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In this paper, we present a new module to predict the potential surface reconstruction configurations of given surface structures in the framework of our machine learning and graph theory assisted universal structure searcher. In addition to random structures generated with specific lattice symmetry, we made full use of bulk materials to obtain a better distribution of population energy, namely, randomly appending atoms to a surface cleaved from bulk structures or moving/removing some of the atoms on the surface, which is inspired by natural surface reconstruction processes. In addition, we borrowed ideas from cluster predictions to spread structures better between different compositions, considering that surface models of different atom numbers usually have some building blocks in common. To validate this newly developed module, we tested it with studies on the surface reconstructions of Si (100), Si (111), and 4H–SiC (1 1 ̄ 02) − c (2 × 2) , respectively. We successfully gave the known ground states, as well as a new SiC surface model, in an extremely Si-rich environment. [ABSTRACT FROM AUTHOR]

Electromagnetic (EM) metasurfaces comprising artificially designed subwavelength unit cells have drawn considerable attention due to the EM properties beyond the limits of natural materials. As one of the representative structures, active polarization conversion metasurface (APCM) is switchable by loading active components. It provides great freedom to manipulate the polarization state of EM waves. However, the current research mainly focused on the application of communication and paid less attention to the radar effect of APCM. APCM redistributes electromagnetic wave energy in multi-polarization channels, so it will have great application potential in polarimetric radar. Herein, based on the fully polarimetric radar one-dimensional high resolution range profile, the radar effect of time-modulated metasurface is studied. For this purpose, a method of target scattering mechanisms manipulation and a polarization-insensitive structure of APCM are proposed. The amplitude-phase joint modulation method is specifically analyzed in detail. The distance transformation and virtual multi-target phenomena are further discovered. Virtual targets along the distance dimension are generated in multi-polarization channels, while the scattering mechanisms of k-order targets are effectively manipulated. The relationship between the target scattering matrix and the modulation parameters is obtained. It may provide an effective method for the application of active metasurface in fully polarimetric radars. [ABSTRACT FROM AUTHOR]

Accurate measurement of trans-stenotic pressure drop is vital for risk stratification in coronary artery disease. Currently, in vivo pressure measurement relies mostly on a pressure guidewire inserted into the artery, which inevitably alters local hemodynamics. To precisely assess the impact of guidewire insertion on the accuracy of pressure measurement, this study conducts numerical simulations with both an idealized straight-tube model and a patient-specific model. Results with and without a guidewire model are compared and analyzed. Results from the idealized model reveal that the insertion of a guidewire shifts velocity distribution, increases resistance, and amplifies the pressure drop across the stenosis. The patient-specific model also demonstrates that the guidewire causes non-negligible flow redistribution among the arterial branches, but the influence on pressure drop remains mostly localized. An analytical model for trans-stenotic pressure drop that takes the guidewire into consideration is also proposed and validated against the 3D simulation results. It is observed that the maximum relative error is around 3.0% in the patient-specific model, indicating the effectiveness of the analytical model in physiologically accurate settings. The proposed model can be used to align computed tomography-derived fractional flow reserve values with clinically recognized FFR standards established through guidewire measurements. [ABSTRACT FROM AUTHOR]

Spectrum conversion involves the movement of a signal from one frequency to another, and it is well-studied in materials interacting with electromagnetic waves from microwave to optical frequencies. In this paper, a time-modulated active frequency selective surface (AFSS) absorber/reflector is proposed to manipulate the spectral distribution of the echo signal. The discrete harmonics based on the period modulation and the continuous spectrum based on the random coding modulation are analyzed in detail. In addition, a simple digital-coding control network based on field programmable gate array (FPGA) in the time-modulated AFSS absorber/reflector (TMAAR) is used to switch between absorbing and reflecting states flexibly. By coding "0" and "1" elements with the controlled sequences, the TMAAR is able to achieve the predefined modulation function for spectrum conversion. On this basis, a prototype of the designed TMAAR is manufactured. The linear frequency modulation radar echo experiments are performed to verify the required function of spectrum conversion. [ABSTRACT FROM AUTHOR]

Machine-learning (ML) potentials are promising in atomic simulations due to their comparable accuracy to density functional theory but much lower computational cost. The descriptors to represent atomic environments are of high importance to the performance of ML potentials. Here, we implemented the descriptor in a differentiable way and found that ML potentials with optimized descriptors have some advantages compared with the ones without descriptor optimization, especially when the training dataset is small. Taking aluminum as an example, the trained potentials with proper descriptors can not only predict energies and forces with high accuracy of the first-principles calculations but also reproduce the statistical results of dynamical simulations. These predictions validate the efficiency of our method, which can be applied to improving the performance of machine learning interatomic potentials and will also strongly expand its applications. [ABSTRACT FROM AUTHOR]

The WiFi technology has become a world-wide everyday service, and the accompanying electromagnetic pollution has attracted more and more attentions. This paper presents a transparent and flexible broadband metamaterial absorber (MMA) covering the whole WiFi waveband. The transparent and conducting material Indium Tin Oxide (ITO) was used for both the frequency select surface and the backplane ground layer, and polydimethylsiloxane (PDMS) was used as the dielectric layer for the microwave MMA. The absorber featured >80% absorption covering a wide frequency range of (2.2-5.85) GHz, which includes all the frequency bands of WiFi signal. The absorber was insensitive to the polarization angles and dropped little for incident angles less than 30°. Signal shielding experiments in 2.4 GHz and 5 GHz were carried out to demonstrate the application potential of the absorber. With additional advantages of optical transparency and mechanical flexibility, the broadband MMA can be useful in WiFi related applications in the future. [ABSTRACT FROM AUTHOR]

The propulsive wing propulsor (PWP), which means an underwater thruster equipped with a wing, a cross-flow fan (CFF), and a deflector, is capable of generating both horizontal thrust and vertical lift, thus enhancing the maneuverability of underwater vehicles and serving as a propulsion device. The hydrodynamic performance of the PWP is significantly influenced by the blade number it possesses. An unsteady numerical method based on the Reynolds-averaged Navier–Stokes equations was developed to examine the impact mechanism of blade number on the hydrodynamic performance, load fluctuation, and wake evolution of the PWP. The results indicate that as the blade number increases, the hydrodynamic forces, power, and propulsive efficiency of the PWP gradually increase. When the blade number exceeds 26, the performance of the PWP tends to stabilize. Insufficient blades can lead to turbulence in the internal flow of the CFF, intensifying interference between blade vortices, resulting in secondary peaks and frequency-domain bifurcations in hydrodynamics. With an increasing blade number, disturbances to the blade vortices decrease, enhancing the periodicity of PWP hydrodynamic fluctuations, but there may be an increase in high-frequency noise levels. The wake modes of the PWP undergo four transitions: double vortex pair mode, single vortex pair mode, single vortex pair + single vortex mode, and vortex strip mode. Disturbed blade vortices promote the transition of vortex pair shedding modes in the PWP wake, thereby causing variations in the periodicity of PWP hydrodynamics. Excessive amplitude and frequency may lead to structural fatigue damage in the PWP. [ABSTRACT FROM AUTHOR]

Inspired by the single-bonded nitrogen chains stabilized by tetravalent cerium, pentavalent tantalum, and hexavalent tungsten atoms, we explored the possibility of single-bonded nitrogen polymorphs stabilized by trivalent lanthanum ions. To achieve this, we utilized the crystal structure search method on the phase diagram of binary La–N compounds. We identified three novel thermodynamically stable phases, the C2/c LaN3, P-1 LaN4, and P-1 LaN8. Among them, the C2/c phase with infinite helical poly-N6 chains becomes thermodynamically stable above 50 GPa. Each nitrogen atom in the poly-N6 chain acquires one extra electron, and the spiral chain is purely single-bonded. The C2/c phase has an indirect band gap of ∼1.6 eV at 60 GPa. Notably, the band gap exhibits non-monotonic behavior, decreases first and then increases with increasing pressure. This abnormal behavior is attributed to the significant bonding of two La–N bonds at around 35 GPa. Phonon spectrum calculations and AIMD simulations have confirmed that the C2/c phase can be quenched to ambient conditions with slight distortion, and it exhibits excellent detonation properties. Additionally, we also discovered armchair-like nitrogen chains in LaN4 and the armchair and zigzag-like mixed nitrogen chains in LaN8. These results provide valuable insights into the electronic and bonding properties of nitrides under high pressure and may have important implications for the design and development of novel functional materials. [ABSTRACT FROM AUTHOR]

The crucial role of interface engineering in controlling functionality has been well documented in the (001)-orientated perovskite oxide heterostructures; however, limited attention has been given to other epitaxial orientations. In this study, the (La,Sr)MnO3 heterostructure is selected to present the epitaxial-orientation-dependent interface engineering strategies based on magnetoelectric coupling and oxygen octahedral coupling. When capped by a ferroelectric PbTiO3 layer for interlayer magnetoelectric coupling, the (001) (La,Sr)MnO3 layer exhibits the most significant magnetic modulations compared to (110) and (111) counterparts. This can be attributed to the PbTiO3's out-of-plane polarization that depends on the epitaxial orientations. Also, if considering the interfacial oxygen octahedral structure in terms of epitaxial orientations, the buffer (or cap) layer hosts the most part of interfacial oxygen octahedra and, thus, plays a dominant role in controlling oxygen octahedral coupling of (001)-orientated [or (110)-orientated] perovskite heterostructures. This is consistent with our observations that for the (001) [or (110)] (La,Sr)MnO3 heterostructures, the DyScO3 buffer layer offers the more (or less) effective magnetic modulations compared to the cap one. These findings underscore the orientation-dependent nature of interface engineering in manganite heterostructures as well as in other perovskite heterostructures for tailoring functionalities. [ABSTRACT FROM AUTHOR]

We present our latest advancements of machine-learned potentials (MLPs) based on the neuroevolution potential (NEP) framework introduced in Fan et al. [Phys. Rev. B 104, 104309 (2021)] and their implementation in the open-source package gpumd. We increase the accuracy of NEP models both by improving the radial functions in the atomic-environment descriptor using a linear combination of Chebyshev basis functions and by extending the angular descriptor with some four-body and five-body contributions as in the atomic cluster expansion approach. We also detail our efficient implementation of the NEP approach in graphics processing units as well as our workflow for the construction of NEP models and demonstrate their application in large-scale atomistic simulations. By comparing to state-of-the-art MLPs, we show that the NEP approach not only achieves above-average accuracy but also is far more computationally efficient. These results demonstrate that the gpumd package is a promising tool for solving challenging problems requiring highly accurate, large-scale atomistic simulations. To enable the construction of MLPs using a minimal training set, we propose an active-learning scheme based on the latent space of a pre-trained NEP model. Finally, we introduce three separate Python packages, viz., gpyumd, calorine, and pynep, that enable the integration of gpumd into Python workflows. [ABSTRACT FROM AUTHOR]

Palatal defect is a common maxillofacial defect after maxillectomy that can be repaired by obturator prostheses, which can effectively improve patients' speech. However, comprehensive evaluation methods for speech recovery are still controversial and remain undefined. A prospective cohort study on 34 patients with palatal defect and 34 healthy controls was performed. Patients received obturator prostheses and their speech was recorded without and then with obturators. Participants pronounced six Chinese vowels and 100 syllables for recording. This paper evaluated the recovery of speech function of patients through the combination of subjective and objective assessment methods. Recruited listeners evaluated the speech intelligibility (SI) of 100 syllables. Vowel formant frequency and quantified vowel nasalization were measured using analysis software. The SI of patients improved significantly after wearing obturators. F2 values of six vowels in patients with obturators were higher than patients without obturators and close to the corresponding values in normal controls. The differences in F2 of /i/ and /u/, (A1-P1) of /i/ and /u/ for patients without and with obturator use were significant. Patients' ability to control the pronunciation of /i/ and /u/ improved greatly with obturators. These provide clinical evidence of the treatment outcomes in patients with palatal defect who received obturators. [ABSTRACT FROM AUTHOR]

The propulsive wing is a new concept wing of automatic propulsion with high lift coefficients and has great application value in plant protection and forest fire control. The propulsive wing wake is a reverse Bénard–von Kármán (RBvK) vortex street, which is considered a thrust-generating wake. The wake structure will change greatly at high angles of attack and lead to changes in the aerodynamic performance of the propulsive wing. To explore the optimal working range and the wake characteristics of the propulsive wing, the wake transition and aerodynamic efficiency of the propulsive wing in cruise are numerically studied. The results indicate that there are three types of structures for the propulsive wing wake. When α ≤ 20°, the wake transits from the RBvK vortex street to the critical state with the increase in cruise speed, and the Strouhal number approaches 1.9. The critical wake region decreases gradually with the increase in the angle of attack. The maximum propulsive efficiency is 0.17 at a cruise speed of 15 m/s. When α > 20°, the wake transits directly from the RBvK vortex street to the Bénard–von Kármán (BvK) vortex street and the Strouhal number approaches 0.34. The maximum propulsive efficiency appears at a cruise speed of 10 m/s, which is close to the BvK vortex street boundary. Before entering the stall state, the lift efficiency of the propulsive wing increases with the increase in cruise speed and angle of attack, up to 3–5. [ABSTRACT FROM AUTHOR]

Spallation is a typical dynamic fracture mode under shock loading and has attracted the attention of most researchers. However, due to the difficulty in measuring temperature in dynamic experiments, the effect of initial temperature on spalling response has been rarely investigated. Molecular dynamics simulation perfectly corresponds to the short duration and high strain rate of the spalling process. Therefore, in this work, molecular dynamics simulations are used to study the spalling reaction of single-crystal aluminum at different initial temperatures. The research has shown that the evolution of spallation is related to dislocation and hole nucleation. First, the spall strength of the material decreases as initial temperature increases, while the dislocation density gradually increases. However, when the initial temperature increases to 750 K, the dislocation density decreases. Then, the number of holes and the degree of damage change as initial temperature increases. However, at the low impact strength (v < 2.0 km/s), the changes in the number of holes and the degree of damage are highly dependent on the initial temperature. In the case of high impact strength, the opposite is true. Finally, the thermodynamic path of the material during impact compression is studied. It is found that melting may occur during compression, release or tension, and damage stages, depending on the initial temperature and impact strength. The discovery and research of these systems have laid a solid foundation for subsequent studies. [ABSTRACT FROM AUTHOR]

Spherical microcavities with low optical loss and high quality factor are attractive candidates for functional applications, including whispering gallery mode lasers, intracellular lasers, ultrasensitive biosensors, etc. In particular, electrically driven devices are beneficial for several applications such as optoelectronic integration. However, it is difficult to realize the electrically driven optoelectronic functions. In this study, the high-quality ZnO microspheres are fabricated by using a laser ablation method, and an excellent ultraviolet lasing system is achieved by optical pumping. Moreover, ultraviolet electroluminescence analysis demonstrates that ZnO microspheres are bonded to the p-GaN substrate. Furthermore, distinct resonant modes are obtained after inserting an appropriate AlN buffer layer at the n-ZnO/p-GaN interface. The optoelectronic behavior, charge carrier transportation, and recombination mechanism of the structure are systematically analyzed, and related interface modification, energy band alignment, and optical gain/loss contrast are also discussed. Overall, the results provide useful insights on further exploration of electrically pumped ZnO microsphere lasers. [ABSTRACT FROM AUTHOR]

Cryocoolers play a vital role in ground low-temperature experiments and space explorations. The regenerator is an important part of the cryocooler, which is used to store the cold produced by the expansion of the working fluid helium and absorb the heat produced during the next cycle of compression. However, the specific heat of helium at temperatures below 10 K is higher than that of almost all currently known regenerator materials, resulting in insufficient cold storage capacity and severely restricting the improvement of the cryocooler cooling performance. In this paper, the study of using porous material to absorb helium as the regenerator material for cryocoolers is carried out. An experimental system was built to test the helium adsorption capacity of super activated carbon in the temperature range of 3–10 K, and it was found that the activated carbon has a significant increase in specific heat compared with the current conventional materials. A thermodynamic calculation model was further established to investigate the dynamic cryogenic adsorption characteristics and the effect of adsorption/desorption heat on the performance of the regenerator. This work is an exploration in the research of low-temperature regenerators, which can provide useful help for the selection and use of regenerator materials for cryocoolers working below 10 K. It also has great application prospects in the fields of energy storage, low-temperature superconductivity, and sub-K refrigeration systems. [ABSTRACT FROM AUTHOR]

The stability of solid–liquid interfaces during solidification is a physical phenomenon of fundamental interest with a wide range of practical applications, including the freezing of biological matter for medical and agricultural purposes. Much of the classical research in this field treats solidification in isobaric (constant-pressure) systems in which the phase transition typically occurs under constant atmospheric pressure. Recent research has found, however, that freezing in isochoric (constant-volume) systems in which the solidifying material is confined within a high-strength constant-volume chamber held at subfreezing temperatures gives rise to many atypical physical phenomena, and understanding of the solid–liquid interface behavior under isochoric conditions is currently lacking. In this work, we study the stability and propagation of the solid–liquid interface during isochoric freezing of aqueous solutions. Using a mathematical model of heat and mass transfer during solidification coupled with multiple criteria for predicting the emergence of interfacial instabilities based on temperature and concentration gradients in the phase transition region, we find that isochoric freezing significantly stabilizes the solid–liquid interface relative to isobaric freezing, suggesting the potential for extended growth of planar, non-dendritic interfaces. [ABSTRACT FROM AUTHOR]

This study analyzes intrapulse nonuniform amplitude distortion of the LFM (linear frequency modulated) pulse propagating through a plasma by conducting theoretical analysis and experiments. The FDTD-IFFT method is proposed to calculate the propagation of the linear frequency modulated (LFM) signal in plasma, and the nonuniform pulse amplitude distortion is verified by a comparison of experimental and theoretical results. The plasma will cause amplitude distortion of the signal which is characterized by two metrics: (1) the pulse amplitude difference Δ G and (2) the pulse amplitude nonuniformity degree ψ values. The degree of nonuniform distortion is determined by the carrier frequency, the bandwidth, electron density, and the collision frequency. The results shows that when the carrier frequency is close to the cut-off frequency of electron density, Δ G and ψ reach positive peak values, the amplitude shows an increasing trend, and pulse amplitude nonuniformity distortion is most serious. Moreover, Δ G is negative when the cut-off frequency of electron density is far greater than the carrier frequency, and the collision frequency is high. The pulse amplitude shows a decreasing trend under this condition. The higher the electron density and the collision frequency, the larger the ψ value and the absolute value of negative Δ G and the more serious intrapulse nonuniform distortion. The intrapulse distortion will cause a signal to noise ratio variation in a pulse which will seriously affect the signal detection. The conclusions of this study are crucial for the application of electronic systems on vehicles under a plasma and for deepening the understanding of the effect of plasma on electromagnetic waves. [ABSTRACT FROM AUTHOR]

The Stirling-type cryocooler with its compact size and high efficiency is always expected to obtain its temperature limit of below 3 K. However, the pressure drop losses caused by high-frequency oscillation create large obstacles for this objective. This paper reports a novel thermal-driven Stirling-type cryocooler to obtain the lowest temperature of a Stirling-type cryocooler. The advantages of a thermal-driven cryocooler (Vuilleumier cryocooler) and pulse tube cryocooler are combined with a new type of cryocooler, called the Vuilleumier gas-coupling pulse tube hybrid cryocooler (VM-PT). A prototype of the VM-PT was recently developed and optimized in our laboratory. By using helium-4 as the working gas and magnetic regenerative materials (HoCu2 and Er3Ni), the lowest temperature of 2.5 K was obtained, which can be regarded as an important breakthrough for the Stirling-type cryocooler to achieve its limit temperature of below 3 K. It can supply >30 mW cooling power at 4.2 K and >500 mW cooling power at 20 K simultaneously. Theoretically, it is feasible to use this VM-PT for cooling the superconducting devices in space applications. [ABSTRACT FROM AUTHOR]