Theory-assisted determination of nano-rippling and impurities in atomic resolution images of angle-mismatched bilayer graphene

Ripples and impurity atoms are universally present in 2D materials, limiting carrier mobility, creating pseudo–magnetic fields, or affecting the electronic and magnetic properties. Scanning transmission electron microscopy (STEM) generally provides picometer-level precision in the determination of the location of atoms or atomic 'columns' in the in-image plane (xy plane). However, precise atomic positions in the z-direction as well as the presence of certain impurities are difficult to detect. Furthermore, images containing moiré patterns such as those in angle-mismatched bilayer graphene compound the problem by limiting the determination of atomic positions in the xy plane. Here, we introduce a reconstructive approach for the analysis of STEM images of twisted bilayers that combines the accessible xy coordinates of atomic positions in a STEM image with density-functional-theory calculations. The approach allows us to determine all three coordinates of all atomic positions in the bilayer and establishes the presence and identity of impurities. The deduced strain-induced rippling in a twisted bilayer graphene sample is consistent with the continuum model of elasticity. We also find that the moiré pattern induces undulations in the z direction that are approximately an order of magnitude smaller than the strain-induced rippling. A single substitutional impurity, identified as nitrogen, is detected. The present reconstructive approach can, therefore, distinguish between moiré and strain-induced effects and allows for the full reconstruction of 3D positions and atomic identities

Secure Wireless Communications Based on Compressive Sensing: A Survey

IEEE Compressive sensing (CS) has become a popular signal processing technique and has extensive applications in numerous fields such as wireless communications, image processing, magnetic resonance imaging, remote sensing imaging, and anology to information conversion, since it can realize simultaneous sampling and compression. In the information security field, secure CS has received much attention due to the fact that CS can be regarded as a cryptosystem to attain simultaneous sampling, compression and encryption when maintaining the secret measurement matrix. Considering that there are increasing works focusing on secure wireless communications based on CS in recent years, we produce a detailed review for the state-of-the-art in this paper. To be specific, the survey proceeds with two phases. The first phase reviews the security aspects of CS according to different types of random measurement matrices such as Gaussian matrix, circulant matrix, and other special random matrices, which establishes theoretical foundations for applications in secure wireless communications. The second phase reviews the applications of secure CS depending on communication scenarios such as wireless wiretap channel, wireless sensor network, internet of things, crowdsensing, smart grid, and wireless body area networks. Finally, some concluding remarks are given

Internal stresses in steel plate generated by shape memory alloy inserts

Neutron strain scanning was employed to investigate the internal stress fields in steel plate coupons with embedded prestrained superelastic NiTi shape memory alloy inserts. Strain fields in steel were evaluated at T = 21 °C and 130 °C on virgin coupons as well as on mechanically and thermally fatigued coupons. Internal stress fields were evaluated by direct calculation of principal stress components from the experimentally measured lattice strains as well as by employing an inverse finite element modeling approach. It is shown that if the NiTi inserts are embedded into the elastic steel matrix following a carefully designed technological procedure, the internal stress fields vary with temperature in a reproducible and predictable way. It is estimated that this mechanism of internal stress generation can be safely applied in the temperature range from −20 °C to 150 °C and is relatively resistant to thermal and mechanical fatigue. The predictability and fatigue endurance of the mechanism are of essential importance for the development of future smart metal matrix composites or smart structures with embedded shape memory alloy components

Roadmap on optical security

Postprint (author's final draft

Towards Baseline-Independent Analysis of Compressive Sensed Functional Magnetic Resonance Image Data

The main task of Functional Magnetic Resonance Imaging (fMRI) is the localisation of brain activities, which depends on the detection of hemodynamic responses in the Blood Oxygenation-Level Dependent (BOLD) signal. While compressive sensing has been widely applied to improve the quality and resolution of MRI in general, its reconstruction noise overwhelms the small magnitude of hemodynamic responses. We propose a new reconstruction algorithm for the compressive sensing fMRI that exploits the temporal redundancy of the data, called Referenced Compressive Sensing, which works well in preserving fMRI analytical features. We also propose the use of the baseline-independent signal for analysis of reconstructed data. It is shown that the baseline-independent reconstructed data from Referenced Compressive Sensing is highly correlated to the lossless data, thus preserving more of the analytical features

Performance evaluations of latex-modified and silica fume modified concrete overlays for bridge decks

Most of the concrete bridge decks in the cold regions undergo severe reinforcement corrosion due to the transport of chloride ions within the concrete by application of deicing salts on bridge decks in the winter. As a result, protective concrete overlays of about 2-inch thickness are applied on bridge decks. Concrete overlays provide: (1) protection against heavy traffic and the further infiltration of the chloride ions; (2) skid resistance surface; and (3) uniform appearance to extend the service life of bridge decks. Despite these advantages, concrete overlays undergo premature delaminations, edge curling, and corner lifting due to expansion/shrinkage of concrete, temperature changes, and repetitive truck loading.;This research work is a part of Phase-II component of a large-scale project sponsored by WVDOH which is focused on the performance evaluation of Latex Modified Concrete (LMC) and Silica Fume Modified Concrete (SFMC) overlays on Type K substrate concrete deck prototypes. This work evaluated four prototype slabs in two stages with each slab of plan size 1829 mm. (6 ft.) by 2438 mm. (8 ft.). In Stage-I, two bi-layer deck slabs were constructed with the same LMC overlay but with two different bonding conditions. Considering the better bonding condition from Stage-I, Stage-II consisted of two bi-layer deck slabs that were constructed with LMC overlay on one slab and SFMC overlay on the other. Pull-Off testing was conducted at different ages for assessing the bond performance of the overlays. The differential length change and differential temperature developed at the interface were continuously monitored using concrete embedment gages and thermocouple loggers, respectively. Debonding due to corner lifting was monitored by installing displacement transducers (LVDT) connected to a data acquisition system. Ultrasonic Pulse Velocity (UPV) testing was conducted to compare the delamination profile at the interface between different types of slabs. Simultaneously with the UPV tester, an oscilloscope was connected to record the time-domain waveform, which was converted into a power spectrum for analysis.;The Stage-I study showed that at 5% level of significance, both the interface bond strength of the LMC slab with and without bonding slurry were similar. However, the other critical delamination parameters such as differential length changes at the interface and vertical displacement due to corner lifting were much less when bonding slurry was incorporated. Also, the time-domain waveform of LMC with bonding slurry showed less attenuation of wave through the interface compared to no-slurry LMC. Further, the frequency spectrum analysis displayed that LMC with bonding slurry had higher peak magnitude compared to LMC without bonding slurry. Based on the results, the use of bonding slurry was preferred for the Stage-II study. This study showed that in-situ pull-off/bond strength results and nature of failure varied widely due to presence of local voids, compaction, and consolidations.;In Stage-II study where the bonding slurries were used for each case, the results showed that the bond strength values of slab with LMC overlay were higher than those of the slab with SFMC overlay, at 5% level of significance as analyzed by three-way ANOVA. The vertical displacement due to corner lifting was found to be lower for the slab with LMC overlay. The time-domain signal indicated that the amplitudes of LMC overlay were higher compared to those of SFMC overlay. Further, the power spectrum analysis showed that LMC had higher magnitude of peaks both at center and edge compared to SFMC overlays indicating the strength of the signals were stronger for LMC compared to SFMC.;The overall conclusion of this study is that the bonding slurry has positive effect to reduce delamination and corner lifting; and when slurry is used, the LMC has advantages over SFMC, although both overlay types are viable options. A full-scale study is required in order to conclude the findings and finally develop a performance based specifications for the overlays for the state of West Virginia