Journal of Physics D: Applied Physics

Rahaman, Mainur; Longchar, Lanuakum A; Lalita, ; Basheed, G A; Reddy, V Raghavendra; Raja, M Manivel; Kaul, S N; Srinath, S

doi: 10.1088/1361-6463/ad4561pmid: N/A

Ferromagnetic resonance and magneto-optic Kerr effect (MOKE) techniques are employed to unravel the nature of ‘in-plane’ (IP) magnetic anisotropy and magnetization reversal (MR) processes in magnetron-sputtered 100 nm Co2Fe0.5Ti0.5Si (CFTS) thin films, deposited (and subsequently annealed) at different substrate temperatures (Ts) ranging from 200 °C to 550 °C. By varying TS, the CFTS films are produced with different amounts of anti-site (AS) atomic disorder. Irrespective of the degree of AS disorder, the IP uniaxial magnetic anisotropy (UMA) is prevalent in all the CFTS films. The TS450 and TS500 films, deposited at TS = 450 °C and 500 °C, stand out as they have (i) the least AS disorder, (ii) lowest value (α = 0.0055) of the Gilbert damping constant, (iii) high saturation magnetization (≅770 G) at 300 K, (iv) UMA energy density as high as ≅1.6×104erg/ccat 120 K dropping to ≅1.0×104erg/ccat 300 K, (v) spin-wave stiffness (D) at T = 0 K, D0≅175 meVÅ2and, as in other CFTS films, the electron–magnon interaction is primarily responsible for the thermal renormalization of D. Furthermore, in the TS450 and TS500 films, MOKE hysteresis loops at various angles (ψH) that the field makes with the easy axis in the film plane, reveals two mutually exclusive UMAs, UMA1≫UMA2, with easy axes perpendicular to each other. By contrast, only a single UMA is observed in all the remaining films. When ψH=0∘, MOKE domain images reveal that, consistent with the uniaxial nature of magnetic anisotropy, an abrupt reversal of magnetization is accompanied by the formation of 180° domains. When ψH= 90°, due to the presence of two UMAs in TS450 and TS500 films, following the field reversal, first the reverse domains nucleate and then grow at the expense of the fundamental domains through field-induced domain wall motion.

Tian, Shuangshuang; Wang, Jiahao; Wu, Yingyu; Hu, Feng; Luo, Yongchao; Zhang, Chaohai; Gao, Shen; Yuan, Zian

doi: 10.1088/1361-6463/ad436apmid: N/A

Phase change cooling technology offers high cooling efficiency, safety, and reliability, representing a novel approach to achieving efficient heat dissipation for high-power and large-capacity electrical equipment. The formulation of the cooling medium is pivotal to phase change cooling technology. However, current media exhibit compatibility, stability, economy, and environmental friendliness deficiencies. Consideration could be given to implementing the C6F12O medium due to its superior overall performance and ability to meet the latent heat requirements in phase change cooling equipment. This paper employs a numerical simulation approach that combines the phase field method based on the Cahn-Hilliard equation with the theory of electrohydrodynamics. It investigates the impact of temperature, electric field intensity, and electric field direction on the evolution of bubble motion and the boiling state of the C6F12O medium, considering the interaction of electric-fluid-heat-phase fields. Numerical results indicate that the system undergoes initial nucleate boiling, nucleate boiling, and film boiling stages at T = 330–335 K, T = 335–350 K, and T ⩾ 355 K, respectively. The introduction of an appropriate electric field can enhance the motion evolution of C6F12O bubbles. However, attention must be paid to the formation of bubble channels under high field strength to prevent potential decreases in insulation performance. An inhomogeneous electric field in the vertical direction proves more effective in improving the bubble release rate compared to a uniform electric field. To some extent, an inhomogeneous electric field in the horizontal direction can prevent the mass accumulation of bubbles in regions of high field intensity. This research has the potential to offer theoretical guidance for the engineering application of the C6F12O phase change cooling medium.

Martín Valderrama, Carmen; Prieto, Irene; Quintana, Mikel; Berger, Andreas

doi: 10.1088/1361-6463/ad4560pmid: N/A

We have experimentally studied the relationship in between non-collinear magnetization states in ferromagnetic (FM) multilayers and their resulting magneto-optical (MO) properties. Hereby, we observe that the phase of the complex-valued MO parameters are especially sensitive towards non-collinear magnetization states and enable their unambiguous detection. For the purpose of our experimental study, we designed, fabricated and characterized a set of epitaxial FM/NM/FM multilayers with in-plane uniaxial anisotropy, in which the non-magnetic (NM) interlayer thickness t was varied, so that tunable FM interlayer exchange coupling strength in between the two FM layers could be achieved. Furthermore, the two FM layers were made from different alloys, so that they exhibit different levels of magnetocrystalline anisotropy, which enables a collinear to non-collinear magnetization state transition upon applying a magnetic field Haway from the in-plane easy axis for samples with sufficiently large t. Utilizing generalized MO ellipsometry, we determined the full reflection matrix Ras a function of Hand we observed that the phases of the complex-valued MO coefficients in Rchange with Hin multilayers that have sufficiently weak interlayer coupling strength, i.e. large t, which can only happen if non-collinear magnetization states of varying non-collinearity occur in those samples. For samples with small t, corresponding to strongly exchange coupled FM layers, this effect is absent, consistent with the existence of collinear magnetization states in those multilayers for all Hvalues.

Li, Shangru; Ding, Fan; Yang, Yuejie; Cheng, Houyuan; Fu, Yang; Yang, Helin

doi: 10.1088/1361-6463/ad4715pmid: N/A

A hybrid mechanism water-based metamaterial (HMWM) with polarization conversion, absorption and phase cancellation mechanisms is proposed in this paper. The absorption and polarization conversion mechanisms are integrated by combining the water layer with polarization conversion structure, and the absorption conversion rate reaches more than 90% in the dual band (4.0–4.7 GHz and 8.2–15.7 GHz). Based on the above mechanism, the phase cancellation mechanism makes use of the opposite phase between HMWM and its mirror structure for checkerboard configuration to reduce the wideband radar cross section (RCS) by 3–18 GHz, achieving wide-angle RCS reduction and polarization insensitivity. In addition, the model realizes stealth control by adjusting RCS reduction capability under different water layer conditions. The results of simulation and experiment agree well, which fully demonstrates that the HMWM has scattering suppression capability and has potential application in multifunctional metamaterial.

Zhou, Yafen; Yu, Tieliang; Shen, Zhaoyang; Liu, Xinqiong; Zhang, Qinghe

doi: 10.1088/1361-6463/ad465epmid: N/A

This paper proposes, measures, and investigates a bifunctional metamaterial capable of achieving absorption and reflective linear-to-linear polarization conversion simultaneously, which both exhibit the characteristics of broadband. The unit cell consists of a metal pattern with resistors, a dielectric plate, an air layer, and a metal backplate. The simulation results demonstrate that the designed metamaterial acquires over 90% absorption in the microwave band of 6.5–9.3 GHz. Within the frequency range of 12.7 GHz–17.2 GHz, the polarization conversion rate exceeds 90%, effectively converting y-polarized incident waves into x-polarized reflected waves. The experimental results align with the simulation data. The surface current and electric field distributions are utilized to analyze the absorption and polarization conversion phenomena. This bifunctional metamaterial exhibits potential application in radar imaging, enhancing data transmission rates, and wireless communication.

Hui, Zhan; Wang, Min; Wang, Jiacheng; Chen, Jialu; Yin, Xiang; Yue, Yunliang

doi: 10.1088/1361-6463/ad460fpmid: N/A

Understanding the unique properties of perovskite materials is crucial in advancing solar energy technologies. Factors like heat of formation and bandgap significantly influence the light absorption capability and stability of perovskite solar cells. However, it is time-consuming and labor-intensive to obtain the properties of perovskites using traditional experimental or high-throughput computational methods. As a prospective method, machine learning can find regularities in the given training data and give accurate prediction results. In this article, we use deep learning models based on attention mechanisms and elemental features to predict the heat of formation and bandgap of perovskite materials. Random Forest and Gradient Boosted Regression Tree models have also been used for interpretable predictions of properties. The compositionally restricted attention-based network was improved by introducing a densely connected network and optimizing the network structure to increase data processing capabilities. The experiment results show that the mean absolute errors of the heat of formation and bandgap on the test sets are decreased by 5.77% and 3.37% respectively. The optimized model also shows better performance when used for classification tasks. In addition, we use the gradient boosting regression tree model and the shapley additive explanations tool to conduct an interpretable analysis, explaining the impact of different features on the predictions of the properties.

Liu, Yunpeng; Ibrahim, Suriani; Majid, Nazia Abdul; Mohd Sabri, Mohd Faizul; Sun, Jianwen; Zhuo, Qiming; Liu, Wei

doi: 10.1088/1361-6463/ad4565pmid: N/A

Metamaterial absorbers allow electromagnetic waves to be converted into heat energy based on impedance matching. However, passive metamaterial absorbers exhibit fixed absorption characteristics, limiting their flexibility. This work demonstrates tunable microwave and terahertz absorbers by integrating adjustable resistors into the metamaterial units. First, a microwave absorber from 1 to 5 GHz was designed by embedding PIN diodes with voltage-controlled resistance. Calculations, simulations, and measurements verified two separate absorption peaks over 90% when optimized to a resistance of 250 Ω. The absorption frequencies shifted based on the resistor tuning. Building on this, a terahertz absorber was modeled by substituting gallium nitride high electron mobility transistors (GaN HEMTs) as the adjustable resistor component. The GaN HEMTs were controlled by an integrated gate electrode to modify the two-dimensional electron gas density, allowing resistance changes without external voltage terminals. Simulations revealed two absorption peaks exceeding 90% absorption at 0.34 THz and 1.06 THz by adjusting the equivalent resistance from 180 Ω to 380 Ω, and the tunable resistance is verified by DC measurement of single GaN HEMT in the unit. This work demonstrates how integrating adjustable resistors enables dynamic control over the absorption frequencies and bandwidths of metamaterial absorbers. The proposed geometries provide blueprints for tunable microwave and terahertz absorbers.

Mi, Yan; Chen, Yong; Liu, Canhui; Liu, Wentao; Peng, Yiqin; Liao, Ruijin

doi: 10.1088/1361-6463/ad465cpmid: N/A

This paper aims to study the effect of rise rate (dV/dt) on the cumulative breakdown characteristics of biaxially oriented polypropylene (BOPP) film under nanosecond pulse voltage. The lifetime (NL) of BOPP film under nanosecond pulse voltage with different pulse voltage amplitude and dV/dt were carried out, respectively. It is found that the NL decreases with the increase of the pulse voltage amplitude, and the NL decreases with the increase of the dV/dt. Meanwhile, the quantitative mathematical relationship between NL and dV/dt, and pulse voltage amplitude are established, respectively, and it is concluded that both the rise rate and pulse voltage amplitude on the lifetime of BOPP film by the inverse power model. The results of the study are expected to provide the experimental basis and mechanistic explanation for the evaluation of film capacitor lifetime under extreme conditions.

Sun, Xueqin; Huang, Le; Liao, Haijun; Xiao, Wenbo; Li, Jingbo

doi: 10.1088/1361-6463/ad465bpmid: N/A

The utilization of hydrogen energy has emerged as a promising solution for clean and sustainable energy sources. The development of cost-effective catalysts with high activity and stability is crucial for efficient hydrogen production. In this work, we investigated the hydrogen evolution reaction (HER) catalytic activity of single transition metal atom (TM = V, Cr, Mn, Fe, Co, Ni) on slab SnO2 by using density functional theory. Our results revealed that the catalytic activity of the slab SnO2 can be significantly enhanced by loading the transition atom. By calculating the Gibbs free energies and exchange current densities in different adsorption configurations of TM-SnO2, single-atom catalyst (SAC) of Mn-loaded SAC exhibits excellent catalytic performance, characterized by a low Gibbs free energy barrier (−0.05 eV). Introducing a TM on the SnO2 surface breaks its local symmetry, while the strong coupling between the metal and H atoms enhances catalytic performance. The synergetic effect of symmetry breaking and metal-H interaction boosts overall catalytic activity. This work not only proposes a novel non-platinum HER catalyst based on SnO2 but also lays a solid foundation for future applications of SnO2-based catalysts.

Ouali, Anthony; Sebih, Lyes; Herrmann, Antoine; Valensi, Flavien; Hamdan, Ahmad

doi: 10.1088/1361-6463/ad44a3pmid: N/A

The plasma-water interface is a complex medium characterized by interesting physical and chemical phenomena useful for many applications such as water processing or material synthesis. In this context, optimizing the transport of reactive species from plasma to water is crucial, and it may be achieved by increasing the surface-to-volume ratio of the processed object. Herein, we study the characteristics of a streamer produced by nanosecond discharge in air gap with a droplet of deionized water. The discharge is characterized experimentally by electrical measurements as well as by 1 ns-intergated ICCD images. To report plasma properties that are not accessible through experiment, such as the spatio-temporal evolution of electron density, electric field, and space charge density, a 2D fluid model is developed and adapted to the experimental geometry. Due to the fast propagation of the ionization front, the droplet is considered as a solid dielectric. The model solves Poisson’s equation as well as the drift-diffusion equation for electrons, positive ions, and negative ions. The utilized transport coefficients are tabulated as a function of the reduced electric field. Helmholtz equations are also included in the model to account for photoionization. The electron impact ionization source obtained from the model is compared to experimental 1 ns-integrated ICCD images, and a good agreement is observed. Finally, the model is used to investigate the influence of droplet dielectric permittivity and wetting angle (the angle between a liquid surface and a solid surface) on the properties of the discharge. Overall, the data reported herein demonstrate that the model can be used to investigate plasma properties under different conditions.