bluuebunny/crossref_metadata_2025 huggingface dataset:
High time resolution diagnosis of electron density in helium plasma jets with impurity gas
Authors: Li Xu , Wang Lanping , Nie Lanlan , Lu Xinpei
Published: February 2024

Atmospheric pressure helium plasma jets are widely used in biomedical applications. Researchers normally introduce small amounts of nitrogen and oxygen (0.2–1.0%) into helium to enhance the electron density and electron energy, thus increasing the concentration of active species in plasma. To further explore why the combination of impurity gases N2/O2 leads to an increase in the electron density from the discharge mechanism, we used a microwave Rayleigh scattering method with excellent time-varying characteristics to monitor the temporal electron density changes when different concentrations of N2/O2 were mixed. The research revealed that even trace amounts of N2/O2 (0.2%) can increase the peak electron density, with this effect being more pronounced when N2 is added, increasing from 3.3 × 1019 to 4.6 × 1019 m−3 in pure helium. As the concentration increases, the introduction of O2 leads to a rapid decrease in the electron density. When 1.0% oxygen is mixed, the electron density decreases from 3.3 × 1019 to 2.4 × 1019 m−3. However, the situation is different when N2 is added, at 0.5% proportion of nitrogen, the electron density increases to its maximum at 6.5 × 1019 m−3. These effects are due to the electronegativity of the oxygen-containing particles or the Penning ionization related to excited nitrogen species.
Effect of mixed basalt fibers and nano-silica on mechanical properties and microstructure of recycled aggregate concrete
Authors: Chen Wei , Chen Yueshun
Published: February 2024

In this study, basalt fibers (BFs) and nano-silica (NS) were innovatively combined to reinforce recycled aggregate concrete (RAC) to prepare recycled aggregate concrete (named NBRAC) with better mechanical and microstructural properties as an alternative to ordinary concrete. Different NBRAC specimens were prepared by adjusting the contents of NS and BF; the damage mechanism and mechanical properties of NBRAC were investigated; and the compressive, split tensile, and flexural strengths of NBRAC were evaluated. The microstructure of NBRAC was analyzed by scanning electron microscopy observation. The results showed that the densification of NBRAC was improved under the condition of 50% RA substitution rate and that its compressive, split tensile, and flexural strengths were increased by 6.8%, 16.3%, and 32.7%, respectively, compared with that of natural concrete, which proved that the method was feasible for the preparation of high-performance RAC. Scanning electron microscope image analysis confirmed the improvement effects of NS and BF on RAC, and combined with the experimental data, a composite explanation for the improvement effects of NS and BF on RAC was proposed for the first time.
Gettering of iron by aluminum oxide thin films on silicon wafers: Kinetics and mechanisms
Authors: Le Tien Trong , Yang Zhongshu , Liang Wensheng , Macdonald Daniel , Liu AnYao
Published: February 2024

Metallic impurities in the silicon wafer bulk are one of the major efficiency-limiting factors in silicon solar cells. Gettering can be used to significantly lower the bulk metal concentrations. Aluminum oxide thin films from plasma-enhanced atomic layer deposition (PE-ALD) have been reported to getter iron from silicon wafers. However, its gettering mechanism and kinetics remain unclear. In this study, by experimentally monitoring the kinetics of iron reduction in the silicon wafer bulk, aluminum oxide gettering of iron is shown to be caused by a segregation mechanism. Fitting the experimental iron reduction kinetics by the simulation of a segregation gettering process based on various diffusion scenarios suggests that the gettering kinetics is limited by both the diffusivities of iron in the silicon wafer bulk and in the aluminum oxide film. The activation energy of the segregation gettering process (negative meaning exothermic reaction) is estimated to be −0.47 ± 0.16 eV for the investigated as-deposited PE-ALD aluminum oxide film at 550–900 °C, and −0.35 ± 0.06 eV at 400–900 °C for the same film after a 400 °C forming gas anneal (FGA), i.e., after activating the passivation effect of the film. Capacitance–voltage measurements of the films indicate a higher surface defect density in the as-deposited films as compared to the FGA-activated films, which suggests a possible correlation between the surface defect density and gettering.
Enhanced near-field radiative heat transfer between nanostructure emitter and GaSb absorber by surface plasmon polaritons and hyperbolic modes
Authors: Li Bowen , Lu Lu , Zhang Kun , Zhou Yulong , Luo Zixue , Cheng Qiang
Published: February 2024

Near-field radiative heat transfer (NFRHT) can overcome the blackbody radiation limit and holds great promise in radiative energy conversion devices such as near-field thermophotovoltaics (NF-TPV). However, NF-TPV is not yet ready for practical applications at larger scales due to the challenges of maintaining the nanoscale gap and the lack of optimized NFRHT with nanostructure emitters. Here, we measure the enhanced near-field radiative heat flux between GaSb absorbers and nanostructure emitters that can be applied in NF-TPV systems, with a cm2-scale plate–plate structure and a temperature difference of 100 K at a gap down to 200 nm. It is demonstrated that the radiative heat flux between the bulk W emitter and the absorber at a 400 nm gap is about an order of magnitude larger than that in the far field, and the heat flux can be further enhanced by 1.6 times at a 200 nm gap. Moreover, the enhanced NFRHT from the surface plasmon polaritons supported by a indium tin oxide film and the hyperbolic modes supported by a W/SiO2 multilayer is also experimentally verified and theoretically analyzed.
Origin of the parasitic luminescence of 235 nm UVC LEDs grown on different AlN templates
Authors: Hagedorn Sylvia , Kolbe Tim , Schmidt Gordon , Bertram Frank , Netzel Carsten , Knauer Arne , Veit Peter , Christen Jürgen , Weyers Markus
Published: February 2024

AlN layers annealed at high temperatures offer low threading dislocation densities of mid 108 cm−2 and are therefore increasingly used as base layers in ultraviolet (UV) light emitting diode (LED) heterostructure growth. These LEDs, just like those grown on conventional metalorganic vapor phase epitaxy (MOVPE) AlN templates, often suffer from long-wavelength parasitic luminescence. In this work, luminescence properties of far-UVC LED heterostructures grown on MOVPE-AlN/sapphire templates and high-temperature annealed AlN/sapphire templates are compared. To investigate the origin of parasitic emission with high spatial resolution, cross section scanning transmission electron microscopy was combined with cathodoluminescence measurements. As a result, the main origin of the parasitic luminescence band centered at 3.5 eV (354 nm) for the heterostructure grown on annealed AlN is assigned to point defects related to oxygen in the AlN template layer. The defect band centered at 3.0 eV (413 nm) for the heterostructure grown on MOVPE-AlN was found to be related to self-compensating VAl-Si point defect complexes in the n-AlGaN layer and oxygen incorporation close to the AlN/sapphire interface. The results also suggest that the type of AlN template determines the kind of parasitic luminescence from the n-AlGaN layer.
Exceptional points in non-Hermitian systems: Applications and recent developments
Authors: Meng Haiyu , Ang Yee Sin , Lee Ching Hua
Published: February 2024

Exceptional points are complex branching singularities of non-Hermitian bands that have lately attracted considerable interest, particularly in non-Hermitian photonics. In this article, we review some recent developments in non-Hermitian photonic platforms such as waveguides, photonic crystals, Fabry–Pérot resonators, and plasmonic systems and suggest how optical non-linearities and exceptional bound states can further inspire the development of non-Hermitian photonics in the near future.
A multi-reflection time-of-flight setup for the study of atomic clusters produced by magnetron sputtering
Authors: Giesel Paul F. , Fischer Paul , Schweikhard Lutz
Published: February 2024

The Greifswald multi-reflection time-of-flight setup has been extended with a magnetron sputtering gas aggregation source for the production of atomic cluster ions with sizes ranging from a single to thousands of atoms. This source, combined with a newly added quadrupole mass filter and a linear Paul trap, opens up the possibility of many new atomic-cluster studies not feasible with the setup before. The new components and their interfacing with the previous setup are described, and benchmarking as well as the first experimental results are presented. The capability of the system to handle singly charged ions with masses of several ten thousand atomic mass units is demonstrated.
Impact of unintentionally formed compositionally graded layer on carrier injection efficiency in AlGaN-based deep-ultraviolet laser diodes
Authors: Zhang Ziyi , Yoshikawa Akira , Kushimoto Maki , Aoto Koji , Sasaoka Chiaki , Amano Hiroshi
Published: February 2024

Increasing the injection efficiency, a critical factor constraining the reduction in threshold current in AlGaN-based deep-ultraviolet laser diodes, represents one of the paramount remaining technical challenges. In this study, the impact of compositionally graded layers that were unintentionally formed at the interface between the p-cladding and the core layer on carrier injection efficiency was analyzed. Experimental evaluations using laser diodes have shown that the elimination of an unintentionally formed layer increases the injection efficiency above the threshold current, from the conventional 3% to 13%. It has been postulated that the electron overflow toward the p-side exerts a substantial deleterious effect on the injection efficiency. An improvement in this aspect is achieved by increasing the electron-blocking capability due to the improved interface abruptness between the p-cladding layer and the core layer. The lasing threshold was strongly reduced, and characteristic temperature increased from 76 to 107 K for the improved devices.
Impact of deposition temperature on electrical properties of HZO-based FeRAM
Authors: Yeh Yu-Hsuan , Tan Yung-Fang , Huang Yen-Che , Lin Chao Cheng , Wu Chung-Wei , Zhang Yong-Ci , Lee Ya-Huan , Chang Ting-Chang , Sze Simon M.
Published: February 2024

This study presents a comprehensive investigation of the impact of the deposition temperature on the HfxZr1−xO2 (HZO) ferroelectric layer of ferroelectric random access memory with TaN electrodes. This investigation mainly focuses on its electrical characteristics and compares the differences. It is revealed that the deposition temperature plays a crucial role in determining the crystal structure of HZO, which can exhibit a combination of tetragonal and orthorhombic phases or exist solely in one of the two phases. Furthermore, the grain size of HZO varies with the deposition temperature. These findings correspond well to the electrical measurement results, including leakage current, polarization, capacitance, and reliability tests. The study tracks the phase transition process during the operation of switching cycles when the phase transition process can be monitored as well. To better understand the observed differences, physical models that shed light on the underlying mechanisms affected by deposition temperature are proposed at the end of the article.
Analysis of the combustion modes in a rocket-based combined cycle combustor using unsupervised machine learning methodology
Authors: An Bin , Sun Mingbo , Zhao Qingjun , Yang Leichao , Yang Daoning , Huang Yuhui , Li Peibo , Wang Jiaoru , Yang Yifan
Published: February 2024

Combustion mode analysis is essential to a rocket-based combined cycle (RBCC) combustor because it may experience multiple combustion modes during the operation. In this study, a method based on an autoencoder and a K-means algorithm was proposed for combustion mode analysis. Flame chemiluminescence images and schlieren images of three combustion modes observed in an RBCC combustor were used to evaluate this method. Two autoencoders that followed the same encoder–decoder architecture were developed separately to generate the latent space representations of flame chemiluminescence images and schlieren images. In the latent space, the centroids and boundaries of different combustion modes were determined using the K-means algorithm. Each autoencoder was trained using 750 images and tested using another 3000 images. The method achieved an accuracy up to 99% on both flame chemiluminescence images and schlieren images. The images generated by the decoder suggested that the autoencoder captured the important features (e.g., primary reaction zone and shock wave) of the reacting flow field. The autoencoder developed for flame chemiluminescence images also successfully detected the combustion mode transition during an ignition process, which suggested that it had the potential to monitor the combustion mode in a real time manner. However, the autoencoder failed on monitoring combustion mode transition when it came to the schlieren images because the optical access of the training data was not exactly the same. Therefore, it is essential to ensure that the optical accesses of different combustion modes are exactly the same when schlieren images are employed for combustion mode analysis.
Effects of preoperative aortic tortuosity and postoperative hypertension on patient-specific hemodynamics of abdominal aortic aneurysm
Authors: Zhu Jing , Han Xiao , Liu Ye , Liu Chen , Zhang Xuelan , Si Xinhui , Shu Chang , Luo Mingyao
Published: February 2024

Abdominal aortic aneurysm (AAA) is a serious dilated vascular disease. The risk factors of aneurysm rupture and postoperative blood pressure are the major clinical concerns. The purpose of this work is to analyze the hemodynamic difference between preoperative and postoperative AAA using patient-specific boundary conditions and explore the rupture risk of AAA with different torsion and to simulate the blood flow of different degrees of hypertension. A three-element Windkessel model is utilized as the outlet boundary condition. 20-sim software and self-developed user-defined functions are used to calibrate the parameters. To analyze the influence of aortic tortuosity on hemodynamics, five AAA geometries with different torsion degrees are modified and obtained. We also grade the hypertension and explore the effect of hypertension after operation. The analysis results are consistent with the patient-specific situation. The results show that stent implantation for aneurysms reduces the pressure at the lesion site and increase the speed and wall shear stress. Vessels with high torsion have lower time-averaged wall shear stress (TAWSS), higher oscillatory shear stress index (OSI) and relative retention time, and more disordered blood flow. After stent implantation, with the increase in blood pressure, TAWSS and OSI continue to increase, and the blood flow speed is also faster. AAA with high torsion is more likely to rupture. In clinical practice, attention should be paid to the degree of torsion of the lesion site. Furthermore, blood pressure should be monitored and controlled in time to prevent postoperative complications.
Measurement of Kerr rotation and ellipticity in magnetic thin films by MOKE magnetometry
Authors: Suzuki Daniel H. , Beach Geoffrey S. D.
Published: February 2024

When polarized light is incident on a magnetic material, the magneto-optical Kerr effect (MOKE) rotates the polarization and induces ellipticity in the reflected light, which allows the magnetization direction to be probed optically. The Kerr rotation and ellipticity determine the magnitude of the effect and are usually measured using dedicated ellipsometers. Here, we demonstrate a simple method for extracting Kerr rotation and ellipticity in magnetic thin films using a conventional MOKE magnetometer consisting of two polarizers and a quarter waveplate. Using this technique, we report the longitudinal Kerr angle of BiYIG, GdCo, and TbCo. We additionally observe a linear decrease in polar complex Kerr angle magnitude in 3 nm GdCo films as the atomic fraction of Gd is increased.
Current status and challenges for hole-selective poly-silicon based passivating contacts
Authors: Basnet Rabin , Yan Di , Kang Di , Shehata Mohamed M. , Phang Pheng , Truong Thien , Bullock James , Shen Heping , Macdonald Daniel
Published: February 2024

Doped polysilicon (poly-Si) passivating contacts have emerged as a key technology for the next generation of silicon solar cells in mass production, owing to their excellent performance and high compatibility with the existing passivated emitter and rear cell technology. However, the current solar cell architecture based on a rear-side electron-selective (n+) poly-Si contact is also approaching its practical limit (∼26%) in mass production. The full potential of doped poly-Si passivating contacts can only be realized through incorporation of both electron-selective and hole-selective (p+) poly-Si contacts. While studies of both p+ and n+ poly-Si contacts commenced simultaneously, significant performance differences have arisen. Phosphorus-doped poly-Si contacts consistently outperform boron-doped counterparts, displaying typically lower recombination current density (J0) values (1–5fA/cm2 vs 7–15fA/cm2). This discrepancy can be attributed to inadequate optimization of p+ poly-Si contacts and fundamental limitations related to boron doping. The poorer passivation of p+ poly-Si contacts can be at least partly attributed to boron segregation into the interfacial oxide layers, compromising the interfacial oxide integrity and reducing the chemical passivation effectiveness. This review critically examines the progress of p+ poly-Si contacts characterized by cell efficiency and J0 values, delves into existing challenges, identifies potential solutions, and explores some potential solar cell architectures to enhance efficiency by incorporating p+ poly-Si contacts.
Detailed dynamical features of the slow hydration water in the vicinity of poly(ethylene oxide) chains
Authors: Kikuchi T. , Tominaga T. , Murakami D. , de Souza N. R. , Tanaka M. , Seto H.
Published: February 2024

Poly(ethylene oxide) (PEO) is a well-known biocompatible polymer and has widely been used for medical applications. Recently, we have investigated the dynamic behavior of hydration water in the vicinity of PEO chains at physiological temperature and shown the presence of slow water with diffusion coefficient one order of magnitude less than that of bulk water. This could be evidence for the intermediate water that is critical for biocompatibility; however, its detailed dynamical features were not established. In this article, we analyze the quasi-elastic neutron scattering from hydration water through mode distribution analysis and present a microscopic picture of hydration water as well as its relation to cold crystallization.
Numerical investigation of a pair of in-line bubbles rising in Newtonian and non-Newtonian fluids with interfacial passive scalar transfer
Authors: Kazemi Koorosh , Vernet Anton , Grau Francesc X. , Pallarès Jordi , Fabregat Alexandre , Cito Salvatore
Published: February 2024

We employ three-dimensional, fully resolved numerical simulations using the volume-of-fluid method to study the motion and interaction of two in-line bubbles ascending in both Newtonian and shear-thinning fluids. Additionally, we explore passive scalar transfer between the fluid phases across a variety of fluidic conditions, modeling shear-thinning behavior in non-Newtonian fluids through the Carreau model. The impact of the Galilei (Ga) and Bond (Bo) numbers, the bubble pair radius ratio, the inelastic time constant (λ), and the flow index (n) on the bubbles dynamics and the transient Sherwood number (Sht) and the surface-averaged Sherwood number (⟨Sh⟩) are reported. Using the well-known Ga–Bo regime phase diagram for a single rising bubble in a Newtonian ambient fluid, the present numerical experiments are used to study the departure from this reference case due to the presence and characteristics of a second bubble and the non-Newtonian nature of the ambient fluid. When categorized based on the single bubble phase diagram, we found that in regimes I (axisymmetric) and III (oscillatory), a pair of bubbles does not breakup or merge during our simulations. However, their behaviors vary due to the second bubble and change in non-Newtonian fluid parameters like the inelastic time constant and flow index. Likewise, we explored this parameter space for regime II (skirted), where the two bubbles eventually merge, and regimes IV (peripheral breakup) and V (central breakup), known for multiple bubble breakups. Additionally, we present results on differently sized bubbles, showing that their merging tendency depends on their arrangement as leading or trailing positions in the pair.
Wettability-modulated behavior of polymers under varying degrees of nano-confinement
Authors: Arya Vinay , Chaudhuri Abhirup , Bakli Chirodeep
Published: February 2024

Extreme confinement in nanochannels results in unconventional equilibrium and flow behavior of polymers. The underlying flow physics dictating such paradigms remains far from being understood and more so if the confining substrate is composed of two-dimensional materials, such as graphene. In this study, we conducted systematic molecular dynamics simulations to explore the effect of wettability, confinement, and chain length on polymer flow through graphene-like nanochannels. Altering the wetting properties of these membranes that structurally represent graphene results in substantial changes in the behavior of polymers of disparate chain lengths. Longer hydrocarbon chains (n-dodecane) exhibit negligible wettability-dependent structuring in narrower nanochannels compared to shorter chains (n-hexane) culminating in higher average velocities and interfacial slippage of n-dodecane under less wettable conditions. We demonstrate that the wettability compensation comes from chain entanglement attributed to entropic factors. This study reveals a delicate balance between wettability-dependent enthalpy and chain-length-dependent entropy, resulting in a unique nanoscale flow paradigm, thus not only having far-reaching implications in the superior discernment of polymeric flow in sub-micrometer regimes but also potentially revolutionizing various applications in the oil industry, including innovative oil transport, oil extraction, ion transport polymers, and separation membranes.
Effects of Pd atom vibration in the Sr–Te octahedral interstitial space in Zintl compound SrPdTe on lattice anharmonicity and thermoelectric properties
Authors: Wang Yue , Zhao Yinchang , Meng Sheng , Ni Jun , Dai Zhenhong
Published: February 2024

Based on first-principles calculations, the current study deeply explores the thermoelectric properties of the Zintl compound SrPdTe. We found that the anharmonic vibration of Pd atoms plays an important role in the quartic anharmonic effect and the temperature dependence of the thermal conductivity. In the crystalline structure, Sr atoms form octahedra with eight surrounding Te atoms, while Pd atoms are located in the gaps between the octahedra. This structure makes the strong atomic mean square displacement of Pd atoms the main factor leading to the ultralow thermal conductivity. The study also reveals the effects of phonon frequency renormalization and four-phonon scattering on heat transfer performance. Even considering the spin–orbit coupling effect, multiple secondary valence band tops maintain the power factor of the material at high temperatures, providing a potential opportunity for achieving excellent thermoelectric performance.
Mixing enhancement of transverse jets in supersonic crossflow using an actively controlled novel fluidic oscillator
Authors: Maikap Spandan , Rajagopal Arun Kumar
Published: February 2024

This study investigates the fluid dynamics and mixing characteristics of an oscillating sonic jet injected into a supersonic cross flow of Mach 2.1 using experimental and computational techniques. The oscillating jet is produced by a novel fluidic oscillator, which consists of a primary rectangular duct that expands into an outer duct with sudden expansion. Control jets are injected in the lateral direction from the side walls of the sudden expansion in an out-of-phase manner to oscillate the injected jet in the spanwise direction of the crossflow. Experimental and numerical investigations based on wall static pressure and mass fraction fluctuations, respectively, revealed that the injected jet oscillation frequency matches the control jet frequency. The iso-surface of lambda-2 criterion showed the presence of various dominant vortex structures, such as counter-rotating vortex pairs, horseshoe vortex, sidewall vortices, and trailing vortices. Helicity contour plots showed that the streamwise vortices oscillate in the spanwise direction with the control strategy and promote the spread of the injected jet in the spanwise direction. The spatiotemporal reconstruction (z–t plot) of the density gradients at a particular streamwise location revealed that the bow shock produced by the interaction of the injected jet and the crossflow oscillates with the actuation of the control strategy. The power spectral density of the z–t plot revealed that the shock wave oscillation frequency matches the control jet frequency. The oscillating jet produced by the control strategy showed significant mixing enhancement in supersonic crossflow compared to a simple rectangular injection.
Improved filters for angular filter refractometry
Authors: Heuer P. V. , Haberberger D. , Ivancic S. T. , Dorrer C. , Walsh C. A. , Davies J. R.
Published: February 2024

Angular filter refractometry is an optical diagnostic that measures the absolute contours of a line-integrated density gradient by placing a filter with alternating opaque and transparent zones in the focal plane of a probe beam, which produce corresponding alternating light and dark regions in the image plane. Identifying transitions between these regions with specific zones on the angular filter (AF) allows the line-integrated density to be determined, but the sign of the density gradient at each transition is degenerate and must be broken using other information about the object plasma. Additional features from diffraction in the filter plane often complicate data analysis. In this paper, we present an improved AF design that uses a stochastic pixel pattern with a sinusoidal radial profile to minimize unwanted diffraction effects in the image caused by the sharp edges of the filter bands. We also present a technique in which a pair of AFs with different patterns on two branches of the same probe beam can be used to break the density gradient degeneracy. Both techniques are demonstrated using a synthetic diagnostic and data collected on the OMEGA EP (extended performance) laser.
Electromagnetic induction with time-varying magductance under constant magnetic field
Authors: Qin Wei , Cheng Ming , Zhu Xinkai , Wang Zheng , Hua Wei
Published: February 2024

Electrical energy is becoming more popular due to the fact that it is economic, practical, clean, and easy to control and convert energy compared to other forms of energy. Currently, however, most of the electrical energy is converted from or to mechanical energy based on the well-known Faraday’s law of electromagnetic induction, which states that a current will be induced in a closed conductive coil exposed to a changing magnetic field. For that purpose, either the magnetic field itself must be changing or the magnetic field must be moving relative to the closed conductive coil. While investigating the physical significance of magductance (or magnetic-inductance), we unearthed a new electromagnetic induction mechanism that the electrical current is induced in a closed conductive coil with the invariant magnetic field by changing the magnetic parameter of magductance of the closed coil. Time-varying magductance is achieved by changing the conductivity of the coil. To demonstrate the feasibility of the electromagnetic induction, both theoretical validation and experimental verification are undertaken. We built a closed superconducting coil and successfully measured the induced electrical currents in the cooling process of the closed superconducting coil under an invariant magnetic field, thus verifying the new law of electromagnetic induction.
Derivation of shock front evolution with rarefaction wave and its verification in dusty plasma simulations
Authors: Chen Xin , Liang Chen , Lu Shaoyu , Huang Dong , Feng Yan
Published: February 2024

The evolution of unsupported shocks is theoretically investigated using the method of characteristics. It is found that the location and the speed of the generated non-uniform shock (NUS) front vary with the propagation time and the initial compression strength. The relationship between the NUS front location and the propagation time is asymptotically parabolic, while the speed of the NUS front decreases gradually with the propagation time. These analytical derivations are verified using computer simulations of unsupported shocks in 2D dusty plasmas performed here. The transition of the NUS front speed found previously [Sun et al., Phys. Plasmas 28, 103703 (2021)] using data fitting with the simulation data is re-investigated and further confirmed with the theoretical derivation of the NUS front in the current investigation.
The statistical characteristics and auto-regeneration of backflow in non-Newtonian turbulent pipe flow
Authors: Chen Xue , Chung Yongmann M. , Wan Minping
Published: February 2024

The backflow phenomenon in shear-thinning and shear-thickening fluids is investigated in pipe flows at friction Reynolds number Reτ=180 via direct numerical simulations. Conditional average results show that the extreme fluctuation of wall shear stress around the backflow regions is more abrupt under the shear-thinning effect. The statistical characteristics of the backflow at different flow indices from 0.5 to 1.5 show remarkable differences. The probability of the backflow events at the wall increases in both the shear-thinning and the shear-thickening fluids under different mechanisms. The backflow occurs more frequently and exists further away from the wall in the shear-thinning fluids owing to the suppressed near-wall turbulent structures and the laminarization at low flow indices. The increase in the probability of the backflow events in the shear-thickening fluids is caused by increased Q2 and Q4 events in the near-wall region. The variation in the size and the lifespan of the backflow regions with the flow index is very prominent which both increase with the shear-thinning effect and decrease as the flow becomes dilatant. In the weakly turbulent flow of shear-thinning fluid, large backflow regions appear near the leading edge of the turbulent spots where the off-axial turbulent fluctuations are significantly lowered. Observations show the linked evolution between the hairpin vortices and the backflow regions induced underneath the strong spanwise rotations. The backflow follows the auto-regeneration process of the hairpin vortices in a packet which results in coherent streamwise-aligned backflow regions under the hairpin packets confined closer to the wall.
Omega motion, rolling, and active standing of a worm-inspired robot under the action of the magnetic field
Authors: Jin Yuchen , Li Jing , Liu Shiyang , Cao Gongqi , Liu Jianlin
Published: February 2024

With the rapid development of origami technologies, worm-inspired robots have attracted a great deal of attention due to their flexible locomotion characteristics. In the present work, we have prepared a soft robot inspired by the worms, which can achieve various locomotion patterns under the actuation of magnetic field. First, the origami technique is used to form the backbone of the robot, and two NdFeB discs are adhered on its two ends. Next, the experiments for controlling the Omega motion and rolling of the robot are performed, and the mechanical analyses are given. In the experiments, the Omega locomotion speed and rolling speed can reach ∼5 mm/s and 2π rad/s, respectively. Then, two typical examples on the composite motion, including the Omega motion and rolling, are demonstrated, where the robot can realize the tasks of sweeping objects and obstacle crossing in unstructured environments. We further design a system to mimic the situation when the worm-like robot detects and responds to the dangerous signal, and the power of the electromagnet can be accurately controlled. These findings cast a new light on engineering intelligent robots and devices originating from the inspirations of living creatures.
Photoinduced patterning of oxygen vacancies to promote the ferroelectric phase of Hf0.5Zr0.5O2
Authors: Beechem Thomas E. , Vega Fernando , Jaszewski Samantha T. , Aronson Benjamin L. , Kelley Kyle P. , Ihlefeld Jon. F.
Published: February 2024

Photoinduced reductions in the oxygen vacancy concentration were leveraged to increase the ferroelectric phase fraction of Hf0.5Zr0.5O2 thin-films. Modest (∼2−77 pJ/cm2) laser doses of visible light (488 nm, 2.54 eV) spatially patterned the concentration of oxygen vacancies as monitored by photoluminescence imaging. Local, tip-based, near-field, nanoFTIR measurements showed that the photoinduced oxygen vacancy concentration reduction promoted formation of the ferroelectric phase (space group Pca21), resulting in an increase in the piezoelectric response measured by piezoresponse force microscopy. Photoinduced vacancy tailoring provides, therefore, a spatially prescriptive, post-synthesis, and low-entry method to modify phase in HfO2-based materials.
Observation of a super-tetrahedral cluster of acetonitrile-solvated dodecaborate dianion via dihydrogen bonding
Authors: Peng Xiaogai , Cao Wenjin , Hu Zhubin , Yang Yan , Sun Zhenrong , Wang Xue-Bin , Sun Haitao
Published: February 2024

We launched a combined negative ion photoelectron spectroscopy and multiscale theoretical investigation on the geometric and electronic structures of a series of acetonitrile-solvated dodecaborate clusters, i.e., B12H122−·nCH3CN (n = 1–4). The electron binding energies of B12H122−·nCH3CN are observed to increase with cluster size, suggesting their enhanced electronic stability. B3LYP-D3(BJ)/ma-def2-TZVP geometry optimizations indicate each acetonitrile molecule binds to B12H122− via a threefold dihydrogen bond (DHB) B3–H3 ⁝⁝⁝ H3C–CN unit, in which three adjacent nucleophilic H atoms in B12H122− interact with the three methyl hydrogens of acetonitrile. The structural evolution from n = 1 to 4 can be rationalized by the surface charge redistributions through the restrained electrostatic potential analysis. Notably, a super-tetrahedral cluster of B12H122− solvated by four acetonitrile molecules with 12 DHBs is observed. The post-Hartree–Fock domain-based local pair natural orbital- coupled cluster singles, doubles, and perturbative triples [DLPNO-CCSD(T)] calculated vertical detachment energies agree well with the experimental measurements, confirming the identified isomers as the most stable ones. Furthermore, the nature and strength of the intermolecular interactions between B12H122− and CH3CN are revealed by the quantum theory of atoms-in-molecules and the energy decomposition analysis. Ab initio molecular dynamics simulations are conducted at various temperatures to reveal the great kinetic and thermodynamic stabilities of the selected B12H122−·CH3CN cluster. The binding motif in B12H122−·CH3CN is largely retained for the whole halogenated series B12X122−·CH3CN (X = F–I). This study provides a molecular-level understanding of structural evolution for acetonitrile-solvated dodecaborate clusters and a fresh view by examining acetonitrile as a real hydrogen bond (HB) donor to form strong HB interactions.
Magnon-mediated spin Hall magnetoresistance and unidirectional magnetoresistance in Pt/NiO/NiFe structures
Authors: Wang Bo , Zhang Quanzhi , Guo Yonghai , Li Wangda , Zhang Bo , Cao Jiangwei
Published: February 2024

Spin–orbit torque provides an efficient strategy for electric manipulation of magnetization. However, Joule heat accompanying with electron motion in the electron-mediated spin current result in unavoidable power dissipation. Moreover, the spin diffusion length in electron-mediated spin current is relatively short, preventing the transmission of spin information over long distances. Magnon-mediated spin current, without moving electrons, can be an excellent alternative to the conventional spin current. Magnon-mediated transfer torque effect has been reported in several previous works. Here, we report the magnon-mediated spin Hall magnetoresistance (SMR) and unidirectional magnetoresistance (UMR) in Pt/NiO/NiFe structures. The significant SMR and UMR were observed in the samples with the NiO thickness up to 60 nm, demonstrating the efficient transmission of magnon-mediated spin current over long distances in the NiO layer. In addition, we observed current-induced in-plane magnetization switching in the NiFe layer via the UMR measurement. These results demonstrated the possibility for developing the efficient spintronic devices operated by magnons.
Space-charge-induced colossal dielectric constant and large flexoelectricity in Nb-doped BaTiO3 ceramics
Authors: Li Hao , Wu Hao , Wang Zhiguo , Xie Zhengqiu , Shu Shengwen , Liu Zhiyong , Ke Shanming , Shu Longlong
Published: February 2024

Donor doping can increase the dielectric constant of a material by several orders of magnitude due to induced space charge causing interfacial polarization. Giant dielectric properties and interface polarization are also both expected to greatly enhance the flexoelectric behavior of a material. In this work, a typical flexoelectric ceramic material, BaTiO3, was selected and donor doped using elemental Nb. Compared with the nominal BaTiO3 ceramic, the dielectric constant and flexoelectric coefficient of the Nb-doped BaTiO3 ceramics were significantly improved. The transverse flexoelectric coefficient of 0.3 mol. % Nb-BaTiO3 was found to increase to nearly 40 times the nominal value, reaching 387 μC/m. The results indicate that the giant dielectric response, and therefore the giant flexoelectric response, is the result of the combined effects of internal barrier-layer capacitance and surface barrier-layer capacitance. This study not only deepens the understanding of the semiconductor macro-dielectric effect and the flexoelectric mechanism caused by doping, but it also provides a feasible strategy for the design of giant dielectric/flexoelectric response materials and related devices with high dielectric constants and flexoelectric coefficients.
Time evolution of the defect states at the surface of MoS2
Authors: Rana Dhan , Dahal Saroj , Sinkovic Boris
Published: February 2024

MoS2 has generated significant attention due to its unique electronic properties and versatile applications. Being a van der Waals material, MoS2 is expected to exhibit an inert surface due to lack of dangling bond. However, our photoemission study finds MoS2 to be highly sensitive toward residual gases. The position of the valence band maximum (VBM) shifts even in a vacuum of 10−10 Torr. We find this to be due to CO adsorption causing unintentional electron doping. The time evolution of the position of VBM is exponential, and it reaches two different saturation points, depending on whether the sample is exposed to ultraviolet (UV) radiation or not. Our XPS (x-ray photoemission spectroscopy) study shows no time-dependent escape of sulfur, which was in a previous study attributed to a VBM shift. The VBM shift can be reversed by annealing, sputtering, and UV light, which desorb CO gases. The study shows that the MoS2 surface is easily doped, which offers the possibility of using it as a sensor but in many other applications could diminish device performance and needs to be considered.
Raman-based mapping and depth-profiling of the relaxation state in amorphous silicon
Authors: Lussier A. W. , Bourbonnais-Sureault D. , Chicoine M. , Martel R. , Martinu L. , Roorda S. , Schiettekatte F.
Published: February 2024

We show that the micro-scale variations in the relaxation state of amorphous silicon (a-Si) can be well-identified by Raman mapping over hundreds or thousands of μm2 in 1–2 h. Pure and relaxed a-Si is obtained by self-implantation in crystalline silicon (c-Si) followed by anneal at 500 °C. It is then locally re-implanted over micro-sized patterns to produce unrelaxed a-Si zones. Raman mappings are obtained by pointwise confocal μ-Raman and hyperspectral Raman imaging. We also measure the depth profiles of the relaxation state in re-implanted a-Si by scanning the edge of a re-implanted sample. We infer from the depth profiles that the minimal damage dose to fully de-relax a-Si is 0.04 displacements per atoms, which is an order of magnitude smaller than the fluence needed to fully amorphize c-Si.
Epitaxial growth and characterization of multi-layer site-controlled InGaAs quantum dots based on the buried stressor method
Authors: Limame Imad , Shih Ching-Wen , Koltchanov Alexej , Heisinger Fabian , Nippert Felix , Plattner Moritz , Schall Johannes , Wagner Markus R. , Rodt Sven , Klenovsky Petr , Reitzenstein Stephan
Published: February 2024

We report on the epitaxial growth, theoretical modeling, and structural as well as optical investigation of multi-layer, site-controlled quantum dots fabricated using the buried stressor method. This deterministic growth technique utilizes the strain from a partially oxidized AlAs layer to induce site-selective nucleation of InGaAs quantum dots. By implementing strain-induced spectral nano-engineering, we achieve spectral control of emission and a local increase in the emitter density. Furthermore, we achieve a threefold increase in the optical intensity and reduce the inhomogeneous broadening of the ensemble emission by 20% via stacking three layers of site-controlled emitters, which is valuable for using the SCQDs as a gain medium in microlaser applications. Our optimization of site-controlled growth of quantum dots enables the development of high-β microlasers with increased confinement factor.
On the shock wave approximation to fractional generalized Burger–Fisher equations using the residual power series transform method
Authors: El-Tantawy S. A. , Matoog R. T. , Shah Rasool , Alrowaily Albandari W. , Ismaeel Sherif M. E.
Published: February 2024

The time-fractional generalized Burger–Fisher equation (TF-GBFE) has various applications across various scientific and engineering disciplines. It is used for investigating various phenomena, including the dynamics of fluid flow, gas dynamics, shock-wave formation, heat transfer, population dynamics, and diffusion transport, among other areas of research. By incorporating fractional calculus into these models, researchers can more effectively represent the non-local and memory-dependent effects frequently observed in natural phenomena. Due to the importance of the family of TF-GBFEs, this work introduces a changed iterative method for analyzing this family analytically to gain a deep understanding of many nonlinear phenomena described by this family (e.g., shock waves). The proposed approach combines two algorithms: the Laplace transform and the residual power series method. The suggested technique is thoroughly discussed. Two numerical problems are discussed to check the effectiveness and accuracy of the proposed method. The approximations for integer and fractional orders are compared with the exact solution for integer-order problems. Finally, to investigate how the fractional order affects these problems, the obtained results are discussed graphically and numerically in the tables.
Li-doping-modulated gelatin electrolyte for biodegradable electric-double-layer synaptic transistors
Authors: Chen Zhengquan , He Gang , Yang Bing , Zhu Minmin , Jiang Shanshan
Published: February 2024

Recent advances in the fabrication of neuromorphic electronic devices using biomaterials have received increasing attention for their prospect in biologically compatible and “green” electronic devices. Here, we proposed a novel electric-double-layer (EDL) synaptic transistor gated with Li-doped natural gelatin membranes, demonstrating larger capacitance at low frequency and extremely strong electrostatic modulation behavior. Experimental results have shown that a concentration of 2 wt. % Li-doping can obtain better electrical performance of EDL synaptic transistor. Ion/Ioff of 4.86 × 105 and threshold voltage of 0.68 V were obtained. Meanwhile, some important synaptic functions such as excitatory postsynaptic current, paired-pulse facilitation, and high-pass filtering characteristic have been successfully implemented. As a result, it can be inferred that the proposed gelatin-based natural polymer EDL electrolyte has demonstrated potential applications in “green” neuromorphic platforms.
Flow over a single dimple recessed in a flat plate
Authors: Zhu Jianxun , Tian Cai , Holmedal Lars Erik
Published: February 2024

Direct numerical simulations have been conducted to investigate a zero-pressure-gradient boundary layer flow over a single shallow dimple. Here, the dimple depth to dimple diameter ratio (d/D) as well as the Reynolds number (based on D and free-stream velocity) are fixed at 0.05 and 20 000, respectively. The effect of inlet boundary layer thickness δ on a given dimple is investigated by considering δ/D∈[0.023,0.1]. The flow within the dimple exhibits either a horseshoe vortex (a continuous core line through the two spirals within the dimple) or a tornado-like vortex pair (discontinuous core line). For the given parameter range, four different flow patterns have been identified within the single dimple: (i) a steady symmetric horseshoe vortex pattern for δ/D∈[0.053,0.1], (ii) a steady asymmetric horseshoe vortex pattern for δ/D=0.04, (iii) a quasi-periodic asymmetric horseshoe vortex pattern for δ/D=0.033, and (iv) a mixed horseshoe and tornado-like vortex pattern for δ/D=0.023. The growth of the streamwise vorticity, mainly caused by the tilting of the vertical vorticity, plays a key role in the transition between the different flow patterns. Dimple-induced velocity streaks above the single dimple have been investigated in detail for the first time, showing four different streaks: (i) a high-speed streak above the dimple, (ii) two side-low-speed streaks located outside the dimple span, (iii) two side-high-speed streaks, and (iv) a mid-low-speed streak in between them. These are mainly caused by a flow acceleration effect and a flow diffuser effect over the dimple, as well as a “lift-up” mechanism within the downstream part of the dimple, tilting the boundary layer upward.
Ridge instability in dense suspensions caused by the second normal stress difference
Authors: Xiong Zhongqiang , Angerman Peter , Ellero Marco , Sandnes Bjornar , Seto Ryohei
Published: February 2024

A dense suspension of the cornstarch flowing on a very inclined wall finally forms some ridge-like patterns of the free surface. The onset of pattern formation is the primary target to elucidate the mechanism. In this work, based on the continuity of fluids and the force balance, we show that the flat free surface is unstable when the second normal stress difference N2 is negatively proportional to shear stress and the gravity component perpendicular to the wall is weak enough. Such instability is inevitable for the growth of a ridge-like surface profile oriented parallel to the flow direction. We use the instability criterion to predict the critical slope angle for the formation of ridge patterns. The estimated critical angle was found to be in agreement with experimental observations for a cornstarch suspension.