Flat Region Research Articles

Raman imaging of strained bubbles and their effects on charge doping in monolayer WS2 encapsulated with hexagonal boron nitride

Interfacial defects significantly affect the optical and electronic properties of two-dimensional (2D) materials. Particularly, bubbles inevitably formed during the layer-by-layer fabrication of 2D heterostructures can cause spatially inhomogeneous distributions in charge density and strain, leading to modifications in emission efficiency and electronic structures that are markedly different depending on which interface the bubbles form. Here, we report spatially resolved Raman results of a hBN/WS2/hBN heterostructure over a large area in which microbubbles are present. Spatial variations in the optical phonon characteristics of both WS2 and hBN reveal that the bubbles are formed at the interface between the top hBN layer and the underlying WS2 monolayer. The presence of the hBN bubbles results in a relatively higher electron density of the underlying WS2 than that of the WS2 in the bubble-free surrounding flat region, possibly due to the flexoelectric effect of the bent hBN layer. In addition, the Grüneisen parameter of hBN is obtained using the relationship between E2g phonon frequency shifts and corresponding strain profiles of the hBN bubble layer. Our work inspires a more comprehensive understanding of charge and strain distributions under the effect of interfacial defects.

Comparison of heteroepitaxial diamond nucleation and growth on roughened and flat Ir/SrTiO3 substrates

Heteroepitaxial diamond deposition on iridium substrates allows production of single crystal diamond layers on large area that is difficult to achieve with homoepitaxy. Critically important is the first stage of the process when diamond nucleates on Ir upon biasing (a voltage applied to the substrate). Here, we compared the heteroepitaxial nucleation and successive growth of diamond thin films in a microwave plasma CVD reactor on Ir/SrTiO3 substrates with different surfaces modified by bias voltage. The roughened surface consists of furrows and ridges while the flat surface is a little wrinkled. The film morphology, crystal quality and stress are characterized with SEM, AFM, TEM, X-ray diffraction and Raman spectroscopy. It is found that the film on the roughened Ir surface has a lower roughness, smaller mosaic spread, but a larger residual stress than the film deposited on the flat Ir surface, which mainly originates from the difference in the domain density. By comparing diamond (111) lattice spacings of different positions from the Ir-diamond interface, it is inferred that diamond nucleation could occur at the furrows and then overgrow the ridges for Ir-roughened sample while diamond may nucleate at the wrinkle and flat regions for Ir-flat sample. These finding are helpful in understanding of details of diamond film heteroepitaxy, and in searching the ways to improve their crystallinity.

Phase Space Transport in a Symmetric Caldera Potential with Three Index-1 Saddles and No Minima

We apply the method of Lagrangian Descriptors (LDs) to a symmetric Caldera-type potential energy surface which has three index-1 saddles surrounding a relatively flat region that contains no minimum. Using this method we show the phase space transport mechanism that is responsible for the existence and nonexistence of the phenomenon of dynamical matching for this form of Caldera potential energy surface.

Second-Harmonic Response in Magnetic Nodal-Line Semimetal Fe3GeTe2

We experimentally investigate second-harmonic transverse voltage response to ac electrical current for a magnetic nodal-line semimetal Fe3GeTe2 (FGT). For zero magnetic field, the observed second-harmonic voltage behaves as a square of the longitudinal current, as it should be expected for nonlinear Hall effect. The magnetic field behavior is found to be sophisticated: while the first-harmonic response shows the known anomalous Hall hysteresis in FGT, the second-harmonic Hall voltage is characterized by the pronounced high-field hysteresis and flat (B-independent) region with curves touching at low fields. The high-field hysteresis strongly depends on the magnetic field sweep rate, so it reflects some slow relaxation process. For the lowest rates, it is also accomplished by multiple crossing points. Similar shape of the second-harmonic hysteresis is known for skyrmion spin textures in nonlinear optics. Since skyrmions have been demonstrated for FGT by direct visualization techniques, we can connect the observed high-field relaxation with deformation of the skyrmion lattice. Thus, the second-harmonic Hall voltage response can be regarded as a tool to detect spin textures in transport experiments.

Spatiotemporal Characteristics, Decoupling Effect and Driving Factors of Carbon Emission from Cultivated Land Utilization in Hubei Province.

The carbon emission level and spatiotemporal characteristics in Hubei Province were estimated and studied using the Intergovernmental Panel on Climate Change (IPCC) carbon emission coefficient technique based on county data from Hubei Province from 2000 to 2020. The relationship between carbon emissions from cultivated land utilization and agricultural economic growth was examined using the Tapio decoupling index, and the factors influencing carbon emissions in Hubei Province were further examined using the Logarithmic Mean Divisia Index (LMDI model). The results demonstrate that: (1) Spatiotemporal variations in carbon emissions are evident. In terms of time, the volume of carbon emissions in Hubei Province is still substantial, and the transition to low-carbon land use is quite gradual. Geographically, the high-value region of the middle east coexists with the low-value zone of the west, with apparent regional contrasts. (2) The decoupling between carbon emissions and agricultural economic growth is becoming more and more obvious in Hubei Province. The number of counties and cities in a negative decoupling state has significantly decreased, and the majority of counties are now in a strong decoupling condition. (3) Agricultural production efficiency is the most significant driving factor for restricting carbon emission, according to the decomposition results of carbon emission driving factors based on the LMDI model. In addition, the results of sample decomposition based on topographic characteristics indicate that agricultural production efficiency is primarily responsible for the suppression of carbon emissions in flat regions. The increase in carbon emissions in hilly regions is primarily influenced by agricultural productivity. The increase in carbon emissions in mountainous regions is mostly influenced by agricultural labor intensity. This study′s finding has enlightening implications for the high-quality growth of agriculture.

Compression-induced buckling of thin films bonded to viscous substrates: Uniform wrinkles vs localized ridges

We examine the buckling of a thin elastic film bonded to a much thicker viscous substrate undergoing compression at a fixed rate. Experiments show two distinct buckling modes. At high rate of compression or at large liquid thickness, the buckles take on the form of approximately-sinusoidal wrinkles. At low rate of compression or at small liquid thickness, the buckles are highly localized into tall ridges separated nearly flat regions. Ridge growth is accompanied by lateral motion of the film in the near-ridge region, and the liquid in that region undergoes severe shear flow. Such ridge formation is entirely distinct from other examples of curvature localization such as fold localization of films on liquids in static equilibrium or ridge formation of films bonded to hyperelastic substrates. We quantify how the two lengthscales that arise from the buckling process, the wrinkle amplitude and the interridge distance, depend on compression rate and liquid thickness. We show that wrinkles formed during compression can also transform into ridges under quiescent conditions which suggests that buckle localization reduces bending energy of the film. We speculate that ridge localization appears due to a competition between two effects: a buckle mode with a few well-spaced ridges offers a lower energy state than uniform wrinkles, but wrinkles can develop faster because they require the viscous fluid to move over shorter distances.

The Legendre Polynomial Axial Expansion Method

This work presents a new formulation of the axial expansion transport method explicitly using Legendre polynomials for arbitrarily high-order expansions. This new formulation also features an alternative method of axial leakage calculation to allow for nonextruded flat source region meshes. This alternative axial leakage is introduced alongside a balance equation requirement to ensure that neutron balance is preserved in the coarse mesh for a given axial leakage formulation, which allows for effective coarse mesh finite difference acceleration. A matrix exponential table method is derived to allow for fast computations of arbitrarily high-order matrix exponentials for this work and precludes the need for further research into matrix exponential calculations for this method. Numerical results are presented that demonstrate the stability of the axial expansion method in systems with voidlike regions, showcase the speedup from matrix exponential tables, and investigate the axial convergence of the method in terms of both expansion order and mesh size.

Conformally-rescaled Schwarzschild metrics do not predict flat galaxy rotation curves

For conformally invariant gravity theories defined on Riemannian spacetime and having the Schwarzschild–de-Sitter (SdS) metric as a solution in the Einstein gauge, we consider whether one may conformally rescale this solution to obtain flat rotation curves, such as those observed in galaxies, without the need for dark matter. Contrary to recent claims in the literature, we show that if one works in terms of quantities that can be physically measured, then in any conformal frame the trajectories followed by ‘ordinary’ matter particles are merely the timelike geodesics of the SdS metric, as one might expect. This resolves the apparent frame dependence of physical predictions and unambiguously yields rotation curves with no flat region. We also show that attempts to model rising rotation curves by fitting the coefficient of the quadratic term in the SdS metric individually for each galaxy are precluded, since this coefficient is most naturally interpreted as proportional to a global cosmological constant. We further extend our analysis beyond static, spherically-symmetric systems to show that the invariance of particle dynamics to the choice of conformal frame holds for arbitary metrics, again as expected. Moreover, we show that this conclusion remains valid for conformally invariant gravity theories defined on more general Weyl–Cartan spacetimes, which include Weyl, Riemann–Cartan and Riemannian spacetimes as special cases.

FLAT LIKELIHOODS: THE SKEW NORMAL DISTRIBUTION CASE

Several references argue in favor of alternative estimation methods, rather than the likelihood one, when the likelihood function exhibits flat regions. However, in the case of the skew normal distribution we present a discussion describing the interpretation of those flat likelihoods. This distribution is widely used in several interesting applications and contains the normal distribution as a nested model and the half-normal as an embedded model. Here, we show that flat likelihoods provide relevant information that should be carefully analyzed before discarding its use and proposing other estimation methods. Two well-known examples, that have been reported as troublesome, are analyzed here, including also an exhaustive computational study. The analysis of different scenarios allows to understand not only the reason of this likelihood function shape, but also to discover the information this behavior provides.

Edge preserving range image smoothing using hybrid locally kernel-based weighted least square

In this paper, we propose the hybrid locally kernel-based weighted least square (HKLS) method to reduce the noise of the range images captured from the Microsoft Kinect sensor. Although removing the noise of Kinect sensor is inevitable in mobile robot applications, preserving the main structures of an indoor scene is a key point of the smoothing methods for range data. Our method uses a situation estimation module to apply an appropriate method for denoising the input data. In this paper, situations are classified into two categories including flat regions and edge regions. The locally kernel-based weighted least square method is used for the edge regions that have significant changes in the surface normals and the weight of each point is assigned with the Gaussian mixture model. The Bilateral filtering is applied for the flat regions with no sharp change in surface normals. We provided a solution for the noise data issue and proof of uncertainty reduction in presence of noise data. There is a fusion module for assigning the proportion of each method according to the different situations. We evaluated our method on the NYUv2 dataset that contains different RGB-D indoor scenes with other state-of-the-art methods. We achieved the value of 49.48 for the Z-MAE and 0.502 for the N-MAE error metrics.

Effect of surface irregularities on diamond Schottky barrier diode with threading dislocations

Pyramidal and flat-type hillocks, both known for having a threading dislocation in the center, are utilized to examine the effect of surface irregularities on the characteristics of Schottky barrier diodes (SBDs). By performing RAMAN mapping on both hillocks, crystalline imperfections in three dimensions with clear strain are observed. Surface scanning images obtained via atomic force microscopy show an extremely large surface protrusion with a maximum height of approximately 15 nm in the pyramidal hillock. Owing to crystalline imperfections in the threading dislocation region, the SBDs fabricated on the dislocations exhibit inferior forward characteristics. However, high reverse leakage currents are only observed in dislocations on pyramidal hillocks with surface protrusions, whereas low reverse currents are observed in the flat hillock region without surface irregularities. The phenomenon above is consistent with the characteristics of wide bandgap semiconductors such as silicon carbide.

Classical and Bayesian estimations of improved Weibull–Weibull distribution for complete and censored failure times data

AbstractIn this article, we demonstrate how to enhance the Weibull–Weibull (WW) distribution introduced in the earlier literature into a better form for fitting monotone and non‐monotone failure rate data, especially the bathtub‐shaped failure rate data with or without a flat region. The model is referred to as an improved WW distribution. The model's flexibility enables it to describe lifetime data with various failure rate functions, including increasing, decreasing, U or V‐shaped, and bathtub‐shaped with a comparatively low and long‐flat segment. We provide a thorough Bayesian analysis of the modified model for complete and right‐censored data. Additionally, we developed maximum likelihood estimators for the model's parameters for both complete and right‐censored data. The Bayesian credible and asymptotic confidence intervals of the estimators are defined, and simulation results validate the estimators' consistency. To illustrate the applications of the improved distribution with the WW and other generalized distributions, we apply one censored and one uncensored failure times data, each with bathtub‐shaped failure rates. The numerical results demonstrate that the improved WW model outperforms the WW distribution and other existing models, as indicated by goodness‐of‐fit statistics and supported by the fitted models' survival and failure rate curves and P‐P plots.

Unifying inflation with early and late dark energy with multiple fields: Spontaneously broken scale invariant two measures theory

A unified multi scalar field model with three flat regions is discussed. The three flat regions are the inflation, early and late dark energy epochs. The potential is obtained by a spontaneous breaking of scale invariance generated by Non Riemannian Measures of integration (or Two Measures Theories (TMT)).We define the scale invariant couplings of the scalar fields to the different measures through exponential potentials. Spontaneous breaking of scale invariance takes place when integrating the fields that define the measures. When going to the Einstein frame we obtain: (i) An effective potential for the scalar fields with three flat regions which allows for a unified description of both early universe inflation (in the higher energy density flat region) as well as of present dark energy epoch which can be realized with a double phase, i.e., in two flat regions. (ii) In the slow roll inflation, only one field combination the ``dilaton", which transforms under scale transformations, has non trivial dynamics, the orthogonal one, which is scale invariant remains constant. (iv) In the late universe we define scale invariant couplings of Dark Matter to the dilaton. These couplings define a matter induced potential for the dilaton and extremizing this potential determines the scale invariant scalar field, while all exotic non canonical behavior of the Dark Matter as well as any possible $5^{th}$ force disappear. (v) We calculate the evolution of the late universe under these conditions with the realization of two different possible realizations of $\Lambda$CDM type scenarios depending of the flat region in the late universe. These two phases could appear at different times in the history of the universe.(vi) From the Planck data, we find the constraints on the parameters during the inflationary epoch and these values are used to obtain constraints relevant to the present epoch.

New Framework of Combining Observations with Topographic Slope to Estimate VS30 and Its Application on Building a VS30 Map for Mainland China

ABSTRACTWe propose a new framework of VS30 proxy based on Cokriging method and apply the framework to build a VS30 map for mainland China. This framework utilizes the VS30–topographic slope correlation in the cross-semivariogram to benefit VS30 estimation and has the following benefits: (1) the estimation results are consistent with the measurement data; (2) the estimation uncertainty can be represented by error variance at each unsampled location according to the spatial structure of VS30 and topographic slope; (3) the result map does not have artificial boundaries; and (4) the estimation results can reflect the spatial relation between VS30 and topographic slope and the spatial relation of local spatial environment of topographic slope. We quantify the performance of this framework and compare it with that of three other topographic slope-based VS30 proxy models, including original models developed from exogenous data and models developed from China local data. The result shows that the framework proposed in this article has the best performance. The framework is applied to 7797 borehole VS30 measurements to build a VS30 map for mainland China. The map can capture the high VS30 values in regions where the geological conditions are dominated by flat-lying rocks. Moreover, we consider the effect of sample bias that comes from oversampling of borehole profiles in flat terrain regions when applying borehole data in the proposed framework. We utilize the relation of VS30 and topographic slope to quantify this bias, and use a distance-related data spatial declustering method to eliminate it.

A primary proton integral depth dose calculation model corrected with straight scattering track approximation

PurposeProton therapy requires a highly accurate dose calculation model. The Bortfeld analytical formula (BAF) is a classical depth-dose calculation model that exhibits a dose deviation compared to the Monte Carlo simulation. This study focuses on the lateral scattering effect on depth dose deviation between model calculations and Monte Carlo simulations and provides a modified model to correct. MethodsOnly the dose contributed by the primary particles was studied in this work. The Bethe stopping power formula was applied to reduce the deviation in the flat region. The proton track lateral scattering is considered to reduce the deviation in the Bragg peak region. Lateral scattering tracks were approximated as straight lines and assumed to have a Gaussian slope distribution. The tracks were projected to the central axis to correct the depth-dose curve. The calculated dose was compared with a Geant4 simulation. Four proton beams with energies of 50 MeV, 100 MeV, 150 MeV, and 200 MeV were calculated and verified. The dose deviation between the model calculation and Monte Carlo simulation was quantified under a dose and depth tolerance of 2%/1 mm with gamma analysis. ResultsThe straight scattering track approximation significantly reduced the primary dose deviation between the model calculation and the Geant4 simulation. The gamma values of all verified cases are below 1, which means that the deviation is acceptable under a given tolerance of 2%/1 mm. ConclusionsThis study proposes a modification of the primary depth-dose calculation model involving scattering-track correction. Good dose agreement between the calculation and Geant4 simulation was obtained. The modified model shows good accuracy in primary depth dose calculation and will be helpful for clinical application together with further secondary dose models, which are still under investigation.

Aberrant uterine folding in mice disrupts implantation chamber formation and alignment of embryo-uterine axes.

ABSTRACTThe uterine luminal epithelium folds characteristically in mammals, including humans, horses and rodents. Improper uterine folding in horses results in pregnancy failure, but the precise function of folds remains unknown. Here, we uncover dynamic changes in the 3D uterine folding pattern during early pregnancy with the entire lumen forming pre-implantation transverse folds along the mesometrial-antimesometrial axis. Using a time course, we show that transverse folds are formed before embryo spacing, whereas implantation chambers form as the embryo begins attachment. Thus, folds and chambers are two distinct structures. Transverse folds resolve to form a flat implantation region, after which an embryo arrives at its center to attach and form the post-implantation chamber. Our data also suggest that the implantation chamber facilitates embryo rotation and its alignment along the uterine mesometrial-antimesometrial axis. Using WNT5A- and RBPJ-deficient mice that display aberrant folds, we show that embryos trapped in longitudinal folds display misalignment of the embryo-uterine axes, abnormal chamber formation and defective post-implantation morphogenesis. These mouse models with disrupted uterine folding provide an opportunity to understand uterine structure-based mechanisms that are crucial for implantation and pregnancy success. This article has an associated ‘The people behind the papers’ interview.

Einstein–Dirac–Maxwell wormholes: ansatz, construction and properties of symmetric solutions

We present a discussion of the traversable wormholes in Einstein–Dirac–Maxwell theory recently reported in e-Print: 2010.07317. This includes a detailed description of the ansatz and junction condition, together with an investigation of the domain of existence of the solutions. In this study, we assume symmetry under interchange of the two asymptotically flat regions of a wormhole. Possible issues and limitations of the approach are also discussed.

Group quantization of the black hole minisuperspace

The emergence of nontrivial symmetries for black holes minisuperspaces has been recently pointed out. These Noether symmetries possess non-null charges and hence map physical solutions to different ones. The symmetry group is isomorphic to the finite-dimensional Poincar\'e group $\mathrm{ISO}(2,1)$, whose irreducible representations are well known. This structure is used to build a consistent quantum theory of black hole minisuperspace. This has, among other consequences, the striking consequence of implying a continuous spectrum for the mass operator. Following loop quantum cosmology, we obtain a regularization scheme compatible with the symmetry structure. It is possible to study the evolution of coherent states following the classical trajectories in the low curvature regime. We show that this produces an effective metric where the singularity is replaced by a Killing horizon merging two asymptotically flat regions. The quantum correction comes from a fundamental discreteness of spacetime, and the uncertainty on the energy of the system. Remarkably, the effective evolution of semiclassical states is described by an effective Hamiltonian, related to the original one through a canonical transformation.

Crustal structure on west-central Baja California peninsula, México, inferred by 3D linear inverse modeling of gravity data

Gravity data from the west-central portion of the Baja California Peninsula, in NW Mexico, were processed on the ellipsoidal reference frame to obtain isostatic residual gravity anomalies. This is a nearly flat region close to sea level and blanked by sediments, where a gravity high bounded by two prominent minima occurs. The gravity high is oriented in a NE-SW direction, and it is possibly the most prominent gravity anomaly transverse to the Baja California Peninsula. The gravity minima, located towards the NW and SE from the gravity high, are respectively related with the Sebastián Vizcaíno and Purísima sedimentary basins. These basins were developed since Early Cretaceous times during the subduction of the Farallon Plate under the North American plate. To infer the crustal structure and the basement geometry in this region, inverse linear modeling is carried on a multilevel data set of upward continued isostatic residual gravity data. Assuming that compaction is the primary diagenetic process on the sediments filling the basins, the initial model is constrained by the surface geology expressed as a density contrast that decreases with depth. The outcome of the inversion is density contrast in a cuboid model, constituted by 67,379 prisms, and its three-dimensional representation suggests that these two basins accommodate as much as five km-thick sedimentary sequences –formed by shallow siliciclastic and deep volcaniclastic rocks penetrated by wells drilled in the region– lying above the inferred basement, which shallows beneath the Lagunitas gravity high, consistently with depth to crystalline rocks penetrated by wells.

Glacier Mass Balance Pattern and Its Variation Mechanism in the West Kunlun Mountains in Tibetan Plateau

Mass balance observations are beneficial for assessing climate change in different world regions. This study analyzed the glacier elevation change, ice flux divergence, and surface mass balance (SMB) in the West Kunlun Mountains (WKM) on the Tibetan Plateau using remote sensing data, including satellite altimetry, glacier surface velocity, and thickness fields. Seventeen local glaciers were examined in detail and showed varying surface elevation changes from −0.39 ± 0.11 to 0.83 ± 0.10 m/a. Overall, we obtained a reasonably rapid elevation trend of 0.21 ± 0.14 m/a. By combining the ice flux divergence and surface mass balance, the overall thickness change of the WKM glacier over time is almost zero, and the WKM glacier shows a positive mass balance of 0.21 ± 0.98 m/a. Moreover, the ice flux divergence is more significant on the ice tongue than in the flat region due to the more considerable gradient of surface velocity and thickness fields. We found that glacier heterogeneity dynamics were associated with a surging dynamic mechanism concentrated in the glacier tongue and were induced by inner terrain instabilities. The glacier surging causes a drastic drop in glacier elevation but does not cause a glacier mass gain or loss, and it has an enhanced effect on the ice flux divergence. Therefore, glacier surging is the main reason for the decline of the two glaciers monitored. In addition, the long-term meteorological data analysis found that, since 2000, the air temperature warming hiatus may have balanced the three glaciers, and significantly increasing precipitation variation may cause the glacier to thicken the most.