Anisotropic Mechanical Properties of Orthorhombic SiP2 Monolayer: A First-Principles Study.

In recent years, the two-dimensional (2D) orthorhombic SiP2 flake has been peeled off successfully by micromechanical exfoliation and it exhibits an excellent performance in photodetection. In this paper, we investigated the mechanical properties and the origin of its anisotropy in an orthorhombic SiP2 monolayer through first-principles calculations, which can provide a theoretical basis for utilizing and tailoring the physical properties of a 2D orthorhombic SiP2 in the future. We found that the Young's modulus is up to 113.36 N/m along the a direction, while the smallest value is only 17.46 N/m in the b direction. The in-plane anisotropic ratio is calculated as 6.49, while a similar anisotropic ratio (~6.55) can also be observed in Poisson's ratio. Meanwhile, the in-plane anisotropic ratio for the fracture stress of the orthorhombic SiP2 monolayer is up to 9.2. These in-plane anisotropic ratios are much larger than in black phosphorus, ReS2, and biphenylene. To explain the origin of strong in-plane anisotropy, the interatomic force constants were obtained using the finite-displacement method. It was found that the maximum of interatomic force constant along the a direction is 5.79 times of that in the b direction, which should be considered as the main origin of the in-plane anisotropy in the orthorhombic SiP2 monolayer. In addition, we also found some negative Poisson's ratios in certain specific orientations, allowing the orthorhombic SiP2 monolayer to be applied in next-generation nanomechanics and nanoelectronics.

Magnon-phonon coupling: from fundamental physics to applications.

In recent decades, there are immense applications for bulk and few-layer magnetic insulators in biomedicine, data storage, and signal transfer. In these applications, the interaction between spin and lattice vibration has significant impacts on the device performance. In this article, we systematically review the fundamental physical aspects of magnon-phonon coupling in magnetic insulators. We first introduce the fundamental physics of magnons and magnon-phonon coupling in magnetic insulators and then discuss the influence of magnon-phonon coupling on the properties of magnons and phonons. Finally, a summary is presented, and we also discuss the possible open problems in this field. This article presents the advanced understanding of magnon-phonon coupling in magnetic insulators, which provides new opportunities for improving various possible applications.

Improved Adaptive Multipoint Optimal Minimum Entropy Deconvolution and Application on Bearing Fault Detection in Random Impulsive Noise Environments.

Random impulsive noise is a special kind of noise, which has strong impact features and random disturbances with large amplitude, short duration, and long intervals. This type of noise often displays nonGaussianity, while common background noise obeys Gaussian distribution. Hence, random impulsive noise greatly differs from common background noise, which renders many commonly used approaches in bearing fault diagnosis inapplicable. In this work, we explore the challenge of bearing fault detection in the presence of random impulsive noise. To deal with this issue, an improved adaptive multipoint optimal minimum entropy deconvolution (IAMOMED) is introduced. In this IAMOMED, an envelope autocorrelation function is used to automatically estimate the cyclic impulse period instead of setting an approximate period range. Moreover, the target vector in the original MOMED is rearranged to enhance its practical applicability. Finally, particle swarm optimization is employed to determine the optimal filter length for selection purposes. According to these improvements, IAMOMED is more suitable for detecting bearing fault features in the case of random impulsive noise when compared to the original MOMED. The contrast experiments demonstrate that the proposed IAMOMED technique is capable of effectively identifying fault characteristics from the vibration signal with strong random impulsive noise and, in addition, it can accurately diagnose the fault types. Thus, the proposed method provides an alternative fault detection tool for rotating machinery in the presence of random impulsive noise.

Enhancing the Curie Temperature in Cr2Ge2Te6 via Charge Doping: A First-Principles Study.

In this work, we explore the impacts of charge doping on the magnetism of a Cr2Ge2Te6 monolayer using first-principles calculations. Our results reveal that doping with 0.3 electrons per unit cell can enhance the ferromagnetic exchange constant in a Cr2Ge2Te6 monolayer from 6.874 meV to 10.202 meV, which is accompanied by an increase in the Curie temperature from ~85 K to ~123 K. The enhanced ratio of the Curie temperature is up to 44.96%, even higher than that caused by surface functionalization on monolayer Cr2Ge2Te6, manifesting the effectiveness of charge doping by improving the magnetic stability of 2D magnets. This remarkable enhancement in the ferromagnetic exchange constant and Curie temperature can be attributed to the increase in the magnetic moment on the Te atom, enlarged Cr-Te-Cr bond angle, reduced Cr-Te distance, and the significant increase in super-exchange coupling between Cr and Te atoms. These results demonstrate that charge doping is a promising route to improve the magnetic stability of 2D magnets, which is beneficial to overcome the obstacles in the application of 2D magnets in spintronics.

Opening a Band Gap in Biphenylene Monolayer via Strain: A First-Principles Study.

A biphenylene network is a novel 2D allotropy of carbon with periodic 4-6-8 rings, which was synthesized successfully in 2021. In recent years, although the mechanical properties and thermal transport received a lot of research attention, how to open the Dirac cone in the band structure of a biphenylene network is still a confused question. In this work, we utilized uniaxial and biaxial lattice strains to manipulate the electronic properties and phonon frequencies of biphenylene, and we found an indirect band gap under 10% biaxial strain through the first-principles calculations. This indirect band gap is caused by the competition between the band-edge state A and the Dirac cone for the conduction band minimum (CBM). Additionally, the lightest carrier's effective mass in biphenylene is 0.184 m0 for electrons along x (Γ→X) direction, while the effective mass for holes shows a remarkable anisotropy, suggesting the holes in the tensile biphenylene monolayer are confined within a one-dimensional chain along x direction. For phonon dispersion, we discovered that the Raman-active Ag3 phonon mode shows a robust single phonon mode character under both compressive and tensile strain, but its frequency is sensitive to lattice strain, suggesting the lattice strain in biphenylene can be identified by Raman spectroscopy.

Calibration method for parabolic reflector measurement by using reverse Hartmann test.

Careful calibration of system geometric position and camera errors is crucial to achieve high testing accuracy for large-scale aspherical surfaces. We propose a geometrical optical calibration method for increasing the overall accuracy of a large-scale reverse Hartmann test system to realize the parabolic reflector measurement. By using a flat crystal, a mask with uniform holes and an external pinhole aperture in front of the camera, camera lens distortions and keystone distortion, as well as system geometry correspondence between LCD screen, camera, and reflecting point on the test surface, are determined accurately. We experimentally demonstrated that the proposed calibration method achieves high slope detection accuracy for a large-scale parabolic reflector: tens of microradians within 1620(H)×850(V)mm2.

Collimation testing using slit Fresnel diffraction.

A simple collimation testing method based on slit Fresnel diffraction is proposed. The method needs only a CMOS and a slit with no requirement in dimensional accuracy. The light beam to be tested diffracts across the slit and forms a Fresnel diffraction pattern received by CMOS. After analysis, the defocusing amount and the distance between the primary peak point and secondary peak point of diffraction pattern fulfill an expression relationship and then the defocusing amount can be deduced from the expression. The method is applied to both the coherent beam and partially coherent beam, and these two beams are emitted from a laser and light-emitting diode (LED) with a spectrum width of about 50 nm in this paper. Simulations show that the wide spectrum of LED has the effect of smooth filtering to provide higher accuracy. Experiments show that the LED with a spectrum width of about 50 nm has a lower limitation error than the laser and can achieve up to 58.1601 µm with focal length 200 mm and slit width 15 mm.

Validation of reverse Hartmann test for mirror shape measurement of parabolic trough concentrator.

A method for the surface shape qualification of a parabolic trough solar concentrator, based on the reflection of sinusoidal fringes in the mirror surface and their distortions due to the surface deviations from ideal geometry, has been developed. Without complex calibration, accurate surface slope data were collected and reduced by using sixteen-step phase shifting methods. As the gradient data may not be ideally available for the entire surface, surface deviations and geometrical parameters were obtained through Zernike slope polynomials iterative fitting and the Southwell integration algorithm. Additionally, an absolute reference measurement for calibration of the measurement accuracy using three standard mirrors is presented. It is shown that the proposed method can easily test the surface shape and concentration efficiency of a solar concentrator with high slope measurement accuracy (less than 0.05 mrad in the x-direction) and high spatial resolution (more than 2.5 × 106 points per mirror facet).