Browsing by Author "Das, Sujit"

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    Emergent chirality in a polar meron to skyrmion phase transition
    (Springer Nature, 2023) Shao, Yu-Tsun; Das, Sujit; Hong, Zijian; Xu, Ruijuan; Chandrika, Swathi; Gómez-Ortiz, Fernando; García-Fernández, Pablo; Chen, Long-Qing; Hwang, Harold Y.; Junquera, Javier; Martin, Lane W.; Ramesh, Ramamoorthy; Muller, David A.
    Polar skyrmions are predicted to emerge from the interplay of elastic, electrostatic and gradient energies, in contrast to the key role of the anti-symmetric Dzyalozhinskii-Moriya interaction in magnetic skyrmions. Here, we explore the reversible transition from a skyrmion state (topological charge of −1) to a two-dimensional, tetratic lattice of merons (with topological charge of −1/2) upon varying the temperature and elastic boundary conditions in [(PbTiO3)16/(SrTiO3)16]8 membranes. This topological phase transition is accompanied by a change in chirality, from zero-net chirality (in meronic phase) to net-handedness (in skyrmionic phase). We show how scanning electron diffraction provides a robust measure of the local polarization simultaneously with the strain state at sub-nm resolution, while also directly mapping the chirality of each skyrmion. Using this, we demonstrate strain as a crucial order parameter to drive isotropic-to-anisotropic structural transitions of chiral polar skyrmions to non-chiral merons, validated with X-ray reciprocal space mapping and phase-field simulations.
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    Optical Control of Adaptive Nanoscale Domain Networks
    (Wiley, 2024) Zajac, Marc; Zhou, Tao; Yang, Tiannan; Das, Sujit; Cao, Yue; Guzelturk, Burak; Stoica, Vladimir; Cherukara, Mathew J.; Freeland, John W.; Gopalan, Venkatraman; Ramesh, Ramamoorthy; Martin, Lane W.; Chen, Long-Qing; Holt, Martin V.; Hruszkewycz, Stephan O.; Wen, Haidan; Rice Advanced Materials Institute
    Adaptive networks can sense and adjust to dynamic environments to optimize their performance. Understanding their nanoscale responses to external stimuli is essential for applications in nanodevices and neuromorphic computing. However, it is challenging to image such responses on the nanoscale with crystallographic sensitivity. Here, the evolution of nanodomain networks in (PbTiO3)n/(SrTiO3)n superlattices (SLs) is directly visualized in real space as the system adapts to ultrafast repetitive optical excitations that emulate controlled neural inputs. The adaptive response allows the system to explore a wealth of metastable states that are previously inaccessible. Their reconfiguration and competition are quantitatively measured by scanning x-ray nanodiffraction as a function of the number of applied pulses, in which crystallographic characteristics are quantitatively assessed by assorted diffraction patterns using unsupervised machine-learning methods. The corresponding domain boundaries and their connectivity are drastically altered by light, holding promise for light-programable nanocircuits in analogy to neuroplasticity. Phase-field simulations elucidate that the reconfiguration of the domain networks is a result of the interplay between photocarriers and transient lattice temperature. The demonstrated optical control scheme and the uncovered nanoscopic insights open opportunities for the remote control of adaptive nanoscale domain networks.
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    The emergence of three-dimensional chiral domain walls in polar vortices
    (Springer Nature, 2023) Susarla, Sandhya; Hsu, Shanglin; Gómez-Ortiz, Fernando; García-Fernández, Pablo; Savitzky, Benjamin H.; Das, Sujit; Behera, Piush; Junquera, Javier; Ercius, Peter; Ramesh, Ramamoorthy; Ophus, Colin
    Chirality or handedness of a material can be used as an order parameter to uncover the emergent electronic properties for quantum information science. Conventionally, chirality is found in naturally occurring biomolecules and magnetic materials. Chirality can be engineered in a topological polar vortex ferroelectric/dielectric system via atomic-scale symmetry-breaking operations. We use four-dimensional scanning transmission electron microscopy (4D-STEM) to map out the topology-driven three-dimensional domain walls, where the handedness of two neighbor topological domains change or remain the same. The nature of the domain walls is governed by the interplay of the local perpendicular (lateral) and parallel (axial) polarization with respect to the tubular vortex structures. Unique symmetry-breaking operations and the finite nature of domain walls result in a triple point formation at the junction of chiral and achiral domain walls. The unconventional nature of the domain walls with triple point pairs may result in unique electrostatic and magnetic properties potentially useful for quantum sensing applications.