• Development of VO2 -Nanoparticle-Based Metal-Insulator Transition Electronic Ink

      Vaseem, Mohammad; Zhen, Su; Yang, Shuai; Li, Weiwei; Shamim, Atif (Advanced Electronic Materials, Wiley, 2019-03-13) [Article]
      The metal–insulator transition (MIT) phase change of vanadium dioxide (VO2) materials has facilitated many exciting applications. Among the various crystal phases of VO2, the monoclinic (M) phase is the only one that demonstrates low-temperature (≈68 °C) MIT behavior. However, the synthesis of pure VO2 (M) is challenging because various polymorphs, such as VO2 (A), VO2 (B), and VO2 (D), are also typically formed during the process. Furthermore, to achieve pure crystalline VO2 (M) phase, very long reaction times, up to 2–4 days, are typically required. In this work, an additional annealing step is introduced post nanoparticle preparation, which not only reduces the complete synthesis time from days to only 6 h but also removes the impure phases and helps in achieving the desired pure VO2 (M) phase. This work covers the complete synthesis and characterization details of such as-prepared nanoparticles. A VO2 (M)-nanoparticle-based ink is formulated for the inkjet printing of films with controlled thicknesses. The inkjet-printed films are investigated for their electrical conductivity with external stimuli such as temperature and electrical current. Finally, a fully printed antenna is devised that can change its frequency based on the different states of the VO2 film.
    • A hybrid finite-difference/lowrank solution to anisotropy acoustic wave equations

      Zhang, Zhendong; Alkhalifah, Tariq Ali; Wu, Zedong (GEOPHYSICS, Society of Exploration Geophysicists, 2018-12-06) [Article]
      P-wave extrapolation in anisotropic media suffers from SVwave artifacts and computational dependency on the complexity of anisotropy. The anisotropic pseudodifferential wave equation cannot be solved using an efficient time-domain finite-difference (FD) scheme directly. The wavenumber domain allows us to handle pseudodifferential operators accurately; however, it requires either smoothly varying media or more computational resources. In the limit of elliptical anisotropy, the pseudodifferential operator reduces to a conventional operator. Therefore, we have developed a hybrid-domain solution that includes a spacedomain FD solver for the elliptical anisotropic part of the anisotropic operator and a wavenumber-domain low-rank scheme to solve the pseudodifferential part. Thus, we split the original pseudodifferential operator into a second-order differentiable background and a pseudodifferential correction term. The background equation is solved using the efficient FD scheme, and the correction term is approximated by the low-rank approximation. As a result, the correction wavefield is independent of the velocity model, and, thus, it has a reduced rank compared with the full operator. The total computation cost of our method includes the cost of solving a spatial FD time-step update plus several fast Fourier transforms related to the rank. The accuracy of our method is of the order of the FD scheme. Applications to a simple homogeneous tilted transverse isotropic (TTI) medium and modified BP TTI models demonstrate the effectiveness of the approach.
    • Unlocking the Effect of Trivalent Metal Doping in All-Inorganic CsPbBr3 Perovskite

      Yin, Jun; Ahmed, Ghada H.; Bakr, Osman; Brédas, Jean-Luc; Mohammed, Omar F. (ACS Energy Letters, American Chemical Society (ACS), 2019-02-27) [Article]
      Metal-ion doping is one of the most efficient approaches to precisely control the electronic and optical properties of perovskite nanocrystals (NCs). However, the origin of the dramatic contrast in the photoluminescence (PL) behavior of CsPbBr3 NCs incorporating bismuth (Bi3+) and cerium (Ce3+) ion dopants remains unclear. Here, we demonstrate dominant PL quenching/enhancing centers both in the bulk and on the surface of Bi3+/Ce3+-doped CsPbBr3 by calculating the dopant defect formation energies and charge-transition levels using high-level density functional theory (DFT). We show that the Bi3+ dopants introduce deep trap states (antisite BiPb and interstitial Bii) that are responsible for PL quenching. In sharp contrast, the Ce3+ dopants enhance the CsPbBr3 lattice order and enrich the conduction band-edge states through antisite CePb, causing PL enhancement. Our findings not only provide new physical insights into the mechanism of the trivalent metal-ion doping effect but also suggest a new strategy to control the dopant defect states for improving the optical performance of perovskite NCs.
    • Temperature Dependence of the Urbach Energy in Lead Iodide Perovskites

      Ledinsky, Martin; Schönfeldová, Tereza; Holovský, Jakub; Aydin, Erkan; Hájková, Zdeňka; Landová, Lucie; Neyková, Neda; Fejfar, Antonín; De Wolf, Stefaan (The Journal of Physical Chemistry Letters, American Chemical Society (ACS), 2019-03-06) [Article]
      To gain insight into the properties of photovoltaic and light-emitting materials, detailed information about their optical absorption spectra is essential. Here, we elucidate the temperature dependence of such spectra for methylammonium lead iodide (CH3NH3PbI3), with specific attention to its sub-band gap absorption edge (often termed Urbach energy). On the basis of these data, we first find clear further evidence for the universality of the correlation between the Urbach energy and open-circuit voltage losses of solar cells. Second, we find that for CH3NH3PbI3 the static, temperature-independent, contribution of the Urbach energy is 3.8 ± 0.7 meV, which is smaller than that of crystalline silicon (Si), gallium arsenide (GaAs), indium phosphide (InP), or gallium nitride (GaN), underlining the remarkable optoelectronic properties of perovskites.
    • Numerical Approximation of a Phase-Field Surfactant Model with Fluid Flow

      Zhu, Guangpu; Kou, Jisheng; Sun, Shuyu; Yao, Jun; Li, Aifen (Journal of Scientific Computing, Springer Nature, 2019-03-07) [Article]
      Modeling interfacial dynamics with soluble surfactants in a multiphase system is a challenging task. Here, we consider the numerical approximation of a phase-field surfactant model with fluid flow. The nonlinearly coupled model consists of two Cahn–Hilliard-type equations and incompressible Navier–Stokes equation. With the introduction of two auxiliary variables, the governing system is transformed into an equivalent form, which allows the nonlinear potentials to be treated efficiently and semi-explicitly. By certain subtle explicit-implicit treatments to stress and convective terms, we construct first and second-order time marching schemes, which are extremely efficient and easy-to-implement, for the transformed governing system. At each time step, the schemes involve solving only a sequence of linear elliptic equations, and computations of phase-field variables, velocity and pressure are fully decoupled. We further establish a rigorous proof of unconditional energy stability for the first-order scheme. Numerical results in both two and three dimensions are obtained, which demonstrate that the proposed schemes are accurate, efficient and unconditionally energy stable. Using our schemes, we investigate the effect of surfactants on droplet deformation and collision under a shear flow, where the increase of surfactant concentration can enhance droplet deformation and inhibit droplet coalescence.
    • Enhancing the Near-Surface Image Using Duplex-Wave Reverse Time Migration

      Sindi, Ghada; Alkhalifah, Tariq Ali; Fei, Tong; Luo, Yi (SPE Middle East Oil and Gas Show and Conference, Society of Petroleum Engineers, 2019-03-13) [Conference Paper]
      Reverse time migration (RTM) involves zero-lag cross-correlation of forward extrapolated source function wavefields and backward extrapolated receiver wavefields. For a near surface with complex structures and velocity anomalies, forward propagating the source wavelet generates wavefields containing reflections, near-surface multiples, and scattered direct arrivals. The wavefields are recorded as upgoing arrivals contaminated by the same reflections, near-surface multiples, and scattered signals, which can be critical for imaging near-surface structures and scatterers. Here, we develop a new depth migration, duplex reverse time migration (DRTM) technique to improve imaging of complex near-surface structures. DRTM uses the direct arrival as a source to forward propagate and generate source wavefields, and reversely extrapolated recorded data in a zero-lag cross-correlation imaging condition to generate the final section. The interaction between the data components during cross- correlation can use primaries and multiples to image the near-surface structure correctly. Cross-talk artifacts may exist, but they are comparatively weak. DRTM is demonstrated on both synthetic and field data examples showing an enhanced image in areas with complex near-surface structures compared to conventional RTM imaging methods. The new algorithm can significantly enhance shallow imaging without additional computation costs compared with conventional RTM. It can produce an image with higher resolution and signal-to-noise (S/N) ratio by replacing the source wavelet with the recorded direct arrivals, which include near-surface information necessary to boost the image in areas with near-surface complexity. Since the direct arrivals are one of the most energetic events recorded, the resultant image is typically of high S/N. The wave can also illuminate shallow zones better than primaries in marine environments.
    • Localization in Adiabatic Shear Flow Via Geometric Theory of Singular Perturbations

      Lee, Min-Gi; Katsaounis, Theodoros; Tzavaras, Athanasios (Journal of Nonlinear Science, Springer Nature, 2019-03-04) [Article]
      We study localization occurring during high-speed shear deformations of metals leading to the formation of shear bands. The localization instability results from the competition between Hadamard instability (caused by softening response) and the stabilizing effects of strain rate hardening. We consider a hyperbolic–parabolic system that expresses the above mechanism and construct self-similar solutions of localizing type that arise as the outcome of the above competition. The existence of self-similar solutions is turned, via a series of transformations, into a problem of constructing a heteroclinic orbit for an induced dynamical system. The dynamical system is in four dimensions but has a fast–slow structure with respect to a small parameter capturing the strength of strain rate hardening. Geometric singular perturbation theory is applied to construct the heteroclinic orbit as a transversal intersection of two invariant manifolds in the phase space.
    • Detecting cyber-attacks using a CRPS-based monitoring approach

      Harrou, Fouzi; Bouyeddou, Benamar; Sun, Ying; Kadri, Benamar (2018 IEEE Symposium Series on Computational Intelligence (SSCI), IEEE, 2019-02-28) [Conference Paper]
      Cyber-attacks can seriously affect the security of computers and network systems. Thus, developing an efficient anomaly detection mechanism is crucial for information protection and cyber security. To accurately detect TCP SYN flood attacks, two statistical schemes based on the continuous ranked probability score (CRPS) metric have been designed in this paper. Specifically, by integrating the CRPS measure with two conventional charts, Shewhart and the exponentially weighted moving average (EWMA) charts, novel anomaly detection strategies were developed: CRPS-Shewhart and CRPS-EWMA. The efficiency of the proposed methods has been verified using the 1999 DARPA intrusion detection evaluation datasets.
    • Monitoring land-cover changes by combining a detection step with a classification step

      Harrou, Fouzi; Zerrouki, Nabil; Sun, Ying; Hocini, Lotfi (2018 IEEE Symposium Series on Computational Intelligence (SSCI), IEEE, 2019-02-28) [Conference Paper]
      An approach merging the HotellingT 2 control scheme with weighted random forest classifier is proposed and used in the context of detecting land cover changes via remote sensing and radiometric measurements. HotellingT 2 procedure is introduced to identify features corresponding to changed areas. However, T 2 scheme is not able to separate real from false changes. To tackle this limitation, the weighted random forest algorithm, which is an efficient classification technique for unbalanced problems, has been successfully applied on features of the detected pixels to recognize the type of change. The performance of the algorithm is evaluated using SZTAKI AirChange benchmark data, results show that the proposed detection scheme succeeds to appropriately identify changes to land cover. Also, we compared the proposed approach to that of the conventional algorithms (i.e., neural network, random forest, support vector machine and k-nearest neighbors) and found improved performance.
    • A Salty Coral Secret: How High Salinity Helps Corals To Be Stronger

      Gegner, Hagen M.; Voolstra, Christian R. (Frontiers for Young Minds, Frontiers Media SA, 2019-03-07) [Article]
      Corals are mysterious animals that have been around for ages. They are the creators of beautiful reefs. Sadly, the reefs that we love to look at are in danger of overheating and are disappearing from our planet. While this sounds bad, not all corals are affected the same way by warm seawater. Corals from the Red Sea seem to be more resistant to higher temperatures than are corals from other regions. Red Sea reefs are thriving in seawater that is hotter than that in other places. But what is their secret? What makes Red Sea corals stronger and more heat resistant? We know that Red Sea corals not only handle the incredibly high temperatures, but also deal with high salinity (saltiness). This connection between high salinity and high temperature made us wonder: can we find evidence that high salinity makes corals stronger?
    • Multilevel Monte Carlo in approximate Bayesian computation

      Jasra, Ajay; Jo, Seongil; Nott, David; Shoemaker, Christine; Tempone, Raul (Stochastic Analysis and Applications, Informa UK Limited, 2019-02-01) [Article]
      In the following article, we consider approximate Bayesian computation (ABC) inference. We introduce a method for numerically approximating ABC posteriors using the multilevel Monte Carlo (MLMC). A sequential Monte Carlo version of the approach is developed and it is shown under some assumptions that for a given level of mean square error, this method for ABC has a lower cost than i.i.d. sampling from the most accurate ABC approximation. Several numerical examples are given.
    • A Survey of Channel Modeling for UAV Communications

      Khuwaja, Aziz Altaf; Chen, Yunfei; Zhao, Nan; Alouini, Mohamed-Slim; Dobbins, Paul (IEEE Communications Surveys & Tutorials, Institute of Electrical and Electronics Engineers (IEEE), 2018-07-16) [Article]
      Unmanned aerial vehicles (UAVs) have attracted great interest in rapid deployment for both civil and military applications. UAV communication has its own distinctive channel characteristics compared to the widely used cellular or satellite systems. Accurate channel characterization is crucial for the performance optimization and design of efficient UAV communication. However, several challenges exist in UAV channel modeling. For example, the propagation characteristics of UAV channels are under explored for spatial and temporal variations in non-stationary channels. Additionally, airframe shadowing has not yet been investigated for small size rotary UAVs. This paper provides an extensive survey of the measurement methods proposed for UAV channel modeling that use low altitude platforms and discusses various channel characterization efforts. We also review from a contemporary perspective of UAV channel modeling approaches, and outline future research challenges in this domain.
    • Improving Shadow Suppression for Illumination Robust Face Recognition

      Zhang, Wuming; Zhao, Xi; Morvan, Jean-Marie; Chen, Liming (IEEE Transactions on Pattern Analysis and Machine Intelligence, Institute of Electrical and Electronics Engineers (IEEE), 2018-02-07) [Article]
      2D face analysis techniques, such as face landmarking, face recognition and face verification, are reasonably dependent on illumination conditions which are usually uncontrolled and unpredictable in the real world. The current massive data-driven approach, e.g., deep learning-based face recognition, requires a huge amount of labeled training face data that hardly cover the infinite lighting variations that can be encountered in real-life applications. An illumination robust preprocessing method thus remains a very interesting but also a significant challenge in reliable face analysis. In this paper we propose a novel model driven approach to improve lighting normalization of face images. Specifically, we propose to build the underlying reflectance model which characterizes interactions between skin surface, lighting source and camera sensor, and elaborate the formation of face color appearance. The proposed illumination processing pipeline enables generation of the Chromaticity Intrinsic Image (CII) in a log chromaticity space which is robust to illumination variations. Moreover, as an advantage over most prevailing methods, a photo-realistic color face image is subsequently reconstructed, which eliminates a wide variety of shadows whilst retaining the color information and identity details. Experimental results under different scenarios and using various face databases show the effectiveness of the proposed approach in dealing with lighting variations, including both soft and hard shadows, in face recognition.
    • Resource Allocation and Cluster Formation for Imperfect NOMA in DL/UL Decoupled HetNets

      Celik, Abdulkadir; Radaydeh, Redha Mahmoud Mesleh; Al-Qahtani, Fawaz S.; El-Malek, Ahmed H.Abd; Alouini, Mohamed-Slim (2017 IEEE Globecom Workshops (GC Wkshps), IEEE, 2018-01-25) [Conference Paper]
      Being capable of serving multiple users with the same radio resource, non-orthogonal multiple access (NOMA) can provide desirable performance enhancements in a fair and spectral efficient manner. In this paper, we investigate the resource allocation (RA) and cluster formation (CF) aspects of NOMA for downlink (DL) uplink (UL) decoupled (DUDe) heterogeneous networks (HetNets). A non-ideal NOMA scheme is considered with power disparity and sensitivity constraints (PDSCs), delay tolerance, and residual interference after cancellation. Taking the PDSCs into account, we analytically show that using the DL decoding order limits UL-NOMA performance by that of OMA, while employing an inverse order result in a performance gain that is mainly determined by the channel gain disparity of users. Thereafter, a generic CF method is proposed for any type of user graph, which iteratively forms clusters using Blossom algorithm. Finally, highly non-convex RA problem is converted into a convex form by employing geometric programming (GP) where power and bandwidth are optimized to maximize network sumrate and max-min fairness objectives.
    • Engineering resistance against Tomato yellow leaf curl virus via the CRISPR/Cas9 system in tomato

      Tashkandi, Manal; Ali, Zahir; Aljedaani, Fatimah R.; Shami, Ashwag; Mahfouz, Magdy M. (Plant Signaling & Behavior, Informa UK Limited, 2019-01-29) [Article]
      CRISPR/Cas systems confer molecular immunity against phages and conjugative plasmids in prokaryotes. Recently, CRISPR/Cas9 systems have been used to confer interference against eukaryotic viruses. Here, we engineered Nicotiana benthamiana and tomato (Solanum lycopersicum) plants with the CRISPR/Cas9 system to confer immunity against the Tomato yellow leaf curl virus (TYLCV). Targeting the TYLCV genome with Cas9-single guide RNA at the sequences encoding the coat protein (CP) or replicase (Rep) resulted in efficient virus interference, as evidenced by low accumulation of the TYLCV DNA genome in the transgenic plants. The CRISPR/Cas9-based immunity remained active across multiple generations in the N. benthamiana and tomato plants. Together, our results confirmed the efficiency of the CRISPR/Cas9 system for stable engineering of TYLCV resistance in N. benthamiana and tomato, and opens the possibilities of engineering virus resistance against single and multiple infectious viruses in other crops.
    • Entropy stable modeling of non-isothermal multi-component diffuse-interface two-phase flows with realistic equations of state

      Kou, Jisheng; Sun, Shuyu (Computer Methods in Applied Mechanics and Engineering, Elsevier BV, 2018-07-05) [Article]
      In this paper, we consider mathematical modeling and numerical simulation of non-isothermal compressible multi-component diffuse-interface two-phase flows with realistic equations of state. A general model with the general reference velocity is derived rigorously through thermodynamical laws and Onsager’s reciprocal principle, and it is capable of characterizing compressibility and partial miscibility between multiple fluids. We prove a novel relation between the pressure, temperature and chemical potentials, which results in a new formulation of the momentum conservation equation indicating that the gradients of chemical potentials and temperature become the primary driving force of the fluid motion except for the external forces. A key challenge in numerical simulation is to develop entropy stable numerical schemes preserving the laws of thermodynamics. Based on the convex–concave splitting of Helmholtz free energy density with respect to molar densities and temperature, we propose an entropy stable numerical method, which solves the total energy balance equation directly, and thus, naturally satisfies the first law of thermodynamics. Unconditional entropy stability (the second law of thermodynamics) of the proposed method is proved by estimating the variations of Helmholtz free energy and kinetic energy with time steps. Numerical results validate the proposed method.
    • Laser-Induced Reversion of δ′ precipitates in an Al-Li Alloy

      Khushaim, Muna S.; Gemma, Ryota; Al-Kassab, Talaat (Microscopy and Microanalysis, Cambridge University Press (CUP), 2017-08-09) [Article]
    • A Randomized Exchange Algorithm for Computing Optimal Approximate Designs of Experiments

      Harman, Radoslav; Filová, Lenka; Richtarik, Peter (Journal of the American Statistical Association, Informa UK Limited, 2018-12-15) [Article]
      We propose a class of subspace ascent methods for computing optimal approximate designs that covers existing algorithms as well as new and more efficient ones. Within this class of methods, we construct a simple, randomized exchange algorithm (REX). Numerical comparisons suggest that the performance of REX is comparable or superior to that of state-of-the-art methods across a broad range of problem structures and sizes. We focus on the most commonly used criterion of D-optimality, which also has applications beyond experimental design, such as the construction of the minimum-volume ellipsoid containing a given set of data points. For D-optimality, we prove that the proposed algorithm converges to the optimum. We also provide formulas for the optimal exchange of weights in the case of the criterion of A-optimality, which enable one to use REX and some other algorithms for computing A-optimal and I-optimal designs.
    • Towards Early Detection of Red Palm Weevil Using Optical Fiber Distributed Acoustic Sensor

      Mao, Yuan; Ashry, Islam; Ng, Tien Khee; Ooi, Boon S. (Optical Fiber Communication Conference (OFC) 2019, OSA, 2019-02-25) [Conference Paper]
      Red palm weevil (RPW) is a severe danger to the dates farming. We use optical fiber distributed acoustic sensor (DAS) as a solution to the detection of RPW via sensing the RPW activities sound.
    • Investigation of high contrast and reversible luminescence thermochromism of the quantum confined Cs4PbBr6 perovskite solid

      Choi, Jin Woo; Cho, Namchul; Woo, Hee Chul; Oh, Byeong M; Almutlaq, Jawaher; Bakr, Osman; Kim, Sung-Hoon; Lee, Chang-Lyoul; Kim, Jong H (Nanoscale, Royal Society of Chemistry (RSC), 2019-02-16) [Article]
      Thermochromism of organic/inorganic halide perovskites has attracted particular interest due to their potential applications as photoluminescence (PL)-based temperature sensors. However, despite the outstanding PL characteristics, their use as a thermochromic material in practical temperature ranges has been limited because of their poor thermal stability. In this study, we used the quantum confinement effect and exceptional PL quantum efficiency of the Cs4PbBr6 perovskite to demonstrate their high on/off ratio (20) and reversible PL thermochromism in the solid state in practical temperature ranges including room temperature (RT). Systematic photophysical and optical characterization studies, including exciton-phonon scattering, exciton binding energy, exciton decay dynamics, and crystal structure change, were performed to investigate the origin of this unique thermochromic PL property. The results showed that the efficient and highly reversible thermochromic PL emission of the Cs4PbBr6 perovskite is due to its desirable optical properties such as highly luminescent emission, efficient PL quenching at high temperatures, and thermally reversible structural changes.