-
Mind the Gap -- Imaging Buried Interfaces in Twisted Oxide Moirés
Authors:
Harikrishnan KP,
Xin Wei,
Chia-Hao Lee,
Dasol Yoon,
Yonghun Lee,
Kevin J. Crust,
Yu-Tsun Shao,
Ruijuan Xu,
Jong-Hoon Kang,
Ce Liang,
Jiwoong Park,
Harold Y. Hwang,
David A. Muller
Abstract:
The ability to tune electronic structure in twisted stacks of layered, two-dimensional (2D) materials has motivated the exploration of similar moiré physics with stacks of twisted oxide membranes. Due to the intrinsic three-dimensional (3D) nature of bonding in many oxides, achieving atomic-level coupling is significantly more challenging than in 2D van der Waals materials. Although clean interfac…
▽ More
The ability to tune electronic structure in twisted stacks of layered, two-dimensional (2D) materials has motivated the exploration of similar moiré physics with stacks of twisted oxide membranes. Due to the intrinsic three-dimensional (3D) nature of bonding in many oxides, achieving atomic-level coupling is significantly more challenging than in 2D van der Waals materials. Although clean interfaces with atomic level proximity have been demonstrated in ceramic bicrystals using high-temperature and high-pressure processing to facilitate atomic diffusion that flattens rough interfaces, such conditions are not readily accessible when bonding oxide membranes. This study shows how topographic mismatch due to surface roughness of the membranes can restrict atomic-scale proximity at the interface to isolated patches even after obvious issues of contaminants and amorphous interlayers are eliminated. In hybrid interfaces between a chemically inert 2D material and an oxide membrane, the reduced ability of the 2D material to conform to the membrane's step-terrace topography also limits atomic-scale contact. In all these material systems, the interface morphology is best characterized using cross-sectional imaging and is necessary to corroborate investigations of interlayer coupling. When imaging the bicrystal in projection, conventional through-focal imaging is found to be relatively insensitive to the buried interface, whereas electron ptychography reliably resolves structural variations on the order of a nanometer. These findings highlight interface roughness as a key challenge for the field of oxide twistronics and emphasizes the need for reliable characterization methods.
△ Less
Submitted 27 October, 2025;
originally announced October 2025.
-
Cepstral Strain Mapping for Small Pixel-Count Detectors
Authors:
Harikrishnan KP,
Dasol Yoon,
Yu-Tsun Shao,
Zhaslan Baraissov,
Luigi Mele,
Christoph Mitterbauer,
Erik Kieft,
Stefano Vespucci,
David A. Muller
Abstract:
With the decreasing sizes of integrated-circuit components, the semiconductor industry is in growing need of high-throughput strain mapping techniques that offer high precision and spatial resolution, with desired industry goals of 0.01-0.1% and 1 nm respectively. As the fundamental limitation on the measurement precision is set by the Poisson noise, pixel array detectors with high saturation curr…
▽ More
With the decreasing sizes of integrated-circuit components, the semiconductor industry is in growing need of high-throughput strain mapping techniques that offer high precision and spatial resolution, with desired industry goals of 0.01-0.1% and 1 nm respectively. As the fundamental limitation on the measurement precision is set by the Poisson noise, pixel array detectors with high saturation current, high dynamic range and fast readout are ideally suited for this purpose. However, due to the limited pixel count on these detectors, they do not work well with traditional strain mapping algorithms that were optimized to work on datasets with a large pixel count. Here, we evaluate the cepstral transform that was designed to address this problem, with the precision determined by the convergence, collection angles and dose while remaining insensitive to the pixel count. We test the performance of our method on silicon wedges and Si-SiGe multilayers, and using datasets collected at different conditions, we show how the measured strain precision scales as a function of dose, aperture size and sample thickness. Using precession gives a further improvement in precision by about 1.5-2x, whereas energy filtering does not have a significant impact on the cepstral method for device-relevant sample thickness ranges.
△ Less
Submitted 10 September, 2025;
originally announced September 2025.
-
Mic-hackathon 2024: Hackathon on Machine Learning for Electron and Scanning Probe Microscopy
Authors:
Utkarsh Pratiush,
Austin Houston,
Kamyar Barakati,
Aditya Raghavan,
Dasol Yoon,
Harikrishnan KP,
Zhaslan Baraissov,
Desheng Ma,
Samuel S. Welborn,
Mikolaj Jakowski,
Shawn-Patrick Barhorst,
Alexander J. Pattison,
Panayotis Manganaris,
Sita Sirisha Madugula,
Sai Venkata Gayathri Ayyagari,
Vishal Kennedy,
Ralph Bulanadi,
Michelle Wang,
Kieran J. Pang,
Ian Addison-Smith,
Willy Menacho,
Horacio V. Guzman,
Alexander Kiefer,
Nicholas Furth,
Nikola L. Kolev
, et al. (48 additional authors not shown)
Abstract:
Microscopy is a primary source of information on materials structure and functionality at nanometer and atomic scales. The data generated is often well-structured, enriched with metadata and sample histories, though not always consistent in detail or format. The adoption of Data Management Plans (DMPs) by major funding agencies promotes preservation and access. However, deriving insights remains d…
▽ More
Microscopy is a primary source of information on materials structure and functionality at nanometer and atomic scales. The data generated is often well-structured, enriched with metadata and sample histories, though not always consistent in detail or format. The adoption of Data Management Plans (DMPs) by major funding agencies promotes preservation and access. However, deriving insights remains difficult due to the lack of standardized code ecosystems, benchmarks, and integration strategies. As a result, data usage is inefficient and analysis time is extensive. In addition to post-acquisition analysis, new APIs from major microscope manufacturers enable real-time, ML-based analytics for automated decision-making and ML-agent-controlled microscope operation. Yet, a gap remains between the ML and microscopy communities, limiting the impact of these methods on physics, materials discovery, and optimization. Hackathons help bridge this divide by fostering collaboration between ML researchers and microscopy experts. They encourage the development of novel solutions that apply ML to microscopy, while preparing a future workforce for instrumentation, materials science, and applied ML. This hackathon produced benchmark datasets and digital twins of microscopes to support community growth and standardized workflows. All related code is available at GitHub: https://github.com/KalininGroup/Mic-hackathon-2024-codes-publication/tree/1.0.0.1
△ Less
Submitted 27 June, 2025; v1 submitted 9 June, 2025;
originally announced June 2025.
-
Microscopic mechanisms of flexoelectricity in oxide membranes
Authors:
Harikrishnan KP,
Varun Harbola,
Jaehong Choi,
Kevin J. Crust,
Yu-Tsun Shao,
Chia-Hao Lee,
Dasol Yoon,
Yonghun Lee,
Gregory D. Fuchs,
Cyrus E. Dreyer,
Harold Y. Hwang,
David A. Muller
Abstract:
Modern electromechanical actuators and sensors rely on the piezoelectric effect that linearly couples strain and electric polarization. However, this effect is restricted to materials that lack inversion symmetry. In contrast, the flexoelectric effect couples strain gradients to electric polarization, and is a universal property in insulating materials of arbitrary symmetry. Flexoelectricity becom…
▽ More
Modern electromechanical actuators and sensors rely on the piezoelectric effect that linearly couples strain and electric polarization. However, this effect is restricted to materials that lack inversion symmetry. In contrast, the flexoelectric effect couples strain gradients to electric polarization, and is a universal property in insulating materials of arbitrary symmetry. Flexoelectricity becomes prominent at the nanoscale from the inverse scaling of strain gradients with material dimensions. Here, we measure the strain-gradient-induced structural distortions in strontium titanate using multislice electron ptychography. This technique enables reliable picometer-scale measurements of the dominant oxygen-titanium distortions, correcting for artifacts that limited conventional imaging methods. This enables us to directly measure the sign of the net ionic contribution to the flexoelectric polarization. Guided by the experimental measurements, first-principles calculations show how the sign and magnitude of the bulk contribution to the flexoelectric coefficient in strontium titanate can be switched by tuning the strain state. Hybridization between the optical soft phonon and acoustic phonon modes drives this transition, yielding a large response and a polarity switch across the resonance. This strain-dependence might explain the sign discrepancy and orders of magnitude variation in the values of previously reported flexoelectric coefficients for strontium titanate. As the strain state of curved membranes can be tuned, our approach also suggests an approach to engineer nanoscale flexoelectric polarization using strain as a control parameter.
△ Less
Submitted 17 March, 2025;
originally announced March 2025.
-
Improper Ferroelectricity at the Monolayer Limit
Authors:
Yilin Evan Li,
Harikrishnan KP,
Haidong Lu,
Rachel A. Steinhardt,
Megan E. Holtz,
Mario Brützam,
Matthew M. Dykes,
Elke Arenholz,
Sankalpa Hazra,
Adriana LaVopa,
Xiaoxi Huang,
Wenwen Zhao,
Piush Behera,
Maya Ramesh,
Evan Krysko,
Venkatraman Gopalan,
Ramamoorthy Ramesh,
Craig J. Fennie,
Robert J. Cava,
Christo Guguschev,
Alexei Gruverman,
David A. Muller,
Darrell G. Schlom
Abstract:
Ultrathin ferroelectric films with out-of-plane polarization and high Curie temperatures are key to miniaturizing electronic devices. Most ferroelectrics employed in devices are proper ferroelectrics, where spontaneous polarization is the primary order parameter. Unfortunately, the Curie temperature of proper ferroelectrics is drastically reduced as the ferroelectric becomes thin; nearly all prope…
▽ More
Ultrathin ferroelectric films with out-of-plane polarization and high Curie temperatures are key to miniaturizing electronic devices. Most ferroelectrics employed in devices are proper ferroelectrics, where spontaneous polarization is the primary order parameter. Unfortunately, the Curie temperature of proper ferroelectrics is drastically reduced as the ferroelectric becomes thin; nearly all proper ferroelectrics need to be thicker than several unit cells. The absence of an ultrathin limit has been predicted, but not verified for improper ferroelectrics. These are ferroelectrics where the polarization emerges secondary to the primary order parameter, such as a structural distortion. Here we report improper ferroelectricity with an undiminished Curie temperature in a 0.75-unit-cell-thick hexagonal LuFeO3 (h-LuFeO3) film grown on a SrCo2Ru4O11 bottom electrode with an atomically engineered monolayer bridging layer. Our results demonstrate the absence of a critical thickness for improper ferroelectricity and provide a methodology for creating ultrathin improper ferroelectrics by stabilizing their primary order parameters.
△ Less
Submitted 8 March, 2025;
originally announced March 2025.
-
Electron ptychography reveals a ferroelectricity dominated by anion displacements
Authors:
Harikrishnan KP,
Ruijuan Xu,
Kinnary Patel,
Kevin J. Crust,
Aarushi Khandelwal,
Chenyu Zhang,
Sergey Prosandeev,
Hua Zhou,
Yu-Tsun Shao,
Laurent Bellaiche,
Harold Y. Hwang,
David A. Muller
Abstract:
Sodium niobate, a lead-free ferroic material, hosts delicately-balanced, competing order parameters, including ferroelectric states that can be stabilized by epitaxial strain. Here, we show that the resulting macroscopic ferroelectricity exhibits an unconventional microscopic structure using multislice electron ptychography. This technique overcomes multiple scattering artifacts limiting conventio…
▽ More
Sodium niobate, a lead-free ferroic material, hosts delicately-balanced, competing order parameters, including ferroelectric states that can be stabilized by epitaxial strain. Here, we show that the resulting macroscopic ferroelectricity exhibits an unconventional microscopic structure using multislice electron ptychography. This technique overcomes multiple scattering artifacts limiting conventional electron microscopy, enabling both lateral spatial resolution beyond the diffraction limit and recovery of three-dimensional structural information. These imaging capabilities allow us to separate the ferroelectric interior of the sample from the relaxed surface structure and identify the soft phonon mode and related structural distortions with picometer precision. Unlike conventional ferroelectric perovskites, we find that the polar distortion in this material involves minimal distortions of the cation sublattices and is instead dominated by anion displacements relative to the niobium sublattice. We establish limits on film thickness for interfacial octahedral rotation engineering and directly visualize a random octahedral rotation pattern, arising from the flat dispersion of the associated phonon mode.
△ Less
Submitted 14 January, 2025; v1 submitted 27 August, 2024;
originally announced August 2024.