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Open Access Publications from the University of California

Scholarly Works (6 results)

    Recent Work (1991)
      Cover page: Topics in Particle Physics and Cosmology
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      • Peer Reviewed
      UC Berkeley Previously Published Works (2017)
      Flexoelectricity refers to electric polarization generated by heterogeneous mechanical strains, namely strain gradients, in materials of arbitrary crystal symmetries. Despite more than 50 years of work on this effect, an accurate identification of its coupling strength remains an experimental challenge for most materials, which impedes its wide recognition. Here, we show the presence of flexoelectricity in the recently discovered polar vortices in PbTiO3/SrTiO3 superlattices based on a combination of machine-learning analysis of the atomic-scale electron microscopy imaging data and phenomenological phase-field modeling. By scrutinizing the influence of flexocoupling on the global vortex structure, we match theory and experiment using computer vision methodologies to determine the flexoelectric coefficients for PbTiO3 and SrTiO3. Our findings highlight the inherent, nontrivial role of flexoelectricity in the generation of emergent complex polarization morphologies and demonstrate a viable approach to delineating this effect, conducive to the deeper exploration of both topics.
        Cover page: Quantification of flexoelectricity in PbTiO<sub>3</sub>/SrTiO<sub>3</sub> superlattice polar vortices using machine learning and phase-field modeling.
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        • Peer Reviewed
        UCLA Previously Published Works (2004)

        The 1999 Chi-Chi, Taiwan, earthquake provides case histories of ground failure and non-ground failure that are valuable to the ongoing development of liquefaction susceptibility, triggering and surface manifestation models because the data occupy sparsely populated parameter spaces (i.e. high cyclic stress ratio and high fines content with low to moderate soil plasticity). In this paper, we document results from several large site investigation programs conducted in Nantou, Wufeng and Yuanlin, Taiwan. The seismic performance of the investigated sites include non-ground failure building and free-field sites, building sites with partial foundation hearing failures, free-field lateral spread sites, and free-field level ground sites with sediment boils. Field and laboratory investigation protocols for the sites are described, including cone penetration testing (some with pore pressure and shear wave velocity measurements) and rotary wash borings with standard penetration testing (including energy measurements). Implications of the SPT energy measurements with respect to established guidelines for the estimation of SPT energy ratio (including short rod corrections) are presented. Finally, data for three example sites are shown that illustrate potential applications of the data set, and which also demonstrate a condition where existing liquefaction analysis procedures fail to predict the observed field performance. (C) 2004 Elsevier Ltd. All rights reserved.

          Cover page: Documentation of soil conditions at liquefaction and non-liquefaction sites from 1999 Chi-Chi (Taiwan) earthquake
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          • Peer Reviewed
          Recent Work (2019)
          Complex topological configurations are fertile ground for exploring emergent phenomena and exotic phases in condensed-matter physics. For example, the recent discovery of polarization vortices and their associated complex-phase coexistence and response under applied electric fields in superlattices of (PbTiO3)n/(SrTiO3)n suggests the presence of a complex, multi-dimensional system capable of interesting physical responses, such as chirality, negative capacitance and large piezo-electric responses1-3. Here, by varying epitaxial constraints, we discover room-temperature polar-skyrmion bubbles in a lead titanate layer confined by strontium titanate layers, which are imaged by atomic-resolution scanning transmission electron microscopy. Phase-field modelling and second-principles calculations reveal that the polar-skyrmion bubbles have a skyrmion number of +1, and resonant soft-X-ray diffraction experiments show circular dichroism, confirming chirality. Such nanometre-scale polar-skyrmion bubbles are the electric analogues of magnetic skyrmions, and could contribute to the advancement of ferroelectrics towards functionalities incorporating emergent chirality and electrically controllable negative capacitance.
            Cover page: Observation of room-temperature polar skyrmions.
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            • Peer Reviewed
            UC Berkeley Previously Published Works (2017)
            Systems that exhibit phase competition, order parameter coexistence, and emergent order parameter topologies constitute a major part of modern condensed-matter physics. Here, by applying a range of characterization techniques, and simulations, we observe that in PbTiO3/SrTiO3 superlattices all of these effects can be found. By exploring superlattice period-, temperature- and field-dependent evolution of these structures, we observe several new features. First, it is possible to engineer phase coexistence mediated by a first-order phase transition between an emergent, low-temperature vortex phase with electric toroidal order and a high-temperature ferroelectric a1/a2 phase. At room temperature, the coexisting vortex and ferroelectric phases form a mesoscale, fibre-textured hierarchical superstructure. The vortex phase possesses an axial polarization, set by the net polarization of the surrounding ferroelectric domains, such that it possesses a multi-order-parameter state and belongs to a class of gyrotropic electrotoroidal compounds. Finally, application of electric fields to this mixed-phase system permits interconversion between the vortex and the ferroelectric phases concomitant with order-of-magnitude changes in piezoelectric and nonlinear optical responses. Our findings suggest new cross-coupled functionalities.
              Cover page: Phase coexistence and electric-field control of toroidal order in oxide superlattices.
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              • Peer Reviewed
              UC Irvine Previously Published Works (2016)
                Cover page: Anatomy of the sign-problem in heavy-dense QCDCreative Commons 'BY' version 4.0 license
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