Localized Inhomogeneities Research Articles

Imaging Valley Excitons in a 2D Semiconductor with Scanning Tunneling Microscope-Induced Luminescence.

Valley excitons dominate the optoelectronic response of transition-metal dichalcogenides and are drastically affected by structural and environmental inhomogeneities localized in these materials. Critical to understanding and controlling these nanoscale excitonic changes is the ability to correlate the imaging of excitonic states with crystalline structures on the atomic scale. Here, we apply scanning tunneling microscope-induced luminescence microscopy to image valley excitons in a semiconducting transition-metal dichalcogenide monolayer decoupled by a 10 nanometer-thick hexagonal-boron-nitride flake incorporated in a lateral homojunction on an Au electrode surface. This design enables the observation of chiral excitonic emission arising from neutral and charged valley excitons of the monolayer semiconductor at ambipolar voltages with a quantum efficiency up to ∼10-5 photon/electron. The measured light helicity demonstrates considerable circular polarization dependent on the sample voltage, reaching as much as 40%. The real-space luminescence imaging maps─at subnanometer resolution─of the valley excitons reveal striking spatial variations associated with localized inhomogeneities, including surface impurities and possibly nanoscale dielectric and/or potential disorders in the monolayer. Our study introduces a promising format for 2D materials to explore and tailor their optoelectronic processes at the atomic scale.

A fast boundary node method for transient scalar waves in domains with localized inhomogeneities

A fast boundary node method for transient scalar waves in domains with localized inhomogeneities

Resolving the Subsurface Structure and Elastic Modulus of Layered Films via Contact Resonance Atomic Force Microscopy.

Since its discovery, atomic force microscopy (AFM) has become widely used for surface characterization, evolving from a tool for probing surface topography to a versatile method for characterizing mechanical, electrical, chemical, magnetic, and electro-optical properties of surfaces at the nanoscale. Developments of several AFM-based techniques have enabled even subsurface imaging, which is routinely being carried out at the qualitative level of feature detection for localized subsurface inhomogeneities. We surmise, however, that a quantitative three-dimensional (3D) subsurface characterization can emerge from the AFM mechanical response of flat buried interfaces, and present here a methodology for determining the depth of a film and its mechanical properties. Using load-dependent contact resonance atomic force microscopy (CR-AFM) and accurate modeling of the contact between the AFM tip and a layered sample, we determine the relationship between the measured resonance frequency of the AFM probe and the contact stiffness. Our subsequent statistical analysis reveals an intrinsic and sample-specific interdependence between the depth and modulus sensitivities of CR-AFM. This interdependence prevents the simultaneous accurate determination of both depth and modulus from measurements on a single-layered sample. If the elastic moduli of the sample components are predetermined from separate investigations of bulk samples (or otherwise known), then this methodology accurately yields the location of the interface between the layers of the sample; as such, it can serve as a nondestructive and robust technique for probing layer thickness, subsurface features, and elastic properties of materials used in semiconductor electronics, additive manufacturing, or biomaterials.

The crust structure of the Aldano-Stanovoy shield according to the wide-angle seismic data (southern part of profile 3-DV)

Significant differences in the layered-block structure of the earth's crust and the upper mantle, expressed in the distribution of localized seismic inhomogeneities of the Aldan and Stanovoi blocks and the zone of their junction, are revealed. A laterally inhomogeneous near-surface two-layer structure with a velocity of 3.8-4.5 and 5.0-5.9 km/s at thicknesses of 0.3-1.0 and up to 2-4 km has been established, which overlaps the Early Precambrian basement of the shield with a velocity of 6.0-6.3 km/s. The crust in the Stanovoi block and the adjacent Central Aldan folded belt is represented by a two-layer, and in the Aldan block by a three-layer models. The zone of the Stanovoi Ridge separating them is characterized by exceptionally contrasting velocity inhomogeneities, which correlate with a step-like increase in the depth of the Moho from 40 to 50 km.

Overcoming Nanoscale Inhomogeneities in Thin-Film Perovskites via Exceptional Post-annealing Grain Growth for Enhanced Photodetection.

Antisolvent-assisted spin coating has been widely used for fabricating metal halide perovskite films with smooth and compact morphology. However, localized nanoscale inhomogeneities exist in these films owing to rapid crystallization, undermining their overall optoelectronic performance. Here, we show that by relaxing the requirement for film smoothness, outstanding film quality can be obtained simply through a post-annealing grain growth process without passivation agents. The morphological changes, driven by a vaporized methylammonium chloride (MACl)–dimethylformamide (DMF) solution, lead to comprehensive defect elimination. Our nanoscale characterization visualizes the local defective clusters in the as-deposited film and their elimination following treatment, which couples with the observation of emissive grain boundaries and excellent inter- and intragrain optoelectronic uniformity in the polycrystalline film. Overcoming these performance-limiting inhomogeneities results in the enhancement of the photoresponse to low-light (<0.1 mW cm–2) illumination by up to 40-fold, yielding high-performance photodiodes with superior low-light detection.

Numerical material testing based on homogenization method for non-periodic media (Formulation using microscopic fluctuation displacement and macroscopic strain)

We propose a method for numerical material testing that does not explicitly assume boundary conditions in homogenization analyses for microstructures with random or localized inhomogeneities. Specifically, instead of the periodic constraint condition on the fluctuation displacement in the RVE, a condition is imposed such that the domain integral of the gradient is zero, which preserves the definition of macroscopic strain in the homogenization theory while also suppressing rigid body rotation. Then, a scale-separated variational problem is defined with this condition introduced as a constraint. Next, the governing equations of the two-variable boundary value problem (BVP) are derived by applying Lagrange multiplier method to this variational problem. The macroscopic BVP is degenerated to a material point, and the microscopic BVP is transformed into a form specialized for numerical material testing and then discretized by the finite element method (FEM). The discretized equations are extended by adding the degrees of freedom corresponding to macroscopic stress and strain. Since the proposed method does not require any restrictions on the external shape of the microstructure and the extraction of external boundaries, it is relatively easy to implement into the FEM. Several numerical examples are presented to demonstrate the validity of the proposed by comparing the results with those of the conventional approach.

Application of an integrated radio-frequency/shim coil technology for signal recovery in fMRI.

Gradient-echo echo-planar imaging (EPI), which is typically used for blood oxygenation level-dependent (BOLD) functional MRI (fMRI), suffers from distortions and signal loss caused by localized B0 inhomogeneities. Such artifacts cannot be effectively corrected for with the low-order spherical harmonic (SH) shim coils available on most scanners. The integrated parallel reception, excitation, and shimming (iPRES) coil technology allows radiofrequency (RF) and direct currents to flow on each coil element, enabling imaging and localized B0 shimming with one coil array. iPRES was previously used to correct for distortions in spin-echo EPI and is further developed here to also recover signal loss in gradient-echo EPI. The cost function in the shim optimization, which typically uses a single term representing the B0 inhomogeneity, was modified to include a second term representing the signal loss, with an adjustable weight to optimize the trade-off between distortion correction and signal recovery. Simulations and experiments were performed to investigate the shimming performance. Slice-optimized shimming with iPRES and the proposed cost function substantially reduced the signal loss in the inferior frontal and temporal brain regions compared to shimming with iPRES and the original cost function or 2nd -order SH shimming with either cost function. In breath-holding fMRI experiments, the ΔB0 and signal loss root-mean-square errors decreased by -34.3% and -56.2%, whereas the EPI signal intensity and number of activated voxels increased by 60.3% and 174.0% in the inferior frontal brain region. iPRES can recover signal loss in gradient-echo EPI, which is expected to improve BOLD fMRI studies in brain regions suffering from signal loss.

Solitary waves dynamic for Davydov α-helical protein model: Effects of localized and periodic inhomogeneities.

We describe the dynamic of excitons through single inhomogeneous α-helical proteins with off-diagonal and diagonal couplings. Inhomogeneities considered are either localized or periodic. Intensive numerical simulations carried show stable structures and allow us to single out important features of excitons dynamic. In the absence of inhomogeneities, the interplay between off-diagonal and diagonal couplings leads to two distinct types of solitary waves. Bright solitary waves correspond to an off-diagonal coupling constant lower than a critical diagonal coupling constant, while dark solitary waves are obtained in the opposite case. Inclusion of inhomogeneities profoundly affects the profiles, amplitudes, and energies transported by the waves. For relatively small strength of inhomogeneities, only the profiles of the waves significantly change, the other properties remaining almost unchanged. Large strength inhomogeneities sensitively twist the profiles and increase amplitudes and energies of the waves. Our study suggests that small strength inhomogeneities allow a coherent transport of energy and the biological functions remain unchanged, but large strengths of inhomogeneities affect the biological functioning of the α-helical protein chains. Hence, large strengths of inhomogeneities may amplify the energy of the molecule and could be used to treat some diseases.

Quantum Effects on the Mesoscale

In different areas of mechanics, highly non-equilibrium processes are accompanied by self-organization of various type turbulent structures and localized inhomogeneities at intermediate scale between macro and micro levels. In order to describe the self-organization of the new dynamic structures on the mesoscale, a new problem formulation based on the results of non-equilibrium statistical mechanics, control theory of adaptive systems, and theory of a special type nonlinear operator sets is proposed. Determination of the turbulent structure parameters through constraints imposed on the system in the form of nonlinear functionals is an inverse problem similar to problems on spectra in quantum mechanics. Like in quantum mechanics, the bounded system in response to impact forms a discrete spectrum of the turbulent structure sizes and lifetimes which goes into continuous spectrum close-to-equilibrium. The proposed description of the structure evolution on the intermediate scale level which is valid far from thermodynamic equilibrium bridges the gap between macroscopic theories and quantum mechanics and affirms the unity of the physical laws of nature.

Guided wave time-reversal imaging of macroscopic localized inhomogeneities in anisotropic composites

Estimation of damage position and extent in the inspected structure is among the emerging problems of active structural health monitoring (SHM) with elastic guided waves. Unlike conventional non-destructive testing (NDT) techniques, which assume continuous surface scanning, SHM systems operate only with data from a limited number of sensors. Nevertheless, in the case of isotropic (metal) plate-like structures, computational time-reversal techniques have proved to be effective for estimating in situ the location and size of wave sources and/or local scatterers within the SHM concept. With composite plates, the reconstruction procedure faces additional difficulties associated with the complexity of wave phenomena caused by their lamination and anisotropy. In this article, we present an extension of the time reversal technique for the case of composite laminate plate-like structures. This technique relies on the simulation of the reversed guided waves generated by reciprocal surface loads applied at a sparse set of measurement points of a real sensor network. The proposed implementation is based on the far-field asymptotics for guided waves generated in arbitrarily anisotropic laminate waveguides by a prescribed source, which have been derived from the path integral representations in terms of Green’s matrix for the structures considered. The performance of this approach has been experimentally tested on cross-ply carbon fiber-reinforced plastic plates showing reliable and adequate results for both active (piezoactuators) and passive (artificial defects) source characterization.

Wideband Diagnostics of Inhomogeneous Media Using Linear Antenna Arrays

The information content of the coupled impedance of linear antenna arrays in the microwave band as a function of the sounding conditions and electrophysical properties of the monitored horizontally-stratified media is analyzed. The applicability of the method of wideband sounding for diagnostics of stratified media from the measured impedance of linear antenna arrays is estimated. The arrangement geometry of the transceiver linear antennas is optimized; new algorithms for remote control and detection of localized inhomogeneities of dielectric and poorly conducting media are proposed.

Integrated radio‐frequency/wireless coil design for simultaneous MR image acquisition and wireless communication

An innovative radio-frequency (RF) coil design that allows RF currents both at the Larmor frequency and in a wireless communication band to flow on the same coil is proposed to enable simultaneous MRI signal reception and wireless data transfer, thereby minimizing the number of wired connections in the scanner without requiring any modifications or additional hardware within the scanner bore. As a first application, the proposed integrated RF/wireless coil design was further combined with an integrated RF/shim coil design to perform not only MR image acquisition and wireless data transfer, but also localized B0 shimming with a single coil. Proof-of-concept phantom experiments were conducted with such a coil to demonstrate its ability to simultaneously perform these three functions, while maintaining the RF performance, wireless data integrity, and B0 shimming performance. Performing wirelessly controlled shimming of localized B0 inhomogeneities with the coil substantially reduced the B0 root-mean-square error (>70%) and geometric distortions in echo-planar images without degrading the image quality, signal-to-noise ratio (<1.7%), or wireless data throughput (maximum variance = 0.04Mbps) of the coil. The RF/wireless coil design can provide a solution for wireless data transfer that can be easily integrated into existing MRI scanners for a variety of applications.

Statistical projection effects in a hydrodynamic pilot-wave system

Millimetric liquid droplets can walk across the surface of a vibrating fluid bath, self-propelled through a resonant interaction with their own guiding or ‘pilot’ wave fields. These walking droplets, or ‘walkers’, exhibit several features previously thought to be peculiar to the microscopic, quantum realm. In particular, walkers confined to circular corrals manifest a wave-like statistical behaviour reminiscent of that of electrons in quantum corrals. Here we demonstrate that localized topological inhomogeneities in an elliptical corral may lead to resonant projection effects in the walker’s statistics similar to those reported in quantum corrals. Specifically, we show that a submerged circular well may drive the walker to excite specific eigenmodes in the bath that result in drastic changes in the particle’s statistical behaviour. The well tends to attract the walker, leading to a local peak in the walker’s position histogram. By placing the well at one of the foci, a mode with maxima near the foci is preferentially excited, leading to a projection effect in the walker’s position histogram towards the empty focus, an effect strongly reminiscent of the quantum mirage. Finally, we demonstrate that the mean pilot-wave field has the same form as the histogram describing the walker’s statistics. Droplets moving on the surface of a vibrating fluid bath mimic the behaviour of electrons in quantum corrals. Introducing submerged features in the bath can even drive the droplets to excite modes that induce effects reminiscent of quantum mirages.

Adaptive integrated parallel reception, excitation, and shimming (iPRES-A) with microelectromechanical systems switches.

Integrated parallel reception, excitation, and shimming coil arrays with N shim loops per radio-frequency (RF) coil element (iPRES(N)) allow an RF current and N direct currents (DC) to flow in each coil element, enabling simultaneous reception/excitation and shimming of highly localized B0 inhomogeneities. The purpose of this work was to reduce the cost and complexity of this design by reducing the number of DC power supplies required by a factor N, while maintaining a high RF and shimming performance. In the proposed design, termed adaptive iPRES(N) (iPRES(N)-A), each coil element only requires one DC power supply, but uses microelectromechanical systems switches to adaptively distribute the DC current into the appropriate shim loops to generate the desired magnetic field for B0 shimming. Proof-of-concept phantom experiments with an iPRES(2)-A coil and simulations in the human abdomen with an 8-channel iPRES(4)-A body coil array were performed to demonstrate the advantages of this innovative design. The iPRES(2)-A coil showed no loss in signal-to-noise ratio and provided a much more effective correction of highly localized B0 inhomogeneities and geometric distortions than an equivalent iPRES(1) coil (88.2% vs. 32.2% lower B0 root-mean-square error). The iPRES(4)-A coil array showed a comparable shimming performance as that of an equivalent iPRES(4) coil array (52.6% vs. 54.2% lower B0 root-mean-square error), while only requiring 8 instead of 32 power supplies. The iPRES(N)-A design retains the ability of the iPRES(N) design to shim highly localized B0 inhomogeneities, while drastically reducing its cost and complexity. Magn Reson Med 80:371-379, 2018. © 2017 International Society for Magnetic Resonance in Medicine.

Multifrequency near-field acoustic tomography and holography of 3D subbottom inhomogeneities

A method of coherent multifrequency acoustic tomography and holography of spatially localized subbottom inhomogeneities in shallow seas is proposed. This method is based on solving of the near-field inverse scattering problem that makes it possible to realize a subwavelength resolution. It involves the analysis of measurement data obtained by the 2D transversal scanning with the source-receiver system along the sea bottom, over the area of sounded inhomogeneities. The solution begins with the Born approximation, where the original 3D integral equation for the scattered field is reduced to the 1D Fredholm equation of the first kind relative to the depth profile of the lateral spectrum of inhomogeneities. When solving this integral equation for each pair of spectral components, the generalized discrepancy method is in use. Then, corrections to the Born approximation can be obtained in the proposed iterative procedure. For distributed inhomogeneities, the inverse Fourier transform of the retrieved spectrum gives their 3D distribution that can be visualized as tomography images. For solid targets, this spectrum is used to obtain their shape (i.e. to solve the problem of computer holography). Corresponding results of the numerical simulation are presented.

Field distributions in piezoelectric composites with semi-infinite cracks

A method is offered for the prediction of the electromechanical field in periodic piezoelectric composites with embedded semi-infinite cracks. It is based on the knowledge of the K-field in piezoelectric materials in which the material constants are replaced by the effective moduli of the piezoelectric composite. In addition to the existing semi-infinite crack, the proposed method can analyze localized inhomogeneities near the crack tip. The established effective K-field is applied at the boundaries of a rectangular domain that should be sufficiently far away from the crack tip and the other inhomogeneities. The proposed approach is based on the combined utilization of a micromechanical analysis, the representative cell method and the higher-order theory. The micromechanical analysis establishes the effective electromechanical constants of the piezoelectric composite, and the representative cell method reduces the periodic composite that is discretized into numerous identical cells to a single cell problem in the Fourier transform domain. The governing equations and constitutive relations that are formulated in this single cell are solved by employing the higher-order theory where discretization into subcells is employed. The inverse of the Fourier transform provides the electromechanical field at any point in the composite. The proposed approach is verified for crack fronts that are parallel and perpendicular to the poling direction (axis of symmetry). Applications are given for a cracked porous piezoelectric material, cracks that have been arrested by cavities and for a periodically bilayered composite with a semi-infinite crack.

Integrated parallel reception, excitation, and shimming (iPRES) with multiple shim loops per radio-frequency coil element for improved B0 shimming.

Integrated parallel reception, excitation, and shimming (iPRES) coil arrays allow radio-frequency currents and direct currents to flow in the same coils, which enables excitation/reception and localized B0 shimming with a single coil array. The purpose of this work was to improve their shimming performance by adding the capability to shim higher-order local B0 inhomogeneities that are smaller than the radio-frequency coil elements. A novel design was proposed in which each radio-frequency/shim coil element is divided into multiple direct current loops, each using an independent direct current current, to increase the number of magnetic fields available for shimming while maintaining the signal-to-noise ratio of the coil. This new design is termed iPRES(N), where N represents the number of direct current loops per radio-frequency coil element. Proof-of-concept phantom and human experiments were performed with an 8-channel body coil array to demonstrate its advantages over the original iPRES(1) design. The average B0 homogeneity in various organs before shimming and after shimming with the iPRES(1) or iPRES(3) coil arrays was 0.24, 0.11, and 0.05 ppm, respectively. iPRES(3) thus reduced the B0 inhomogeneity by 53% and further reduced distortions in echo-planar images of the abdomen when compared with iPRES(1). iPRES(N) can correct for localized B0 inhomogeneities more effectively than iPRES(1) with no signal-to-noise ratio loss, resulting in a significant improvement in image quality. Magn Reson Med 77:2077-2086, 2017. © 2016 International Society for Magnetic Resonance in Medicine.

Fast silicon photomultiplier improves signal harvesting and reduces complexity in time-domain diffuse optics.

We present a proof of concept prototype of a time-domain diffuse optics probe exploiting a fast Silicon PhotoMultiplier (SiPM), featuring a timing resolution better than 80 ps, a fast tail with just 90 ps decay time-constant and a wide active area of 1 mm2. The detector is hosted into the probe and used in direct contact with the sample under investigation, thus providing high harvesting efficiency by exploiting the whole SiPM numerical aperture and also reducing complexity by avoiding the use of cumbersome fiber bundles. Our tests also demonstrate high accuracy and linearity in retrieving the optical properties and suitable contrast and depth sensitivity for detecting localized inhomogeneities. In addition to a strong improvement in both instrumentation cost and size with respect to legacy solutions, the setup performances are comparable to those of state-of-the-art time-domain instrumentation, thus opening a new way to compact, low-cost and high-performance time-resolved devices for diffuse optical imaging and spectroscopy.

Excitation of magnetic inhomogeneities in three-layer ferromagnetic structure with different parameters of the magnetic anisotropy and exchange

The generation and evolution of magnetic inhomogeneities, emerging in a thin flat layer with the parameters of the magnetic anisotropy and exchange interaction, with the parameters different from other two thick layers of the three-layer ferromagnetic structure, were investigated. The parameters ranges that determine the possibility of their existence were found. The possibility of the external magnetic field influence on the structure and dynamic properties of localized magnetic inhomogeneities was shown.

Strong scintillation of GNSS signals in the inhomogeneous ionosphere: 1. Theoretical background

The analytical theory of the Global Navigation Satellite Systems (GNSS) signal propagation through the inhomogeneous ionosphere with time-varying embedded localized random inhomogeneities of electron density is constructed in order to describe the regime of strong scintillation of the wave field propagating in the inhomogeneous ionospheric layer. The theory is based on the solutions of the appropriate Markov's parabolic moment equations, extended to the case of the inhomogeneous background medium. The solutions are validated by the comparison with some rigorous (numerical) solutions known for some limiting cases. The developed theory forms the theoretical background for constructing the physically based software simulator of the random transionospheric GNSS signals.