arXiv subjects

Harikrishnan KP

Publications and source records attributed to Harikrishnan KP.

7 recordsLinked to original sources

arXiv2026

Defect-engineered scaling of lead-free ferroelectrics with ultralow-voltage switching

Scaling ferroelectrics to nanometer thicknesses remains a central challenge for low-power, nonvolatile electronics, as leakage currents increasingly dominate with reduced dimensions. Alkali-based, lead-free ferroelectrics offer an environmentally sustainable alternative to lead-based systems, yet their scaling is severely limited by leakage arising from volatile alkali constituents. Here, we show that this intrinsic limitation can be transformed into an advantageous degree of freedom through defect engineering. By precisely modulating alkali deficiency during thin-film synthesis, we engineer clustered defect complexes that function as deep trap states, strongly suppressing leakage and enabling robust ferroelectric operation in ultrathin films down to the sub-10 nm regime at voltages below 100 mV. Our results establish defect-enabled scaling as a viable pathway for advancing environmentally benign ferroelectrics toward ultra-low-power, non-volatile electronic technologies.

cond-mat.mtrl-sci↗

arXiv2025

Mind the Gap -- Imaging Buried Interfaces in Twisted Oxide Moir\'es

The ability to tune electronic structure in twisted stacks of two-dimensional (2D) materials has motivated the exploration of similar moir\'e physics with twisted oxide membranes. Due to the intrinsic three-dimensional nature of bonding in many oxides, achieving atomic-level coupling is significantly more challenging than with 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 contaminants and amorphous interlayers are eliminated. In interfaces between 2D materials and oxide membranes the reduced ability of the 2D material to conform to the membrane's step-terrace topography also limits atomic-scale contact. When imaging stacked membranes in projection, we find conventional through-focal imaging to be relatively insensitive to the buried interface, whereas electron ptychography detects structural variations on the order of a nanometer. These findings highlight interface roughness as a key challenge for the field of oxide twistronics and emphasize the need for reliable characterization methods, both in cross-section and projection.

cond-mat.mtrl-sci↗

arXiv2025

Cepstral Strain Mapping for Small Pixel-Count Detectors

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.

physics.ins-det↗

arXiv2025

Mic-hackathon 2024: Hackathon on Machine Learning for Electron and Scanning Probe Microscopy

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

cond-mat.mtrl-sci↗

arXiv2025

Microscopic mechanisms of flexoelectricity in oxide membranes

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.

cond-mat.mtrl-sci↗

arXiv2025

Improper Ferroelectricity at the Monolayer Limit

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.

cond-mat.mtrl-sci↗

arXiv2024

Electron ptychography reveals a ferroelectricity dominated by anion displacements

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.

cond-mat.mtrl-sci↗