bluuebunny/crossref_metadata_2025 huggingface dataset:
Generalized slab universal instability and its appearance in pair plasma
Authors: Zhu Ben , Francisquez Manaure , Rogers Barrett N. , Xu Xue-qiao
Published: October 2020

A generalized linear dispersion relation of electromagnetic slab universal modes is derived, taking into account arbitrary ion charge state, electron finite Larmor radius (FLR) effects, and Debye shielding effects. As a consequence, it provides more accurate predictions and is applicable to a wider range of plasmas. We find that electron FLR effects have a weakly stabilizing effect on the slab universal instability in electron–ion plasma, while Debye shielding strongly stabilizes this instability when λD approaches ρi (λD is the Debye length and ρi is the ion gyroradius). In particular, we examine the stability criterion for this instability in electron–positron pair plasmas and find that the instability persists in this simplest plasma system as long as the pair plasma number density exceeds the critical value nc=B2/(8πmec2).
An extremely ultrathin flexible Huygens’s transformer
Authors: Ghaneizadeh Alireza , Mafinezhad Khalil , Joodaki Mojtaba
Published: October 2020

The current study aims to present the physical perception of a meta-surface energy harvester’s (MEH’s) design based on space-time physics of a traveling wave. Regarding the relation between the wave-velocity and field-impedance, the balance condition in Huygens’s meta-atoms is provided. Accordingly, it was demonstrated that MEH behaves as a transformer at far-field. It was observed that the location of the metallic-via is mimicked by the number of loop coils in the secondary of the transformer in the unit-cell. In addition, the impedance matching between the wave impedance in a lossless medium and MEH’s load was to be tuned by adjusting the size parameters of the unit-cell at a desired resonance frequency. For this purpose, the present study developed a simple design framework to achieve the resonance frequency at a more optimum pace based on surrogate modeling. The theoretical analyses are validated by the results of full-wave and circuit simulations. Finally, a recently developed flexible MEH was further extended to a multi-polarization structure using more compact cells. The fabricated flexible MEH has 10 × 10 number of deep subwavelength thick cells (≈0.004λ0), while traditional MEH was basically designed only to fit on the planar surface. The new design paves the way for the multi-polarized MEH to wrap around the cylindrical surface as a 2D-isotropic MEH. The results of the data analyses show that the simulation and experimental results enjoy an acceptable agreement.
A Cerenkov microwave generator with cross-band frequency hopping based on magnetic field tuning
Authors: Zhao Chenyu , Ge Xingjun , Song Lili , Deng Rujin , Huang Chao , Zhang Peng , Zhang Jun , He Juntao
Published: October 2020

Frequency hopping is one of the most significant technological advancements in high power microwave (HPM) sources. Mechanical alteration is generally adopted in traditional cross-band microwave generation, but it has the detriments of a complex adjustment strategy. In this paper, a cross-band frequency hopping Cerenkov microwave generator with an advantageous adjustment strategy, high beam–wave interaction efficiency, and pure output mode is developed. Two-segment slow wave structures (SWSs), which work in C and X-bands separately, are adopted. Additionally, by utilizing the cyclotron resonance absorption phenomenon reasonably, only the relative band microwave is energized under different magnetic fields, which adequately represses mode competition and realizes cross-band frequency hopping. After the electron beam passes through the first SWS section, the bunching center is consistently at the acceleration or deceleration phase of the subsequent SWS section, which a guarantees high beam–wave interaction efficiency in the two bands. The preliminary results of the particle-in-cell (PIC) simulation are as follows: when the guiding magnetic field is 0.7 T, the microwave output comprising only the X-band is obtained, with an efficiency of 42%; when the guiding magnetic field is 1.5 T, the microwave output comprising only the C-band is obtained, with an efficiency of 30%.
Bond-level imaging of organic molecules using Q-controlled amplitude modulation atomic force microscopy
Authors: Martin-Jimenez Daniel , Ihle Alexander , Ahles Sebastian , Wegner Hermann A. , Schirmeisen Andre , Ebeling Daniel
Published: October 2020

The bond imaging atomic force microscopy (AFM) technique has become an invaluable tool for studying organic molecules on surfaces. The key feature of this technique is the functionalization of the AFM-tip with a single CO molecule, which improves the lateral resolution and allows us to visualize the chemical structure of adsorbed organic molecules. Such experiments are usually performed at low temperatures in an ultrahigh vacuum environment in the frequency modulation (FM) mode. Here, we use the Q-controlled amplitude modulation (AM) mode for imaging organic molecules with CO-functionalized tips in constant-height mode. By reducing the effective quality factor (Qeff) of the sensor from about 20 000–30 000 to 1500–4000, we are able to image molecules with atomic resolution. Detailed instructions for determining the optimum Qeff and oscillation amplitude are given. To compare the phase and frequency shift images of the Q-controlled AM and the FM mode, we define an effective signal-to-noise ratio (SNR) that relates the observed contrast between the bonds and centers of imaged carbon rings to the noise in the respective image regions. This effective SNR is systematically analyzed in different regions of the imaged molecule for different oscillation amplitudes and average tip-substrate distances. By using appropriate imaging parameters, an increased effective SNR is achieved in the Q-controlled AM mode (on the order of 30–60%). This advantage over the conventional FM mode might, e.g., be used for increasing the experimental throughput.
Carrier density dependence of localized carrier recombination dynamics in orange-emitting InGaN/GaN nanocolumns
Authors: Shimosako N. , Inose Y. , Kinjo K. , Nakaoka T. , Oto T. , Kishino K. , Ema K.
Published: October 2020

The carrier density dependence of photoluminescence (PL) spectra, PL efficiency, and time-resolved PL (TRPL) in InGaN/GaN nanocolumns with orange emission has been measured over a wide range of carrier densities to investigate the carrier recombination dynamics at low and room temperatures. The TRPL decay curves were analyzed by stretched exponential functions to obtain a distribution of the decay times. The results indicate that the emission origin in InGaN with orange emission under low excitation intensity is the recombination of localized electrons and holes, not from localized excitons. Our analysis shows that the total density of localized states is an important factor to increase PL efficiency, which implies that red-emitting InGaN with a high density of localized states due to large indium fluctuation has the potential to achieve high PL efficiency. In addition, we have shown that the stretched exponential function is an effective approach to analyze several carrier relaxation processes with different decay times.
Crucial effect of transverse vibrations on the transport through polymer chains
Authors: Boulatov Alexei , Burin Alexander L.
Published: October 2020

The low temperature transport of electron, or vibrational or electronic exciton toward polymer chains, turns out to be dramatically sensitive to its interaction with transverse acoustic vibrations. We show that this interaction leads to a substantial polaron effect and decoherence, which are generally stronger than those associated with longitudinal vibrations. For site-dependent interactions, transverse phonons form subohmic bath leading to the quantum phase transition accompanied by full suppression of the transport at zero temperature and fast decoherence characterized by temperature dependent rate k2 ∝ T3/4 at low temperature, while k2 ∝ T2 for site-independent interactions. The latter dependence was used to interpret recent measurements of temperature dependent vibrational energy transport in polyethylene glycol oligomers.
Accurate prediction of core-level spectra of radicals at density functional theory cost via square gradient minimization and recoupling of mixed configurations
Authors: Hait Diptarka , Haugen Eric A. , Yang Zheyue , Oosterbaan Katherine J. , Leone Stephen R. , Head-Gordon Martin
Published: October 2020

State-specific orbital optimized approaches are more accurate at predicting core-level spectra than traditional linear-response protocols, but their utility had been restricted due to the risk of “variational collapse” down to the ground state. We employ the recently developed square gradient minimization [D. Hait and M. Head-Gordon, J. Chem. Theory Comput. 16, 1699 (2020)] algorithm to reliably avoid variational collapse and study the effectiveness of orbital optimized density functional theory (DFT) at predicting second period element 1s core-level spectra of open-shell systems. Several density functionals (including SCAN, B3LYP, and ωB97X-D3) are found to predict excitation energies from the core to singly occupied levels with high accuracy (≤0.3 eV RMS error) against available experimental data. Higher excited states are, however, more challenging by virtue of being intrinsically multiconfigurational. We thus present a configuration interaction inspired route to self-consistently recouple single determinant mixed configurations obtained from DFT, in order to obtain approximate doublet states. This recoupling scheme is used to predict the C K-edge spectra of the allyl radical, the O K-edge spectra of CO+, and the N K-edge of NO2 with high accuracy relative to experiment, indicating substantial promise in using this approach for the computation of core-level spectra for doublet species [vs more traditional time dependent DFT, equation of motion coupled cluster singles and doubles (EOM-CCSD), or using unrecoupled mixed configurations]. We also present general guidelines for computing core-excited states from orbital optimized DFT.
Strong solutions to the 2D Cauchy problem of nonhomogeneous magnetohydrodynamic equations with vacuum
Authors: Lü Boqiang , Wang Xiang , Zhong Xin
Published: October 2020

In this paper, we study the strong solutions to the Cauchy problem of nonhomogeneous incompressible magnetohydrodynamic (MHD) equations on the whole two-dimensional (2D) space with vacuum as far field density. In particular, the initial density can have compact support. If both the initial density and the initial magnetic field decay not too slow at infinity, the 2D Cauchy problem of the nonhomogeneous incompressible MHD equations admits a unique local strong solution.
Hybridization of plasmonic and dielectric metasurfaces with asymmetric absorption enhancement
Authors: Yang Jhen-Hong , Chen Kuo-Ping
Published: October 2020

Surface plasmon resonance (SPR) is widely utilized to improve the capabilities of nanophotonic devices, such as nanoantennas, plasmonic photovoltaics, and photocatalysts. In order to further enhance the absorption efficiency, high refractive index (HRI) materials are used to enhance absorbability. The desired absorber properties can be realized by combining the SPR and Mie resonance of HRI nanostructures. In this study, the asymmetric absorption enhancements are demonstrated by combining the Mie resonance of silicon nanopillars and the localized SPR of gold nanodisks. In order to increase the possibility of this design in future applications, TiO2 and Ge also present the same phenomenon. For mass production, the nanostructure design of absorbers can be simplified and made suitable for the current semiconductor process technology.
Terahertz transient stimulated emission from doped silicon
Authors: Pavlov S. G. , Deßmann N. , Pohl A. , Zhukavin R. Kh. , Klaassen T. O. , Abrosimov N. V. , Riemann H. , Redlich B. , van der Meer A. F. G. , Ortega J.-M. , Prazeres R. , Orlova E. E. , Muraviev A. V. , Shastin V. N. , Hübers H.-W.
Published: October 2020

Transient-type stimulated emission in the terahertz (THz) frequency range has been achieved from phosphorus doped silicon crystals under optical excitation by a few-picosecond-long pulses generated by the infrared free electron lasers FELIX and CLIO. The analysis of the lasing threshold and emission spectra indicates that the stimulated emission occurs due to combined population inversion based lasing and stimulated Raman scattering. Giant gain has been obtained in the optically pumped silicon due to large THz cross sections of intracenter impurity transitions and resonant intracenter electronic scattering. The transient-type emission is formed under conditions when the pump pulse intervals exceed significantly the photon lifetime in the laser resonator.
Effects of the collision model in interface-resolved simulations of particle-laden turbulent channel flows
Authors: Xia Yan , Xiong Hongbing , Yu Zhaosheng , Zhu Chenlin
Published: October 2020

The effects of the particle collision model in a direct-forcing fictitious domain method on the fluid and particle statistics of a fully developed turbulent channel flow laden with finite-size neutrally buoyant particles are numerically investigated. The particle collisions are described by a combination of the discrete element method and the lubrication force correction model. We first validate our code via several benchmark tests, including the normal particle–wall collisions at different impact Stokes numbers and the oblique collisions with varied incidence angles. Subsequently, the effects of the lubrication correction and the particle stiffness on the fluid and particle statistics of the particle-laden turbulent flows are examined. The results show that the lubrication force correction has an important effect on the particle pair statistics at the near-contact regime. Both the lubrication force between the particles and the decrease in the particle stiffness result in the decrease in the flow friction mainly due to the increase in the fluid Reynolds stress. The flow friction is always larger for smaller particles at the same particle volume fraction irrespective of the lubrication correction. The particle–particle lubrication force decreases the near-wall particle concentration, whereas the particle–wall lubrication force has the opposite effect.
Effect of annealing on the electrophysical properties of CdTe/HgCdTe passivation interface by the capacitance–voltage characteristics of the metal–insulator–semiconductor structures
Authors: Wang Xi , He Kai , Chen Xing , Li Yang , Lin Chun , Zhang Qinyao , Ye Zhenhua , Xin Liwei , Gao Guilong , Yan Xin , Wang Gang , Liu Yiheng , Wang Tao , Tian Jinshou
Published: October 2020

The capacitance–voltage characteristics of metal–insulator–semiconductor structures based on Hg1−xCdxTe (x = 0.218) with CdTe passivation are studied before and after the passivation annealing process. We found that after vacuum annealing at 300 °C for 24 h, the micromorphology of the passivation layer was significantly improved, and as the fixed charge density decreased from 1.3 × 1012 cm−2 to 1.0 × 1010 cm−2, the fast surface state density decreased from 2 × 1013 cm−2 eV−1 to 3 × 1012 cm−2 eV−1, with a minimum value of 1.2 × 1011 cm−2 eV−1. From these findings, combined with the secondary ion mass spectroscopy analysis, we conclude that the annealing process propagates an equivalent electrical surface for CdTe/HgCdTe uniformly from the principal physical interface to the inside of the bulk material, effectively improving the characteristics of the CdTe passivation layer.
Local gas transfer rate through the free surface in spatially accelerated open-channel turbulence
Authors: Sanjou M.
Published: October 2020

It is important to determine the transport mechanisms of dissolved gases through free surfaces in open channels. Even though many physical and phenomenological models have been proposed, not much is known about the local distribution of the gas transfer velocity. Thus, this study formulated a theoretical equation for developing a concentration boundary layer in an open-channel flow. Additionally, the free-surface velocity component and the concentration of dissolved oxygen were measured in a spatially accelerated open-channel flow. In particular, the streamwise profile of concentration boundary layer thickness was measured using an extra-fine needle-type dissolved oxygen probe, and the local gas transfer rate was experimentally obtained. The present theory suggests that the local gas transfer is controlled by two significant terms: the streamwise gradient of the mean velocity and the relative intensity of turbulent diffusion. By focusing on accelerated open-channel flows with bottom-situated wedges, the formation mechanism of the concentration boundary layer was explained by a comparison with the presented theory. The comparison of the theoretical model and measurement data indicated that the contributions of both the mean velocity and turbulence diffusion are comparable and significant in the acceleration zone. The thickness of the oxygen concentration boundary layer began to decrease a bit downstream of the entrance, corresponding to the variation in the free-surface turbulence factors. The inflectional point of the concentration thickness appeared downstream of the acceleration zone and continued to decrease at the exit related to relaminarization. These interesting features are explained by the streamwise profiles of the terms in the presented theory.
Monte Carlo simulation for development of electron avalanches in nitrogen at moderate and high reduced electric field
Authors: Grishkov A. A. , Korolev Y. D. , Shklyaev V. A.
Published: October 2020

This paper addresses the simulation of electron avalanches in nitrogen by means of the Monte Carlo method. The data have been obtained for the conditions of classical measurements of the impact ionization coefficient in a wide range of reduced electric fields E/p. It is shown that at moderate E/p values, the results are in a good agreement with the generally accepted approaches in the description of the avalanches based on the drift motion of the electrons and on the diffusion. The high E/p is achieved in the vicinity of the minimum of Paschen's curve due to decreasing the gas pressure. Then the process of development of the single electron avalanche changes radically. The reason is that the runaway electrons appear, which leads to essential longitudinal expansion of the electron cloud. It is revealed that the average electron energy at the front of the cloud turns out to be larger than that at the backside of the cloud.
A new capillary force model implemented in lattice Boltzmann method for gas–liquid–solid three-phase flows
Authors: Zhang Xitong , Liu Haihu , Zhang Jinggang
Published: October 2020

A lattice Boltzmann method is developed for the direct numerical simulation of gas, liquid, and solid three-phase flows. The liquid–gas two-phase flow with a high density ratio is solved using a phase-field model where the interface evolution is described by the conservative Allen–Cahn equation, and the dynamics of the solid particle is captured by the momentum exchange method. By distributing the surface tension over the entire diffuse interface, a new model is proposed to account for the capillary force exerted on the particle, which not only is suited for curved boundaries but can also be implemented in a simple and accurate manner. Several typical benchmark cases, including the wetting behavior of a particle on the liquid–gas interface, a bubble adhering to a particle that can move freely, and the sinking of a horizontal cylinder through an air–water interface, are used to validate the present method. Results show the necessity to incorporate the capillary force on the contact lines, especially when the surface tension is a dominant factor, and that the new capillary force model is able to calculate the capillary force accurately and suppress the oscillations of the capillary force. In addition, the capability of the present method for particle interactions is further demonstrated by studying the self-assembling behavior of three hydrophilic particles on a liquid–gas interface.
Three-dimensional deflecting oscillation of turbulent planar opposed jets confined in an open cavity under crossflow
Authors: Carnero David , Treviño César , Martínez-Suástegui Lorenzo
Published: October 2020

In this work, the flow dynamics of two isothermal turbulent opposed water jets issuing from rectangular slot nozzles and confined inside an open cubical cavity with vertical crossflow in a rectangular duct are studied experimentally using time-resolved stereo particle image velocimetry. The turbulent structure and the deflecting oscillation of the colliding jets have been investigated for fixed nozzle separation and a crossflow Reynolds number of 3000 for three jets’ Reynolds numbers of Rej = 1500, 3000, and 5000. The probability density function of the fluctuations for the three nondimensional velocity components measured at the mean impingement height of the jets has been calculated for three different positions along the cavity span. The experimental results confirm that as Rej increases, departure from Gaussian behavior has been noted, with stronger skewness and larger scatters observed for Rej = 5000. The oscillation dynamics of the flapping jets has been characterized and examined with high spatial and temporal resolution by identifying phase-averaged velocity and vorticity fields and Reynolds stresses at several specific phases. Analysis shows that the value of Rej plays a significant role in the motion and behavior of the switching jets. A proper orthogonal decomposition (POD) analysis of the coherent structure organization of the turbulent flow has been performed to construct a reduced-order model of the turbulent flow. The results show that the first four POD modes contain about 17% of the total kinetic energy (the first 120 modes up to 79%) and that the complexity of the most energetic flow structures increases with an increase in the Rej number.
Efficient organic solar cells with the active layer fabricated from glovebox to ambient condition
Authors: Li Hengyue , Huang Keqing , Dong Yanan , Guo Xiaotong , Yang Yu , Luo Qun , Ma Chang-Qi , Li Dongfan , Lu Guanghao , Xiong Jian , Zhang Jian , Yang Yingguo , Gao Xingyu , Yang Junliang
Published: October 2020

Organic solar cells (OSCs) have been attracting considerable interest due to their unique advantages of low cost, light weight, and especially mechanical flexibility. The low-cost and high-throughput techniques matching with the large-scale and roll-to-roll (R2R) process for fabricating efficient OSCs in the ambient condition would greatly accelerate the potential commercialization of OSCs. Herein, we demonstrate that the fabrication processes of OSCs using the bulk heterojunction (BHJ) composed of poly[(2,6–(4,8-bis(5–(2-ethylhexy)thiophen-2-yl)-benzo[1,2-b:4,5-b′]dithiophene))-alt-(5,5–(1′,3′-di-2-thienyl-5′,7′-bis(2-ethylhexyl)benzo[1′,2′-c:4′5′-c′]dithiophene-4,8-dione))] (PBDB-T) and 3,9-bis(2-methylene-(3–(1,1-dicyanomethylene)-5-methylindanone)-5,5,11,11-tetrakis(4-hexylphenyl)-dithieno[2,3-d:2′,3′-d′]-s-indaceno[1,2-b:5,6-b′]-dithiophene (IT-M) are transferred from a glovebox to the ambient condition, where the deposition of doctor blading instead of conventional spin coating is investigated. The morphology, microphase separation, and crystallinity of BHJ PBDB-T:IT-M are dramatically influenced by the fabrication processes. The OSCs with a structure of ITO/ZnO/PBDB-T:IT-M/MoO3/Ag fabricated via doctor-blading in the ambient condition show a power conversion efficiency (PCE) of 9.0% as compared to conventional spin-coated OSCs in a glovebox with a PCE of 11.91% and in the ambient condition with a PCE of 9.91%. These results suggest that efficient OSCs could be processed in the ambient condition by large-scale and low-cost doctor-blading, which can be compatible with the R2R manufacturing process.
Aerodynamics of multiple freely falling plates
Authors: Kushwaha Vivek Kr. , De Arnab K.
Published: October 2020

Numerical simulations of the complex fluid structure interaction between multiple freely falling plates in an otherwise quiescent medium are carried out in this work. The non-vertical descending motion of the plates shows a wide variety of dynamical behavior that depends not only on the shape of the plates but also on the relative initial orientation of release. The collision between the plates and subsequent movement through a varied vorticity field cause significant deviations in the trajectory from the single plate behavior. The interaction of vortices shed from closely moving plates is seen to either support, in flutter, or oppose, during the initial transient of tumble, rotational motion. In the case of parallel fluttering, a combination of the forces exerted by the surrounding fluid and the wake of the leading plate causes the trailing plate to emerge faster at the later stage of its descent. In comparison to the single plate case, a stable tumbling motion can be achieved at a higher non-dimensional moment of inertia when the plate moves in the proximity of another one. While in some case a denser vorticity field exhibits vortex merging with a range of scales present near the plates, scattered large scale flow is mostly observed away from the plates. Initial collision and vortex interaction combinedly decide the height at which the plates settle for steady gliding motion in the case of tumble. This preferential selection of the level height for tumbling is found to be the key for subsequent similar stable trajectories. The rate of energy transfer to the surrounding fluid decides the duration and direction of flight of the plates. Even a small contribution in energy transfer from the surrounding fluid to the plates amplifies and causes significant changes in energy interaction.
Diversity-induced resonance for optimally suprathreshold signals
Authors: Liang Xiaoming , Zhang Xiyun , Zhao Liang
Published: October 2020

Recent research has revealed that a system of coupled units with a certain degree of parameter diversity can generate an enhanced response to a subthreshold signal compared to that without diversity, exhibiting a diversity-induced resonance. We here show that diversity-induced resonance can also respond to a suprathreshold signal in a system of globally coupled bistable oscillators or excitable neurons, when the signal amplitude is in an optimal range close to the threshold amplitude. We find that such diversity-induced resonance for optimally suprathreshold signals is sensitive to the signal period for the system of coupled excitable neurons, but not for the coupled bistable oscillators. Moreover, we show that the resonance phenomenon is robust to the system size. Furthermore, we find that intermediate degrees of parameter diversity and coupling strength jointly modulate either the waveform or the period of collective activity of the system, giving rise to the resonance for optimally suprathreshold signals. Finally, with low-dimensional reduced models, we explain the underlying mechanism of the observed resonance. Our results extend the scope of the diversity-induced resonance effect.
Plasmas generated by electron beam passing through diamond window
Authors: Zhu B. L. , Ma Y. , Yan S. Q. , Ke W. , Yang K. , Zhu X. D.
Published: October 2020

In this paper, we report the generation of Ar plasmas by using an electron beam passing through a diamond window. It is observed that plasma brightness varies with the electron beam energy and the duty cycle. The transmission properties of the electrons in a 5-μm-thick diamond film are investigated by both experiments and Monte Carlo simulation. The measured transmittance of the electron beam increases from 20% to 80% by increasing the incident energy of the electron beam on the diamond window from 32.5 keV to 50 keV. The diamond window was checked using a CCD camera and SEM after plasma generation experiments. It is found that the focused electron beam passes through the central region of the diamond window, from the incident plane to the exit plane, and the cross section of the diamond window gradually changes from textured morphology to a featureless structure, indicating that the main energy loss of electrons in the diamond window occurs in the later stage of the transmitting journey. The interaction between the electron and the diamond causes the structure change of the diamond. The simulated transmittances of the electron beam with respect to its incident energy are in agreement with the experimental results.
Competition of natural convection and thermal creep in a square enclosure
Authors: Zhang Jun , Yao Siqi , Fei Fei , Ghalambaz Mohammad , Wen Dongsheng
Published: October 2020

Although natural convection and thermal creep have been well recognized in the continuum and rarefied regimes, respectively, the study of the competition of them in a wide flow regime is very scarce. From a theoretical point of view, natural convection can be described by Navier–Stokes–Fourier (NSF) equations at the macroscopic level, while thermal creep needs descriptions at the molecular level. Therefore, it is quite challenging to capture these two effects simultaneously. In this work, we employ the unified stochastic particle Bhatnagar–Gross–Krook (USP-BGK) method to investigate thermally driven gas flow in a square enclosure. The simulation results obtained by the USP-BGK method are validated by comparing to those from NSF solutions and direct simulation Monte Carlo method for the continuum and transitional regimes, respectively. We find that the flow patterns in the whole flow regime cannot be determined by just one nondimensional parameter, i.e., the Rayleigh number (Ra), but needs two nondimensional parameters, i.e., the Knudsen number (Kn) and the Froude number (Fr), or Kn and Ra. Specifically, small Knudsen and Froude numbers tend to generate natural convection, while large Knudsen and Froude numbers tend to cause thermal creep. Moreover, our simulation results and analyses demonstrate that when Kn < 0.12, thermal creep is dominant if Ra < 1.0, while natural convection is dominant if Ra/Fr > 0.28, or equivalently, L/L* > 1.0, where L is the characteristic length of the system and L* is the equivalent characteristic length of molecules. These findings provide useful guidance for better understanding of the complex gas flows resulting from the competition of natural convection and thermal creep under microscale or low-density conditions such as on Mars.
AlScN-based MEMS magnetoelectric sensor
Authors: Su Jingxiang , Niekiel Florian , Fichtner Simon , Thormaehlen Lars , Kirchhof Christine , Meyners Dirk , Quandt Eckhard , Wagner Bernhard , Lofink Fabian
Published: October 2020

MEMS sensors based on magnetoelectric composites have attracted great interest due to their capability to detect weak magnetic fields, showing high potential in applications like biomagnetic field detection and magnetic particle imaging. This paper reports on a scandium aluminum nitride thin film-based MEMS magnetoelectric sensor. The sensor consists of a polycrystalline silicon cantilever with a size of 1000 μm × 200 μm covered by a piezoelectric Al0.73Sc0.27N and a magnetostrictive (Fe90Co10)78Si12B10 thin film. The performance of the presented sensor is investigated based on the magnetoelectric (ME) voltage coefficient, voltage noise density, and limit of detection and compared to the characteristics of the aluminum nitride thin film-based ME sensor with the same layout and fabrication technology. By using an Al0.73Sc0.27N thin film with a higher piezoelectric activity instead of AlN in MEMS ME sensors, the ME voltage coefficient of (1334 ± 84) V/cm Oe in resonance is almost double, thereby lowering the requirements for the electronic system. The limit of detection of (60 ± 2) pT/Hz0.5 remains unchanged due to the dominant thermomechanical noise in resonance.
Thermoacoustic instability in a two-dimensional dusty plasma: A study in the weakly and strongly coupled regime
Authors: Garai Sudip , Ghose-Choudhury Anindya , Sain Sharmistha
Published: October 2020

Thermoacoustic instability in a two-dimensional unmagnetized dusty plasma has been investigated with the implementation of the general hydrodynamic model both in strongly and weakly coupled regimes. It has been found that the thermoacoustic modes are unstable in the long wavelength limit both in strongly and weakly coupled regimes although the modes literally die out in the weakly coupled regime due to viscous damping. The amplification of the thermoacoustic mode is triggered by the positive feedback response of the system via density and temperature fluctuations. The higher the thermal diffusivity, the higher the chances of mode stabilization.
Instability of electroconvection in viscoelastic fluids subjected to unipolar injection
Authors: Su Zheng-Gang , Zhang Yi-Mo , Luo Kang , Yi Hong-Liang
Published: October 2020

In this paper, a two-dimensional numerical study on the nonlinear behaviors of electrohydrodynamic flows of Oldroyd-B viscoelastic dielectric liquid subjected to unipolar injection is performed via the finite volume method. The entire set of coupled equations, which includes the Navier–Stokes equations, simplified Maxwell’s equations, and conformation transport equations, is solved for the first time. The effects of elasticity on the nonlinear evolution of electroconvection and instability patterns are mainly investigated. Various physical models including free and rigid boundary cases are simulated entirely, and detailed analyses of stability parameters are performed. Convection and fluid motion instability are investigated and explained in detail, with a focus on the onset of motion transitions from a purely conducted state to losing its stability. It is found that the coupling of the electric field with the elasticity field gives rise to new instability and completely new mechanisms. In addition to instabilities such as subcritical bifurcation in electroconvection of Newtonian fluids, supercritical bifurcation and Hopf bifurcation are also possible as the first instability in electroconvection of viscoelastic fluids under free boundary conditions. Under rigid boundary conditions, the system with a large Weissenberg number can also lose its stability via earlier Hopf bifurcation. The stability threshold is not affected by the elastic effect if the Weissenberg number is small enough but decreases when the first instability of the system becomes Hopf bifurcation. Moreover, elasticity promotes the transition from a steady state flow to unsteady convection after the onset of convection. These phenomena are closely related to the elastic parameters.
Unusual behavior of coercivity in Hf/GdFeCo bilayer with MgO cap layer by electric current
Authors: Hai Ngo Trong , Kindiak Ivan , Yurlov Vladislav , Chandra Bhatt Ramesh , Liao Chun-Ming , Ye Lin-Xiu , Wu Te-ho , Zvezdin K. A. , Wu Jong-Ching
Published: October 2020

We investigate the Hf/GdFeCo bilayer with the MgO cap layer for both rare earth (RE)-rich and transition metal (TM)-rich configurations of the ferrimagnetic sublattice in the presence of the perpendicular field. We study the coercivity using the anomalous Hall effect (AHE) technique by multiple measurements on the same sample. In the first set of measurements and at low electric currents, coercivity sharply drops because of the oxygen diffusion at the interface between MgO and GdFeCo when the AHE probe current is applied. During the subsequent measurements on the RE-rich sample, we observe a moderate decrease in coercivity at low currents and the coercivity increases in a high current range. Such nonlinear dependence of coercivity on electric current can be explained by the competing interplay of the spin–orbit torque (SOT) and the Joule heating effects. On the other hand, for the TM-rich case, the SOT effect is observed over a widely applied current range.
Observation of nitrogen species at Al2O3/NO2/H-diamond interfaces by synchrotron radiation x-ray photoemission spectroscopy
Authors: Saha Niloy Chandra , Takahashi Kazutoshi , Imamura Masaki , Kasu Makoto
Published: October 2020

Two nitrogen-related peaks were successfully detected by synchrotron radiation x-ray photoemission spectroscopy (XPS) at the interface of the Al2O3/NO2/H-diamond structure, which is used for metal–oxide–semiconductor field-effect transistors. The 399-eV peak was attributed to the C‒NH2 or C‒N bond, which was formed by electron transfer from the NO2 molecule to the H-diamond surface and subsequent decomposition of NO2 molecules at the H-diamond surface. The 407-eV peak was attributed to NO3− bond, which was formed by the decomposition of N2O4 molecules. We confirmed that N species are localized at the interface by changing the photoemission angle in XPS. The N interface density at the (111) interface was about twice that at the (001) interface. This difference is caused by C‒H bond density on different surface orientations. The band alignments were determined to be type II (staggered type), and the valence band offset (ΔEV) was determined to be 3.9 ± 0.1 eV for (001) and 4.3 ± 0.1 eV for (111) surface orientations. These results agree well with the higher hole sheet concentration on the (111) interface than on the (001) interface.
Tunneling splittings of vibrationally excited states using general instanton paths
Authors: Eraković Mihael , Cvitaš Marko T.
Published: October 2020

A multidimensional semiclassical method for calculating tunneling splittings in vibrationally excited states of molecules using Cartesian coordinates is developed. It is an extension of the theory by Mil’nikov and Nakamura [J. Chem. Phys. 122, 124311 (2005)] to asymmetric paths that are necessary for calculating tunneling splitting patterns in multi-well systems, such as water clusters. Additionally, new terms are introduced in the description of the semiclassical wavefunction that drastically improves the splitting estimates for certain systems. The method is based on the instanton theory and builds the semiclassical wavefunction of the vibrationally excited states from the ground-state instanton wavefunction along the minimum action path and its harmonic neighborhood. The splittings of excited states are thus obtained at a negligible added numerical effort. The cost is concentrated, as for the ground-state splittings, in the instanton path optimization and the hessian evaluation along the path. The method can thus be applied without modification to many mid-sized molecules in full dimensionality and in combination with on-the-fly evaluation of electronic potentials. The tests were performed on several model potentials and on the water dimer.
A mechanochemical model for the simulation of molecules and molecular crystals under hydrostatic pressure
Authors: Stauch Tim
Published: October 2020

A novel mechanochemical method for the simulation of molecules and molecular crystals under hydrostatic pressure, the eXtended Hydrostatic Compression Force Field (X-HCFF) approach, is introduced. In contrast to comparable methods, the desired pressure can be adjusted non-iteratively and molecules of general shape retain chemically reasonable geometries even at high pressure. The implementation of the X-HCFF approach is straightforward, and the computational cost is practically the same as for regular geometry optimization. Pressure can be applied by using any desired electronic structure method for which a nuclear gradient is available. The results of the X-HCFF for pressure-dependent intramolecular structural changes in the investigated molecules and molecular crystals as well as a simple pressure-induced dimerization reaction are chemically intuitive and fall within the range of other established computational methods. Experimental spectroscopic data of a molecular crystal under pressure are reproduced accurately.
Splitting of the surface modes for bubble oscillations near a boundary
Authors: Maksimov A.
Published: October 2020

The aim of this study is to describe the effect of the interface between media with different mechanical properties on the behavior of a gas bubble. The presence of a boundary leads to the breaking of the degeneracy of the surface modes, which are the perturbations of the spherically symmetric shape of the equilibrium bubble. The splitting of the surface modes was determined by using perturbation and numerical techniques. The results obtained can be applied to elucidate the physical mechanisms underlying a new ultrasonic cleaning technique, the ultrasonically activated stream. An experimental observation of the described effect is possible when analyzing a scattered signal under conditions of two-frequency excitation: a pump wave with a frequency close to the resonant frequency of the bubble parametrically generates distortion modes and a high-frequency signal wave that effectively scatters from distortion modes.
A theoretical study of parietal vortex shedding in Taylor–Culick flow via linear stability analysis
Authors: Li Yang , Wang Zhuopu , Liu Peijin , He Wei , Li Larry K. B.
Published: October 2020

In this theoretical study, we use linear stability analysis to investigate the cause of parietal vortex shedding in Taylor–Culick flow, which is representative of the flow in solid rocket motors. We focus on the effects of the lateral-injection Reynolds number and the length-to-radius ratio of the combustion chamber. Through a comparison with pipe flow, we find that flow turning is a major contributor to parietal vortex shedding. We explore the role of amphidromic points and find that they can divide the flow field into two distinct regions, an outer region with strong perturbations and an inner region with weak perturbations. In the outer region, we find that the velocity perturbations develop advection patterns with axial (streamwise) periodicity, while the pressure perturbations induce flow gradients that enhance shear stresses. Collectively, these effects are thought to combine to induce parietal vortex shedding in solid rocket motors.
Enhanced space charge limited current for curved electron emitters
Authors: Singh Gaurav , Kumar Raghwendra , Biswas Debabrata
Published: October 2020

The maximum current that can be transported across a vacuum diode is limited by forces arising due to space charge. In a planar thermionic or explosive emission diode, the space charge limited current density from an emitting patch is given by the Child–Langmuir law JCL∼Vg3/2/D2, where Vg is the potential difference across the diode and D is the separation between the anode and the cathode. We show here, analytically using the nonlinear line charge model, that for a curved emitter in a planar diode configuration, the limiting current obeys the scaling relationship JSCL∼γaVg3/2/D2, where γa is the apex field enhancement factor of the curved emitter. For an emitter with a large height (h) to the apex radius of curvature (Ra) ratio, the limiting current far exceeds the planar value. The result is verified using the particle-in-cell code PASUPAT for two curved emitter shapes.
PhD student–adviser pairing is critical, but in US physics departments it’s often haphazard
Authors: Feder Toni
Published: October 2020

Scientific interests, personal chemistry, and mentoring styles go into choosing an adviser. The best way to know if a match will work is to try it out.
How to measure the economic health cost of wildfires – A systematic review of the literature for northern America
Authors: Dittrich Ruth , McCallum Stuart
Published: October 2020

There has been an increasing interest in the economic health cost from smoke exposure from wildfires in the past 20 years, particularly in the north-western USA that is reflected in an emergent literature. In this review, we provide an overview and discussion of studies since 2006 on the health impacts of wildfire smoke and of approaches for the estimation of the associated economic cost. We focus on the choice of key variables such as cost estimators for determining the economic impact of mortality and morbidity effects. In addition, we provide an in-depth discussion and guidance on the functioning, advantages and challenges of BenMAP-CE, freely available software of the US Environmental Protection Agency (EPA) that has been used in a growing number of studies to assess cost from wildfire smoke. We highlight what generates differences in outcomes between relevant studies and make suggestions for increasing the comparability between studies. All studies, however, demonstrate highly significant health cost from smoke exposure, in the millions or billions of US dollars, often driven by increases in mortality. The results indicate the need to take health cost into account for a comprehensive analysis of wildfire impacts.
Three-dimensional digital PCR through light-sheet imaging of optically cleared emulsion
Authors: Liao Peiyu , Jiang Mengcheng , Chen Zitian , Zhang Fangli , Sun Yue , Nie Jun , Du Meijie , Wang Jianbin , Fei Peng , Huang Yanyi
Published: October 2020

Significance

Digital PCR (dPCR) has long been recognized as a powerful tool in nucleic acid quantitative analysis, but there still remain problems that hinder its wide popularity. Sample loss is one major challenge that compromises the certainty of dPCR quantification and handicaps its ability to detect and quantify nucleic samples of low copy number. Another main difficulty is to prevent this single-copy-sensitivity experiment from possible contaminations of PCR product exposure. The CLEAR-dPCR introduced herein solves these two problems by integrating optically cleared monodisperse droplet generation and light-sheet fluorescence microscopy and fulfills a genuinely lossless and contamination-free digital PCR approach. Such an emulsion clearing method pioneers a way of high-throughput in situ signal capture in mass droplets.
Subunit-selective iGluR antagonists can potentiate heteromeric receptor responses by blocking desensitization
Authors: Pollok Stefan , Reiner Andreas
Published: October 2020

Significance

Ionotropic glutamate receptors (iGluRs) are a diverse family of tetrameric ligand-gated ion channels. Recent structural information has provided valuable insight into the conformational dynamics and assembly properties of iGluRs, most importantly on heteromeric iGluRs, which are prevalent in the nervous system. However, little is known about how channel gating is controlled by the 4 subunits within the tetramers, different ligand occupancies, and intersubunit cooperativity. In this study, we probe how subunit-selective ligands affect activation and desensitization of heteromeric iGluRs and we provide mechanistic insight into ligand dissociation. Our findings have direct pharmacologic implications, as we show that subunit-selective antagonists can block desensitization of heteromeric iGluRs, thereby causing potentiation instead of inhibition.
Fusidic acid resistance through changes in the dynamics of the drug target
Authors: Tomlinson Jennifer H. , Kalverda Arnout P. , Calabrese Antonio N.
Published: October 2020

Significance

The World Health Organization has declared antimicrobial resistance one of the greatest threats to human health. Understanding the molecular mechanisms by which bacteria resist the effects of antibiotic therapies is central to understanding antimicrobial resistance and to informing the development of new treatments that can circumvent these resistance mechanisms. This study reveals the molecular details of the mechanism of FusB-mediated resistance to fusidic acid, an important clinical treatment for
Staphylococcus aureus
infections, including methicillin-resistant
Staphylococcus
aureus
. Here we elucidate an antibiotic resistance mechanism driven by an allosteric effect on the conformational flexibility of the drug target.