ALBERT

Description: MnBi 2 n Te 3 n + 1 (MBT) is the first intrinsic magnetic topological insulator and is promising to host emergent phenomena such as quantum anomalous Hall effect. They can be made ferromagnetic by having n   ⩾   4 or with Sb doping. We studied the magnetic dynamics in a few selected ferromagnetic (FM) MBT compounds, including MnBi8Te13 and Sb doped MnBi 2 n Te 3 n + 1 ( n = 2 , 3 ) using AC susceptibility and magneto-optical imaging. Slow relaxation behavior is observed in all three compounds, suggesting its universality among FM MBT. We attribute the origin of the relaxation behavior to the irreversible domain movements since they only appear below the saturation fields when ferromagnetic domains form. The very soft ferromagnetic domain nature is revealed by the low-field fine-structured domains and high-field sea-urchin-shaped remanent-state domains imaged via our magneto-optical measurements. Finally, we ascribe the rare ‘double-peak’ behavior observed in the AC susceptibility under small DC bias fields to the very soft ferromagnetic domain formations.

Description: Although many studies have been done and advanced progress has been made in understanding partial discharge (PD) behavior in the void, this is not the case firinception of PD, especially its stochastic nature. The statistical behaviors of PD inception voltage (PDIV) and inception time delay (PDTD) inside the void were investigated in this study through repeated tests to observe the stochastic nature of PD inception. The results show that the PDIV and PDTD of the void are highly dispersed and obey Weibull and exponential distributions, respectively. The significant dispersion of PDIV can be attributed to the statistical time delay of PD inception. The lengthy inception delay is attributable to a lack of free electrons. The exponential distribution of PDTD indicates that free-electron generation is completely random; further, the stochastic nature of void PD inception is determined by the supply of free electrons. The test method (voltage rise rate, test time, and test time interval), void parameters (size, material, and surface condition), and background radiation determine PD inception by affecting the volume ionization or surface-emission process providing free electrons. Enhanced background ionization or significant increase in test voltage and test time allow for the effective detection of void defects during PD tests. This work contributes to an empirical understanding of the physical process of PD inception in voids and improving existing PD testing technologies.

Description: The anisotropic droplet formulation is generalized from hydrophilic to superhydrophobic surfaces. An experimental method to calibrate the ellipsoidal droplet volume on both hydrophilic and hydrophobic surfaces is presented. A broad range of contact angles (CAs) is produced on the copper and stainless-steel surfaces using femtosecond laser patterning. The effects of line spacing between the laser scanning on the formation of anisotropic CAs are discussed. The comparative study of the evolution of anisotropic CAs and droplet’s spreading dynamics are studied on both surfaces. According to the triple contact line (TCL) theory, CAs are determined by the TCL between droplet and surface rather than the contact area. We presented the mathematical formalism and the experimental validity of the TCL theory on ellipsoidal droplets over a broad range of CAs, from as low as 37°–172°. This work experimentally validated the TCL theory over a broad range of CAs with good confidence.

Description: The generation of a large cold plasma jet while maintaining the reproducibility and homogeneity of the discharge is one of the major challenges encountered by the plasma community to efficiently apply this technology in the industry. Here, we report on the discharge in a recently developed device called the plasma candle (PC), wherein a stable plasma jet with a diameter of 20 mm can be generated at atmospheric pressure and temperature. Unlike the discharge morphology previously reported for conventional plasma jet devices, the unique configuration of PC device resulted in distinctive discharge patterns. Homogenous discharge was generated in the electrode gap and followed by a swirling discharge toward the tube nozzle. Fast photography and electrical measurements revealed that filament propagation and its morphology form the visually observable swirl discharge. Detailed analysis indicated that residual helium metastable species (Hem) and their penning ionization play an essential role in the discharge mode and its transition, which was verified by changing the feeding gas and the frequency of the applied voltage. For instance, it is found that only filamentary discharge was observed along the entire tube at frequencies less than 3 kHz, at which the time between consecutive discharges was long enough for Hem decay. Consequently, the homogenous discharge pattern was recovered by increasing the pre-ionization levels by adding a trace of impurities (N2, O2 or H2O) to the feeding gas. However, the level of these impurities must be carefully adjusted to achieve a homogenous discharge without negatively affecting the jet properties. A trivial change in the gas impurity, in the range of adsorption and desorption of water from the gas tubing, is sufficient to cause a noticeable change and instability in the discharge mode. This finding is critical to predicting the production of reactive species and plasma-surface interaction for different applications.

Description: Electronic skin (E-skin) has attracted much attention in smart wearables, prosthetics, and robotics. The capacitive-type pressure sensor is generally regarded as one good option to design tactile sensing devices owing to its superior sensitivity in low-pressure region, fast response time and convenient manufacturing. Introducing microstructures on electrode surface is an effective approach to achieve highly sensitive capacitive pressure sensors. In this work, an electromechanical model is proposed to build the relationship between capacitance change and compressive force. The present model can predict the sensitivity of capacitive pressure sensor with microstructured electrodes, where each cellular microstructure is modeled using the contact mechanics theory. It is the first time in the literature that based on Hertz theory framework, one rigorous electromechanical theory framework is established to model flexible capacitive pressure sensor, and the model can be extended to other microstructures, such as micro-pyramid, micro-pillar, and micro-dome array. The validation indicates that the analytical results well agree with the experimental data from our previous work and other literatures. Moreover, the present model can well capture the sensitivity of pressure sensor on the beginning range of small pressure. The sensitivity on this range is the most significant for the E-skin due to its robust linearity for one pressure sensor. Besides, we analyzed the compressive force-displacement relationship, the compressive force-contact radius relationship and the influences of the geometrical and material parameters on the electromechanical coupling effect. The results show that the height and the Young’s modulus of the soft dielectric layer are regarded as the dominant influencing factors in the sensitivity of capacitive pressure sensors.

Description: The wide variety of silicon materials used by various groups to investigate LeTID make it difficult to directly compare the defect concentrations (Nt) using the typical normalised defect density (NDD) metric. Here, we propose a new formulation for a relative defect concentration (β) as a correction for NDD that allows flexibility to perform lifetime analysis at arbitrary injection levels (Δn), away from the required ratio between Δn and the background doping density (Ndop) for NDD of Δn/N dop = 0.1. As such, β allows for a meaningful comparison of the maximum degradation extent between different samples in different studies and also gives a more accurate representative value to estimate the defect concentration. It also allows an extraction at the cross-over point in the undesirable presence of iron, or flexibility to reduce the impact of modulation in surface passivation. Although the accurate determination of β at a given Δn requires knowledge of the capture cross-section ratio (k), the injection-independent property of the β formulation allows a self-consistent determination of k. Experimental verification is also demonstrated for boron-oxygen related defects and LeTID defects, yielding k-values of 10.6 ± 3.2 and 30.7 ± 4.0, respectively, which are within the ranges reported in the literature. With this, when extracting the defect density at different Δn ranging between 1014 /cm3 to 1015 /cm3 with Ndop = 9.1 ×1015 /cm3, the error is less than 12% using β, allowing for a greatly improved understanding of the defect concentration in a material.

Description: As an efficient approach to improve the visibility, defogging technology is essential for the operation of ports and airports. This paper proposes a new and hybrid defogging technology, i.e. electric–acoustic defogging method. Specifically, the droplets are charged by corona discharge, which is beneficial to overcome the hydrodynamic interaction force to improve the droplet collision efficiency. Meanwhile, sound waves (especially acoustic turbulence) promote the relative movement of droplets to increase the collision probability. In this study, the effects of acoustic frequency ( f ), sound pressure level (SPL), and voltage (V) on the droplet growth ratio were studied by orthogonal design analysis. The results of difference analysis and multi-factor variance analysis show that frequency and sound pressure level are the dominant factors that affect the collision of droplets, and the effect of voltage is relatively weak. And f = 400 Hz, SPL = 132 dB, and V = -7.2 kV are the optimal parameters in our experiment. In addition, we further studied the impact of single factor on droplet growth ratio. The results show that there is an optimal frequency of 400 Hz. That is, the impact of frequency is non-linear. The droplet growth ratio increases with sound pressure level and voltage level. The new technology proposed in this paper can provide a new approach for defogging in open space.

Description: Measurements of frequency dependent ferromagnetic resonance (FMR) and spin pumping driven dc voltage (Vdc) are reported for amorphous films of Fe78Ga13B9 (FeGaB) alloy to address the phenomenon of self-induced inverse spin Hall effect (ISHE) in plain films of metallic ferromagnets. The Vdc signal, which is antisymmetric on field reversal, comprises of symmetric and asymmetric Lorentzians centered around the resonance field. Dominant role of thin film size effects is seen in setting the magnitude of static magnetization, Vdc and dynamics of magnetization precession in thinner films (≤ 8 nm). The film thickness dependence of magnetization parameters indicates the presence of a magnetically disordered region at the film – substrate interface, which may promote preferential flow of spins generated by the precessing magnetization towards the substrate. However, the Vdc signal also draws contributions from rectification effects of a ≈ 0.4 % anisotropic magnetoresistance and a large (≈ 54 nΩ.m) anomalous Hall resistivity (AHR) of these films which ride over the effect of spin – orbit coupling driven spin-to-charge conversion near the film – substrate interface. We have addressed these data in the framework of the existing theories of electrodynamics of a ferromagnetic film subjected to radio-frequency field in a coplanar waveguide geometry. Our estimation of the self-induced ISHE for the sample with 54 nΩ.m AHR shows that it may contribute significantly (≈ 90%) to the measured symmetric voltage. This study is expected to be very useful for fully understanding the spin pumping induced dc voltages in metallic ferromagnets with disordered interfaces and large anomalous Hall effect.

Description: Broadband sound energy enhancement is essential in practical scenarios, such as acoustic positioning and acoustic communication. In this paper, a dual anisotropic metamaterial composed of an inner Mie resonator and an outer acoustic grating is proposed, aiming to achieve enhanced broadband monopole emission and acoustic energy harvesting (AEH) via the coupling of the first and second monopole resonances. Considering thermo-viscous dissipation, numerical simulations and experimental results demonstrate that the dual anisotropic metamaterial can realize omnidirectional enhanced broadband monopole emission at 795 Hz–1511 Hz, the maximum sound pressure level (SPL) gain is 16.4 dB and the SPL gain fluctuation is 3 dB. Furthermore, simulation results reveal that the broadband AEH can be achieved by the dual anisotropic metamaterial, the fluctuation of the SPL gain at 794 Hz–1537 Hz is 3 dB and the maximum is 14.7 dB. Based on the results, the dual anisotropic metamaterial is expected to show significant potentials in acoustic positioning and acoustic communication.

Description: Two-dimensional (2D) materials, due to their unique electronic, optical and structural properties, have attracted extensive attention of researchers in the world. However, most of 2D materials have low optical absorption efficiencies in the visible and near-infrared regimes, which leads to the weak light–matter interaction and limits their further applications in optoelectronic devices. Thus, enhancing the light–matter interaction of various 2D materials in the visible and near-infrared regimes, has been a key topic for many optoelectronic equipment and related applications. In this topical review, we summarized the recent developments of the 2D materials-based optical absorbers in the visible and near infrared regimes, focusing mainly on the methods and relevant physical mechanisms of several typical perfect absorbers, such as narrowband perfect absorbers, dual-band perfect absorbers, and broadband perfect absorbers. Finally, several prospective research directions from our perspectives are presented at the end.