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Register a new userA coherent way to image dislocations
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The use of coherent x-ray beams has been greatly developing for the past decades. They are now used by a wide scientific community to study biological materials, phase transitions in crystalline materials, soft matter, magnetism, strained structures, or nano-objects. Different kinds of measurements can be carried out: x-ray photon correlation spectroscopy allowing studying dynamics in soft and hard matter, and coherent diffraction imaging enabling to reconstruct the shape and strain of some objects by using methods such as holography or ptychography. In this article, we show that coherent x-ray diffraction (CXRD) brings a new insight in another scientific field: the detection of single phase defects in bulk materials. Extended phase objects such as dislocations embedded in the bulk are usually probed by electron microscopy or X-ray topography. However, electron microscopy is restricted to thin samples, and x-ray topography is resolution-limited. We show here that CXRD brings much more accurate information about dislocation lines (DLs) in bulk samples and opens a route for a better understanding of the fine structure of the core of bulk dislocations.
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Competitive mechanisms contribute to image contrast from dislocations in annular dark field scanning transmission electron microscopy ADF STEM. A clear theoretical understanding of the mechanisms underlying the ADF STEM contrast is therefore essential for correct interpretation of dislocation images. This paper reports on a systematic study of the ADF STEM contrast from dislocations in a GaN specimen, both experimentally and computationally. Systematic experimental ADF STEM images of the edge character dislocations revealed a number of characteristic contrast features that are shown to depend on both the angular detection range and specific position of the dislocation in the sample. A theoretical model based on electron channelling and Bloch wave scattering theories, supported by multislice simulations using Grillo s strain channelling equation, is proposed to elucidate the physical origin of such complex contrast phenomena.
We develop a non-singular theory of three-dimensional dislocation loops in a particular version of Mindlins anisotropic gradient elasticity with up to six length scale parameters. The theory is systematically developed as a generalization of the classical anisotropic theory in the framework of linearized incompatible elasticity. The non-singular version of all key equations of anisotropic dislocation theory are derived as line integrals, including the Burgers displacement equation with isolated solid angle, the Peach-Koehler stress equation, the Mura-Willis equation for the elastic distortion, and the Peach-Koehler force. The expression for the interaction energy between two dislocation loops as a double line integral is obtained directly, without the use of a stress function. It is shown that all the elastic fields are non-singular, and that they converge to their classical counterparts a few characteristic lengths away from the dislocation core. In practice, the non-singular fields can be obtained from the classical ones by replacing the classical (singular) anisotropic Greens tensor with the non-singular anisotropic Greens tensor derived by cite{Lazar:2015ja}. The elastic solution is valid for arbitrary anisotropic media. In addition to the classical anisotropic elastic constants, the non-singular Greens tensor depends on a second order symmetric tensor of length scale parameters modeling a weak non-locality, whose structure depends on the specific class of crystal symmetry. The anisotropic Helmholtz operator defined by such tensor admits a Greens function which is used as the spreading function for the Burgers vector density. As a consequence, the Burgers vector density spreads differently in different crystal structures.
Dislocation pinning plays a vital role in the plastic behaviour of a crystalline solid. Here we report the first observation of the damped oscillations of a mobile dislocation after it gets pinned at an obstacle in the presence of a constant static shear load. These oscillations are found to be inertial, instead of forced as obtained in the studies of internal friction of solid. The rate of damping enables us to determine the effective mass of the dislocation. Nevertheless, the observed relation between the oscillation frequency and the link length is found to be anomalous, when compared with the theoretical results in the framework of Koehlers vibrating string model. We assign this anomaly to the improper boundary conditions employed in the treatment. Finally, we propose that the inertial oscillations may offer a plausible explanation of the electromagnetic emissions during material deformation and seismic activities.
Studies of individual quantum systems, which have led to considerable progress in our understanding of quantum physics, have traditionally been associated with atomic gases. In the last decades however, the emphasis has shifted towards solid-state systems, which are much more practical for applications. In particular, a new field has recently emerged that is concerned with the study of quantum systems based on single spins localized near point defects in crystalline solids. One such system is the nitrogen-vacancy (NV) defect in diamond. Initially used as an experimental breadboard for testing concepts of quantum physics and quantum computation, the NV defect was soon proposed as a sensitive magnetometer, capable of detecting minute magnetic fields, down to ultimate level of single spins. This atomic-sized magnetometer can be used as a standalone sensor, or integrated into an imaging system providing spatial resolution down to the atomic scale. Diamond-based instruments thus offer new pathways to probe the magnetism of matter from the mesoscale down to the nanoscale. This book chapter gives an overview of the field of diamond-based magnetic sensing and imaging, with an emphasis on already demonstrated applications of this technology. The chapter is divided into three main sections. In Section 2, the underlying physics and methods of diamond-based magnetometry are described. Section 3 is devoted to various experimental implementations that employ this new class of sensors for magnetic sensing and imaging. Finally, some recent applications are presented in Section 4.
We introduce an approach to exploit the existence of multiple levels of description of a physical system to radically accelerate the determination of thermodynamic quantities. We first give a proof of principle of the method using two empirical interatomic potential functions. We then apply the technique to feed information from an interatomic potential into otherwise inaccessible quantum mechanical tight-binding calculations of the reconstruction of partial dislocations in silicon at finite temperature. With this approach, comprehensive ab initio studies at finite temperature will now be possible.
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