that shifts and distorts the Raman peaks. In this work, we apply a method to obtain nonresonant background-free spectra based on time-delayed, broadband CARS (TD-BCARS) using an intrapulse excitation scheme. In particular, this method can measure the phononic dephasing times across the full phonon spectrum at once. We test the methodology on amorphous SiO2 (glass), which is used to characterize the setup-specific and material-independent response times, and then apply TD-BCARS to the analysis of single crystals of diamond and ferroelectrics of potassium titanyl phosphate (KTP) and potassium titanyl arsenate (KTA). For diamond, we determine a dephasing time of 𝜏=7.81 ps for the single 𝑠⁢𝑝3 peak.}},
  author       = {{Hempel, F. and Rüsing, Michael and Vernuccio, F. and Spychala, K. J. and Buschbeck, R. and Cerullo, G. and Polli, D. and Eng, L. M.}},
  issn         = {{2469-9950}},
  journal      = {{Physical Review B}},
  number       = {{22}},
  publisher    = {{American Physical Society (APS)}},
  title        = {{{Phonon dephasing times determined with time-delayed broadband coherent anti-Stokes Raman scattering}}},
  doi          = {{10.1103/1ctr-csjy}},
  volume       = {{112}},
  year         = {{2025}},
}

@article{62713,
  abstract     = {{Periodically poled thin-film lithium niobate (TFLN) crystals are the fundamental building block for highly-efficient quantum light sources and frequency converters. The efficiency of these devices is strongly dependent on the interaction length between the light and the nonlinear material, scaling quadratically with this parameter. Nevertheless, the fabrication of long, continuously poled areas in TFLN remains challenging, the length of continuously poled areas rarely exceeds 10 mm. In this work, we demonstrate a significant progress in this field achieving the periodic poling of continuous poled areas of 70 mm length with a 3 μm poling period and a close to 50 % duty cycle. We compare two poling electrode design approaches to fabricate long, continuous poled areas. The first approach involves the poling of a single, continuous 70 mm long electrode. The second utilize a segmented approach including the poling of more than 20 individual sections forming together a 70 mm long poling area with no stitching errors. While the continuous electrode allows for faster fabrication, the segmented approach allows to individually optimize the poling resulting in less duty cycle variation. A detailed analysis of the periodic poling results reveals that the results of both are consistent with previously reported poling outcomes for shorter devices. Thus, we demonstrate wafer-scale periodic poling exceeding chiplet-size without any loss in the periodic poling quality. Our work presents a key step towards highly-efficient, narrow-bandwidth and low-pump power nonlinear optical devices.}},
  author       = {{Bollmers, Laura and Spiegelberg, Noah and Rüsing, Michael and Eigner, Christof and Padberg, Laura and Silberhorn, Christine}},
  issn         = {{2192-8606}},
  journal      = {{Nanophotonics}},
  pages        = {{4761}},
  publisher    = {{Walter de Gruyter GmbH}},
  title        = {{{Segmented finger electrodes to optimize ultra-long continuous wafer-scale periodic poling in thin-film lithium niobate}}},
  doi          = {{10.1515/nanoph-2025-0461}},
  volume       = {{14}},
  year         = {{2025}},
}

@article{60566,
  author       = {{Bocchini, Adriana and Rüsing, Michael and Bollmers, Laura and Lengeling, Sebastian and Mues, Philipp and Padberg, Laura and Gerstmann, Uwe and Silberhorn, Christine and Eigner, Christof and Schmidt, Wolf Gero}},
  issn         = {{2475-9953}},
  journal      = {{Physical Review Materials}},
  number       = {{7}},
  publisher    = {{American Physical Society (APS)}},
  title        = {{{Mg dopants in lithium niobate: Defect models and impact on domain inversion}}},
  doi          = {{10.1103/5wz1-bjyr}},
  volume       = {{9}},
  year         = {{2025}},
}

@article{51156,
  abstract     = {{Ferroelectric domain wall (DW) conductivity (DWC) can be attributed to two separate mechanisms: (a) the injection/ejection of charge carriers across the Schottky barrier formed at the (metal-)electrode-DW junction and (b) the transport of those charge carriers along the DW. Current-voltage (I-U) characteristics, recorded at variable temperatures from LiNbO3 (LNO) DWs, are clearly able to differentiate between these two contributions. Practically, they allow us to directly quantify the physical parameters relevant to the two mechanisms (a) and (b) mentioned above. These are, for example, the resistance of the DW, the saturation current, the ideality factor, and the Schottky barrier height of the electrode-DW junction. Furthermore, the activation energies needed to initiate the thermally activated electronic transport along the DWs can be extracted. In addition, we show that electronic transport along LNO DWs can be elegantly viewed and interpreted in an adapted semiconductor picture based on a double-diode, double-resistor equivalent-circuit model, the R2D2 model. Finally, our R2D2 model was checked for its universality by successfully fitting the I-U curves of not only z-cut LNO bulk DWs, but equally of z-cut thin-film LNO DWs, and of x-cut thin-film DWs as reported in literature.}},
  author       = {{Zahn, Manuel and Beyreuther, Elke and Kiseleva, Iuliia and Lotfy, Ahmed Samir and McCluskey, Conor J. and Maguire, Jesi R. and Suna, Ahmet and Rüsing, Michael and Gregg, J. Marty and Eng, Lukas M.}},
  issn         = {{2331-7019}},
  journal      = {{Physical Review Applied}},
  keywords     = {{General Physics and Astronomy}},
  number       = {{2}},
  publisher    = {{American Physical Society (APS)}},
  title        = {{{Equivalent-circuit model that quantitatively describes domain-wall conductivity in ferroelectric lithium }}},
  doi          = {{10.1103/physrevapplied.21.024007}},
  volume       = {{21}},
  year         = {{2024}},
}

@article{55085,
  abstract     = {{The lithium niobate–lithium tantalate solid solution’s phase diagram was investigated using experimental data from differential thermal analysis (DTA) and crystal growth. We used XRF analysis to determine the elemental composition of the crystals. The Neumann–Kopp rule provided essential data for the end members lithium niobate (LN) and lithium tantalate (LT). The heats of fusion of the end members, given by DTA measurements, are 103 kJ/mol at 1531 K for LN and 289 kJ/mol at 1913 K for LT. These values were used as input parameters to generate the data. This data served as the basis for calculating a phase diagram for LN-LT solid solutions. Finally, based on the experimental data and a thermodynamic solution model, the Calphad Factsage module optimized the phase diagram. We also generated thermodynamic parameters for Gibbs’ excess energy of the solid solution. A plot of the segregation coefficient as a function of Ta concentration was derived from the phase diagram.}},
  author       = {{Bashir, Umar and Klimm, Detlef and Rüsing, Michael and Bickermann, Matthias and Ganschow, Steffen}},
  issn         = {{0022-2461}},
  journal      = {{Journal of Materials Science}},
  publisher    = {{Springer Science and Business Media LLC}},
  title        = {{{Evaluation and thermodynamic optimization of phase diagram of lithium niobate tantalate solid solutions}}},
  doi          = {{10.1007/s10853-024-09932-7}},
  year         = {{2024}},
}

@article{49652,
  abstract     = {{Broadband coherent anti-Stokes Raman scattering (BCARS) is a powerful spectroscopy method combining high signal intensity with spectral sensitivity, enabling rapid imaging of heterogeneous samples in biomedical research and, more recently, in crystalline materials. However, BCARS encounters spectral distortion due to a setup-dependent non-resonant background (NRB). This study assesses BCARS reproducibility through a round robin experiment using two distinct BCARS setups and crystalline materials with varying structural complexity, including diamond, 6H-SiC, KDP, and KTP. The analysis compares setup-specific NRB correction procedures, detected and NRB-removed spectra, and mode assignment. We determine the influence of BCARS setup parameters like pump wavelength, pulse width, and detection geometry and provide a practical guide for optimizing BCARS setups for solid-state applications.}},
  author       = {{Hempel, Franz and Vernuccio, Federico and König, Lukas and Buschbeck, Robin and Rüsing, Michael and Cerullo, Giulio and Polli, Dario and Eng, Lukas M.}},
  issn         = {{1559-128X}},
  journal      = {{Applied Optics}},
  keywords     = {{Atomic and Molecular Physics, and Optics, Engineering (miscellaneous), Electrical and Electronic Engineering}},
  number       = {{1}},
  publisher    = {{Optica Publishing Group}},
  title        = {{{Comparing transmission- and epi-BCARS: a round robin on solid-state materials}}},
  doi          = {{10.1364/ao.505374}},
  volume       = {{63}},
  year         = {{2024}},
}

@article{57028,
  abstract     = {{<jats:p>Lithium niobate and lithium tantalate are among the most widespread materials for nonlinear, integrated photonics. Mixed crystals with arbitrary Nb–Ta ratios provide an additional degree of freedom to not only tune materials properties, such as the birefringence but also leverage the advantages of the singular compounds, for example, by combining the thermal stability of lithium tantalate with the larger nonlinear or piezoelectric constants of lithium niobate. Periodic poling allows to achieve phase-matching independent of waveguide geometry and is, therefore, one of the commonly used methods in integrated nonlinear optics. For mixed crystals, periodic poling has been challenging so far due to the lack of homogeneous, mono-domain crystals, which severely inhibit domain growth and nucleation. In this work, we investigate surface-near (&amp;lt;1μm depth) domain inversion on x-cut lithium niobate tantalate mixed crystals via electric field poling and lithographically structured electrodes. We find that naturally occurring head-to-head or tail-to-tail domain walls in the as-grown crystal inhibit domain inversion at a larger scale. However, periodic poling is possible if the gap size between the poling electrodes is of the same order of magnitude or smaller than the average size of naturally occurring domains. This work provides the basis for the nonlinear optical application of lithium niobate tantalate mixed crystals.</jats:p>}},
  author       = {{Bollmers, Laura and Babai-Hemati, Tobias and Koppitz, Boris and Eigner, Christof and Padberg, Laura and Rüsing, Michael and Eng, Lukas M. and Silberhorn, Christine}},
  issn         = {{0003-6951}},
  journal      = {{Applied Physics Letters}},
  number       = {{15}},
  publisher    = {{AIP Publishing}},
  title        = {{{Surface-near domain engineering in multi-domain x-cut lithium niobate tantalate mixed crystals}}},
  doi          = {{10.1063/5.0210972}},
  volume       = {{125}},
  year         = {{2024}},
}

@article{59269,
  abstract     = {{Ferroelectric materials play a crucial role in a broad range of technologies due to their unique properties that are deeply connected to the pattern and behavior of their ferroelectric (FE) domains. Chief among them, barium titanate (BaTiO3; BTO) sees widespread applications such as in electronics but equally is a ferroelectric model system for fundamental research, e.g., to study the interplay of such FE domains, the domain walls (DWs), and their macroscopic properties, owed to BTO’s multiple and experimentally accessible phase transitions. Here, we employ Second Harmonic Generation Microscopy (SHGM) to in situ investigate the cubic-to-tetragonal (at ∼126°C) and the tetragonal-to-orthorhombic (at ∼5°C) phase transition in single-crystalline BTO via three-dimensional (3D) DW mapping. We demonstrate that SHGM imaging provides the direct visualization of FE domain switching as well as the domain dynamics in 3D, shedding light on the interplay of the domain structure and phase transition. These results allow us to extract the different transition temperatures locally, to unveil the hysteresis behavior, and to determine the type of phase transition at play (first/second order) from the recorded SHGM data. The capabilities of SHGM in uncovering these crucial phenomena can easily be applied to other ferroelectrics to provide new possibilities for in situ engineering of advanced ferroic devices.}},
  author       = {{Kirbus, Benjamin and Seddon, Samuel D. and Kiseleva, Iuliia and Beyreuther, Elke and Rüsing, Michael and Eng, Lukas M.}},
  issn         = {{0021-8979}},
  journal      = {{Journal of Applied Physics}},
  number       = {{15}},
  publisher    = {{AIP Publishing}},
  title        = {{{Probing ferroelectric phase transitions in barium titanate single crystals via in-situ second harmonic generation microscopy}}},
  doi          = {{10.1063/5.0237769}},
  volume       = {{136}},
  year         = {{2024}},
}

@article{59271,
  abstract     = {{Lithium niobate (LNO) and lithium tantalate (LTO) see widespread use in fundamental research and commercial technologies reaching from electronics over classical optics to integrated quantum communication. The mixed crystal system lithium niobate tantalate (LNT) allows for the dedicate engineering of material properties by combining the advantages of the two parental materials LNO and LTO. Vibrational spectroscopies such as Raman spectroscopy or (Fourier transform) infrared (IR) spectroscopy are vital techniques to provide detailed insight into the material properties, which is central to the analysis and optimization of devices. This work presents a joint experimental–theoretical approach allowing to unambiguously assign the spectral features in the LNT material family through both Raman and IR spectroscopy, as well as providing an in‐depth explanation for the observed scattering efficiencies based on first‐principles calculations. The phononic contribution to the static dielectric tensor is calculated from the experimental and theoretical data using the generalized Lyddane–Sachs–Teller relation and compared with the results of the first‐principles calculations.}},
  author       = {{Bernhardt, Felix and Gharat, Soham and Kapp, Alexander and Pfeiffer, Florian and Buschbeck, Robin and Hempel, Franz and Pashkin, Oleksiy and Kehr, Susanne C. and Rüsing, Michael and Sanna, Simone and Eng, Lukas M.}},
  issn         = {{1862-6300}},
  journal      = {{physica status solidi (a)}},
  number       = {{1}},
  pages        = {{2300968}},
  publisher    = {{Wiley}},
  title        = {{{Lattice Dynamics of LiNb(1–x)Ta(x)O3 Solid Solutions: Theory and Experiment}}},
  doi          = {{10.1002/pssa.202300968}},
  volume       = {{222}},
  year         = {{2024}},
}

@article{59270,
  abstract     = {{Lithium niobate tantalate (LiNb1−xTaxO3, LNT) solid solutions offer exciting new possibilities for applications ranging from optics, piezotronics, and electronics beyond the capabilities of the widely used singular compounds of lithium niobate (LiNbO3, LN) or lithium tantalate (LiTaO3, LT). Crystal growth of homogeneous LNT single crystals by the Czochralski method is still challenging. One key aspect of homogeneous growth is the accurate knowledge of thermal conductivity through the crystal boule during the growth, which is central to control the crystal growth. Therefore, the temperature dependent thermal conductivity of pure LN, LT, and LNT solid solutions, as well as of selected doped LN and LT crystals (Mg, Zn) was investigated across the temperature range from 300 to 1300 K. The results that span across the whole composition range can directly be applied for optimizing growth conditions of both LNT solid solutions as well as doped and undoped LN and LT crystals.}},
  author       = {{Bashir, Umar and Rüsing, Michael and Klimm, Detlef and Blukis, Roberts and Koppitz, Boris and Eng, Lukas M. and Bickermann, Matthias and Ganschow, Steffen}},
  issn         = {{0925-8388}},
  journal      = {{Journal of Alloys and Compounds}},
  publisher    = {{Elsevier BV}},
  title        = {{{Thermal conductivity in solid solutions of lithium niobate tantalate single crystals from 300 K up to 1300 K}}},
  doi          = {{10.1016/j.jallcom.2024.176549}},
  volume       = {{1008}},
  year         = {{2024}},
}

@article{59272,
  abstract     = {{Ferroelectrics such as LiNbO3 (LN) are wide-band-gap insulators that may show a high local electric conductivity at the domain walls (DWs). The latter are interfaces separating regions of noncollinear polarization, which can be manipulated to build integrated nanoelectronic elements. In the present work, we model different DW types in LN from first principles. Our models reveal the DW morphology and shed light on their electronic properties: A strong band bending is predicted for charged DWs, leading to local metallicity. Defect trapping at the DW may further enhance the electric conductivity.}},
  author       = {{Verhoff, Leonard M. and Pionteck, Mike N. and Rüsing, Michael and Fritze, Holger and Eng, Lukas M. and Sanna, Simone}},
  issn         = {{2643-1564}},
  journal      = {{Physical Review Research}},
  number       = {{4}},
  publisher    = {{American Physical Society (APS)}},
  title        = {{{Two-dimensional electronic conductivity in insulating ferroelectrics: Peculiar properties of domain walls}}},
  doi          = {{10.1103/physrevresearch.6.l042015}},
  volume       = {{6}},
  year         = {{2024}},
}

@article{59273,
  abstract     = {{Ferroelectric domain walls (DWs) are promising structures for assembling future nano-electronic circuit elements on a larger scale since reporting domain wall currents of up to 1 mA per single DW. One key requirement hereto is their reproducible manufacturing by gaining preparative control over domain size and domain wall conductivity (DWC). To date, most works on DWC have focused on exploring the fundamental electrical properties of individual DWs within single-shot experiments, with an emphasis on quantifying the origins of DWC. Very few reports exist when it comes to comparing the DWC properties between two separate DWs, and literally nothing exists where issues of reproducibility in DWC devices have been addressed. To fill this gap while facing the challenge of finding guidelines for achieving predictable DWC performance, we report on a procedure that allows us to reproducibly prepare single hexagonal domains of a predefined diameter into uniaxial ferroelectric lithium niobate single crystals of 200 and 300 μm thickness, respectively. We show that the domain diameter can be controlled with an uncertainty of a few percent. As-grown DWs are then subjected to a standard procedure of current-limited high-voltage DWC enhancement, and they repetitively reach a DWC increase of six orders of magnitude. While all resulting DWs show significantly enhanced DWC values, their individual current–voltage (I–V) characteristics exhibit different shapes, which can be explained by variations in their 3D real structure reflecting local heterogeneities by defects, DW pinning, and surface-near DW inclination.}},
  author       = {{Ratzenberger, Julius and Kiseleva, Iuliia and Koppitz, Boris and Beyreuther, Elke and Zahn, Manuel and Gössel, Joshua and Hegarty, Peter A. and Amber, Zeeshan H. and Rüsing, Michael and Eng, Lukas M.}},
  issn         = {{0021-8979}},
  journal      = {{Journal of Applied Physics}},
  number       = {{10}},
  pages        = {{104302}},
  publisher    = {{AIP Publishing}},
  title        = {{{Toward the reproducible fabrication of conductive ferroelectric domain walls into lithium niobate bulk single crystals}}},
  doi          = {{10.1063/5.0219300}},
  volume       = {{136}},
  year         = {{2024}},
}

@article{59274,
  abstract     = {{Recently, ion exchange (IE) has been used to periodically modify the coercive field (Ec) of the crystal prior to periodic poling, to fabricate fine-pitch domain structures in Rb-doped KTiOPO4 (RKTP). Here, we use micro-Raman spectroscopy to understand the impact of IE on the vibrational modes related to the Rb/K lattice sites, TiO octahedra, and PO4 tetrahedra, which all form the basis of the RKTP crystal structure. We analyze the Raman spectra of three different RKTP samples: (1) a RKTP sample that shows a poled domain grating only, (2) a RKTP sample that has an Ec grating only, and (3) a RKTP sample that has both an Ec and a domain grating of the nominally same spacing. This allows us to determine the impact of IE on the vibrational modes of RKTP. We characterize the changes in the lower Raman peaks related to the alkali-metal ions, as well as observe lattice modifications induced by the incorporation of Rb+ that extend further into the crystal bulk than the expected IE depth. Moreover, the influence of IE on the domain walls is also manifested in their Raman peak shift. We discuss our results in terms of the deformation of the PO4and TiO groups. Our results highlight the intricate impact of IE on the crystal structure and how it facilitates periodic poling, paving the way for further development of the Ec-engineering technique.}},
  author       = {{Lee, Cherrie S. J. and Canalias, Carlota and Buschbeck, Robin and Koppitz, Boris and Hempel, Franz and Amber, Zeeshan and Eng, Lukas M. and Rüsing, Michael}},
  issn         = {{2469-9950}},
  journal      = {{Physical Review B}},
  number       = {{21}},
  publisher    = {{American Physical Society (APS)}},
  title        = {{{Impact of ion exchange on vibrational modes in Rb-doped KTiOPO4: A Raman spectroscopy study on the interplay between ion exchange and polarization switching}}},
  doi          = {{10.1103/physrevb.110.214115}},
  volume       = {{110}},
  year         = {{2024}},
}

@article{59275,
  abstract     = {{Studying and understanding many‐body interactions, particularly electron‐boson interactions, is essential for a deeper elucidation of fundamental physical phenomena and the development of novel material functionalities. Here, this aspect is explored in the weak itinerant ferromagnet LaCo2P2 by means of momentum‐resolved photoelectron spectroscopy (ARPES) and first‐principles calculations. The detailed ARPES patterns enable to unveil bulk and surface bands, spin splittings due to Rashba and exchange interactions, as well as the evolution of bands with temperature, which altogether creates a solid foundation for theoretical studies. The latter has allowed to establish the impact of electron‐boson interactions on the electronic structure, that are reflected in its strong renormalization driven by electron‐magnon interaction and the emergence of distinctive kinks of surface and bulk electron bands due to significant electron‐phonon coupling. Our results highlight the distinct impact of electron‐boson interactions on the electronic structure, particularly on the itinerant d states. Similar electronic states are observed in the isostructural iron pnictides, where electron‐boson interactions play a crucial role in the emergence of superconductivity. It is believed that further studies of material systems involving both magnetically active d‐ and f‐sublattices will reveal more advanced phenomena in the bulk and at distinct surfaces, driven by a combination of factors including Rashba and Kondo effects, exchange magnetism, and electron‐boson interactions.}},
  author       = {{Usachov, D. Yu. and Ali, K. and Poelchen, G. and Mende, M. and Schulz, S. and Peters, M. and Bokai, K. and Sklyadneva, I. Yu. and Stolyarov, V. and Chulkov, E. V. and Kliemt, K. and Paischer, S. and Buczek, P. A. and Heid, R. and Hempel, F. and Rüsing, Michael and Ernst, A. and Krellner, C. and Eremeev, S. V. and Vyalikh, D. V.}},
  issn         = {{2751-1200}},
  journal      = {{Advanced Physics Research}},
  publisher    = {{Wiley}},
  title        = {{{Unveiling Electron‐Phonon and Electron‐Magnon Interactions in the Weak Itinerant Ferromagnet LaCo2P2}}},
  doi          = {{10.1002/apxr.202400137}},
  year         = {{2024}},
}

@article{54967,
  abstract     = {{<jats:p>Ferroelectric domain wall conductivity (DWC) is an intriguing and promising functional property that can be elegantly controlled and steered through a variety of external stimuli such as electric and mechanical fields. Optical-field control, as a noninvasive and flexible tool, has rarely been applied so far, but it significantly expands the possibility for both tuning and probing DWC. On the one hand, as known from second-harmonic or Raman micro-spectroscopy, the optical approach provides information on DW distribution and inclination, while simultaneously probing the DW vibrational modes; on the other hand, photons might be applied to directly generate charge carriers, thereby acting as a functional and spectrally tunable probe to deduce the local absorption properties and bandgaps of conductive DWs. Here, we report on investigating the photo-induced DWC (PI-DWC) of three lithium niobate crystals, containing a very different number of DWs, namely: (A) none, (B) one, and (C) many conductive DWs. All three samples are inspected for their current–voltage behavior in darkness and for different illumination wavelengths swept from 500 nm down to 310 nm. All samples show their maximum PI-DWC at 310 nm; moreover, sample (C) reaches PI-DWCs of several microampere. Interestingly, a noticeable PI-DWC is also observed for sub-bandgap illumination, hinting toward the existence and decisive role of electronic in-gap states that contribute to the electronic charge transport along DWs. Finally, complementary conductive atomic force microscopy investigations under illumination proved that the PI-DWC indeed is confined to the DW area and does not originate from photo-induced bulk conductivity.</jats:p>}},
  author       = {{Ding, L. L. and Beyreuther, E. and Koppitz, B. and Kempf, K. and Ren, J. H. and Chen, W. J. and Rüsing, Michael and Zheng, Y. and Eng, L. M.}},
  issn         = {{0003-6951}},
  journal      = {{Applied Physics Letters}},
  number       = {{25}},
  publisher    = {{AIP Publishing}},
  title        = {{{Comparative study of photo-induced electronic transport along ferroelectric domain walls in lithium niobate single crystals}}},
  doi          = {{10.1063/5.0205877}},
  volume       = {{124}},
  year         = {{2024}},
}

@article{54966,
  abstract     = {{Piezoresponse force microscopy (PFM) is one of the most widespread methods for investigating and visualizing ferroelectric domain structures down to the nanometer length scale. PFM makes use of the direct coupling of the piezoelectric response to the crystal lattice, and hence, it is most often applied to spatially map the three-dimensional (3D) near-surface domain distribution of any polar or ferroic sample. Nonetheless, since most samples investigated by PFM are at least semiconducting or fully insulating, the electric ac field emerging from the conductive scanning force microscopy (SFM) tip penetrates the sample and, hence, may also couple to polar features that are deeply buried into the bulk of the sample under investigation. Thus, in the work presented here, we experimentally and theoretically explore the contrast and depth resolution capabilities of PFM, by analyzing the dependence of several key parameters. These key parameters include the depth of the buried feature, i.e., here a domain wall (DW), as well as PFM-relevant technical parameters such as the tip radius, the PFM drive voltage and frequency, and the signal-to-noise ratio. The theoretical predictions are experimentally verified using x-cut periodically poled lithium niobate single crystals that are specially prepared into wedge-shaped samples, in order to allow the buried feature, here the DW, to be “positioned” at any depth into the bulk. This inspection essentially contributes to the fundamental understanding in PFM contrast analysis and to the reconstruction of 3D domain structures down to a 1 μm-penetration depth into the sample.}},
  author       = {{Roeper, Matthias and Seddon, Samuel D. and Amber, Zeeshan H. and Rüsing, Michael and Eng, Lukas M.}},
  issn         = {{0021-8979}},
  journal      = {{Journal of Applied Physics}},
  keywords     = {{Ferroelectrics, lithium niobate, piezoresponse force microscopy}},
  number       = {{22}},
  publisher    = {{AIP Publishing}},
  title        = {{{Depth resolution in piezoresponse force microscopy}}},
  doi          = {{10.1063/5.0206784}},
  volume       = {{135}},
  year         = {{2024}},
}