Structure retrieval at atomic resolution in the presence of multiple scattering of the electron probe

The projected electrostatic potential of a thick crystal is reconstructed at atomic-resolution from experimental scanning transmission electron microscopy data recorded using a new generation fast- readout electron camera. This practical and deterministic inversion of the equations encapsulating multiple scattering that were written down by Bethe in 1928 removes the restriction of established methods to ultrathin ($\lesssim 50$ {\AA}) samples. Instruments already coming on-line can overcome the remaining resolution-limiting effects in this method due to finite probe-forming aperture size, spatial incoherence and residual lens aberrations.Comment: 6 pages, 3 figure

Book Reviews

Book Review 1Book Title: Atlas of Microscopic Structures of Fur Skins Vol. 1Book Authors: Anton Blažej et al.Elsevier Amsterdam and SNTl, Prague, 1989. 378 pages.Book Review 2Book Title: Ornithology for AfricaBook Author: Gordon L. MacleanUniversity of Natal Press, 1990. 270 pages.Book Review 3Book Title: Biology of the Vespine WaspsBook Authors: Makoto Matsuura & Seiki YamaneSpringer-Verlag, Berlin, 1990. 323 pages, numerous figures, tables and photographs. ISBN 3-540-51900-9Book Review 4Book Title: Horns, Pronghorns, and AntlersBook Authors: Edited by G.A. Bubenik & A.B. BubenikSpringer-Verlag, New York. ISBN 0-387-97176-9. 562 pp.Book Review 5Book Title: Ecophysiology of Desert Arthropods and ReptilesBook Author: J.L. Cloudsley-ThompsonSpringer-Verlag, 1991. 203 pagesBook Review 6Book Title: Practical Taxonomic ComputingBook Author: Richard J. PankhurstCambridge University Press. 202 page

Uncovering polar vortex structures by inversion of multiple scattering with a stacked Bloch wave model

Nanobeam electron diffraction can probe local structural properties of complex crystalline materials including phase, orientation, tilt, strain, and polarization. Ideally, each diffraction pattern from a projected area of a few unit cells would produce clear a Bragg diffraction pattern, where the reciprocal lattice vectors can be measured from the spacing of the diffracted spots, and the spot intensities are equal to the square of the structure factor amplitudes. However, many samples are too thick for this simple interpretation of their diffraction patterns, as multiple scattering of the electron beam can produce a highly nonlinear relationship between the spot intensities and the underlying structure. Here, we develop a stacked Bloch wave method to model the diffracted intensities from thick samples with structure that varies along the electron beam. Our method reduces the large parameter space of electron scattering to just a few structural variables per probe position, making it fast enough to apply to very large fields of view. We apply our method to SrTiO$_3$/PbTiO$_3$/SrTiO$_3$ multilayer samples, and successfully disentangle specimen tilt from the mean polarization of the PbTiO$_3$ layers. We elucidate the structure of complex vortex topologies in the PbTiO$_3$ layers, demonstrating the promise of our method to extract material properties from thick samples

Book Reviews

Book Review 1Book Title: Megaherbivores: the influence of very large body size on ecologyBook Author: R.N. Owen-SmithCambridge University Press, Cambridge, 1988. 369 pages.Book Review 2Book Title: Comparative Physiology of the Vertebrate KidneyBook Author: W.H. Dantzler Springer-Verlag 1989.198 pp. (Hardcover).Book Review 3Book Title: Atlas on the Biology of Soil ArthropodsBook Authors: G. Esenbeis & W. Wichard Springer-Verlag, Berlin 1987. 437 pp.Book Review 4Book Title: Primate Vocal CommunicationBook Authors: Edited by D. Todt, P. Goedeking & D. SymmesSpringer Verlag, Berlin (1988)Book Review 5Book Title: The Honey BeeBook Authors: J.L. Gould & C. G. GouldScientific American Ubrary, W.H.Freeman, New York. 239 pp.Book Review 6Book Title: Insect Development Photoperiodic and Temperature ControlBook Author: Victor A. ZaslavskiSpringer-Verlag, Berlin. 187 pp.Book Review 7Book Title: OrganellesBook Author: Mark CarrollMacmillan 1989. 202 pp.Book Review 8Book Title: Comparative Protozoology Ecology, Physiology, Life HistoryBook Author: O. Roger AndersonSpringer-Verlag, Berlin, 1988. 482pp.Book Review 9Book Title: Ecotoxicology: Problems and ApproachesBook Authors: Edited by S.A. Levin, M.A. Harwell, J.R. Kelly & K.D. KimballSpringer Verlag, New York. 547 pp

Strain fields in twisted bilayer graphene

Van der Waals heteroepitaxy allows deterministic control over lattice mismatch or azimuthal orientation between atomic layers to produce long wavelength superlattices. The resulting electronic phases depend critically on the superlattice periodicity as well as localized structural deformations that introduce disorder and strain. Here, we introduce Bragg interferometry, based on four-dimensional scanning transmission electron microscopy, to capture atomic displacement fields in twisted bilayer graphene with twist angles < 2{\deg}. Nanoscale spatial fluctuations in twist angle and uniaxial heterostrain are statistically evaluated, revealing the prevalence of short-range disorder in this class of materials. By quantitatively mapping strain tensor fields we uncover two distinct regimes of structural relaxation -- in contrast to previous models depicting a single continuous process -- and we disentangle the electronic contributions of the rotation modes that comprise this relaxation. Further, we find that applied heterostrain accumulates anisotropically in saddle point regions to generate distinctive striped shear strain phases. Our results thus establish the reconstruction mechanics underpinning the twist angle dependent electronic behaviour of twisted bilayer graphene, and provide a new framework for directly visualizing structural relaxation, disorder, and strain in any moir\'e material.Comment: 29 pages, 6 figures plus supporting information (42 pages, 28 figures

py4DSTEM: a software package for multimodal analysis of four-dimensional scanning transmission electron microscopy datasets

Scanning transmission electron microscopy (STEM) allows for imaging, diffraction, and spectroscopy of materials on length scales ranging from microns to atoms. By using a high-speed, direct electron detector, it is now possible to record a full 2D image of the diffracted electron beam at each probe position, typically a 2D grid of probe positions. These 4D-STEM datasets are rich in information, including signatures of the local structure, orientation, deformation, electromagnetic fields and other sample-dependent properties. However, extracting this information requires complex analysis pipelines, from data wrangling to calibration to analysis to visualization, all while maintaining robustness against imaging distortions and artifacts. In this paper, we present py4DSTEM, an analysis toolkit for measuring material properties from 4D-STEM datasets, written in the Python language and released with an open source license. We describe the algorithmic steps for dataset calibration and various 4D-STEM property measurements in detail, and present results from several experimental datasets. We have also implemented a simple and universal file format appropriate for electron microscopy data in py4DSTEM, which uses the open source HDF5 standard. We hope this tool will benefit the research community, helps to move the developing standards for data and computational methods in electron microscopy, and invite the community to contribute to this ongoing, fully open-source project

Curriculum Making as the Enactment of Dwelling in Places

This article uses an account of dwelling to interrogate the concept of curriculum making. Tim Ingold's use of dwelling to understand culture is productive here because of his implicit and explicit interest in intergenerational learning. His account of dwelling rests on a foundational ontological claim-that mental construction and representation are not the basis upon which we live in the world-which is very challenging for the kinds of curriculum making with which many educators are now familiar. It undermines assumptions of propositional knowledge and of the use of mental schemas to communicate and share. At the level of critique, then, dwelling destabilizes contemporary ideas of curriculum as textual, pre-specified content for transmission or pre-defined objectives or standardized activity. The positive claims of dwelling are equally challenging, for these are that the world is a domain of relational entanglement in which an organism can be no more than a point of growth for an emergent &lsquo;environment', and meaning only inheres in these relations. The paper articulates how differentiation (of learner, salient meanings, knowledge, skill and place) are possible in such an ontology, and how curriculum making can be understood from this perspective as being the remaking of relationships between these