bluuebunny/crossref_metadata_2025 huggingface dataset:
Renewable generation monitoring platform and its applications
Authors: Kou Gefei , Markham Penn , Purcell Thomas , Reynolds Aaron , Valdez Ariel , Orndorff Robert , Starling Brian , VanSant Philip
Published: August 2020

Monitoring a large and geographically dispersed solar generation fleet can be a challenging undertaking. At Dominion Energy, there is currently over 1.7 GW of transmission and distribution‐connected solar generation. Performing the post‐mortem analysis and ensuring the safe and reliable operation on more than 100 solar facilities can take substantial engineering efforts. In this study, a renewable generation data collection platform is presented to address the increasing need for solar generation monitoring. The data platform consists of data collection, event storage, real‐time alerts, and data analytics applications. The data analysis system can support various technical aspects in solar integration, including grounding monitoring, inverter fault response and model validation, power quality monitoring, and equipment overvoltage protection. The proposed data platform greatly improves the visibility of solar generation and ensures its successful integration.
Decision tree‐based classifiers for root‐cause detection of equipment‐related distribution power system outages
Authors: Dehbozorgi Mohammad Reza , Rastegar Mohammad , Dabbaghjamanesh Morteza
Published: August 2020

Recently, power system reliability has been challenged due to the increment of electrical demand. When an outage occurs, locating the outage may take a long time because of the distribution system's radial structure and the presence of various elements. To decrease the outage detection time, this study proposes to classify the equipment‐related outage causes to diagnose the faulty equipment at the time of outage occurrence. To this end, available historical outage, load and weather data sets are integrated, and various features are defined. Then, binary classifiers are developed to classify each equipment's failures against others'. To enhance classifiers' performance, this study also proposes to use cost function and ensemble models. The results of applying proposed classifiers show the accuracy of the proposed method and improvements in outcomes.
Topology identification in distribution networks based on alternating optimisation
Authors: Feng Renhai , Yuan Wanqi , Xiao Meng , Zhao Zheng , Wang Qiulin
Published: August 2020

Topology identification (TI) is an essential problem in the distribution network due to exponentially growing power grid size in recent years. In this study, it is reformulated as a regularised alternating convex optimisation problem. Then an application based on the current injection model is proposed. Compared to the traditional algorithm optimising ‐norm, which may lead to overfitting, a new ‐norm minimisation problem with ‐norm regularisation is proposed to solve the trade‐off problem with non‐convex constraints. The proposed method reduces the size of the training data set compared with the traditional TI method. Simulation results show that the recovery performance of the proposed algorithm is superior to the traditional one in additive white Gaussian noise scenario.
Research on dynamic control of low‐voltage distribution network with high penetration of electric heat pumps based on μPMU measurements and data‐driven approach
Authors: Yu Kun , Guo Wenxin , Chen Xingying , Shi Di , Wang Jixiang , Gan Lei
Published: August 2020

Electric heat pump (EHP) is a kind of high‐power load, which causes power impact and voltage sag during its start‐up process. With the increasing penetration of EHP loads, serious low‐voltage and overload problems occur in the low‐voltage distribution network, resulting in malfunction of voltage‐sensitive load. This study analyses the start‐up characteristics of the EHP and proposes a data‐driven approach for identifying the starting state and model of EHP based on high‐frequency micro‐phasor measurement unit (μPMU) measurements. The state criterion based on the starting model can be used to quickly determine whether there is a starting impact at the present time. The identification method based on superposition fitting can be used to identify the number of EHPs that cause a start‐up impact. Then, a dynamic control algorithm is proposed to solve the problem of low voltage and voltage sag of EHP loads. Simulation results show that the proposed quadratic model can better reflect the starting characteristics of the EHP, and the proposed dynamic control method of EHP based on data‐driven approach can identify the state of EHP using µPMU measurement accurately in a very fast manner, effectively avoiding the overshoot of EHP start‐up impact.
Towards affect‐aware vehicles for increasing safety and comfort: recognising driver emotions from audio recordings in a realistic driving study
Authors: Requardt Alicia F. , Ihme Klas , Wilbrink Marc , Wendemuth Andreas
Published: August 2020

For vehicle safety, the in‐time monitoring of the driver and assessing his/her state is a demanding issue. Frustration can lead to aggressive driving behaviours, which play a decisive role in up to one‐third of fatal road accidents. Consequently, the authors present the automatic analysis of the emotional driver states of frustration, anxiety, positive and neutral. Based on experiments with normal drivers within cars in real‐world (low expressivity) situations, they use speech data, as speech can be recorded with zero invasiveness and comes naturally in driving situations. A careful selection of speech features, subject data identification, hyper‐parameter optimisation, and machine learning algorithms was applied for this difficult 4‐emotion‐class detection problem, where the literature hardly reports results above chance level. In‐car assistance demands real‐time computing. A very detailed analysis yields best results with relatively small random forests, and with an optimal feature set containing only 65 features (6.51% of the standard emobase feature set) which outperformed all other feature sets, producing 35.38% unweighted average recall (53.26% precision) with low computational effort, and also reducing the inevitably high confusion of ‘neutral’ with low‐expressed emotions. This result is comparable to and even outperforming other reported studies of emotion recognition in the wild. Their work, therefore, triggers adaptive automotive safety applications.
Linking ice and gas in the Serpens low-mass star-forming region
Authors: Perotti G. , Rocha W. R. M. , Jørgensen J. K. , Kristensen L. E. , Fraser H. J. , Pontoppidan K. M.
Published: August 2020

Context. The interaction between dust, ice, and gas during the formation of stars produces complex organic molecules. While observations indicate that several species are formed on ice-covered dust grains and are released into the gas phase, the exact chemical interplay between solid and gas phases and their relative importance remain unclear.

Aims. Our goal is to study the interplay between dust, ice, and gas in regions of low-mass star formation through ice- and gas-mapping and by directly measuring gas-to-ice ratios. This provides constraints on the routes that lead to the chemical complexity that is observed in solid and gas phases.

Methods. We present observations of gas-phase methanol (CH3OH) and carbon monoxide (13CO and C18O) at 1.3 mm towards ten low-mass young protostars in the Serpens SVS 4 cluster from the SubMillimeter Array (SMA) and the Atacama Pathfinder EXperiment (APEX) telescope. We used archival data from the Very Large Telescope (VLT) to derive abundances of ice H2O, CO, and CH3OH towards the same region. Finally, we constructed gas-ice maps of SVS 4 and directly measured CO and CH3OH gas-to-ice ratios.

Results. The SVS 4 cluster is characterised by a global temperature of 15 ± 5 K. At this temperature, the chemical behaviours of CH3OH and CO are anti-correlated: larger variations are observed for CH3OH gas than for CH3OH ice, whereas the opposite is seen for CO. The gas-to-ice ratios (Ngas/Nice) range from 1–6 for CO and 1.4 × 10−4–3.7 × 10−3 for CH3OH. The CO gas-maps trace an extended gaseous component that is not sensitive to the effect of freeze-out. Because of temperature variations and dust heating around 20 K, the frozen CO is efficiently desorbed. The CH3OH gas-maps, in contrast, probe regions where methanol is predominantly formed and present in ices and is released into the gas phase through non-thermal desorption mechanisms.

Conclusions. Combining gas- and ice-mapping techniques, we measure gas-to-ice ratios of CO and CH3OH in the SVS 4 cluster. The CH3OH gas-to-ice ratio agrees with values that were previously reported for embedded Class 0/I low-mass protostars. We find that there is no straightforward correlation between CO and CH3OH gas with their ice counterparts in the cluster. This is likely related to the complex morphology of SVS 4: the Class 0 protostar SMM 4 and its envelope lie in the vicinity, and the outflow associated with SMM 4 intersects the cluster. This study serves as a pathfinder for future observations with ALMA and the James Webb Space Telescope (JWST) that will provide high-sensitivity gas-ice maps of molecules more complex than methanol. Such comparative maps will be essential to constrain the chemical routes that regulate the chemical complexity in star-forming regions.
Binary asteroid (31) Euphrosyne: ice-rich and nearly spherical
Authors: Yang B. , Hanuš J. , Carry B. , Vernazza P. , Brož M. , Vachier F. , Rambaux N. , Marsset M. , Chrenko O. , Ševeček P. , Viikinkoski M. , Jehin E. , Ferrais M. , Podlewska-Gaca E. , Drouard A. , Marchis F. , Birlan M. , Benkhaldoun Z. , Berthier J. , Bartczak P. , Dumas C. , Dudziński G. , Ďurech J. , Castillo-Rogez J. , Cipriani F. , Colas F. , Fetick R. , Fusco T. , Grice J. , Jorda L. , Kaasalainen M. , Kryszczynska A. , Lamy P. , Marciniak A. , Michalowski T. , Michel P. , Pajuelo M. , Santana-Ros T. , Tanga P. , Vigan A. , Witasse O.
Published: August 2020

Aims. Asteroid (31) Euphrosyne is one of the biggest objects in the asteroid main belt and it is also the largest member of its namesake family. The Euphrosyne family occupies a highly inclined region in the outer main belt and contains a remarkably large number of members, which is interpreted as an outcome of a disruptive cratering event.

Methods. The goals of this adaptive-optics imaging study are threefold: to characterize the shape of Euphrosyne, to constrain its density, and to search for the large craters that may be associated with the family formation event.

Results. We obtained disk-resolved images of Euphrosyne using SPHERE/ZIMPOL at the ESO 8.2 m VLT as part of our large program (ID: 199.C-0074, PI: Vernazza). We reconstructed its 3D shape via the ADAM shape modeling algorithm based on the SPHERE images and the available light curves of this asteroid. We analyzed the dynamics of the satellite with the Genoid meta-heuristic algorithm. Finally, we studied the shape of Euphrosyne using hydrostatic equilibrium models.

Conclusions. Our SPHERE observations show that Euphrosyne has a nearly spherical shape with the sphericity index of 0.9888 and its surface lacks large impact craters. Euphrosyne’s diameter is 268 ± 6 km, making it one of the top ten largest main belt asteroids. We detected a satellite of Euphrosyne – S/2019 (31) 1 – that is about 4 km across, on a circular orbit. The mass determined from the orbit of the satellite together with the volume computed from the shape model imply a density of 1665 ± 242 kg m−3, suggesting that Euphrosyne probably contains a large fraction of water ice in its interior. We find that the spherical shape of Euphrosyne is a result of the reaccumulation process following the impact, as in the case of (10) Hygiea. However, our shape analysis reveals that, contrary to Hygiea, the axis ratios of Euphrosyne significantly differ from those suggested by fluid hydrostatic equilibrium following reaccumulation.
Atmospheric NLTE models for the spectroscopic analysis of blue stars with winds
Authors: Puls J. , Najarro F. , Sundqvist J. O. , Sen K.
Published: August 2020

Context. Obtaining precise stellar and wind properties and abundance patterns of massive stars is crucial to understanding their nature and interactions with their environments, as well as to constrain their evolutionary paths and end-products.

Aims. To enable higher versatility and precision of the complete ultraviolet (UV) to optical range, we improve our high-performance, unified, NLTE atmosphere and spectrum synthesis code FASTWIND. Moreover, we aim to obtain an advanced description of X-ray emission from wind-embedded shocks, consistent with alternative modeling approaches.

Methods. We include a detailed comoving frame radiative transfer for the essential frequency range, but still apply methods that enable low turnaround times. We compare the results of our updated computations with those from the alternative code CMFGEN, and our previous FASTWIND version, for a representative model grid.

Results. In most cases, our new results agree excellently with those from CMFGEN, both regarding the total radiative acceleration, strategic optical lines, and the UV-range. Moderate differences concern He II λλ4200-4541 and N V λλ4603-4619. The agreement regarding N III λλ4634−4640−4642 has improved, though there are still certain discrepancies, mostly related to line overlap effects in the extreme ultraviolet, depending on abundances and micro-turbulence. In the UV range of our coolest models, we find differences in the predicted depression of the pseudo-continuum, which is most pronounced around Lyα. This depression is larger in CMFGEN, and related to different Fe IV atomic data. The comparison between our new and previous FASTWIND version reveals an almost perfect agreement, except again for N V λλ4603-4619. Using an improved, depth-dependent description for the filling factors of hot, X-ray emitting material, we confirm previous analytic scaling relations with our numerical models.

Conclusions. We warn against uncritically relying on transitions, which are strongly affected by direct or indirect line-overlap effects. The predicted UV-continuum depression for the coolest grid-models needs to be checked, both observationally, and regarding the underlying atomic data. Wind lines from “super-ionized” ions such as O VI can, in principle, be used to constrain the distribution of wind-embedded shocks. The new FASTWIND version v11 is now ready to be used.
Ubiquitous hundred-Gauss magnetic fields in solar spicules
Authors: Kriginsky M. , Oliver R. , Freij N. , Kuridze D. , Asensio Ramos A. , Antolin P.
Published: August 2020

Aims. We aim to study the magnetic field in solar spicules using high-resolution spectropolarimetric observations in the Ca II 8542 Å line obtained with the Swedish 1-m Solar Telescope.

Methods. The equations that result from the application of the weak field approximation (WFA) to the radiative transfer equations were used to infer the line-of-sight (LOS) component of the magnetic field (BLOS). Two restrictive conditions were imposed on the Stokes I and V profiles at each pixel before they could be used in a Bayesian inversion to compute its BLOS.

Results. The LOS magnetic field component was inferred in six data sets totalling 448 spectral scans in the Ca II 8542 Å line and containing both active region and quiet Sun areas, with values of hundreds of Gauss being abundantly inferred. There seems to be no difference, from a statistical point of view, between the magnetic field strength of spicules in the quiet Sun or near an active region. On the other hand, the BLOS distributions present smaller values on the disc than off-limb, a fact that can be explained by the effect of superposition on the chromosphere of on-disc structures. We show that on-disc pixels in which the BLOS is determined are possibly associated with spicular structures because these pixels are co-spatial with the magnetic field concentrations at the network boundaries and the sign of their BLOS agrees with that of the underlying photosphere. We find that spicules in the vicinity of a sunspot have a magnetic field polarity (i.e. north or south) equal to that of the sunspot. This paper also contains an analysis of the effect of off-limb overlapping structures on the observed Stokes I and V parameters and the BLOS obtained from the WFA. It is found that this value is equal to or smaller than the largest LOS magnetic field components of the two structures. In addition, using random BLOS, Doppler velocities, and line intensities of these two structures leads in ≃50% of the cases to Stokes I and V parameters that are unsuitable to be used with the WFA.

Conclusions. Our results present a scarcity of LOS magnetic field components smaller than some 50 G, which must not be taken as evidence against the existence of these magnetic field strengths in spicules. This fact possibly arises as the consequence of signal superposition and noise in the data. We also suggest that the failure of previous works to infer the strong magnetic fields in spicules detected here is their coarser spatial and/or temporal resolution.
Simultaneous transverse oscillations of a coronal loop and a filament excited by a circular-ribbon flare
Authors: Zhang Q. M.
Published: August 2020

Aims. The aim of this study is to investigate the excitation of kink oscillations in coronal loops and filaments, by analyzing a C3.4 circular-ribbon flare associated with a blowout jet in active region 12434 on 2015 October 16.

Methods. The flare was observed in ultraviolet and extreme-ultraviolet wavelengths by the Atmospheric Imaging Assembly on board the Solar Dynamics Observatory (SDO) spacecraft. The line-of-sight (LOS) magnetograms of the photosphere were observed by the Helioseismic and Magnetic Imager on board SDO. Soft X-ray fluxes of the flares in 0.5−4 and 1−8 Å were recorded by the GOES spacecraft.

Results. The flare excited small-amplitude kink oscillation of a remote coronal loop. The oscillation lasted for ≥4 cycles without significant damping. The amplitude and period are 0.3 ± 0.1 Mm and 207 ± 12 s. Interestingly, the flare also excited transverse oscillation of a remote filament. The oscillation lasted for ∼3.5 cycles with decaying amplitudes. The initial amplitude is 1.7−2.2 Mm. The period and damping time are 437−475 s and 1142−1600 s. The starting times of simultaneous oscillations of coronal loop and filament were concurrent with the hard X-ray peak time. Though small in size and short in lifetime, the flare set off a chain reaction. It generated a bright secondary flare ribbon (SFR) in the chromosphere, remote brightening (RB) that was cospatial with the filament, and intermittent, jet-like flow propagating in the northeast direction.

Conclusions. The loop oscillation is most probably excited by the flare-induced blast wave at a speed of ≥1300 km s−1. The excitation of the filament oscillation is more complicated. The blast wave triggers secondary magnetic reconnection far from the main flare, which not only heats the local plasma to higher temperatures (SFR and RB), but produces jet-like flow (i.e., reconnection outflow) as well. The filament is disturbed by the secondary magnetic reconnection and experiences transverse oscillation. These findings provide new insight into the excitation of transverse oscillations of coronal loops and filaments.
Sunspot penumbral filaments intruding into a light bridge and the resultant reconnection jets
Authors: Hou Y. J. , Li T. , Zhong S. H. , Yang S. H. , Guo Y. L. , Li X. H. , Zhang J. , Xiang Y. Y.
Published: August 2020

Context. Penumbral filaments and light bridges are prominent structures inside sunspots and are important for understanding the nature of sunspot magnetic fields and magneto-convection underneath.

Aims. We investigate an interesting event where several penumbral filaments intrude into a sunspot light bridge. In doing so we aim to gain further insight into the magnetic fields of the sunspot penumbral filament and light bridge, as well as their interaction.

Methods. Combining data from the New Vacuum Solar Telescope, Solar Dynamics Observatory, and Interface Region Imaging Spectrograph, we study the emission, kinematic, and magnetic topology characteristics of the penumbral filaments intruding into the light bridge and the resultant jets.

Results. At the west part of the light bridge, the intruding penumbral filaments penetrate into the umbrae on both sides of the light bridge, and two groups of jets are also detected. The jets share the same projected morphology with the intruding filaments and are accompanied by intermittent footpoint brightenings. Simultaneous spectral imaging observations provide convincing evidence for the presences of magnetic-reconnection-related heating and bidirectional flows near the jet bases and contribute to measuring the vector velocities of the jets. Additionally, nonlinear force-free field extrapolation results reveal strong and highly inclined magnetic fields along the intruding penumbral filaments, highly consistent with the results deduced from the vector velocities of the jets. Therefore, we propose that the jets could be caused by magnetic reconnections between emerging fields within the light bridge and the nearly horizontal fields of intruding filaments. The jets are then ejected outward along the stronger filament fields.

Conclusions. Our study indicates that magnetic reconnection could occur between the penumbral filament fields and emerging fields within the light bridge and produce jets along the stronger filament fields. These results further complement the study of magnetic reconnection and dynamic activities within the sunspot.
Quelle stratégie de semis du blé dur en Algérie pour s’adapter au changement climatique ?
Authors: Chourghal Nacira , Hartani Tarik
Published: August 2020

Face à un changement climatique sévère projeté, les agriculteurs dans les pays de l’Afrique du Nord expérimentent habituellement des stratégies d’adaptation basées sur la précocité du semis, l’utilisation de variétés précoces et la fertilisation. Cependant, l’information concernant l’efficacité de la précocité des semis comme stratégie d’adaptation de la culture du blé dur au changement climatique reste très limitée. Cet article a pour objectif d’aider les gestionnaires à proposer des pratiques de semis plus efficaces. Deux options de semis sont examinées ; une date prescrite (fixe) et une date dynamique (dépendant du début des pluies utiles). Un modèle de culture à deux types d’entrées, culturales et climatiques, est utilisé pour simuler, au pas de temps journalier, la phénologie, le bilan hydrique et les rendements du blé dur en Algérie. Le comportement futur du blé dur est simulé en utilisant les projections climatiques du modèle ARPEGE-Climat de Météo-France sous le scénario médium A1B SRES pour le futur lointain (2071–2100). Dans le cas de semis à date prescrite, le réchauffement climatique raccourcit le cycle de 31 jours durant la phase végétative, le bilan hydrique est négatif, l’accumulation de la matière sèche est réduite et le rendement réel diminue de 36 %. En semis à date dynamique, un allongement de la phase reproductive accompagne le raccourcissement de la phase végétative et le cycle est raccourci de seulement 15 jours. La matière sèche totale est réduite, mais le bilan hydrique, favorisé par le semis précoce, est positif. Par conséquent, le rendement dans le probable climat futur est maintenu au même niveau que celui de la situation actuelle. Nos résultats permettent d’outiller les gestionnaires en leur proposant une stratégie de semis basée sur une date dynamique, pour faire face aux défis du changement climatique et de son impact sur la culture du blé dur.
Feature Selection on Maximum Information Coefficient for Underwater Target Recognition
Authors: Zhang Muhang , Shen Xiaohong , He Lei , Wang Haiyan
Published: August 2020

Feature selection is an essential process in the identification task because the irrelevant and redundant features contained in the unselected feature set can reduce both the performance and efficiency of recognition. However, when identifying the underwater targets based on their radiated noise, the diversity of targets, and the complexity of underwater acoustic channels introduce various complex relationships among the extracted acoustic features. For this problem, this paper employs the normalized maximum information coefficient (NMIC) to measure the correlations between features and categories and the redundancy among different features and further proposes an NMIC based feature selection method (NMIC-FS). Then, on the real-world dataset, the average classification accuracy estimated by models such as random forest and support vector machine is used to evaluate the performance of the NMIC-FS. The analysis results show that the feature subset obtained by NMIC-FS can achieve higher classification accuracy in a shorter time than that without selection. Compared with correlation-based feature selection, laplacian score, and lasso methods, the NMIC-FS improves the classification accuracy faster in the process of feature selection and requires the least acoustic features to obtain classification accuracy comparable to that of the full feature set.
Study of Morphology and Secondary Droplet Behaviors during Droplets Impacting on Static Hot Film
Authors: Fang Long , Chen Guoding
Published: August 2020

The phenomenon of droplet impacting on hot film exists widely in nature and engineering applications. Considering limitations in current researches, an experimental setup to record the process of droplets impacting on hot film was built in this paper. Lots of experiments have been done and then the transitional threshold was established to determine impingement outcomes. The number of secondary droplets and the temperature of the secondary droplet are investigated based on the experimental results. The results show that impingement outcomes can be determined by the splashing parameter provided in this paper. The number of secondary droplets increases with the splash parameter, the probability distribution function of secondary droplet diameters follows the Weibull distribution. The temperature of secondary droplets is close to the hot film and almost all secondary droplets' temperature is above 90% of the hot film.
Online Fault Detection of Fixed-Wing UAV Based on DKPCA Algorithm with Multiple Operation Conditions Considered
Authors: Liang Shaojun , Zhang Shirong , Zheng Xing , Lin Dongsheng
Published: August 2020

The mission execution process of a fixed-wing UAV has multiple phases and multiple operation conditions. Its parameters are nonlinear and dynamic. These characteristics make its online fault detection rather complicated. To carry out the fault detection, this paper selects nine key parameters of the transverse, longitudinal and velocity control loops of the UAV to characterize its real-time conditions. The core parameters are dynamically preprocessed to construct an augmented matrix so as to describe the dynamic characteristics of the UAV. Then, the improved k-mediods* algorithm is used to cluster the operation conditions of the UAVs by means of augmented dimensions. Neural networks are used to achieve the online matching of operation conditions. To overcome the nonlinearity of the UAV, the fault detection is performed by using the DKPCA algorithm; the fault monitoring is conducted through constructing the compound indexes of SPE and T2, notated as FAI. Furthermore, the fault separation algorithm is proposed to specify the variables of fault from the augmented high-dimensional data set. In order to deal with the erroneous reporting of faults due to measurement errors, the paper conducts the wavelet denoising of FAI, the compound indexes of the DKPCA algorithm. Finally, the data set collected from a real UAV flight is used to verify the effectiveness of the DKPCA algorithm for operation condition clustering and matching, fault detection and wavelet denoising.
Dynamics of a three species ratio-dependent food chain model with diffusion and double free boundaries
Authors: Zhang Dawei , Duan Beiping , Dai Binxiang
Published: August 2020

This paper focuses on the dynamics of a three species ratio-dependent food chain model with diffusion and double free boundaries in one dimensional space, in which the free boundaries represent expanding fronts of top predator species. The existence, uniqueness and estimates of the global solution are discussed firstly. Then we prove a spreading–vanishing dichotomy, specifically, the top predator species either successfully spreads to the entire space as time t goes to infinity and survives in the new environment, or fails to establish and dies out in the long run. The long time behavior of the three species and criteria for spreading and vanishing are also obtained. Besides, our simulations illustrate the impacts of initial occupying area and expanding capability on the dynamics of top predator for free boundaries.
Optical and interfacial properties of epitaxially fused GaInP/Si heterojunction
Authors: Wang Chengru , Omanakuttan Giriprasanth , Xu Leilei , Liu Tong , Huang Zengli , Lourdudoss Sebastian , Xie Chaoying , Sun Yan-Ting
Published: August 2020

This work investigates the optical and interfacial properties of epitaxially fused direct GaInP/Si heterojunctions realized by the corrugated epitaxial lateral overgrowth (CELOG) approach. To provide a broad analysis of the above heterojunction, photoluminescence (PL), cathodoluminescence (CL), Raman, and high-resolution transmission electron microscopy (TEM) were employed in this study. The enhanced luminescence intensity was observed in the direct GaInP/Si heterojunction in the cross-sectional CL because of the reduced defect density in the CELOG GaInP. The spatial resolution dependent PL and CL spectra of GaInP on Si yielded the composition variation of GaInP arising from the anisotropic growth behavior of CELOG. The Ga composition, x, in GaxIn1−xP/Si at the interface deduced from the lattice constant measured by TEM has a good agreement with the results of PL and CL. Low thermal and lattice mismatch strain in CELOG GaInP on Si were revealed by the Raman spectra. TEM investigation further revealed the atomic structure of some planar defects in CELOG GaInP over Si. It is confirmed that although a thin atomic disorder was observed on the surface of Si substrate, an epitaxially fused GaInP/Si heterojunction with a reduced threading dislocation density of ∼6.4 × 107 cm−2 in comparison to ∼4.8 × 108 cm−2 in the InP seed on Si has been successfully fabricated by the CELOG technique despite about 4% lattice mismatch between GaInP and Si. The findings of this study demonstrate the great potential of the CELOG technique for promoting monolithic integration of III-V/Si-based optoelectronics.
Deformation of Lie–Poisson algebras and chirality
Authors: Yoshida Zensho , Morrison Philip J.
Published: August 2020

Linearization of a Hamiltonian system around an equilibrium point yields a set of Hamiltonian symmetric spectra: If λ is an eigenvalue of the linearized generator, −λ and λ¯ (hence, −λ¯) are also eigenvalues—the former implies a time-reversal symmetry, while the latter guarantees the reality of the solution. However, linearization around a singular equilibrium point (which commonly exists in noncanonical Hamiltonian systems) works out differently, resulting in breaking of the Hamiltonian symmetry of spectra; time-reversal asymmetry causes chirality. This interesting phenomenon was first found in analyzing the chiral motion of the rattleback, a boat-shaped top having misaligned axes of inertia and geometry [Z. Yoshida et al., Phys. Lett. A 381, 2772–2777 (2017)]. To elucidate how chiral spectra are generated, we study the three-dimensional Lie–Poisson systems and classify the prototypes of singularities that cause symmetry breaking. The central idea is the deformation of the underlying Lie algebra; invoking Bianchi’s list of all three-dimensional Lie algebras, we show that the so-called class-B algebras, which are produced by asymmetric deformations of the simple algebra so(3), yield chiral spectra when linearized around their singularities. The theory of deformation is generalized to higher dimensions, including the infinite-dimensional Poisson manifolds relevant to fluid mechanics.
Perspectives on single particle imaging with x rays at the advent of high repetition rate x-ray free electron laser sources
Authors: Bielecki Johan , Maia Filipe R. N. C. , Mancuso Adrian P.
Published: August 2020

X-ray free electron lasers (XFELs) now routinely produce millijoule level pulses of x-ray photons with tens of femtoseconds duration. Such x-ray intensities gave rise to the idea that weakly scattering particles—perhaps single biomolecules or viruses—could be investigated free of radiation damage. Here, we examine elements from the past decade of so-called single particle imaging with hard XFELs. We look at the progress made to date and identify some future possible directions for the field. In particular, we summarize the presently achieved resolutions as well as identifying the bottlenecks and enabling technologies to future resolution improvement, which in turn enables application to samples of scientific interest.
Phase transition in the Kolkata Paise Restaurant problem
Authors: Sinha Antika , Chakrabarti Bikas K.
Published: August 2020

A novel phase transition behavior is observed in the Kolkata Paise Restaurant problem where a large number (N) of agents or customers collectively (and iteratively) learn to choose among the N restaurants where she would expect to be alone that evening and would get the only dish available there (or may get randomly picked up if more than one agent arrive there that evening). The players are expected to evolve their strategy such that the publicly available information about past crowds in different restaurants can be utilized and each of them is able to make the best minority choice. For equally ranked restaurants, we follow two crowd-avoiding strategies: strategy I, where each of the ni(t) number of agents arriving at the ith restaurant on the tth evening goes back to the same restaurant the next evening with probability [ni(t)]−α, and strategy II, with probability p, when ni(t)>1. We study the steady state (t-independent) utilization fraction f:(1−f) giving the steady state (wastage) fraction of restaurants going without any customer at any particular evening. With both strategies, we find, near αc=0+ (in strategy I) or p=1− (in strategy II), the steady state wastage fraction (1−f)∝(α−αc)β or (pc−p)β with β≃0.8,0.87,1.0, and the convergence time τ [for f(t) becoming independent of t] varies as τ∝(α−αc)−γ or (pc−p)−γ, with γ≃1.18,1.11,1.05 in infinite-dimensions (rest of the N−1 neighboring restaurants), three dimensions (six neighbors), and two dimensions (four neighbors), respectively.
The Heun–Racah and Heun–Bannai–Ito algebras
Authors: Bergeron Geoffroy , Crampé Nicolas , Tsujimoto Satoshi , Vinet Luc , Zhedanov Alexei
Published: August 2020

The Heun–Racah and Heun–Bannai–Ito algebras are introduced. Specializations of these algebras are seen to be realized by the operators obtained by applying the algebraic Heun construct to the bispectral operators of the Racah and Bannai–Ito polynomials. The study supplements the results on the Heun–Askey–Wilson algebra and completes the description of the Heun algebras associated with the polynomial families at the top of the Askey scheme, its q-analog, and the Bannai–Ito one.
Multi-scale multi-mode nonlinear interaction in tokamak plasma turbulence with moderate small-scale shear flow
Authors: Li Hui , Li Jiquan , Wang Zhengxiong , Wei Lai , Hu Zhaoqing , Ren Guangzhi
Published: August 2020

Effects of moderate small-scale shear flow, e.g., which may be created by the trapped electron mode, on electromagnetic (EM) ion-scale turbulence in tokamak plasmas are numerically investigated via a self-consistent Landau-fluid model. A modeling analysis is carried out in slab geometry to reveal the underlying mechanism of the multi-scale multi-mode nonlinear interaction. Results show that while a Kelvin–Helmholtz (KH) instability with long wavelengths may be excited by the shear flows to dominate the multi-scale EM fluctuation, shorter wavelength ion temperature gradient (ITG) modes experience multiple quasi-steady (QS) stages with enhanced fluctuation level through different driving and saturation mechanisms. One mechanism is the secondary ITG instability due to the decrease in flow stabilization modified by the zonal flow. Meanwhile, the other one is the modulational interaction between the EM ITG and KH modes through the nonlinear mode coupling. Moreover, the synergism of these two mechanisms may sustain the final QS state near the marginal KH instability threshold. Complex linear and nonlinear interactions among multiple modes and external flow, as well as self-generated zonal flow, result in a weak dependence of the final saturation level of the dominant EM ITG mode on the small-scale flow amplitude. The turbulent heat transport is visibly suppressed by weaker shear flow, but is almost not affected by stronger shear flows. The underlying mechanism is elaborated.
Damage evolution and spall failure in copper under complex shockwave loading conditions
Authors: Xie Puchu , Wang Yonggang , Shi Tongya , Wang Xiaofeng , Hu Changming , Hu Jianbo , Zhang Fengguo
Published: August 2020

The damage evolution and spall behavior of copper under complex shockwave loading conditions were investigated using plate impact experiments with conical targets. Sweeping tensile waves were generated by the interaction of the released waves that were reflected from the free surfaces of the impactor and the cone surface. From the free-surface velocity profiles measured by multi-channel velocimetry, the classic pull-back spall signals were observed in incipient and complete spallation experiments. The spall strength estimated from the pull-back velocity strongly depended on the loading path and the loading wave profile. Post-experiment analysis based on the soft-recovery technique revealed that the damage distributions were very different from the bottom to the top of the conical target, but the corresponding free-surface velocity data measured at different locations suggested that similar responses occurred, which indicated that the spall strength was the critical threshold stress of micro-void nucleation or early growth. The fractography analysis of the fracture surfaces showed that metal micro-spheres were scattered in deep dimples, which indicated that the increase in temperature due to local severe plastic deformation around the voids was important. With the same set of model parameters, the plate impact spallation experiments with plane and conical targets were simulated using a critical damage evolution model. A good agreement was obtained between the simulations and experiments, which demonstrated the model capabilities for predicting the spall responses of metals under complex shockwave loading.
Enhanced performance of solar-blind photodetector of hexagonal boron nitride with bottom-contact electrodes
Authors: Zhang Naifu , Song Yanpeng , Zhou Kaiyao , Wang Wenjun
Published: August 2020

A deep ultraviolet photodetector has wide applications in both military and civil fields, such as the missile early warning system, transparent wireless communication, environmental monitoring, and so on. Because of its wide bandgap, hexagonal boron nitride (h-BN) has been envisioned to be a promising material for deep ultraviolet photodetectors. In this work, we have synthesized high-quality h-BN crystals using the flux growth method at atmospheric pressure. Based on the thin-layered h-BN prepared by mechanical exfoliation, two types of solar-blind photodetectors, with either top-contact or bottom-contact electrodes, were fabricated. It is found that the device with bottom-contact electrodes exhibits excellent performance in the specific detectivity exceeding 3.68 × 108 Jones (cm Hz1/2 W−1) at 215 nm. Moreover, the photodetectors have a cutoff wavelength at about 270 nm, which demonstrates that the device can be an ideal candidate for a solar-blind photodetector.
Pressure-induced phase transition in barium hydride studied with neutron scattering
Authors: Novak E. , Haberl B. , Daemen L. , Molaison J. , Egami T. , Jalarvo N.
Published: August 2020

Barium hydride can undergo a structural phase transition from an orthorhombic phase to a hexagonal phase induced by high temperature or high pressure. This transition causes an immediate increase in the hydrogen diffusion rates by over an order of magnitude, and therefore, understanding the origin and details of such transition is of great interest not only for fundamental reasons but also for improving materials for future applications. In this work, the pressure evolution of the crystal structure was characterized using neutron powder diffraction up to a maximum pressure of 11.3 GPa. The pressure dependence of the unit cell volumes, lattice parameters, atomic sites, and compressibilities were determined for both phases. A structural phase transition occurred over a wide pressure range of P = 1.3 GPa–4.9 GPa. The transition to the higher density hexagonal phase reduced the volume per formula unit of BaD2 by 13.6%, hence increasing the volumetric storage density. In addition, we investigated the hydrogen diffusion process using high pressure quasi-elastic neutron scattering up to 7.1 GPa. Our results show that the hydrogen mobility increases with pressure in the hexagonal phase. This work sheds light on the structural and dynamical aspects of barium hydride caused by the application of high pressure. The results may aid in the development of advanced metal hydride systems with increased hydrogen dynamics.
The deconvolution of FTIR-ATR spectra to five Gaussians for detection of small changes in plant–water clusters
Authors: Laurson Peeter , Raudsepp Piret , Kaldmäe Hedi , Kikas Ave , Mäeorg Uno
Published: August 2020

This article presents a method that efficiently enables us to identify differences in the composition of water clusters. The Fourier transform infrared (FTIR) spectra in the O–H stretching region 2500 cm−1 to 4000 cm−1 of apple juice, raspberry juice, lyophilized water from apples (apple water), and lyophilized water from raspberry juice (raspberry water) were measured at a controlled temperature applying the Attenuated Total Reflection (ATR) sampling technique. The dependence of the IR spectra of ultra-pure water on temperature was also studied. Deconvolution to five Gaussians was used to demonstrate the absorption maxima of the spectra. The analysis of these deconvoluted spectra allows us to reveal the differences in the water clusters of different origin. The comparison of the deconvoluted FTIR-ATR spectra of apple and raspberry juices demonstrates that the application of such a method enables us to successfully evaluate the impact of different soluble solids occurred in juices on the composition of water clusters. The comparison of differences in the deconvoluted FTIR-ATR spectra of “apple water” and “raspberry water” with the deconvoluted spectra of ultra-pure water shows the suitability of such a method in the identification of changes in water clusters, where the total concentration of additives is as low as 20 ppm.
Can surface-transfer doping and UV irradiation during annealing improve shallow implanted nitrogen-vacancy centers in diamond?
Authors: Glaser N. J. , Braunbeck G. , Bienek O. , Sharp I. D. , Reinhard F.
Published: August 2020

It has been reported that the conversion yield and coherence time of ion-implanted NV centers improve if the Fermi level is raised or lowered during the annealing step following implantation. Here, we investigate whether surface transfer doping and surface charging, by UV light, can be harnessed to induce this effect. We analyze the coherence times and the yield of NV centers created by ion implantation and annealing, applying various conditions during annealing. Specifically, we study coating diamond with nickel, palladium, or aluminum oxide, to induce positive surface transfer doping, as well as annealing under UV illumination to trigger vacancy charging. The metal-coated diamonds display a two times higher formation yield than the other samples. The coherence time T2 varies by less than a factor of two between the investigated samples. Both effects are weaker than previous reports, suggesting that stronger modifications of the band structure are necessary to find a pronounced effect. UV irradiation has no effect on the yield and T2 times.
Calibration of T-shaped atomic force microscope cantilevers using the thermal noise method
Authors: Kim Youngkyu , Mandriota Nicola , Goodnight Davis , Sahin Ozgur
Published: August 2020

The tip—sample interaction force measurements in atomic force microscopy (AFM) provide information about materials’ properties with nanoscale resolution. The T-shaped cantilevers used in Torsional-Harmonic AFM allow measuring the rapidly changing tip–sample interaction forces using the torsional (twisting) deflections of the cantilever due to the off-axis placement of the sharp tip. However, it has been difficult to calibrate these cantilevers using the commonly used thermal noise-based calibration method as the mechanical coupling between flexural and torsional deflections makes it challenging to determine the deflection sensitivities from force–distance curves. Here, we present thermal noise-based calibration of these T-shaped AFM cantilevers by simultaneously analyzing flexural and torsional thermal noise spectra, along with deflection signals during a force–distance curve measurement. The calibration steps remain identical to the conventional thermal noise method, but a computer performs additional calculations to account for mode coupling. We demonstrate the robustness of the calibration method by determining the sensitivity of calibration results to the laser spot position on the cantilever, to the orientation of the cantilever in the cantilever holder, and by repeated measurements. We validated the quantitative force measurements against the known unfolding force of a protein, the I91 domain of titin, which resulted in consistent unfolding force values among six independently calibrated cantilevers.
Surface currents in Hall devices
Authors: Creff M. , Faisant F. , Rubì J. M. , Wegrowe J.-E.
Published: August 2020

One hundred and forty years after his discovery, the Hall effect still deserves attention. If it is well-known that the Hall voltage measured in Hall bar devices is due to the electric charges accumulated at the edges in response to the magnetic field, the nature of the corresponding boundary conditions is still problematic. In order to study this out-of-equilibrium stationary state, the Onsager’s least-dissipation principle is applied. It is shown that, beside the well-known expression of the charge accumulation and the corresponding Hall voltage, a longitudinal surface current proportional to the charge accumulation is generated. An expression of the surface current is given. The surface currents allow the Hall voltage to be stabilized at a stationary state, despite, e.g., the presence of leakage of charges at the edges.
A protocol for preparing explicitly solvated systems for stable molecular dynamics simulations
Authors: Roe Daniel R. , Brooks Bernard R.
Published: August 2020

Before beginning the production phase of molecular dynamics simulations, i.e., the phase that produces the data to be analyzed, it is often necessary to first perform a series of one or more preparatory minimizations and/or molecular dynamics simulations in order to ensure that subsequent production simulations are stable. This is particularly important for simulations with explicit solvent molecules. Despite the preparatory minimizations and simulations being ubiquitous and essential for stable production simulations, there are currently no general recommended procedures to perform them and very few criteria to decide whether the system is capable of producing a stable simulation trajectory. Here, we propose a simple and well-defined ten step simulation preparation protocol for explicitly solvated biomolecules, which can be applied to a wide variety of system types, as well as a simple test based on the system density for determining whether the simulation is stabilized.