5,403 research outputs found
Measuring surface dislocation nucleation in defect-scarce nanostructures
Linear defects in crystalline materials, known as dislocations, are central to the understanding of plastic deformation and mechanical strength, as well as control of performance in a variety of electronic and photonic materials. Despite a thorough understanding of dislocation structure and their interactions, measurements of the energetics and kinetics of dislocation nucleation have not been possible, as synthesizing and testing pristine crystals absent of defects has been prohibitively challenging. In this talk, experiments that directly measure the surface dislocation nucleation strengths in high qualityPd nanowhiskers subjected to uniaxial tension will be presented. We find that, whereas nucleation strengths are weakly size- and strain-rate-dependent, a strong temperature dependence is uncovered, corroborating predictions that nucleation is assisted by thermal fluctuations. We measure atomic-scale activation volumes, which explain both the ultrahigh athermal strength as well as the temperature-dependent scatter, evident in our experiments and well captured by a thermal activation model. Modeling of the probabilistic nature of surface dislocation nucleation suggests activation energies consistent with surface self-diffusion as the rate-limiting step needed to promote displacive activity. In this context, approaches allowing for modification of the surface chemistry and structure of metallic nanostructures to either inhibit or enhance surface diffusion will be discussed
Modelling Large Scale Invasion of Aedes aegypti and Aedes albopictus Mosquitoes
The principle aim of this work is to simulate the invasion of two invasive mosquito species Aedes aegypti and Aedes albopictus in central Europe at a landscape scale. The spatial-temporal dynamics of invasion is investigated in dependence of predation pressure, seasonal variation of ambient temperature as well as human population density. The introduction of temperature dependent entomological parameters enables the simulation of seasonal pattern of population dynamics. The influence of temperature, predation pressure and human population density on invasion is studied in one-dimensional cases. In two dimensions, georeferenced parameters such as annual mean temperature and human population density are prepared by a geographical information system and introduced into the finite element tool COMSOL Multiphysics. The results show that under the current temperature, central Europe cannot become a permanent breeding region for Aedes aegypti. However, southwest Germany especially the regions along the Upper Rhine Valley may provide suitable habitats for the permanent establishment of Aedes albopictus. An annual temperature rise of two degrees would lead to dramatic increase of invasion speed and extension range of Aedes albopictus
Dendritic GluN2A Synthesis Mediates Activity-Induced NMDA Receptor Insertion
Long-term synaptic plasticity involves changes in the expression and membrane insertion of cell-surface proteins. Interestingly, the mRNAs encoding many cell-surface proteins are localized to dendrites, but whether dendritic protein synthesis is required for activity-induced surface expression of specific proteins is unknown. Herein, we used microfluidic devices to demonstrate that dendritic protein synthesis is necessary for activity-induced insertion of GluN2A-containing NMDA receptors in rat hippocampal neurons. Furthermore, visualization of activity-induced local translation of GluN2A mRNA and membrane insertion of GluN2A protein in dendrites was directly observed and shown to depend on a 3\u27 untranslated region cytoplasmic polyadenylation element and its associated translation complex. These findings uncover a novel mechanism for cytoplasmic polyadenylation element-mediated posttranscriptional regulation of GluN2A mRNA to control NMDA receptor surface expression during synaptic plasticity
Analog Content-Addressable Memory from Complementary FeFETs
To address the increasing computational demands of artificial intelligence
(AI) and big data, compute-in-memory (CIM) integrates memory and processing
units into the same physical location, reducing the time and energy overhead of
the system. Despite advancements in non-volatile memory (NVM) for matrix
multiplication, other critical data-intensive operations, like parallel search,
have been overlooked. Current parallel search architectures, namely
content-addressable memory (CAM), often use binary, which restricts density and
functionality. We present an analog CAM (ACAM) cell, built on two complementary
ferroelectric field-effect transistors (FeFETs), that performs parallel search
in the analog domain with over 40 distinct match windows. We then deploy it to
calculate similarity between vectors, a building block in the following two
machine learning problems. ACAM outperforms ternary CAM (TCAM) when applied to
similarity search for few-shot learning on the Omniglot dataset, yielding
projected simulation results with improved inference accuracy by 5%, 3x denser
memory architecture, and more than 100x faster speed compared to central
processing unit (CPU) and graphics processing unit (GPU) per similarity search
on scaled CMOS nodes. We also demonstrate 1-step inference on a kernel
regression model by combining non-linear kernel computation and matrix
multiplication in ACAM, with simulation estimates indicating 1,000x faster
inference than CPU and GPU
Crack path instabilities in DCDC experiments in the low speed regime
We studied the low speed fracture regime (0.1mm/s - 1nm/s) in different
glassy materials (soda-lime glass, glass-ceramics) with variable but controlled
length scale of heterogeneity. The chosen mechanical system enabled us to work
in pure mode I (tensile) and at a fixed load on DCDC (double cleavage drilled
compression) specimen. The internal residual stresses of studied samples were
carefully relaxed by appropriate thermal treatment. By means of optical and
atomic force (AFM) microscopy techniques fracture surfaces have been examined.
We have shown for the first time that the crack front line underwent an
out-of-plane oscillating behavior as a result of a reproducible sequence of
instabilities. The wavelength of such a phenomenon is in the micrometer range
and its amplitude in the nanometer range. These features were observed for
different glassy materials providing that a typical length scale characterizing
internal heterogeneities was lower than a threshold limit estimated to few
nanometers. This effect is the first clear experimental evidence of crack path
instabilities in the low speed regime in a uniaxial loading experiment. This
phenomenon has been interpreted by referring to the stability criterion for a
straight crack propagation as presented by Adda-Bedia et al. (Phys. Rev.
Letters (1996) 76} p1497).Comment: 16 pages, 10 figures, submitted to Journal of Non-Crystalline Solid
Manganese‐Doping‐Induced Quantum Confinement within Host Perovskite Nanocrystals through Ruddlesden–Popper Defects
The concept of doping Mn2+ ions into II–VI semiconductor nanocrystals (NCs) was recently extended to perovskite NCs. To date, most studies on Mn2+ doped NCs focus on enhancing the emission related to the Mn2+ dopant via an energy transfer mechanism. Herein, we found that the doping of Mn2+ ions into CsPbCl3 NCs not only results in a Mn2+‐related orange emission, but also strongly influences the excitonic properties of the host NCs. We observe for the first time that Mn2+ doping leads to the formation of Ruddlesden–Popper (R.P.) defects and thus induces quantum confinement within the host NCs. We find that a slight doping with Mn2+ ions improves the size distribution of the NCs, which results in a prominent excitonic peak. However, with increasing the Mn2+ concentration, the number of R.P. planes increases leading to smaller single‐crystal domains. The thus enhanced confinement and crystal inhomogeneity cause a gradual blue shift and broadening of the excitonic transition, respectively
Systematic analysis of interannual and seasonal variations of model-simulated tropospheric NO<sub>2</sub> in Asia and comparison with GOME-satellite data
International audienceSystematic analyses of interannual and seasonal variations of tropospheric NO2 vertical column densities (VCDs) based on GOME satellite data and the regional scale chemical transport model (CTM), Community Multi-scale Air Quality (CMAQ), are presented over eastern Asia between 1996 and June 2003. A newly developed year-by-year emission inventory (REAS) was used in CMAQ. The horizontal distribution of annual averaged GOME NO2 VCDs generally agrees well with the CMAQ results. However, CMAQ/REAS results underestimate the GOME retrievals with factors of 2?4 over polluted industrial regions such as Central East China (CEC), a major part of Korea, Hong Kong, and central and western Japan. For the Japan region, GOME and CMAQ NO2 data show good agreement with respect to interannual variation and show no clear increasing trend. For CEC, GOME and CMAQ NO2 data show good agreement and indicate a very rapid increasing trend from 2000. Analyses of the seasonal cycle of NO2 VCDs show that GOME data have systematically larger dips than CMAQ NO2 during February?April and September?November. Sensitivity experiments with fixed emission intensity reveal that the detection of emission trends from satellite in fall or winter have a larger error caused by the variability of meteorology. Examination during summer time and annual averaged NO2 VCDs are robust with respect to variability of meteorology and are therefore more suitable for analyses of emission trends. Analysis of recent trends of annual emissions in China shows that the increasing trends of 1996?1998 and 2000?2002 for GOME and CMAQ/REAS show good agreement, but the rate of increase by GOME is approximately 10?11% yr?1 after 2000; it is slightly steeper than CMAQ/REAS (8?9% yr?1). The greatest difference was apparent between the years 1998 and 2000: CMAQ/REAS only shows a few percentage points of increase, whereas GOME gives a greater than 8% yr?1 increase. The exact reason remains unclear, but the most likely explanation is that the emission trend based on the Chinese emission related statistics underestimates the rapid growth of emissions
Top quark associated production of the neutral top-pion at high energy colliders
In the context of topcolor-assisted technicolor (TC2) models, we calculate
the associated production of the neutral top-pion with a pair of
top quarks via the process . We
find that the production cross section is larger than that of the process both in the standard model (SM) and in the
minimal supersymmetric SM. With reasonable values of the parameters in TC2
models, the cross section can reach . The neutral top-pion
may be direct observed via this process.Comment: Latex files, 10 pages and 3 figure
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Thiophene-Bridged Donor–Acceptor sp2-Carbon-Linked 2D Conjugated Polymers as Photocathodes for Water Reduction
Photoelectrochemical (PEC) water reduction, converting solar energy into environmentally friendly hydrogen fuel, requires delicate design and synthesis of semiconductors with appropriate bandgaps, suitable energy levels of the frontier orbitals, and high intrinsic charge mobility. In this work, the synthesis of a novel bithiophene-bridged donor–acceptor-based 2D sp2-carbon-linked conjugated polymer (2D CCP) is demonstrated. The Knoevenagel polymerization between the electron-accepting building block 2,3,8,9,14,15-hexa(4-formylphenyl) diquinoxalino[2,3-a:2′,3′-c]phenazine (HATN-6CHO) and the first electron-donating linker 2,2′-([2,2′-bithiophene]-5,5′-diyl)diacetonitrile (ThDAN) provides the 2D CCP-HATNThDAN (2D CCP-Th). Compared with the corresponding biphenyl-bridged 2D CCP-HATN-BDAN (2D CCP-BD), the bithiophene-based 2D CCP-Th exhibits a wide light-harvesting range (up to 674 nm), a optical energy gap (2.04 eV), and highest energy occupied molecular orbital–lowest unoccupied molecular orbital distributions for facilitated charge transfer, which make 2D CCP-Th a promising candidate for PEC water reduction. As a result, 2D CCP-Th presents a superb H2-evolution photocurrent density up to ≈7.9 µA cm−2 at 0 V versus reversible hydrogen electrode, which is superior to the reported 2D covalent organic frameworks and most carbon nitride materials (0.09–6.0 µA cm−2). Density functional theory calculations identify the thiophene units and cyano substituents at the vinylene linkage as active sites for the evolution of H2. © 2020 The Authors. Advanced Materials published by Wiley-VCH Gmb
Solar energetic particle access to distant longitudes through turbulent field-line meandering
Context. Current solar energetic particle (SEP) propagation models describe the effects of interplanetary plasma turbulence on SEPs as diffusion, using a Fokker-Planck (FP) equation. However, FP models cannot explain the observed fast access of SEPs across the average magnetic field to regions that are widely separated in longitude within the heliosphere without using unrealistically strong cross-field diffusion.
Aims. We study whether the recently suggested early non-diffusive phase of SEP propagation can explain the wide SEP events with realistic particle transport parameters.
Methods. We used a novel model that accounts for the SEP propagation along field lines that meander as a result of plasma turbulence. Such a non-diffusive propagation mode has been shown to dominate the SEP cross-field propagation early in the SEP event history. We compare the new model to the traditional approach, and to SEP observations.
Results. Using the new model, we reproduce the observed longitudinal extent of SEP peak fluxes that are characterised by a Gaussian profile with σ = 30 − 50◦ , while current diffusion theory can only explain extents of 11◦ with realistic diffusion coefficients. Our model also reproduces the timing of SEP arrival at distant longitudes, which cannot be explained using the diffusion model.
Conclusions. The early onset of SEPs over a wide range of longitudes can be understood as a result of the effects of magnetic fieldline random walk in the interplanetary medium and requires an SEP transport model that properly describes the non-diffusive early phase of SEP cross-field propagation
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