833 research outputs found
A scheme for simulating multi-level phase change photonics materials
Abstract Chalcogenide phase change materials (PCMs) have been extensively applied in data storage, and they are now being proposed for high resolution displays, holographic displays, reprogrammable photonics, and all-optical neural networks. These wide-ranging applications all exploit the radical property contrast between the PCMs’ different structural phases, extremely fast switching speed, long-term stability, and low energy consumption. Designing PCM photonic devices requires an accurate model to predict the response of the device during phase transitions. Here, we describe an approach that accurately predicts the microstructure and optical response of phase change materials during laser induced heating. The framework couples the Gillespie Cellular Automata approach for modelling phase transitions with effective medium theory and Fresnel equations. The accuracy of the approach is verified by comparing the PCM’s optical response and microstructure evolution with the results of nanosecond laser switching experiments. We anticipate that this approach to simulating the switching response of PCMs will become an important component for designing and simulating programmable photonics devices. The method is particularly important for predicting the multi-level optical response of PCMs, which is important for all-optical neural networks and PCM-programmable perceptrons
An Integrated Constrained Gradient Descent (iCGD) Protocol to Correct Scan-Positional Errors for Electron Ptychography with High Accuracy and Precision
Correcting scan-positional errors is critical in achieving electron
ptychography with both high resolution and high precision. This is a demanding
and challenging task due to the sheer number of parameters that need to be
optimized. For atomic-resolution ptychographic reconstructions, we found
classical refining methods for scan positions not satisfactory due to the
inherent entanglement between the object and scan positions, which can produce
systematic errors in the results. Here, we propose a new protocol consisting of
a series of constrained gradient descent (CGD) methods to achieve better
recovery of scan positions. The central idea of these CGD methods is to utilize
a priori knowledge about the nature of STEM experiments and add necessary
constraints to isolate different types of scan positional errors during the
iterative reconstruction process. Each constraint will be introduced with the
help of simulated 4D-STEM datasets with known positional errors. Then the
integrated constrained gradient decent (iCGD) protocol will be demonstrated
using an experimental 4D-STEM dataset of the 1H-MoS2 monolayer. We will show
that the iCGD protocol can effectively address the errors of scan positions
across the spectrum and help to achieve electron ptychography with high
accuracy and precision
Single-shot pop-out 3D metrology of thin specimens with TEM
Three-dimensional (3D) imaging of thin, extended specimens at nanometer
resolution is critical for applications in biology, materials science, advanced
synthesis, and manufacturing. Many 3D imaging techniques are limited to surface
features, or available only for selective cross-sections, or require a tilt
series of a local region, hence making them unsuitable for rapid,
non-sacrificial screening of extended objects, or investigating fast dynamics.
Here we describe a coherent imaging technique that recovers the 3D volume of a
thin specimen with only a single, non-tomographic, energy-filtered,
bright-field transmission electron microscopy (TEM) image. This technique does
not require physically fracturing or sectioning thin specimens, only needs a
single brief exposures to electron doses of ~100 e {\AA}-2, and can be readily
calibrated for many existing TEMs; thus it can be widely deployed for rapid 3D
metrology that complements existing forms of metrology.Comment: 33 pages, 14 figure
Defects in WS2 monolayer calculated with a nonlocal functional: any difference from GGA?
Density Functional Theory (DFT) with Generalized Gradient Approximation (GGA) functionals is commonly used to predict defect properties in 2D transition metal dichalcogenides (TMDs). Since GGA functionals often underestimate bandgaps of semiconductors and incorrectly describe the character of electron localization in defects and their level positions within the band-gap, it is important to assess the accuracy of these predictions. To this end, we used the non-local density functional PBE0-TC-LRC to calculate the properties of a wide range of intrinsic defects in monolayer WS2. The properties, such as geometry, in-gap states, charge transition levels, electronic structure and the electron/hole localization of the lowest formation energy defects are discussed in detail. They are broadly similar to those predicted by the GGA PBE functional but exhibit numerous quantitative differences caused by the degree of electron and hole localization in charged states. For some anti-site defects, more significant differences are seen, with both changes in defect geometries (differences of up to 0.5 Å) as well as defect level positions within the band gap of WS2. This work provides an insight into the performance of functionals chosen for future DFT calculations of transition metal dichalcogenides with respect to the desired defect properties
Low Thermal Conductivity Phase Change Memory Superlattices
Phase change memory devices are typically reset by melt-quenching a material
to radically lower its electrical conductance. The high power and concomitantly
high current density required to reset phase change materials is the major
issue that limits the access times of 3D phase change memory architectures.
Phase change superlattices were developed to lower the reset energy by
confining the phase transition to the interface between two different phase
change materials. However, the high thermal conductivity of the superlattices
means that heat is poorly confined within the phase change material, and most
of the thermal energy is wasted to the surrounding materials. Here, we
identified Ti as a useful dopant for substantially lowering the thermal
conductivity of Sb2Te3-GeTe superlattices whilst also stabilising the layered
structure from unwanted disordering. We demonstrate via laser heating that
lowering the thermal conductivity by doping the Sb2Te3 layers with Ti halves
the switching energy compared to superlattices that only use interfacial phase
change transitions and strain engineering. The thermally optimized superlattice
has (0 0 l) crystallographic orientation yet a thermal conductivity of just
0.25 W/m.K in the "on" (set) state. Prototype phase change memory devices that
incorporate this Ti-doped superlattice switch faster and and at a substantially
lower voltage than the undoped superlattice. During switching the Ti-doped
Sb2Te3 layers remain stable within the superlattice and only the Ge atoms are
active and undergo interfacial phase transitions. In conclusion, we show the
potential of thermally optimised Sb2Te3-GeTe superlattices for a new generation
of energy-efficient electrical and optical phase change memory.Comment: 4 Figures, 7 Supplementary Figures, 27 pages including a supplemen
Nanoscale mapping of optically inaccessible bound-states-in-the-continuum
Bound-states-in-the-continuum (BIC) is an emerging concept in nanophotonics with potential impact in applications, such as hyperspectral imaging, mirror-less lasing, and nonlinear harmonic generation. As true BIC modes are non-radiative, they cannot be excited by using propagating light to investigate their optical characteristics. In this paper, for the 1st time, we map out the strong near-field localization of the true BIC resonance on arrays of silicon nanoantennas, via electron energy loss spectroscopy with a sub-1-nm electron beam. By systematically breaking the designed antenna symmetry, emissive quasi-BIC resonances become visible. This gives a unique experimental tool to determine the coherent interaction length, which we show to require at least six neighboring antenna elements. More importantly, we demonstrate that quasi-BIC resonances are able to enhance localized light emission via the Purcell effect by at least 60 times, as compared to unpatterned silicon. This work is expected to enable practical applications of designed, ultra-compact BIC antennas such as for the controlled, localized excitation of quantum emitter
Molecular Bridges Link Monolayers of Hexagonal Boron Nitride during Dielectric Breakdown
We use conduction atomic force microscopy (CAFM) to examine the soft breakdown of monocrystalline hexagonal boron nitride (h-BN) and relate the observations to the defect generation and dielectric degradation in the h-BN by charge transport simulations and density functional theory (DFT) calculations. A modified CAFM approach is adopted, whereby 500
7 500 nm2 to 3
7 3 μm2 sized metal/h-BN/metal capacitors are fabricated on 7 to 19 nm-thick h-BN crystal flakes and the CAFM tip is placed on top of the capacitor as an electrical probe. Current-voltage (I-V) sweeps and time-dependent dielectric breakdown measurements indicate that defects are generated gradually over time, leading to a progressive increase in current prior to dielectric breakdown. Typical leakage currents are around 0.3 A/cm2 at a 10 MV/cm applied field. DFT calculations indicate that many types of defects could be generated and contribute to the leakage current. However, three defects created from adjacent boron and nitrogen monovacancies exhibit the lowest formation energy. These three defects form molecular bridges between two adjacent h-BN layers, which in turn "electrically shorts"the two layers at the defect location. Electrical shorting between layers is manifested in charge transport simulations, which show that the I-V data can only be correctly modeled by incorporating a decrease in effective electrical thickness of the h-BN as well as the usual increase in trap density, which, alone, cannot explain the experimental data. An alternative breakdown mechanism, namely, the physical removal of h-BN layers under soft breakdown, appears unlikely given the h-BN is mechanically confined by the electrodes and no change in AFM topography is observed after breakdown. High-resolution transmission electron microscope micrographs of the breakdown location show a highly localized (<1 nm) breakdown path extending between the two electrodes, with the h-BN layers fractured and disrupted, but not removed
A NEW SATELLITE IMAGERY STEREO PIPELINE DESIGNED FOR SCALABILITY, ROBUSTNESS AND PERFORMANCE
Abstract. This paper presents a new Multiview Stereo Pipeline (MVS), called CARS, dedicated to satellite imagery. This pipeline is intended for massive Digital Surface Model (DSM) production and has therefore been designed to maximize scalability robustness and performance. Those two properties have driven the design of the workflow as well as the choice of algorithms and parameter trends, making our pipeline unique with respect to existing solutions in literature. This paper intends to serve as a reference paper for the pipeline implementation, and therefore provides a detailed description of algorithms and workflow. It also demonstrates the pipeline robustness and stability in several use cases, and compares its accuracy with the state-of-the-art pipelines on a reference dataset.
Document type: Articl
Search for the glueball candidates f0(1500) and fJ(1710) in gamma gamma collisions
Data taken with the ALEPH detector at LEP1 have been used to search for gamma
gamma production of the glueball candidates f0(1500) and fJ(1710) via their
decay to pi+pi-. No signal is observed and upper limits to the product of gamma
gamma width and pi+pi- branching ratio of the f0(1500) and the fJ(1710) have
been measured to be Gamma_(gamma gamma -> f0(1500)). BR(f0(1500)->pi+pi-) <
0.31 keV and Gamma_(gamma gamma -> fJ(1710)). BR(fJ(1710)->pi+pi-) < 0.55 keV
at 95% confidence level.Comment: 10 pages, 3 figure
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