Showing 1–50 of 64 results for author: Eng, M

On the temperature dependence of the optical band gap in the material system of lithium niobate and lithium tantalate

Authors: Maximilian Henneke, Michael Ruesing, Nina A. Lange, Timon Schapeler, Noah Spiegelberg, Ernst-Lukas Kuhlmann, Elke Beyreuther, Philipp Mues, Ludmila Eisner, Lukas M. Eng, Laura Padberg, Donat J. As, Klaus-Dieter Becker, Tim J. Bartley, Christine Silberhorn, Christof Eigner

Abstract: Lithium niobate and lithium tantalate see widespread use in optics and electronics, and are increasingly used for cryogenic applications. Despite their broad deployment, their optical band gap and its relation to the crystal stoichiometry are not well characterised as a function of temperature. In this work, we study the optical absorption properties of congruent, stoichiometric, MgO-doped and Er-… ▽ More Lithium niobate and lithium tantalate see widespread use in optics and electronics, and are increasingly used for cryogenic applications. Despite their broad deployment, their optical band gap and its relation to the crystal stoichiometry are not well characterised as a function of temperature. In this work, we study the optical absorption properties of congruent, stoichiometric, MgO-doped and Er-doped lithium niobate as well as congruent lithium tantalate across the temperature range between 7~K and 1000~K by means of optical transmission spectroscopy. Our results demonstrate that the difference of the optical band gap typically observed at room temperature between different stoichiometries is not primarily attributable to the intrinsic electronic structure, but rather to different electron-phonon couplings and the average phonon energies. Additionally, we exemplarily study the temperature shift of the 523 nm absorption line in Er-doped lithium niobate due to the increased interest in optically active dopants. To facilitate future analyses, we present the open-source software suite PhoQS-Treat (Tauc Regression Edge Analysis Tool), which enables automated Tauc regressions alongside additional analytical capabilities. This work advances the development of high-performance lithium niobate-based devices. △ Less

Submitted 20 August, 2026; v1 submitted 19 August, 2026; originally announced August 2026.

Infrared light scattering and emission from epsilon near-zero hyperbolic phonon-polaritons in two-dimensional crystals

Authors: Flavio Henrique Feres, Martin Bearzatto, Adrien Debacq, Gergely Nemeth, Rafael A. Mayer, Yuri B. Marcal, Marcus V. de Paiva, Maximilian Obst, Felix Kaps, Jakob Wetzel, Osama Hatem, Wenjun Zhou, Ran Jing, Heng Whang, Adrian Cernescu, Stephan Winnerl, J. Michael Klopf, Mengkun Liu, Ferenc Borondics, Ingrid D. Barcelos, Raul O. Freitas, Ana I. F. Tresguerres-Mata, Yannick De Wilde, Lukas M. Eng, Michaël Lobet , et al. (2 additional authors not shown)

Abstract: This work presents the near-field super-scattering and the near-field thermal emission from 2D hyperbolic crystals as general optical phenomena that link near-field nanophotonics to far-field photonics. This work presents the near-field super-scattering and the near-field thermal emission from 2D hyperbolic crystals as general optical phenomena that link near-field nanophotonics to far-field photonics. △ Less

Submitted 16 August, 2026; originally announced August 2026.

Comments: 20 pages, 4 figures

A Causal Probabilistic Framework for Perception-Informed Closed-Loop Simulation of Autonomous Driving

Authors: Zhennan Fei, Rickard Johansson, Mikael Andersson, Matthias Eng, Mattias Eriksson, Kaveh Kianfar, Sadegh Rahrovani, Chris van der Ploeg, Michael Borth, Maren Buermann, Michiel Braat, Henk Goossens, Zijian Han, Majid Khorsand Vakilzadeh, Gabriel Rodrigues de Campos

Abstract: Software-in-the-loop (SIL) simulation is a cornerstone for the validation of modern automotive safety functions. However, many current frameworks utilize ideal sensing, which bypasses the functional insufficiencies of perception algorithms, leading to over-optimistic safety assessments. This paper proposes a perception-informed SIL testing methodology that bridges the gap between ground-truth simu… ▽ More Software-in-the-loop (SIL) simulation is a cornerstone for the validation of modern automotive safety functions. However, many current frameworks utilize ideal sensing, which bypasses the functional insufficiencies of perception algorithms, leading to over-optimistic safety assessments. This paper proposes a perception-informed SIL testing methodology that bridges the gap between ground-truth simulation and real-world perception behavior. We present a framework for incorporating causal probabilistic models into standardized, scenario-based simulation toolchains, applicable to both Advanced Driver Assistance Systems (ADAS) and Autonomous Driving Systems (ADS). Our approach enables the systematic injection of realistic perception errors, such as loss of detection, sizing inaccuracies, and positioning offsets, derived from physical triggering conditions like fog, rain, and object-merging scenarios. By evaluating these ``faults'' within a standardized simulation environment, we demonstrate that perception-informed testing reveals latent operational risks that ideal SIL environments fail to capture, providing a scalable pathway for SOTIF (ISO 21448) validation. △ Less

Submitted 5 June, 2026; originally announced June 2026.

Uniaxial strain-driven ferroelastic domain control in LaAlO3

Authors: Matthias Roeper, Robin Buschbeck, Jakob Wetzel, Tobias Ritschel, Anna-Lena Hofmann, Vladyslav Kovtunovych, Mike N. Pionteck, Javier Taboada-Gutiérrez, Alexey B. Kuzmenko, Martina Basini, Vivek Unikandanunni, Iuliia Kiseleva, Jochen Geck, Susanne C. Kehr, Maximilian Lederer, Simone Sanna, Lukas M. Eng, Samuel D. Seddon

Abstract: Multiferroic domain walls in functional oxides exhibit properties distinct from the bulk and are increasingly exploited as active elements in nanoelectronic and photonic devices. Deterministic control of domain populations has typically remained limited to local control, or removal with temperature. Here we demonstrate continuous, reversible manipulation of the ferroelastic domain structure in sin… ▽ More Multiferroic domain walls in functional oxides exhibit properties distinct from the bulk and are increasingly exploited as active elements in nanoelectronic and photonic devices. Deterministic control of domain populations has typically remained limited to local control, or removal with temperature. Here we demonstrate continuous, reversible manipulation of the ferroelastic domain structure in single-crystal LaAlO$_3$ using in-situ uniaxial strain. Combining atomic force microscopy, X-ray diffraction, and Raman spectroscopy with first-principles calculations we map the complete microscopic evolution of the twin domain population through the strain-driven transition from the rhombohedral $R\bar{3}c$ ground state toward the predicted orthorhombic $Fmmm$ phase. Applied strains below $0.5\%$ produce pronounced surface flattening and large-scale domain reorganisation, establishing uniaxial strain as a technically accessible control parameter for ferroelastic domain engineering. These results open a route to active, real-time programming of domain architectures in LaAlO$_3$-based heterostructures, with implications for strain-tunable superconducting interfaces, nanoscale phonon-polariton optics, and ultrafast lattice control. △ Less

Submitted 30 April, 2026; originally announced April 2026.

Universal material basis for biocompatible printed electrolytes in Organic Electrochemical Transistors

Authors: Moritz Flemming, Paul Zechel, Rakesh R. Nair, Emil Mahnke, Markus Löffler, Alyna Ong, Bernd Rellinghaus, Lukas M. Eng, Karl Leo, Hans Kleemann

Abstract: Organic Electrochemical Transistors (OECTs) stand out for their interplay between ionic and electronic conduction, making them ideal analogues to biological synapses for neuromorphic computing and biosensing applications. Furthermore, they can be printed into integrated circuits on flexible substrates, enabling low-cost and high-throughput fabrication of complete electronic systems. However, most… ▽ More Organic Electrochemical Transistors (OECTs) stand out for their interplay between ionic and electronic conduction, making them ideal analogues to biological synapses for neuromorphic computing and biosensing applications. Furthermore, they can be printed into integrated circuits on flexible substrates, enabling low-cost and high-throughput fabrication of complete electronic systems. However, most OECT electrolytes for integrated circuits still lack biocompatibility and suffer from rheology-related printing challenges. This paper presents a novel material basis that can be combined with an ionic liquid to fabricate an electrolyte for OECTs that only contains biocompatible materials. It allows rheological adjustments to enable the use of electrolyte in both inkjet and screen printing. Furthermore, the electrolyte is UV-curable, enabling it to transition into solid-state structures after printing. Extended ink and device lifetimes for screen-printed structures enable the fabrication of advanced OECTs that can operate in ambient air for over 30 days after fabrication. Ultimately, a fully screen-printed transistor using only biocompatible materials on a leaf substrate is shown △ Less

Submitted 28 April, 2026; originally announced April 2026.

The domain-wall/metal-electrode injection barrier in lithium niobate: Which electrical transport model fits best?

Authors: Manuel Zahn, Elke Beyreuther, Iuliia Kiseleva, Julius Ratzenberger, Michael Rüsing, Lukas M. Eng

Abstract: The comprehensive description of both the electrical transport along conductive domain walls (CDWs) in lithium niobate (LNO) single crystals and the charge injection at the interfacing metal electrodes, emerged to be a complex challenge. Recently, a heuristic evaluation allowed to postulate the "R2D2" equivalent-circuit model (consisting of two parallel resistor-diode pairs) to appropriately match… ▽ More The comprehensive description of both the electrical transport along conductive domain walls (CDWs) in lithium niobate (LNO) single crystals and the charge injection at the interfacing metal electrodes, emerged to be a complex challenge. Recently, a heuristic evaluation allowed to postulate the "R2D2" equivalent-circuit model (consisting of two parallel resistor-diode pairs) to appropriately match the DC current-voltage (I-V) characteristics. Here, we carefully revisit the interfacial electrical behavior, i.e., the diode part of the equivalent circuit model, since many more processes beyond the diode-related electron hopping transport (HT) assumed so far, may concurrently occur, such as thermionic emission (TE), Fowler-Nordheim tunneling (FNT), space-charge limited conduction (SCLC), and others more. The "R2D2" model thus needs to be generalized into an "R2X2" circuit model (with X = HT, TE, FNT, and others) to fit to the experimental data. Moreover, to double check for the best I-V curve fitting to the different theories, we apply a higher-harmonic DW current-contribution (HHCC) analysis, i.e., an AC I-V inspection, that allows us to discriminate between all these possible models with much higher precision than from pure DC I-V curve fitting. Both the AC and DC analysis reveal well consistent results, finally finding that the FNT model accounts best for the domain-wall/electrode junctions investigated here. △ Less

Submitted 25 March, 2026; originally announced March 2026.

Comments: 13 pages, 5 figures, with appended Supplemental Material (9 pages, 5 figures)

Journal ref: Physical Review B 114, 045406 (2026)

Ultra-high THz-field-confinement at LaAlO3 twin walls

Authors: Jakob Wetzel, Javier Taboada-Gutiérrez, Matthias Roeper, Felix G. Kaps, Giuliano Esposito, Drini Marchese, Robin Buschbeck, Pauline Lenz, J. Michael Klopf, Hans A. Bechtel, Stephanie N. Gilbert Corder, Jeremie Teyssier, Susanne C. Kehr, Lukas M. Eng, Alexey B. Kuzmenko, Samuel D. Seddon

Abstract: The control and steering of light at nanometre length scales is crucial for the development of both fundamental science and nanophotonic technologies. Recent advancements have been achieved by exploiting various crystalline anisotropies, allowing for subdiffractional and diffraction-less canalisation of energy. These studies in particular benefit from stacking and twisting of 2D materials, whereas… ▽ More The control and steering of light at nanometre length scales is crucial for the development of both fundamental science and nanophotonic technologies. Recent advancements have been achieved by exploiting various crystalline anisotropies, allowing for subdiffractional and diffraction-less canalisation of energy. These studies in particular benefit from stacking and twisting of 2D materials, whereas corresponding capabilities of anisotropic bulk crystals are rather unexplored. In this work, we show that ferroelastic twin walls - crystallographically perfect 2D-sheets that separate regions of differently oriented domains - in the distorted perovskite LaAlO3 provide a natural platform for broadband lateral confinement and superb canalisation of light at the nanoscale. Without fabrication processes, the electromagnetic fields localised at such walls exhibit lateral optical sizes up to 260 times smaller than the free-space wavelength. Depending on the adjacent domain orientation and frequency, the twin wall pattern preferentially concentrates or repels the electromagnetic energy, constituting a natural building block towards broadband MIR and THz nanophotonics for polaritonic circuitry. △ Less

Submitted 16 April, 2026; v1 submitted 23 March, 2026; originally announced March 2026.

Legged Autonomous Surface Science In Analogue Environments (LASSIE): Making Every Robotic Step Count in Planetary Exploration

Authors: Cristina G. Wilson, Marion Nachon, Shipeng Liu, John G. Ruck, J. Diego Caporale, Benjamin E. McKeeby, Yifeng Zhang, Jordan M. Bretzfelder, John Bush, Alivia M. Eng, Ethan Fulcher, Emmy B. Hughes, Ian C. Rankin, Jelis J. Sostre Cortés, Sophie Silver, Michael R. Zanetti, Ryan C. Ewing, Kenton R. Fisher, Douglas J. Jerolmack, Daniel E. Koditschek, Frances Rivera-Hernández, Thomas F. Shipley, Feifei Qian

Abstract: The ability to efficiently and effectively explore planetary surfaces is currently limited by the capability of wheeled rovers to traverse challenging terrains, and by pre-programmed data acquisition plans with limited in-situ flexibility. In this paper, we present two novel approaches to address these limitations: (i) high-mobility legged robots that use direct surface interactions to collect ric… ▽ More The ability to efficiently and effectively explore planetary surfaces is currently limited by the capability of wheeled rovers to traverse challenging terrains, and by pre-programmed data acquisition plans with limited in-situ flexibility. In this paper, we present two novel approaches to address these limitations: (i) high-mobility legged robots that use direct surface interactions to collect rich information about the terrain's mechanics to guide exploration; (ii) human-inspired data acquisition algorithms that enable robots to reason about scientific hypotheses and adapt exploration priorities based on incoming ground-sensing measurements. We successfully verify our approach through lab work and field deployments in two planetary analog environments. The new capability for legged robots to measure soil mechanical properties is shown to enable effective traversal of challenging terrains. When coupled with other geologic properties (e.g., composition, thermal properties, and grain size data etc), soil mechanical measurements reveal key factors governing the formation and development of geologic environments. We then demonstrate how human-inspired algorithms turn terrain-sensing robots into teammates, by supporting more flexible and adaptive data collection decisions with human scientists. Our approach therefore enables exploration of a wider range of planetary environments and new substrate investigation opportunities through integrated human-robot systems that support maximum scientific return. △ Less

Submitted 20 March, 2026; originally announced March 2026.

Uncovering the properties of homo-epitaxial GaN devices through cross-sectional infrared nanoscopy

Authors: Hossein Zandipour, Felix Kaps, Robin Buschbeck, Maximilian Obst, Aditha Senarath, Richarda Niemann, Niclas S. Mueller, Gonzalo Alvarez-Perez, Katja Diaz-Granados, Ryan A Kowalski, Jakob Wetzel, Raghunandan Balasubramanyam Iyer, Matthew Wortel, J. Michael Klopf, Travis Anderson, Alan Jacobs, Mona Ebrish, Lukas M. Eng, Alexander Paarman, Susanne C. Kehr, Joshua D. Caldwell, Thomas G. Folland

Abstract: Validating material performance in electrical devices is crucial to product development. For Gallium Nitride (GaN) devices, evaluating material factors such as defects, dopant concentration, and overall production quality is essential to ensure their performance in advanced electronic and optoelectronic applications. This work demonstrates that scattering-type scanning near-field optical microscop… ▽ More Validating material performance in electrical devices is crucial to product development. For Gallium Nitride (GaN) devices, evaluating material factors such as defects, dopant concentration, and overall production quality is essential to ensure their performance in advanced electronic and optoelectronic applications. This work demonstrates that scattering-type scanning near-field optical microscopy (s-SNOM) can meet the demanding performance requirements for characterizing homoepitaxial GaN devices. Specifically, we show that combining s-SNOM results in the mid-IR and terahertz (THz) spectral ranges can disentangle carrier and lattice changes in a GaN p-i-n diode, which is not possible using one spectral range alone. We observe strong, resonant near-field signals near the LO phonon mode of GaN that correlate well with point-dipole models. This data shows great sensitivity to the local carrier density, with changes on the order of 1018 cm-3 easily resolved experimentally. Further, we demonstrate high sensitivity to sub-surface defects, which remain a significant challenge for other non-destructive techniques. To validate the power of s-SNOM imaging, our results are compared to traditional metrologies, including micro-Raman mapping and Kelvin Probe Force Microscopy (KPFM). Our results show that s-SNOM shows superior resolution and sensitivity to perturbations, highlighting the power of this technique in semiconductor device characterization. △ Less

Submitted 9 March, 2026; originally announced March 2026.

Tailoring phonon-driven responses in α-MoO3 through isotopic enrichment

Authors: Thiago S. Arnaud, Ryan W. Spangler, Johnathan D. Georgaras, Jonah B. Haber, Daniel Hirt, Maximilian Obst, Gonzalo Álvarez-Pérez, Mackey Long III, Felix G. Kaps, Jakob Wetzel, Courtney Ragle, John E. Buchner, Youngji Kim, Aditha S. Senarath, Richarda Niemann, Mingze He, Giulia Carini, Unai Arregui-Leon, Akash C. Behera, Ramachandra Bangari, Nihar Sahoo, Niels C. Brumby, J. Michael Klopf, Martin Wolf, Lukas M. Eng , et al. (7 additional authors not shown)

Abstract: The implementation of polaritonic materials into nanoscale devices requires selective tuning of parameters to realize desired spectral or thermal responses. One robust material is α-MoO3, which as an orthorhombic crystal boasts three distinct phonon dispersions, providing three polaritonic dispersions of hyperbolic phonon polaritons (HPhPs) across the mid-infrared (MIR). Here, the tunability of bo… ▽ More The implementation of polaritonic materials into nanoscale devices requires selective tuning of parameters to realize desired spectral or thermal responses. One robust material is α-MoO3, which as an orthorhombic crystal boasts three distinct phonon dispersions, providing three polaritonic dispersions of hyperbolic phonon polaritons (HPhPs) across the mid-infrared (MIR). Here, the tunability of both optical and thermal responses in isotopically enriched α-MoO3 (98MoO3, Mo18O3 and 98Mo18O3) are explored. A uniform ~5 % spectral redshift from 18O enrichment is observed in both Raman- and IR-active TO phonons. Both the in- and out-of-plane thermal conductivities for the isotopic variations are reported. Ab initio calculations both replicate experimental findings and analyze the select-mode three-phonon scattering contributions. The HPhPs from each isotopic variation are probed with s-SNOM and their Q- factors are reported. A Q-factor maxima increase of ~50 % along the [100] in the RB2 and ~100 % along the [001] in the RB3 are reported for HPhPs supported in 98Mo18O3. Observations in both real and Fourier space of higher-order HPhP modes propagating in single slabs of isotopically enriched α-MoO3 without the use of a subdiffractional surface scatterer are presented here. This work illustrates the tunability of α-MoO3 for thermal and nanophotonic applications. △ Less

Submitted 5 January, 2026; originally announced January 2026.

Real-space observation of the low-temperature Skyrmion lattice in Cu2OSeO3(100) single crystal

Authors: Gerald Malsch, Peter Milde, Dmytro Ivaneiko, Andreas Bauer, Christian Pfleiderer, H. Berger, Lukas M. Eng

Abstract: Cu2OSeO3 is a skyrmion host material in which two distinct thermodynamically stable skyrmion phases were identified. We report magnetic force microscopy imaging of the low-temperature magnetic phases in bulk Cu2OSeO3(100) single crystal. Tuning the external magnetic field over the various phase transition at a temperature of 10 K, we observe the formation of helical, conical, tilted conical and sk… ▽ More Cu2OSeO3 is a skyrmion host material in which two distinct thermodynamically stable skyrmion phases were identified. We report magnetic force microscopy imaging of the low-temperature magnetic phases in bulk Cu2OSeO3(100) single crystal. Tuning the external magnetic field over the various phase transition at a temperature of 10 K, we observe the formation of helical, conical, tilted conical and skyrmion lattice domains in real space. △ Less

Submitted 30 August, 2025; originally announced September 2025.

Spectral tuning of hyperbolic shear polaritons in monoclinic gallium oxide via isotopic substitution

Authors: Giulia Carini, Mohit Pradhan, Elena Gelzinyte, Andrea Ardenghi, Saurabh Dixit, Maximilian Obst, Aditha S. Senarath, Niclas S. Mueller, Gonzalo Alvarez-Perez, Katja Diaz-Granados, Ryan A. Kowalski, Richarda Niemann, Felix G. Kaps, Jakob Wetzel, Raghunandan Balasubramanyam Iyer, Piero Mazzolini, Mathias Schubert, J. Michael Klopf, Johannes T. Margraf, Oliver Bierwagen, Martin Wolf, Karsten Reuter, Lukas M. Eng, Susanne Kehr, Joshua D. Caldwell , et al. (4 additional authors not shown)

Abstract: Hyperbolic phonon polaritons - hybridized modes arising from the ultrastrong coupling of infrared light to strongly anisotropic lattice vibrations in uniaxial or biaxial polar crystals - enable to confine light to the nanoscale with low losses and high directionality. In even lower symmetry materials, such as monoclinic $β$-Ga$_2$O$_3$ (bGO), hyperbolic shear polaritons (HShPs) further enhance the… ▽ More Hyperbolic phonon polaritons - hybridized modes arising from the ultrastrong coupling of infrared light to strongly anisotropic lattice vibrations in uniaxial or biaxial polar crystals - enable to confine light to the nanoscale with low losses and high directionality. In even lower symmetry materials, such as monoclinic $β$-Ga$_2$O$_3$ (bGO), hyperbolic shear polaritons (HShPs) further enhance the directionality. Yet, HShPs are intrinsically supported only within narrow frequency ranges defined by the phonon frequencies of the host material. Here, we report spectral tuning of HShPs in bGO by isotopic substitution. Employing near-field optical microscopy to image HShPs in $^{18}$O bGO films homo-epitaxially grown on a $^{16}$O bGO substrate, we demonstrate a spectral redshift of $\sim~40~$cm$^{-1}$ for the $^{18}$O bGO, compared to $^{16}$O bGO. The technique allows for direct observation and a model-free estimation of the spectral shift driven by isotopic substitution without the need for knowledge of the dielectric tensor. Complementary far-field measurements and ab initio calculations - in good agreement with the near-field data - confirm the effectiveness of this estimation. This multifaceted study demonstrates a significant isotopic substitution induced spectral tuning of HShPs into a previously inaccessible frequency range, creating new avenues for technological applications of such highly directional polaritons. △ Less

Submitted 28 July, 2025; originally announced July 2025.

In-situ SHG microscopy investigation of the domain-wall-conductivity enhancement procedure in lithium niobate

Authors: Iuliia Kiseleva, Boris Koppitz, Elke Beyreuther, Matthias Roeper, Samuel D. Seddon, Lukas M. Eng

Abstract: Conductive domain walls (CDWs) in the uniaxial ferroelectric lithium niobate (LiNbO$_3$, LN) have attracted a lot of interest as potential elements in 2D nanoelectronics, due to their orders-of-magnitude larger electronic AC and DC conductivities as compared to the host material. On the way towards generating standardized CDWs into z-cut bulk LN crystals with controllable geometry and electrical p… ▽ More Conductive domain walls (CDWs) in the uniaxial ferroelectric lithium niobate (LiNbO$_3$, LN) have attracted a lot of interest as potential elements in 2D nanoelectronics, due to their orders-of-magnitude larger electronic AC and DC conductivities as compared to the host material. On the way towards generating standardized CDWs into z-cut bulk LN crystals with controllable geometry and electrical properties, we have encountered setbacks recently: Although the first preparation step, i.e., the established UV-light-assisted liquid-electrode poling, reliably creates fully penetrating hexagonal domains with the DWs being aligned almost parallel to the polarization axis, the second step in the DW 'conductivity-enhancement' process through post-growth voltage ramping, resulted in randomly-shaped DWs as reflected in their different current-voltage (I-V) characteristics even after having applied the same process parameters. To clarify this phenomenon, we present here an \textit{in-situ} and time-resolved second-harmonic-generation (SHG) microscopy investigation of DW samples of different sizes, monitoring the DW evolution during that critical voltage ramping, which allowed us to reconstruct the 3D DW shapes both prior to and after the enhancement process. As a result, we are able to map the temporal changes of the local DW inclination angle $α$, and to quantify the DW velocity. As a consequence, we need to re-assess and re-think the origin of the DW conductivity (DWC) in LN: The hitherto assumed simple connection between $α$ and the DWC can not be generalized, since point defects accumulating along DWs act as extra sources for charge carrier trapping/release, significantly contributing to the DW current. △ Less

Submitted 26 August, 2025; v1 submitted 10 July, 2025; originally announced July 2025.

Comments: 14 pages, 6 figures, with appended supplementary material (11 pages, 9 figures)

Journal ref: Journal of Applied Physics 138, 114101 (2025)

Phonon dephasing times determined with time-delayed, broadband CARS

Authors: Franz Hempel, Michael Rüsing, Federico Vernuccio, Kai J. Spychala, Robin Buschbeck, Giulio Cerullo, Dario Polli, Lukas M. Eng

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 non-resonant contribution to the scattering process that shifts and distort… ▽ More 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 non-resonant contribution to the scattering process that shifts and distorts the Raman peaks. In this work, we apply a method to obtain non-resonant background-free spectra based on time-delayed, broadband CARS (TD-BCARS) using an intra-pulse 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 t = 7.81 ps for the single sp3 peak. △ Less

Submitted 25 October, 2025; v1 submitted 5 June, 2025; originally announced June 2025.

Ga$_2$O$_3$ TCAD Mobility Parameter Calibration using Simulation Augmented Machine Learning with Physics Informed Neural Network

Authors: Le Minh Long Nguyen, Edric Ong, Matthew Eng, Yuhao Zhang, Hiu Yung Wong

Abstract: In this paper, we demonstrate the feasibility of performing automatic Technology Computer Aided Design (TCAD) parameter calibration and extraction using machine learning, with the machine trained solely by TCAD simulation data. The methodology is validated using experimental data. Schottky Barrier Diodes (SBDs) with different effective anode workfunction (WF) are fabricated with emerging ultra-wid… ▽ More In this paper, we demonstrate the feasibility of performing automatic Technology Computer Aided Design (TCAD) parameter calibration and extraction using machine learning, with the machine trained solely by TCAD simulation data. The methodology is validated using experimental data. Schottky Barrier Diodes (SBDs) with different effective anode workfunction (WF) are fabricated with emerging ultra-wide bandgap material, Gallium Oxide (Ga2O3), and are measured at various temperatures (T). Their current voltage curves are used for automatic Ga2O3 Philips Unified Mobility (PhuMob) model parameters calibration. Five critical PhuMob parameters were calibrated. The machine consists of an autoencoder and a neural network and is trained solely by TCAD simulation data with variations in WF, T, and the five PhuMob parameters (seven variations in total). Then, Ga2O3 PhuMob parameters are extracted from the noisy experimental curves. Subsequent TCAD simulation using the extracted parameters shows that the quality of the parameters is as good as an expert's calibration at the pre-turned on regime, but not in the on state regime. By using a simple physics-informed neural network, the machine performs as well as the human expert in all regimes. △ Less

Submitted 27 November, 2025; v1 submitted 3 April, 2025; originally announced April 2025.

Comments: 7 pages, 10 figures

Demonstration of domain wall current in MgO-doped lithium niobate single crystals up to 400 °C

Authors: Hendrik Wulfmeier, Uliana Yakhnevych, Cornelius Boekhoff, Allan Diima, Marlo Kunzner, Leonard M. Verhoff, Jonas Paul, Julius Ratzenberger, Elke Beyreuther, Joshua Gössel, Iuliia Kiseleva, Michael Rüsing, Simone Sanna, Lukas M. Eng, Holger Fritze

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 conductivit… ▽ More 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 \SI{400}{\degreeCelsius} which extends previous studies significantly. Experimental investigation of the DW conductivity of charged, inclined DWs is performed using \SI{5}{\mole\percent} MgO-doped lithium niobate single crystals. \CR{Current-voltage (\IV) 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 \SI{160}{\degreeCelsius}~to~\SI{230}{\degreeCelsius}. 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 modelling 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 \SI{400}{\degreeCelsius}. Our investigations underline the potential to extend \DWC based nanoelectronic and sensor applications even into the so-far unexplored temperature range up to \SI{400}{\degreeCelsius}. △ Less

Submitted 9 September, 2025; v1 submitted 31 March, 2025; originally announced April 2025.

Comments: arXiv admin note: text overlap with arXiv:2404.01214

Journal ref: Solid State Ionics 429 (2025) 116949

Ultraconfined THz Phonon Polaritons in Hafnium Dichalcogenides

Authors: R. A. Kowalski, N. S. Mueller, G. Álvarez-Pérez, M. Obst, K. Diaz-Granados, G. Carini, A. Senarath, S. Dixit, R. Niemann, R. B. Iyer, F. G. Kaps, J. Wetzel, J. M. Klopf, I. I. Kravchenko, M. Wolf, T. G. Folland, L. M. Eng, S. C. Kehr, P. Alonso-Gonzalez, A. Paarmann, J. D. Caldwell

Abstract: The confinement of electromagnetic radiation to subwavelength scales relies on strong light-matter interactions. In the infrared (IR) and terahertz (THz) spectral ranges, phonon polaritons are commonly employed to achieve extremely subdiffractional light confinement, with much lower losses as compared to plasmon polaritons. Among these, hyperbolic phonon polaritons in anisotropic materials offer a… ▽ More The confinement of electromagnetic radiation to subwavelength scales relies on strong light-matter interactions. In the infrared (IR) and terahertz (THz) spectral ranges, phonon polaritons are commonly employed to achieve extremely subdiffractional light confinement, with much lower losses as compared to plasmon polaritons. Among these, hyperbolic phonon polaritons in anisotropic materials offer a highly promising platform for light confinement, which, however, typically plateaus at values of λ0/100, with λ0 being the free-space incident wavelength. In this study, we report on ultraconfined phonon polaritons in hafnium-based dichalcogenides with confinement factors exceeding λ0/250 in the terahertz spectral range. This extreme light compression within deeply sub-wavelength thin films is enabled by the unprecedented magnitude of the light-matter coupling strength in these compounds, and the natural hyperbolicity of HfSe2 in particular. Our findings emphasize the critical role of light-matter coupling for polariton confinement, which for phonon polaritons in polar dielectrics is dictated by the transverse-longitudinal optic phonon energy splitting. Our results demonstrate transition metal dichalcogenides as an enabling platform for THz nanophotonic applications that push the limits of light control. △ Less

Submitted 13 February, 2025; originally announced February 2025.

Two-dimensional talc as a natural hyperbolic material

Authors: Flávio H. Feres, Francisco C. B. Maia, Shu Chen, Rafael A. Mayer, Maximillian Obst, Osama Hatem, Lukas Wehmeier, Tobias Nörenberg, Matheus S. Queiroz, Victor Mazzotti, J. Michael Klopf, Susanne C. Kehr, Lukas M. Eng, Alisson R. Cadore, Rainer Hillenbrand, Raul O. Freitas, Ingrid D. Barcelos

Abstract: This study demonstrates that two-dimensional talc, a naturally abundant mineral, supports hyperbolic phonon-polaritons (HPhPs) at mid-infrared wavelengths, thus offering a low-cost alternative to synthetic polaritonic materials. Using scattering scanning near-field optical microscopy (s-SNOM) and synchrotron infrared nano spectroscopy (SINS), we reveal tunable HPhP modes in talc flakes of a long l… ▽ More This study demonstrates that two-dimensional talc, a naturally abundant mineral, supports hyperbolic phonon-polaritons (HPhPs) at mid-infrared wavelengths, thus offering a low-cost alternative to synthetic polaritonic materials. Using scattering scanning near-field optical microscopy (s-SNOM) and synchrotron infrared nano spectroscopy (SINS), we reveal tunable HPhP modes in talc flakes of a long lifetime. These results highlight the potential of natural 2D talc crystals to constituting an effective platform for establishing scalable optoelectronic and photonic devices. △ Less

Submitted 30 January, 2025; v1 submitted 28 January, 2025; originally announced January 2025.

Comments: 8 pages, 4 figures

Journal ref: Nanoscale, 2025,17, 24151-24160

Probing Ferroelectric Phase Transitions in Barium Titanate Single Crystals via $\it{in-situ}$ Second Harmonic Generation Microscopy

Authors: Benjamin Kirbus, Samuel D. Seddon, Iuliia Kiseleva, Elke Beyreuther, Michael Rüsing, Lukas M. Eng

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 (BaTiO$_3$; BTO) sees widespread applications such as in electronics but equally is a ferroelectric model system for fundamental research, e.g., to study the interplay… ▽ More 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 (BaTiO$_3$; 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 $\it{in-situ}$ investigate the cubic-to-tetragonal (at $\sim$126$^\circ$C) and the tetragonal-to-orthorhombic (at $\sim$5$^\circ$C) phase transition in single-crystalline BTO via 3-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 the 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 (1st/2nd 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 $\it{in-situ}$ engineering of advanced ferroic devices. △ Less

Submitted 8 June, 2024; originally announced June 2024.

Comments: 8 pages, 3 figures, with appended supplementary material (2 pages, 2 figures)

Journal ref: Journal of Applied Physics 136(15), 154102 (2024)

Towards the reproducible fabrication of conductive ferroelectric domain walls into lithium niobate bulk single crystals

Authors: Julius Ratzenberger, Iuliia Kiseleva, Boris Koppitz, Elke Beyreuther, Manuel Zahn, Joshua Gössel, Peter A. Hegarty, Zeeshan H. Amber, Michael Rüsing, Lukas M. Eng

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 th… ▽ More 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 emphasis on quantifying the origins for DWC. Very few reports exist when it comes to compare 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 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 (FE) lithium niobate (LN) single crystals of 200 and 300 micrometers thickness, respectively. We show that the domain diameter can be controlled with an error of a few percent. As-grown DWs are then subjected to a standard procedure of current-controlled high-voltage DWC enhancement, repetitively reaching a DWC increase of 6 orders of magnitude. While all resulting DWs show significantly enhanced DWC values, subtle features in their individual current-voltage (I-V) characteristics hint towards different 3D shapes into the bulk, with variations probably reflecting local heterogeneities by defects, DW pinning, and surface-near DW inclination, which seem to have a larger impact than expected. △ Less

Submitted 13 May, 2024; originally announced May 2024.

Comments: 12 pages, 7 figures, with appended supplement (9 pages, 7 figures)

Journal ref: Journal of Applied Physics 136, 104302 (2024)

High-temperature domain wall current in Mg-doped lithium niobate single crystals up to 400°C

Authors: Uliana Yakhnevych, Marlo Kunzner, Leonard M. Verhoff, Julius Ratzenberger, Elke Beyreuther, Michael Rüsing, Simone Sanna, Lukas M. Eng, Holger Fritze

Abstract: Conductive ferroelectric domain walls (DWs) represent a promising topical system for the development of nanoelectronic components and devices. DWs show very different properties as compared to their bulk counterparts. Of central interest here is the domain wall current (DWC) of charged DWs in 5mol\% Mg-doped lithium niobate single crystals; in contrast to former works, we extend the DWC study here… ▽ More Conductive ferroelectric domain walls (DWs) represent a promising topical system for the development of nanoelectronic components and devices. DWs show very different properties as compared to their bulk counterparts. Of central interest here is the domain wall current (DWC) of charged DWs in 5mol\% Mg-doped lithium niobate single crystals; in contrast to former works, we extend the DWC study here to temperatures as high as 400$^\circ$C. Both the temporal stability and the thermal activation energies of 90 - 160 meV are readily deduced from current-voltage sweeps as recorded over multiple heating cycles. Our experimental work is backed up by atomistic modelling of the DWC. The latter suggests that a large band bending renders head-to-head and tail-to-tail DWs semimetallic. These detailed investigations underline the potential to extend DWC-based nanoelectronic applications even into the so-far unexplored high-temperature regime. △ Less

Submitted 15 April, 2024; v1 submitted 1 April, 2024; originally announced April 2024.

Comments: 5 pages, 4 figures; with included supplementary material (6 pages, 9 figures)

Ferroelastic control of magnetic domain structure: direct imaging by Magnetic Force Microscopy

Authors: S. D. Seddon, C. R. S. Haines, T. P. A. Hase, M. R. Lees, L. M. Eng, M. Alexe, M. A. Carpenter

Abstract: Pyrrhotite, Fe$_7$S$_8$, provides an example of exceptionally strong magnetoelastic coupling through pinning of ferromagnetic domains by ferroelastic twins. Using direct imaging of both magnetic and ferroelastic domains by magnetic force microscopy (MFM), the mechanism by which this coupling controls local magnetic switching behaviour of regions on the pyrrhotite surface is revealed, and leads to… ▽ More Pyrrhotite, Fe$_7$S$_8$, provides an example of exceptionally strong magnetoelastic coupling through pinning of ferromagnetic domains by ferroelastic twins. Using direct imaging of both magnetic and ferroelastic domains by magnetic force microscopy (MFM), the mechanism by which this coupling controls local magnetic switching behaviour of regions on the pyrrhotite surface is revealed, and leads to quantitative fitting of field dependent MFM phase shifts with bulk magnetometry data. It is shown that characteristic inflection points in the magnetometry data along certain direction, in particular $[\overline 120]^*_h$ of the hexagonal parent structure, are in fact caused by ferroelastic pinning of the magnetic moments. △ Less

Submitted 27 March, 2024; originally announced March 2024.

Unidirectional Ray Polaritons in Twisted Asymmetric Stacks

Authors: J. Álvarez-Cuervo, M. Obst, S. Dixit, G. Carini, A. I. F. Tresguerres-Mata, C. Lanza, E. Terán-García, G. Álvarez-Pérez, L. Álvarez-Tomillo, K. Diaz-Granados, R. Kowalski, A. S. Senerath, N. S. Mueller, L. Herrer, J. M. De Teresa, S. Wasserroth, J. M. Klopf, T. Beechem, M. Wolf, L. M. Eng, T. G. Folland, A. Tarazaga Martín-Luengo, J. Martín-Sánchez, S. C. Kehr, A. Y. Nikitin , et al. (3 additional authors not shown)

Abstract: The vast repository of van der Waals (vdW) materials supporting polaritons offers numerous possibilities to tailor electromagnetic waves at the nanoscale. The development of twistoptics - the modulation of the optical properties by twisting stacks of vdW materials - enables directional propagation of phonon polaritons (PhPs) along a single spatial direction, known as canalization. Here we demonstr… ▽ More The vast repository of van der Waals (vdW) materials supporting polaritons offers numerous possibilities to tailor electromagnetic waves at the nanoscale. The development of twistoptics - the modulation of the optical properties by twisting stacks of vdW materials - enables directional propagation of phonon polaritons (PhPs) along a single spatial direction, known as canalization. Here we demonstrate a complementary type of directional propagation of polaritons by reporting the visualization of unidirectional ray polaritons (URPs). They arise naturally in twisted hyperbolic stacks with very different thicknesses of their constituents, demonstrated for homostructures of $α$-MoO$_3$ and heterostructures of $α$-MoO$_3$ and $β$-Ga$_2$O$_3$. Importantly, their ray-like propagation, characterized by large momenta and constant phase, is tunable by both the twist angle and the illumination frequency. Apart from their fundamental importance, our findings introduce twisted asymmetric stacks as efficient platforms for nanoscale directional polariton propagation, opening the door for applications in nanoimaging, (bio)-sensing or polaritonic thermal management. △ Less

Submitted 7 January, 2025; v1 submitted 27 March, 2024; originally announced March 2024.

Comments: 15 pages, 5 figures

Journal ref: Nature Communications 15, 9042 (2024)

Lattice dynamics of LiNb$_{\text{1-x}}$Ta$_{\text{x}}$O$_{\text{3}}$ solid solutions: Theory and experiment

Authors: Felix Bernhardt, Soham Gharat, Alexander Kapp, Florian Pfeiffer, Robin Buschbeck, Franz Hempel, Oleksiy Pashkin, Susanne C. Kehr, Michael Rüsing, Simone Sanna, Lukas M. Eng

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. In recent years, 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… ▽ More 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. In recent years, 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 spectroscopy are vital techniques to provide detailed insight into the material properties, which is central to the analysis and optimization of devices. In this work, we present 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 to provide 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. The joint methodology can be readily expanded to other materials and serves, e.g., as the basis for studying the role of point defects or doping. △ Less

Submitted 26 March, 2024; originally announced March 2024.

Hot electron dynamics in a semiconductor nanowire under intense THz excitation

Authors: Andrei Luferau, Maximilian Obst, Stephan Winnerl, Alexej Pashkin, Susanne C. Kehr, Emmanouil Dimakis, Felix G. Kaps, Osama Hatem, Kalliopi Mavridou, Lukas M. Eng, Manfred Helm

Abstract: We report THz-pump / mid-infrared probe near-field studies on Si-doped GaAs-InGaAs core-shell nanowires utilizing THz radiation from the free-electron laser FELBE. Upon THz excitation of free carriers, we observe a red shift of the plasma resonance in both amplitude and phase spectra, which we attribute to the heating up of electrons in the conduction band. The simulation of heated electron distri… ▽ More We report THz-pump / mid-infrared probe near-field studies on Si-doped GaAs-InGaAs core-shell nanowires utilizing THz radiation from the free-electron laser FELBE. Upon THz excitation of free carriers, we observe a red shift of the plasma resonance in both amplitude and phase spectra, which we attribute to the heating up of electrons in the conduction band. The simulation of heated electron distributions anticipates a significant electron population in both L- and X-valleys. The two-temperature model is utilized for a quantitative analysis of the dynamics of the electron gas temperature under THz pumping at various power levels. △ Less

Submitted 25 March, 2024; originally announced March 2024.

Comments: Manuscript + Supplementary Information

Journal ref: ACS Photonics 11, 3123 (2024)

Surface-near domain engineering in multi-domain x-cut lithium niobate tantalate mixed crystals

Authors: Laura Bollmers, Tobias Babai-Hemati, Boris Koppitz, Christof Eigner, Laura Padberg, Michael Ruesing, Lukas M. Eng, Christine Silberhorn

Abstract: Lithium niobate and lithium tantalate are among the most widespread materials for nonlinear, integrated photonics. Mixed crystals with arbitrary Nb-Ta ratios provide a new degree of freedom to 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 nonlinea… ▽ More Lithium niobate and lithium tantalate are among the most widespread materials for nonlinear, integrated photonics. Mixed crystals with arbitrary Nb-Ta ratios provide a new degree of freedom to 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 is the prerequisite for any nonlinear optical application. For mixed crystals this 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 demonstrate that surface-near ($< 1$~$μ$m depth) periodic poling on x-cut lithium niobate tantalate mixed crystals can be achieved 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. △ Less

Submitted 25 September, 2024; v1 submitted 7 March, 2024; originally announced March 2024.

Depth resolution in piezoresponse force microscopy

Authors: Matthias Roeper, Samuel Dominic Seddon, Zeeshan H. Amber, Michael Rüsing, Lukas M. Eng

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 is most often applied to spatially map the 3-dimensional (3D) near-surface domain distribution of any polar or ferroic sam… ▽ More 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 is most often applied to spatially map the 3-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-$μ$m-penetration depth into the sample. △ Less

Submitted 5 March, 2024; originally announced March 2024.

Comparative study of photo-induced electronic transport along ferroelectric domain walls in lithium niobate single crystals

Authors: Lili Ding, Elke Beyreuther, Boris Koppitz, Konrad Kempf, Jianhua Ren, Weijin Chen, Michael Rüsing, Yue Zheng, Lukas M. Eng

Abstract: Ferroelectric domain wall conductivity (DWC) is an intriguing functional property, that can be controlled through external stimuli such as electric and mechanical fields. Optical-field control, as a non-invasive flexible handle, has rarely been applied so far, but significantly expands the possibility for both tuning and probing DWC. On the one hand, as known from Second-Harmonic, Raman, and CARS… ▽ More Ferroelectric domain wall conductivity (DWC) is an intriguing functional property, that can be controlled through external stimuli such as electric and mechanical fields. Optical-field control, as a non-invasive flexible handle, has rarely been applied so far, but significantly expands the possibility for both tuning and probing DWC. On the one hand, as known from Second-Harmonic, Raman, and CARS micro-spectroscopy, the optical in-and-out approach delivers parameters on the DW distribution, the DW inclination, and probes the DW vibrational modes; on the other hand, photons might be applied also to directly generate charge carriers within the DW, hence acting as a functional and spectrally tunable probe to deduce the integral or local absorption properties and bandgaps of conductive DWs. Here, we report on such an optoelectronic approach by investigating the photo-induced DWC (PI-DWC) in DWs of the model system lithium niobate, a material that is well known for hosting conductive DWs. We compare three different crystals containing different numbers of domain walls: (A) none, (B) one, and (C) many conductive DWs. All samples are inspected for their current-voltage (I-V) behavior (i) in darkness, and (ii) for different illumination wavelengths swept from 500 nm down to 310 nm. All samples show their maximum PI-DWC at 310 nm, i.e., at the optical bandgap of lithium niobate; moreover, sample (C) reaches PI-DWCs of several $μ$A. Interestingly, a noticeable PI-DWC is also observed for sub-bandgap illumination, i.e., wavelengths as high as 500 nm, hinting towards the existence and decisive role of electronic in-gap states that contribute to the electronic transport along DWs. Finally, conductive atomic force microscopy (c-AFM) investigations under illumination proved that the PI-DWC is confined to the DW area, and does not originate from photo-induced bulk conductivity. △ Less

Submitted 27 February, 2024; originally announced February 2024.

Comments: 18 pages, 3 figures, and Supplementary Material (with 10 pages, 4 figures)

Journal ref: Applied Physics Letters 124(25), 252901 (2024)

Towards 3D Magnetic Force Microscopy

Authors: Jori F. Schmidt, Lukas M. Eng, Samuel D. Seddon

Abstract: Magnetic force microscopy (MFM) is long established as a powerful tool for probing the local manifestation of magnetic nanostructures across a range of temperatures and applied stimuli. A major drawback of the technique, however, is that the detection of stray fields emanating from a samples surface rely on a uniaxial vertical cantilever oscillation, and thus are only sensitive to vertically orien… ▽ More Magnetic force microscopy (MFM) is long established as a powerful tool for probing the local manifestation of magnetic nanostructures across a range of temperatures and applied stimuli. A major drawback of the technique, however, is that the detection of stray fields emanating from a samples surface rely on a uniaxial vertical cantilever oscillation, and thus are only sensitive to vertically oriented stray field components. The last two decades have shown an ever-increasing literature fascination for exotic topological windings where particular attention to in-plane magnetic moment rotation is highly valuable when identifying and understanding such systems. Here we present a new method of detecting in-plane magnetic stray field components, by utilizing a home made split-electrode excitation piezo that allows the simultaneous excitation of a cantilever at its fundamental flexural and torsional modes. This allows for the joint acquisition of traditional vertical mode (V-MFM) images and a lateral MFM (L-MFM) where the tip-cantilever system is only sensitive to stray fields acting perpendicular to the torsional axis of the cantilever. △ Less

Submitted 18 August, 2023; v1 submitted 16 August, 2023; originally announced August 2023.

Polarization sensitivity in scattering-type scanning near-field optical microscopy -- towards nanoellipsometry

Authors: Felix G. Kaps, Susanne C. Kehr, Lukas M. Eng

Abstract: Electric field enhancement mediated through sharp tips in scattering-type scanning near-field optical microscopy (s-SNOM) enables optical material analysis down to the 10-nm length scale, and even below. Nevertheless, mostly the out-of-plane electric field component is considered here due to the lightning rod effect of the elongated s-SNOM tip being orders of magnitude stronger as compared to any… ▽ More Electric field enhancement mediated through sharp tips in scattering-type scanning near-field optical microscopy (s-SNOM) enables optical material analysis down to the 10-nm length scale, and even below. Nevertheless, mostly the out-of-plane electric field component is considered here due to the lightning rod effect of the elongated s-SNOM tip being orders of magnitude stronger as compared to any in-plane field component. Nonetheless, the fundamental understanding of resonantly excited near-field coupled systems clearly allows us to take profit from all vectorial components, especially also from the in-plane ones. In this paper, we theoretically and experimentally explore how linear polarization control of both near-field illumination and detection, can constructively be implemented to (non-)resonantly couple to selected sample permittivity tensor components, e.g. explicitly also to the in-plane directions. When applying the point-dipole model, we show that resonantly excited samples respond with a strong near-field signal, to all linear polarization angles. We then experimentally investigate the polarization-dependent responses for both non-resonant (Au) and phonon-resonant (3C-SiC) sample excitations at a 10.6~$μ$m and 10.7~$μ$m incident wavelength using a tabletop CO$_2$ laser. Varying the illumination polarization angle thus allows for quantitatively comparing the scattered near-field signatures for the two wavelengths. Finally, we compare our experimental data to simulation results, and thus gain the fundamental understanding of the polarization's influence on the near-field interaction. As a result, the near-field components parallel and perpendicular to the sample surface can be easily disentangled and quantified through their polarization signatures, connecting them directly to the sample's local permittivity. △ Less

Submitted 11 August, 2023; originally announced August 2023.

Comments: 11 pages, 5 figures

Terahertz Twistoptics -- engineering canalized phonon polaritons

Authors: Maximilian Obst, Tobias Nörenberg, Gonzalo Álvarez-Pérez, Thales V. A. G. de Oliveira, Javier Taboada-Gutiérrez, Flávio H. Feres, Felix G. Kaps, Osama Hatem, Andrei Luferau, Alexey Y. Nikitin, J. Michael Klopf, Pablo Alonso-González, Susanne C. Kehr, Lukas M. Eng

Abstract: The terahertz (THz) frequency range is key to study collective excitations in many crystals and organic molecules. However, due to the large wavelength of THz radiation, the local probing of these excitations in smaller crystalline structures or few-molecular arrangements, requires sophisticated methods to confine THz light down to the nanometer length scale, as well as to manipulate such a confin… ▽ More The terahertz (THz) frequency range is key to study collective excitations in many crystals and organic molecules. However, due to the large wavelength of THz radiation, the local probing of these excitations in smaller crystalline structures or few-molecular arrangements, requires sophisticated methods to confine THz light down to the nanometer length scale, as well as to manipulate such a confined radiation. For this purpose, in recent years, taking advantage of hyperbolic phonon polaritons (HPhP) in highly anisotropic van der Waals (vdW) materials has emerged as a promising approach, offering a multitude of manipulation options such as control over the wavefront shape and propagation direction. Here, we demonstrate the first THz application of twist-angle-induced HPhP manipulation, designing the propagation of confined THz radiation between 8.39 and 8.98 THz in the vdW material alpha-molybdenum trioxide ($α-MoO_{3}$), hence extending twistoptics to this intriguing frequency range. Our images, recorded by near-field optical microscopy, show the frequency- and twist-angle-dependent change between hyperbolic and elliptic polariton propagation, revealing a polaritonic transition at THz frequencies. As a result, we are able to allocate canalization (highly collimated propagation) of confined THz radiation by carefully adjusting these two parameters, i.e. frequency and twist angle. Specifically, we report polariton canalization in $α-MoO_{3}$ at 8.67 THz for a twist angle of 50°. Our results demonstrate an unprecedented control and the manipulation of highly-confined collective excitations at THz frequencies, offering novel possibilities for nanophotonic applications. △ Less

Submitted 7 August, 2023; originally announced August 2023.

Comments: 25 pages, 4 figures, article, submitted

Hall mobilities and sheet carrier densities in a single LiNbO$_3$ conductive ferroelectric domain wall

Authors: Henrik Beccard, Elke Beyreuther, Benjamin Kirbus, Samuel D. Seddon, Michael Rüsing, Lukas M. Eng

Abstract: For the last decade, conductive domain walls (CDWs) in single crystals of the uniaxial model ferroelectric lithium niobate (LiNbO$_3$, LNO) have shown to reach resistances more than 10 orders of magnitude lower as compared to the surrounding bulk, with charge carriers being firmly confined to sheets of a few nanometers in width. LNO thus currently witnesses an increased attention since bearing the… ▽ More For the last decade, conductive domain walls (CDWs) in single crystals of the uniaxial model ferroelectric lithium niobate (LiNbO$_3$, LNO) have shown to reach resistances more than 10 orders of magnitude lower as compared to the surrounding bulk, with charge carriers being firmly confined to sheets of a few nanometers in width. LNO thus currently witnesses an increased attention since bearing the potential for variably designing room-temperature nanoelectronic circuits and devices based on such CDWs. In this context, the reliable determination of the fundamental transport parameters of LNO CDWs, in particular the 2D charge carrier density $n_{2D}$ and the Hall mobility $μ_{H}$ of the majority carriers, are of highest interest. In this contribution, we present and apply a robust and easy-to-prepare Hall-effect measurement setup by adapting the standard 4-probe van-der-Pauw method to contact a single, hexagonally-shaped domain wall that fully penetrates the 200-$μ$m-thick LNO bulk single crystal. We then determine $n_{2D}$ and $μ_{H}$ for a set of external magnetic fields $B$ and prove the expected cosine-like angular dependence of the Hall voltage. Lastly, we present photo-Hall measurements of one and the same DW, by determining the impact of super-bandgap illumination on the 2D charge carrier density $n_{2D}$. △ Less

Submitted 6 November, 2023; v1 submitted 31 July, 2023; originally announced August 2023.

Journal ref: Physical Review Applied 20(6), 064043 (2023)

R2D2 -- An equivalent-circuit model that quantitatively describes domain wall conductivity in ferroelectric LiNbO$_3$

Authors: Manuel Zahn, Elke Beyreuther, Iuliia Kiseleva, Ahmed Samir Lotfy, Conor J. McCluskey, Jesi R. Maguire, Ahmet Suna, Michael Rüsing, J. Marty Gregg, Lukas M. Eng

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 (IU) characteristics, recorded at variable temperatures from LiNbO$_3$ (LNO) DWs, are clearly able to differ… ▽ More 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 (IU) characteristics, recorded at variable temperatures from LiNbO$_3$ (LNO) DWs, are clearly able to differentiate between these two contributions. Practically, they allow us here to directly quantify the physical parameters relevant for the two mechanisms (a) and (b) mentioned above. These are, e.g., 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 LiNbO$_3$ 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 fitting the DWC data not only to z-cut LNO bulk DWs, but equally to z-cut thin-film LNO DWs, and DWC from x-cut DWs as reported in literature. △ Less

Submitted 19 November, 2023; v1 submitted 19 July, 2023; originally announced July 2023.

Comments: 10 pages, 7 figures; with included supporting information (6 pages, 4 figures)

Journal ref: Physical Review Applied 21(2), 024007 (2024)

Comparing Transmission- and Epi-BCARS: A Transnational Round Robin on Solid State Materials

Authors: Franz Hempel, Federico Vernuccio, Lukas König, Robin Buschbeck, Michael Rüsing, Giulio Cerullo, Dario Polli, Lukas M. Eng

Abstract: Broadband coherent anti-Stokes Raman scattering (BCARS) is an advanced Raman spectroscopy method that combines the spectral sensitivity of spontaneous Raman scattering (SR) with the increased signal intensity of single-frequency coherent Raman techniques. These two features make BCARS particularly suitable for ultra-fast imaging of heterogeneous samples, as already shown in biomedicine. Recent stu… ▽ More Broadband coherent anti-Stokes Raman scattering (BCARS) is an advanced Raman spectroscopy method that combines the spectral sensitivity of spontaneous Raman scattering (SR) with the increased signal intensity of single-frequency coherent Raman techniques. These two features make BCARS particularly suitable for ultra-fast imaging of heterogeneous samples, as already shown in biomedicine. Recent studies demonstrated that BCARS also shows exceptional spectroscopic capabilities when inspecting crystalline materials like lithium niobate and lithium tantalate, and can be used for fast imaging of ferroelectric domain walls. These results strongly suggest the extension of BCARS towards new imaging applications like mapping defects, strain, or dopant levels, similar to standard SR imaging. Despite these advantages, BCARS suffers from a spurious and chemically unspecific non-resonant background (NRB) that distorts and shifts the Raman peaks. Post-processing numerical algorithms are then used to remove the NRB and to obtain spectra comparable to SR results. Here, we show the reproducibility of BCARS by conducting an internal Round Robin with two different BCARS experimental setups, comparing the results on different crystalline materials of increasing structural complexity: diamond, 6H-SiC, KDP, and KTP. First, we compare the detected and phase-retrieved signals, the setup-specific NRB-removal steps, and the mode assignment. Subsequently, we demonstrate the versatility of BCARS by showcasing how the selection of pump wavelength, pulse width, and detection geometry can be tailored to suit the specific objectives of the experiment. Finally, we compare and optimize measurement parameters for the high-speed, hyperspectral imaging of ferroelectric domain walls in lithium niobate. △ Less

Submitted 6 September, 2023; v1 submitted 16 June, 2023; originally announced June 2023.

Comments: 12 pages, 8 figures

Modeling nonlinear optical interactions of focused beams in bulk crystals and thin films: A phenomenological approach

Authors: Kai J. Spychala, Zeeshan H. Amber, Lukas M. Eng, Michael Rüsing

Abstract: Coherent nonlinear optical micro-spectroscopy is a frequently used tool in modern material science, as it is sensitive to many different local observables, which comprise, among others, crystal symmetry and vibrational properties. The richness in information, however, may come with challenges in data interpretation, as one has to disentangle the many different effects like multiple reflections, ph… ▽ More Coherent nonlinear optical micro-spectroscopy is a frequently used tool in modern material science, as it is sensitive to many different local observables, which comprise, among others, crystal symmetry and vibrational properties. The richness in information, however, may come with challenges in data interpretation, as one has to disentangle the many different effects like multiple reflections, phase jumps at interfaces, or the influence of the Guoy-phase. In order to facilitate interpretation, the work presented here proposes an easy-to-use semi-analytical modeling ansatz, that bases upon known analytical solutions using Gaussian beams. Specifically, we apply this ansatz to compute nonlinear optical responses of (thin film) optical materials. We try to conserve the meaning of intuitive parameters like the Gouy-phase and the nonlinear coherent interaction length. In particular, the concept of coherence length is extended, which is a must when using focal beams. The model is subsequently applied to exemplary cases of second-harmonic and third-harmonic generation. We observe a very good agreement with experimental data and furthermore, despite the constraints and limits of the analytical ansatz, our model performs similarly well as when using more rigorous simulations. However, it outperforms the latter in terms of computational power, requiring more than three orders less computational time and less performant computer systems. △ Less

Submitted 20 November, 2022; v1 submitted 15 November, 2022; originally announced November 2022.

Vibrational properties of LiNbO$_3$ and LiTaO$_3$ under uniaxial stress

Authors: Ekta Singh, Mike N. Pionteck, Sven Reitzig, Michael Lange, Michael Rüsing, Lukas M. Eng, Simone Sanna

Abstract: Structural strain severely impacts material properties such as the linear and non-linear optical response. Moreover, strain plays a key role, e.g., in the physics of ferroelectrics and in particular of their domain walls. $μ$-Raman spectroscopy is a well-suited technique for the investigation of such strain effects, as it allows to measure the lattice dynamics locally. However, quantifying and rec… ▽ More Structural strain severely impacts material properties such as the linear and non-linear optical response. Moreover, strain plays a key role, e.g., in the physics of ferroelectrics and in particular of their domain walls. $μ$-Raman spectroscopy is a well-suited technique for the investigation of such strain effects, as it allows to measure the lattice dynamics locally. However, quantifying and reconstructing strain fields from Raman maps requires knowledge on the strain dependence of phonon frequencies. In this work, we have analyzed both theoretically and experimentally the phonon frequencies in the widely used ferroelectrics lithium niobate and lithium tantalate as a function of uniaxial strain via density functional theory and $μ$-Raman spectroscopy. Overall, we find a good agreement between our $ab$ $initio$ models and the experimental data performed with a stress cell. The majority of phonons show an increase in frequency under compressive strain, while the opposite is observed for tensile strains. Moreover, for E-type phonons, we observe the lifting of degeneracy already at moderate strain fields (i.e. at $\pm0.2~\%$) along the x and y directions. This work hence allows for the systematic analysis of 3D strains in modern-type bulk and thin-film devices assembled from lithium niobate and tantalate. △ Less

Submitted 25 October, 2022; originally announced October 2022.

Comments: 12 pages, 7 figures, Supplementary Information, submitted to Physical Review Materials

Out-of-equilibrium optomechanical resonance self-excitation

Authors: P. Milde, M. Langenhorst, H. Hölscher, J. Rottmann-Matthes, D. Hundertmark, L. M. Eng, R. Hoffmann-Vogel

Abstract: The fundamental sensitivity limit of atomic force microscopy is strongly correlated to the thermal noise of the cantilever oscillation. A method to suppress this unwanted noise is to reduce the bandwidth of the measurement, but this approach is limited by the speed of the measurement and the width of the cantilever resonance, commonly defined through the quality factor Q. However, it has been show… ▽ More The fundamental sensitivity limit of atomic force microscopy is strongly correlated to the thermal noise of the cantilever oscillation. A method to suppress this unwanted noise is to reduce the bandwidth of the measurement, but this approach is limited by the speed of the measurement and the width of the cantilever resonance, commonly defined through the quality factor Q. However, it has been shown that optomechanical resonances in interferometers might affect the cantilever oscillations resulting in an effective quality factor Q$_{eff}$ . When the laser power is sufficiently increased the cantilever oscillations might even reach the regime of self-oscillation. In this self-oscillation state, the noise of the system is partially determined by the interaction with the laser light far from equilibrium. Here, we show and discuss how tuning of the laser power leads to nonlinear optomechanical effects that can dramatically increase the effective quality factor of the cantilever leading to out-of-equilibrium noise. We model the effects using a fourth order nonlinearity of the damping coefficient. △ Less

Submitted 24 August, 2022; originally announced August 2022.

Comments: 10 pages

Journal ref: J. Appl. Phys. 130, 035303 (2021)

Impact of 3D curvature on the polarization orientation in non-Ising domain walls

Authors: Ulises Acevedo-Salas, Boris Croes, Yide Zhang, Olivier Cregut, Kokou Dodzi Dorkenoo, Benjamin Kirbus, Ekta Singh, Henrik Beccard, Michael Rüsing, Lukas M. Eng, Riccardo Hertel, Eugene A. Eliseev, Anna N. Morozovska, Salia Cherifi-Hertel

Abstract: Ferroelectric domain boundaries are quasi-two-dimensional functional interfaces with high prospects for nanoelectronic applications. Despite their reduced dimensionality, they can exhibit complex non-Ising polarization configurations and unexpected physical properties. Here, the impact of the three-dimensional (3D) curvature on the polarization profile of nominally uncharged 180° domain walls in L… ▽ More Ferroelectric domain boundaries are quasi-two-dimensional functional interfaces with high prospects for nanoelectronic applications. Despite their reduced dimensionality, they can exhibit complex non-Ising polarization configurations and unexpected physical properties. Here, the impact of the three-dimensional (3D) curvature on the polarization profile of nominally uncharged 180° domain walls in LiNbO3 is studied using second-harmonic generation microscopy and 3D polarimetry analysis. Correlations between the domain wall curvature and the variation of its internal polarization unfold in the form of modulations of the Néel-like character, which we attribute to the flexoelectric effect. While the Néel-like character originates mainly from the tilting of the domain wall, the internal polarization adjusts its orientation due to the synergetic upshot of dipolar and monopolar bound charges and their variation with the 3D curvature. Our results show that curved interfaces in solid crystals may offer a rich playground for tailoring nanoscale polar states. △ Less

Submitted 4 July, 2022; originally announced July 2022.

Comments: 35 pages, 4 figures and Supporting Information

Tuning the domain wall conductivity in bulk lithium niobate by uniaxial stress

Authors: Ekta Singh, Henrik Beccard, Zeeshan H. Amber, Julius Ratzenberger, Clifford W. Hicks, Michael Rüsing, Lukas M. Eng

Abstract: Conductive domain walls (CDWs) in insulating ferroelectrics have recently attracted considerable attention due to their unique topological, optical, and electronic properties, and offer potential applications such as in memory devices or re-writable circuitry. The electronic properties of domain walls (DWs) can be tuned by the application of strain, hence controlling the charge carrier density at… ▽ More Conductive domain walls (CDWs) in insulating ferroelectrics have recently attracted considerable attention due to their unique topological, optical, and electronic properties, and offer potential applications such as in memory devices or re-writable circuitry. The electronic properties of domain walls (DWs) can be tuned by the application of strain, hence controlling the charge carrier density at DWs. In this work, we study the influence of uniaxial stress on the conductivity of DWs in the bulk single crystal lithium niobate (LiNbO$_3$). Using conductive atomic force microscopy (cAFM), we observe a large asymmetry in the conductivity of DWs, where only negatively screened walls, so called head-to-head DWs, are becoming increasingly conductive, while positively screened, tail-to-tails DWs, show a decrease in conductivity. This asymmetry of DW conductivity agrees with our theoretical model based on the piezoelectric effect. In addition, we observed that the current in the DW increases up to an order of magnitude for smaller compressive stresses of 100 MPa. This response of DWs remained intact for multiple stress cycles over 2 months, opening a path for future applications. △ Less

Submitted 4 May, 2022; v1 submitted 2 May, 2022; originally announced May 2022.

Large Hall electron mobilities in head-to-head BaTiO$_3$-domain walls

Authors: Henrik Beccard, Benjamin Kirbus, Elke Beyreuther, Michael Rüsing, Petr Bednyakov, Jirka Hlinka, Lukas M. Eng

Abstract: Strongly charged head-to-head (H2H) domain walls (DWs) that are purposely engineered along the [110] crystallographic orientation into ferroelectric BaTiO$_3$ single crystals have been proposed as novel 2-dimensional electron gases (2DEGs) due to their significant domain wall conductivity (DWC). Here, we quantify these 2DEG properties through dedicated Hall-transport measurements in van-der-Pauw 4… ▽ More Strongly charged head-to-head (H2H) domain walls (DWs) that are purposely engineered along the [110] crystallographic orientation into ferroelectric BaTiO$_3$ single crystals have been proposed as novel 2-dimensional electron gases (2DEGs) due to their significant domain wall conductivity (DWC). Here, we quantify these 2DEG properties through dedicated Hall-transport measurements in van-der-Pauw 4-point geometry at room temperature, finding the electron mobility to reach around 400~cm$^2$(Vs)$^{-1}$, while the 2-dimensional charge density amounts to ~7$\times$10$^3$cm$^{-2}$. We underline the necessity to take account of thermal and geometrical-misalignment offset voltages by evaluating the Hall resistance under magnetic-field sweeps, since otherwise dramatic errors of several hundred percent in the derived mobility and charge density values can occur. Apart from the specific characterization of the conducting BaTiO$_3$ DW, we propose the method as an easy and fast way to quantitatively characterize ferroic conducting DWs, complementary to previously proposed scanning-probe-based Hall-potential analyses. △ Less

Submitted 24 April, 2022; originally announced April 2022.

Comments: 8 pages, 4 figures; with included supporting information (4 pages, 5 figures)

Journal ref: ACS Applied Nano Materials 5(7), 8717-8722 (2022)

Antiskyrmions and their electrical footprint in crystalline mesoscale structures of Mn$_{1.4}$PtSn

Authors: Moritz Winter, Francisco J. T. Goncalves, Ivan Soldatov, Yangkun He, Belén E. Zúňiga Céspedes, Peter Milde, Kilian Lenz, Sandra Hamann, Marc Uhlarz, Praveen Vir, Markus König, Philip J. W. Moll, Richard Schlitz, Sebastian T. B. Goennenwein, Lukas M. Eng, Rudolf Schaefer, Jochen Wosnitza, Claudia Felser, Jacob Gayles, Toni Helm

Abstract: Skyrmionic materials hold the potential for future information technologies, such as racetrack memories. Key to that advancement are systems that exhibit high tunability and scalability, with stored information being easy to read and write by means of all-electrical techniques. Topological magnetic excitations such as skyrmions and antiskyrmions, give rise to a characteristic topological Hall effe… ▽ More Skyrmionic materials hold the potential for future information technologies, such as racetrack memories. Key to that advancement are systems that exhibit high tunability and scalability, with stored information being easy to read and write by means of all-electrical techniques. Topological magnetic excitations such as skyrmions and antiskyrmions, give rise to a characteristic topological Hall effect. However, the electrical detection of antiskyrmions, in both thin films and bulk samples has been challenging to date. Here, we apply magneto-optical microscopy combined with electrical transport to explore the antiskyrmion phase as it emerges in crystalline mesoscale structures of the Heusler magnet Mn$_{1.4}$PtSn. We reveal the Hall signature of antiskyrmions in line with our theoretical model, comprising anomalous and topological components. We examine its dependence on the vertical device thickness, field orientation, and temperature. Our atomistic simulations and experimental anisotropy studies demonstrate the link between antiskyrmions and a complex magnetism that consists of competing ferromagnetic, antiferromagnetic, and chiral exchange interactions, not captured by micromagnetic simulations. △ Less

Submitted 15 March, 2023; v1 submitted 3 November, 2021; originally announced November 2021.

Comments: 30 pages, 4 figures, Supplementary Information with additional 22 pages and 16 figures

Journal ref: Commun. Mater. 3, 102 (2022)

Germanium monosulfide as a natural platform for highly anisotropic THz polaritons

Authors: Tobias Nörenberg, Gonzalo Álvarez-Pérez, Maximilian Obst, Lukas Wehmeier, Franz Hempel, J. Michael Klopf, Alexey Y. Nikitin, Susanne C. Kehr, Lukas M. Eng, Pablo Alonso-González, Thales V. A. G. de Oliveira

Abstract: Terahertz (THz) electromagnetic radiation is key to optically access collective excitations such as magnons (spins), plasmons (electrons), or phonons (atomic vibrations), thus bridging between optics and solid-state physics. Confinement of THz light to the nanometer length scale is desirable for local probing of such excitations in low dimensional systems, thereby inherently circumventing the larg… ▽ More Terahertz (THz) electromagnetic radiation is key to optically access collective excitations such as magnons (spins), plasmons (electrons), or phonons (atomic vibrations), thus bridging between optics and solid-state physics. Confinement of THz light to the nanometer length scale is desirable for local probing of such excitations in low dimensional systems, thereby inherently circumventing the large footprint and low spectral density of far-field THz optics. For that purpose, phonon polaritons (PhPs, i.e., light coupled to lattice vibrations in polar crystals) in anisotropic van der Waals (vdW) materials have recently emerged as a promising platform for THz nanooptics; yet the amount of explored, viable materials is still exiguous. Hence, there is a demand for the exploration of novel materials that feature not only THz PhPs at different spectral regimes, but also exhibit unique anisotropic (directional) electrical, thermoelectric, and vibronic properties. To that end, we introduce here the semiconducting alpha-germanium(II) sulfide (GeS) as an intriguing candidate. By employing THz nano-spectroscopy supported by theoretical analysis, we provide a thorough characterization of the different in-plane hyperbolic and elliptical PhP modes in GeS. We find not only PhPs with long life times ($τ$ > 2 ps) and excellent THz light confinement ($λ_0/λ$ > 45), but also an intrinsic, phonon-induced anomalous dispersion as well as signatures of naturally occurring PhP canalization within one single GeS slab. △ Less

Submitted 4 November, 2021; v1 submitted 25 October, 2021; originally announced October 2021.

Journal ref: ACS Nano 2022, 16, 20174-20185

Quantifying the Coherent Interaction Length of Second-Harmonic Microscopy in Lithium Niobate Confined Nanostructures

Authors: Zeeshan Hussain Amber, Benjamin Kirbus, Lukas M. Eng, Michael Rüsing

Abstract: Thin-film lithium niobate (TFLN) in the form of x- or z-cut lithium-niobate-on-insulator (LNOI) has recently popped up as a very promising and novel platform for developing integrated optoelectronic (nano)devices and exploring fundamental research. Here, we investigate the coherent interaction length $l_{c}$ of optical second-harmonic (SH) microscopy in such samples, that are purposely prepared in… ▽ More Thin-film lithium niobate (TFLN) in the form of x- or z-cut lithium-niobate-on-insulator (LNOI) has recently popped up as a very promising and novel platform for developing integrated optoelectronic (nano)devices and exploring fundamental research. Here, we investigate the coherent interaction length $l_{c}$ of optical second-harmonic (SH) microscopy in such samples, that are purposely prepared into a wedge shape, in order to elegantly tune the geometrical confinement from bulk thicknesses down to $\approx$ 50 nm. SH microscopy is a very powerful and non-invasive tool for the investigation of structural properties in the biological and solid-state sciences, especially also for visualizing and analyzing ferroelectric domains and domain walls. However, unlike bulk LN, SH microscopy in TFLN is largely affected by interfacial reflections and resonant enhancement that both rely on film thickness and substrate material. In this paper we show that the dominant SHG contribution measured in back-reflection, is the co-propagating phase-matched SH signal and \textit{not} the counter-propagating SH portion as is the case for bulk LN samples. Moreover, $l_{c}$ dramatically depends also on the incident pump laser wavelength (sample dispersion) but even more on the numerical aperture of the focussing objective in use. These experimental findings on x- and z-cut TFLN are excellently backed up by our advanced numerical simulations. △ Less

Submitted 19 August, 2021; v1 submitted 7 August, 2021; originally announced August 2021.

Atomic Layer Deposition of Yttrium Iron Garnet Thin Films for 3D Magnetic Structures

Authors: M. Lammel, D. Scheffler, D. Pohl, P. Swekis, S. Reitzig, H. Reichlova, R. Schlitz, K. Geishendorf, L. Siegl, B. Rellinghaus, L. M. Eng, K. Nielsch, S. T. B. Goennenwein, A. Thomas

Abstract: A wide variety of new phenomena such as novel magnetization configurations have been predicted to occur in three dimensional magnetic nanostructures. However, the fabrication of such structures is often challenging due to the specific shapes required, such as magnetic tubes and spirals. Furthermore, the materials currently used to assemble these structures are predominantly magnetic metals that do… ▽ More A wide variety of new phenomena such as novel magnetization configurations have been predicted to occur in three dimensional magnetic nanostructures. However, the fabrication of such structures is often challenging due to the specific shapes required, such as magnetic tubes and spirals. Furthermore, the materials currently used to assemble these structures are predominantly magnetic metals that do not allow to study the magnetic response of the system separately from the electronic one. In the field of spintronics, the prototypical material used for such experiments is the ferrimagnetic insulator yttrium iron garnet (Y$_3$Fe$_5$O$_{12}$, YIG). YIG is one of the best materials especially for magnonic studies due to its low Gilbert damping. Here, we report the first successful fabrication of YIG thin films via atomic layer deposition. To that end we utilize a supercycle approach based on the combination of sub-nanometer thin layers of the binary systems Fe$_2$O$_3$ and Y$_2$O$_3$ in the correct atomic ratio on Y$_3$Al$_5$O$_{12}$ substrates with a subsequent annealing step. Our process is robust against typical growth-related deviations, ensuring a good reproducibility. The ALD-YIG thin films exhibit a good crystalline quality as well as magnetic properties comparable to other deposition techniques. One of the outstanding characteristics of atomic layer deposition is its ability to conformally coat arbitrarily-shaped substrates. ALD hence is the ideal deposition technique to grant an extensive freedom in choosing the shape of the magnetic system. The atomic layer deposition of YIG enables the fabrication of novel three dimensional magnetic nanostructures, which in turn can be utilized for experimentally investigating the phenomena predicted in those structures. △ Less

Submitted 18 January, 2022; v1 submitted 20 April, 2021; originally announced April 2021.

Field-induced reorientation of helimagnetic order in Cu$_2$OSeO$_3$ probed by magnetic force microscopy

Authors: Peter Milde, Laura Köhler, Erik Neuber, Philipp Ritzinger, Markus Garst, Andreas Bauer, Christian Pfleiderer, Helmuth Berger, Lukas M. Eng

Abstract: Cu$_2$OSeO$_3$ is an insulating skyrmion-host material with a magnetoelectric coupling giving rise to an electric polarization with a characteristic dependence on the magnetic field $\vec H$. We report magnetic force microscopy imaging of the helical real-space spin structure on the surface of a bulk single crystal of Cu$_2$OSeO$_3$. In the presence of a magnetic field, the helimagnetic order in g… ▽ More Cu$_2$OSeO$_3$ is an insulating skyrmion-host material with a magnetoelectric coupling giving rise to an electric polarization with a characteristic dependence on the magnetic field $\vec H$. We report magnetic force microscopy imaging of the helical real-space spin structure on the surface of a bulk single crystal of Cu$_2$OSeO$_3$. In the presence of a magnetic field, the helimagnetic order in general reorients and acquires a homogeneous component of the magnetization, resulting in a conical arrangement at larger fields. We investigate this reorientation process at a temperature of 10~K for fields close to the crystallographic $\langle 110\rangle$ direction that involves a phase transition at $H_{c1}$. Experimental evidence is presented for the formation of magnetic domains in real space as well as for the microscopic origin of relaxation events that accompany the reorientation process. In addition, the electric polarization is measured by means of Kelvin-probe force microscopy. We show that the characteristic field dependency of the electric polarization originates in this helimagnetic reorientation process. Our experimental results are well described by an effective Landau theory previously invoked for MnSi, that captures the competition between magnetocrystalline anisotropies and Zeeman energy. △ Less

Submitted 22 March, 2021; originally announced March 2021.

Journal ref: Physical Review B102, 024426 (2020)

Surface pinning and triggered unwinding of skyrmions in a cubic chiral magnet

Authors: Peter Milde, Erik Neuber, Andreas Bauer, Christian Pfleiderer, Lukas M. Eng

Abstract: In the cubic chiral magnet Fe$_{1-x}$Co$_{x}$Si a metastable state comprising of topologically nontrivial spin whirls, so-called skyrmions, may be preserved down to low temperatures by means of field cooling the sample. This metastable skyrmion state is energetically separated from the topologically trivial ground state by a considerable potential barrier, a phenomenon also referred to as topologi… ▽ More In the cubic chiral magnet Fe$_{1-x}$Co$_{x}$Si a metastable state comprising of topologically nontrivial spin whirls, so-called skyrmions, may be preserved down to low temperatures by means of field cooling the sample. This metastable skyrmion state is energetically separated from the topologically trivial ground state by a considerable potential barrier, a phenomenon also referred to as topological protection. Using magnetic force microscopy on the surface of a bulk crystal, we show that certain positions are preferentially and reproducibly decorated with metastable skyrmions, indicating that surface pinning plays a crucial role. Increasing the magnetic field allows an increasing number of skyrmions to overcome the potential barrier, and hence to transform into the ground state. Most notably, we find that the unwinding of individual skyrmions may be triggered by the magnetic tip itself, however, only when its magnetization is aligned parallel to the external field. This implies that the stray field of the tip is key for locally overcoming the topological protection. Both the control of the position of topologically nontrivial states as well as their creation and annihilation on demand pose important challenges in the context of potential skyrmionic applications. △ Less

Submitted 23 March, 2021; v1 submitted 22 March, 2021; originally announced March 2021.

Journal ref: Physical Review B 100, 024408 (2019)

Architecture of nanoscale ferroelectric domains in GaMo4S8

Authors: Erik Neuber, Peter Milde, Adam Butykai, Sandor Bordacs, Hiroyuki Nakamura, Takeshi Waki, Yoshikazu Tabata, Korbinian Geirhos, Peter Lunkenheimer, Istvan Kézsmárki, Petr Ondrejkovic, Jirka Hlinka, Lukas M. Eng

Abstract: Local-probe imaging of the ferroelectric domain structure and auxiliary bulk pyroelectric measurements were conducted at low temperatures with the aim to clarify the essential aspects of the orbitally driven phase transition in GaMo4S8, a lacunar spinel crystal that can be viewed as a spin-hole analogue of its GaV4S8 counterpart. We employed multiple scanning probe techniques combined with symmetr… ▽ More Local-probe imaging of the ferroelectric domain structure and auxiliary bulk pyroelectric measurements were conducted at low temperatures with the aim to clarify the essential aspects of the orbitally driven phase transition in GaMo4S8, a lacunar spinel crystal that can be viewed as a spin-hole analogue of its GaV4S8 counterpart. We employed multiple scanning probe techniques combined with symmetry and mechanical compatibility analysis to uncover the hierarchical domain structures, developing on the 10-100 nm scale. The identified domain architecture involves a plethora of ferroelectric domain boundaries and junctions, including primary and secondary domain walls in both electrically neutral and charged configurations, and topological line defects transforming neutral secondary walls into two oppositely charged ones. △ Less

Submitted 11 January, 2021; originally announced January 2021.

Comments: This is the Accepted Manuscript version of an article accepted for publication in Journal of Physics: Condensed Matter. IOP Publishing Ltd is not responsible for any errors or omissions in this version of the manuscript or any version derived from it. The Version of Record is available online at https://doi.org/10.1088/1361-648X/aae448

Journal ref: Journal of Physics: Condensed Matter 30 (2018) 445402 (9pp)

Anisotropic fractal magnetic domain pattern in bulk Mn$_{1.4}$PtSn

Authors: A. S. Sukhanov, B. E. Zuniga Cespedes, P. Vir, A. S. Cameron, A. Heinemann, N. Martin, G. Chaboussant, V. Kumar, P. Milde, L. M. Eng, C. Felser, D. S. Inosov

Abstract: The tetragonal compound Mn$_{1.4}$PtSn with the $D_{2d}$ symmetry recently attracted attention as the first known material that hosts magnetic antiskyrmions, which differ from the so far known skyrmions by their internal structure. The latter have been found in a number of magnets with the chiral crystal structure. In previous works, the existence of antiskyrmions in Mn$_{1.4}$PtSn was unambiguous… ▽ More The tetragonal compound Mn$_{1.4}$PtSn with the $D_{2d}$ symmetry recently attracted attention as the first known material that hosts magnetic antiskyrmions, which differ from the so far known skyrmions by their internal structure. The latter have been found in a number of magnets with the chiral crystal structure. In previous works, the existence of antiskyrmions in Mn$_{1.4}$PtSn was unambiguously demonstrated in real space by means of Lorentz transmission electron microscopy on thin-plate samples ($\sim$100~nm thick). In the present study, we used small-angle neutron scattering and magnetic force microscopy to perform reciprocal- and real-space imaging of the magnetic texture of bulk Mn$_{1.4}$PtSn single-crystals at different temperatures and in applied magnetic field. We found that the magnetic texture in the bulk differs significantly from that of thin-plate samples. Instead of spin helices or an antiskyrmion lattice, we observe an anisotropic fractal magnetic pattern of closure domains in zero field above the spin-reorientation transition temperature, which transforms into a set of bubble domains in high field. Below the spin-reorientation transition temperature the strong in-plane anisotropy as well as the fractal self-affinity in zero field is gradually lost, while the formation of bubble domains in high field remains robust. The results of our study highlight the importance of dipole-dipole interactions in thin-plate samples for the stabilization of antiskyrmions and identify criteria which should guide the search for potential (anti)skyrmion host materials. Moreover, they provide consistent interpretations of the previously reported magnetotransport anomalies of the bulk crystals. △ Less

Submitted 15 July, 2020; originally announced July 2020.

Journal ref: Phys. Rev. B 102, 174447 (2020)

Nanoscale-confined Terahertz Polaritons in a van der Waals Crystal

Authors: Thales V. A. G. de Oliveira, Tobias Nörenberg, Gonzalo Álvarez-Pérez, Lukas Wehmeier, Javier Taboada-Gutiérrez, Maximilian Obst, Franz Hempel, Eduardo J. H. Lee, John M. Klopf, Ion Errea, Alexey Y. Nikitin, Susanne C. Kehr, Pablo Alonso-Gonzaléz, Lukas M. Eng

Abstract: Electromagnetic field confinement is crucial for nanophotonic technologies, since it allows for enhancing light-matter interactions, thus enabling light manipulation in deep sub-wavelength scales. In the terahertz (THz) spectral range, radiation confinement is conventionally achieved with specially designed metallic structures - such as antennas or nanoslits - with large footprints due to the rath… ▽ More Electromagnetic field confinement is crucial for nanophotonic technologies, since it allows for enhancing light-matter interactions, thus enabling light manipulation in deep sub-wavelength scales. In the terahertz (THz) spectral range, radiation confinement is conventionally achieved with specially designed metallic structures - such as antennas or nanoslits - with large footprints due to the rather long wavelengths of THz radiation. In this context, phonon polaritons - light coupled to lattice vibrations - in van der Waals (vdW) crystals have emerged as a promising solution for controlling light beyond the diffraction limit, as they feature extreme field confinements and low optical losses. However, experimental demonstration of nanoscale-confined phonon polaritons at THz frequencies has so far remained elusive. Here, we provide it by employing scattering-type scanning near-field optical microscopy (s-SNOM) combined with a free-electron laser (FEL) to reveal a range of low-loss polaritonic excitations at frequencies from 8 to 12 THz in the vdW semiconductor $α-MoO_3$. We visualize THz polaritons with i) in-plane hyperbolic dispersion, ii) extreme nanoscale field confinement (below $λ_o/75$) and iii) long polariton lifetimes, with a lower limit of > 2 ps. △ Less

Submitted 11 November, 2020; v1 submitted 13 July, 2020; originally announced July 2020.

Journal ref: Adv. Mater. 2021, 33, 2005777

Terahertz signatures of ultrafast Dirac fermion relaxation at the surface of topological insulators at room temperature

Authors: S. Kovalev, K. -J. Tielrooij, J. -C. Deinert, I. Ilyakov, N. Awari, M. Chen, A. Ponomaryov, M. Bawatna, T. V. A. G. de Oliveira, L. M. Eng, K. A. Kuznetsov, G. Kh. Kitaeva, P. I. Kuznetsov, H. A. Hafez, D. Turchinovich, M. Gensch

Abstract: Topologically-protected surface states present rich physics and promising spintronic, optoelectronic and photonic applications that require a proper understanding of their ultrafast carrier dynamics. Here, we investigate these dynamics in topological insulators (TIs) of the bismuth and antimony chalcogenide family, where we isolate the response of Dirac fermions at the surface from the response of… ▽ More Topologically-protected surface states present rich physics and promising spintronic, optoelectronic and photonic applications that require a proper understanding of their ultrafast carrier dynamics. Here, we investigate these dynamics in topological insulators (TIs) of the bismuth and antimony chalcogenide family, where we isolate the response of Dirac fermions at the surface from the response of bulk carriers by combining photoexcitation with below-bandgap terahertz (THz) photons with TI samples with varying Fermi level, including one sample with the Fermi level located within the bandgap. We identify distinctly faster relaxation of charge carriers in the topologically-protected Dirac surface states (few hundred femtoseconds), compared to bulk carriers (few picoseconds). In agreement with such fast cooling dynamics, we observe THz harmonic generation without any saturation effects for increasing incident fields, unlike graphene which exhibits strong saturation. This opens up promising avenues for increased THz nonlinear conversion efficiencies, and high-bandwidth optoelectronic and spintronic information and communication applications. △ Less

Submitted 1 March, 2021; v1 submitted 6 June, 2020; originally announced June 2020.