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The frequencies of magnetostatic wave (MSW) oscillators employing three principal modes of propagation, surface (MSSW), forward (MSFVW), and backward (MSBVW) volume waves, have been investigated. Previous (MSW) oscillator papers dealt with MSSW. Oscillators were fabricated using LPE‐YIG MSW delay lines in a feedback loop of a 2–4 GHz amplifier. Wide and narrow band transducers were employed. Oscillator frequency as a function of biasing field is in agreement with a theoretical analysis. The analysis predicts frequency in terms of material parameters, biasing field, and transducer geometry. With wide band transducers a comb of frequencies is generated. Narrow band transducers for MSSW and MSFVW select a single mode; and MSBVW selects two modes. Spurious modes, attributed to instrumentation, are more than 20 dB below the main response, and bandwidths are less than 0.005 percent. No other spurious modes are observed. MSW oscillators produce clean electronically tunable signals and appear attractive in frequency agile systems.

The problem of the generation of the coherent far ir radiation by a resonant parametric coupling between light waves is taken into consideration in the present paper. The equations of propagation of the interacting waves are given and the effect of the ir activity of the stimulated Raman active resonant mode on the dynamics of the wave interaction is considered. The experimental case of crystals and fluids as suitable media for achieving the effect is taken into account. At last, considerations on the resonant phase matching condition show that the achievement of the effect is not critical in general cases.

Electric and magnetic modes establish the basis of the localized spoof surface plasmons (LSSPs), both of which are transverse electric (TE) modes. In this paper, the concept of transverse magnetic (TM) modes of LSSPs is proposed, and the double-layer planar-circular-grating resonator is demonstrated to support the TM modes in addition to the TE modes. In numerical simulations, the TE and TM modes were excited by a plane wave and an aperture, respectively, and their resonant frequencies and electromagnetic field distributions were demonstrated to be different. The electric field of the TE mode is parallel to the grating, while the magnetic field of the TM mode is parallel to the grating. In experiments, multiple TE and TM LSSPs modes were excited by a magnetic coupler. The magnetic coupler can be rotated to distinguish the TM modes from the TE ones, which provides conclusive evidence of the existence of the TM modes. Compared with the TE LSSPs modes, the TM LSSPs modes have the advantages of high sensitivity to the distance between layers and high quality factors (Q-factors), which hold promise for providing improvements on compact applications of LSSPs. Based on the proposed concept, therefore, the classification standard of the LSSPs is expanded to make it more scientific and comprehensive.

Most of the photonic valley-Hall edge states were constructed by changing structures to break the spatial inversion symmetry, restricting the practical application potential. In this paper, we construct a tunable topological valley-Hall edge state based on the large optical Kerr effect. It is demonstrated that topological phase transition happens by engineering the intensity of the injected pump and that a valley-Hall edge state can be generated at the interface between two regions with different topological properties. In addition, eigenfrequency and transmission characteristics of the edge state as a function of applied pump intensity are investigated. The topological protected valley-dependent transmission is studied under non-uniform distributed pump intensity. This work may open a new path toward designing reconfigurable all-optical metadevices.

The initial states and pressure of detonation products in a bubble have a great impact on bubble pulsation in underwater explosions; particularly, the initial accelerated expansion of a bubble can determine the energy distribution. The energy output and distribution of explosives were obtained on the basis of the underwater explosion experiment in this paper. To study the process of bubble pulsation and energy output, we proposed a gas equation of state (EOS) combining the pvk form, Jones–Wilkins–Lee (JWL) EOS, and the initial states of a bubble to take the effects of the initial bubble pressure and detonation products state transformation into account; furthermore, the bubble radius, velocity, and acceleration vs time were obtained through the Rayleigh–Plesset equation under our experimental condition. The differences of bubble behaviors were compared by adopting the JWL EOS and a polytropic EOS with k = 3. The results showed that the initial bubble pressure and detonation products state transformation influence the accelerating expansion and the subsequent bubble oscillation, respectively. Subsequently, comparisons of the energy output and distribution for different gas EOSs showed that the initial shock wave energy for the JWL EOS was underestimated in accelerating expansion, and the bubble energy was overestimated using the polytropic EOS for k = 3; the obtained energy output and distribution had a better agreement with experimental data when adopting the improved gas EOS. In addition, the energy distribution was determined before the detonation products turned to the explosion gas state in initial expansion based on the relationship of the accelerating expansion characteristics and the initial shock wave energy generation. The research has a great significance to reveal the mechanism of bubble pulsation in underwater explosions.

Acoustic transport through topological edge states in phononic crystals improves the suppression of backscattering, which gives these systems significant potential for controlling sound waves. Recent research shows that only one acoustic edge state caused by topological valley phases can transmit in phononic crystals. This paper proposes a genre of valley phases with one, two, and three topological edge states created by transforming the structure of unit cells. The bulk-edge correspondence indicates that these edge states are topological based on the topological invariant number (i.e., the valley Chern number of one, two, and three) of this system coinciding with the number of topological edge states. Different types of defects are introduced into the phononic crystals, whose transmission spectra show that they can withstand bending defects. These results indicate that these systems have significant potential for application in noise control, acoustic communication, and acoustic-electrical integration.

Recently developed field of all-dielectric nanophotonics allowed for the observation of the Kerker effect, i.e., unidirectional scattering of electromagnetic radiation by a dielectric particle in optical frequency range. In this paper, we consider the analogue of this effect for localized waves which manifests itself as an interference of the evanescent tails of the optical waveguide modes. Specifically, we design a discrete nanophotonic waveguide that supports two degenerate modes characterized by different symmetries with respect to the plane that contains the waveguide axis, leading to an asymmetric field distribution of the propagating wave under appropriate excitation conditions. We perform numerical simulations of the excitation of such a waveguide with a point dipole and predict that its polarization state can be encoded into the field pattern of the signal propagating along the waveguide and transferred for relatively large distances. We also propose a planar directional coupler that consists of a developed discrete waveguide and conventional single-mode dielectric waveguides that exploit the interference effect for routing single photons generated by circularly polarized quantum emitters.

At present, the Kuramoto model is the standard and widely accepted theoretical approach for analyzing the synchronization of spin-torque nano-oscillators (STNOs) coupled by an interaction. Nevertheless, the oscillatory decaying regime and the initial condition (IC)-dependence (hysteretic) that exist in the synchronization of many types of STNOs cannot be explained by this model. In order to more precisely elucidate the physical mechanisms behind the two phenomena, in this paper we develop a generalized pendulum-like model based on the two common features of non-linear auto-oscillators: one is the stability of the amplitude/energy of dynamic states and the other is the non-linear dynamic state energy of oscillators. In this new model, we find that the Newtonian-like particle with sufficient kinetic energy can overcome the barrier of phase-locking potential to evolve into a stable asynchronization state, leading to the IC-dependent synchronization. Furthermore, due to the presence of kinetic energy, this particle can also oscillate around the minima of the phase-locking potential, leading to the oscillatory decaying regime. Thereby, in this work, we adopt this new model to analyze the IC-dependent mutual synchronization of perpendicular-to-plane-STNO pairs, and then we suggest that the initial conditions can be controlled to avoid such a phenomenon by using magnetic dipolar coupling.

Aiming at the problem of the need for trial-and-error in the design of the size of Fabry–Perot (F–P) resonant absorbers, we start from the sound absorption caused by loss and propose a design method to accurately obtain the optimal size of F–P tubes with circular and rectangular cross sections. An innovative loss equation is constructed, which relates the F–P tube's critical loss to the transmission loss of sound waves in the tube. By solving the loss equation, the size of the F–P tube required for perfect sound absorption can be obtained. This method avoids the need for experiments or simulations to find the optimal size, and it is simple, fast, and accurate. Single-frequency perfect sound-absorbing metasurfaces of circular and rectangular cross sections were designed using this method. The performances of these metasurfaces were verified using theoretical, numerical, and experimental models. The three resulting sound absorption coefficient curves had good consistency and achieved perfect sound absorption at the target frequency. The feasibility and accuracy of the design method were established. The essence of the loss equation is to find the size of the F–P tube corresponding to the “zero” point on the real-frequency axis of the complex-frequency plane. The work in this paper is of guiding significance for determining the sizes of F–P tubes.

Considering the manufacturing difficulty and tunability, a one-dimensional phononic crystal-type metastructure is proposed in this paper. By adjusting the distance between the mass oscillators installed on the connecting bar, that is, changing the length of the phononic crystal unit, the phase shift of the flexural wave can span over a full range of 0–2π and then the phenomena such as abnormal refraction, beam focusing, and self-acceleration can be realized based on the generalized Snell’s law. In addition, the method of multiple mass oscillator array design is used to broaden the operating frequency domain by increasing or decreasing the number of mass oscillators. This proposed method provides an innovative concept for realizing broadband tunable metastructures and has potential applications in the fields of vibration control, energy harvesting, and noise isolation.

An improved plane wave expansion (PWE) method is developed based on the governing equations of a lattice unit cell, providing support for calculating the band structures of a locally resonant (LR) plate with multiple arrays of multiple degree-of-freedom resonators. The extended plane wave expansion (EPWE) method is correspondingly presented to obtain the complex band structures of the LR plate. Then the bandgap properties of LR plate systems with single/multiple arrays of single/multiple degree-of-freedom resonators are analyzed, which present a good agreement with the results calculated by the classical theory or finite element method. Compared with the classical theory, the improved PWE and EPWE methods have excellent efficiency and broad applicability for LR plate systems with complex attachments. Furthermore, when the damping of resonators is taken into account, the merging of the multiple bandgaps can be observed using the EPWE method adopted in this paper, which gives a useful tool to analyze the band structures of an LR plate with merged bandgaps for broadband vibration suppression.

Dynamic control of probe absorption spectrum via vortex beams in multilevel systems has attracted much attention because it provides an avenue to observe unique transport properties. In this paper, we employ experimentally accessible parameters to flexibly control the probe absorption spectrum that shows vortex-induced transparency (VIT) and absorption (VIA) phenomena. Specifically, this system can be served as an optical switch while the vortex beams can act as the control knob, which has the ability to switch from a transparency dip to an absorption peak. Differing from a conventional electromagnetically induced transparency technique, our proposed scheme investigates VIT and VIA by tuning the orbital angular momentum of vortex beams in the resonance and non-resonance cases. Moreover, the collective feature caused by constructive or destructive interference is also studied via adjustable frequency detuning. Our results may provide an efficient approach to study similar phenomena in multilevel quantum systems driven by optical vortex beams.

This paper explores the stochastic dynamics of a Hopf adaptive frequency oscillator when driven by noise. Adaptive oscillators are nonlinear oscillators that store information via plastic states. As noise is ubiquitous in physical systems, it is important to gain an understanding of the stochastic effects on adaptive oscillators. Previously, it has been shown that a simplified analysis of the Fokker–Planck equation results in affecting the plastic frequency state of these oscillators. However, when the full Fokker–Planck equation is considered, new behaviors are observed due to changes in oscillation amplitudes in addition to frequencies. The plastic frequency state of these oscillators may benefit from enhanced learning due to small amplitudes of noise, converge to incorrect values for medium amplitudes of noise, and even collapse to zero in the limit of large amplitudes of noise. Interestingly, not all averaged states collapse equally, which leads a two dimensional limit cycle to collapse into single dimensional oscillations when considering the averaged dynamics. These behaviors are compared analytically through the Fokker–Planck equation, numerically using the Euler–Maruyama simulations, and finally validated experimentally using an analog, electronic circuit. These results show that noise can enhance, mislead, or even reduce the dimensionality of the averaged adaptive Hopf oscillator.

We present torsional wave focusing in cylindrical structures integrated with conformal gradient-index phononic crystal (GRIN-PC) lenses. The mechanical lens modifies the refractive characteristics of propagating waves along the axial direction such that the torsional wave energy focuses at a desired point with an increased amplitude. In this paper, our main goal is to exploit the GRIN-PC concept for amplification of the fundamental torsional wave mode, namely, T(0,1), which is especially favorable in the guided wave ultrasonic testing of pipelines and can extend important structural information over long-range pipelines. To this end, we first study d35 piezoelectric transducer arrays through multiphysics simulations coupling piezoelectric and solid mechanics models and determine the optimal number of transducers to achieve the excitation of torsional plane wave. Two-dimensional Fourier transform as well as the tangential displacement histories around the pipe circumference at various axial positions verify the pure T(0,1) excitation, which is validated with experiments. Next, we design, fabricate, and numerically and experimentally study the conformal GRIN-PC for T(0,1) mode focusing in a range of ultrasonic frequencies, from 30 to 50 kHz. Measurements show that the amplitude of the torsional wave mode is amplified by 39% at the focal point with the presence of the GRIN-PC lens.

Metamaterials with multiple resonators have been widely investigated for the purpose of generating multiple stop bands or broadening the attenuation bandwidth. The multiple resonators could be connected end to end in a line, namely, in-series connection, or connected individually to the host structures, namely, in-parallel connection. This paper investigates the influence of the resonator connection methodology on the frequency response functions of metamaterial beams with multiple resonators and exhibits an approach for optimizing their resonator distribution over the structure. The receptance functions of metamaterial beams with various resonator connection architectures are calculated by a transfer matrix model, which is verified through finite element model results. It is demonstrated that resonator interconnection architectures have a great impact on the global dynamic properties of metamaterials. An optimization strategy is subsequently proposed to find out the optimal resonator connection architectures and mass distributions that could minimize the maximal receptance functions in targeted single and multiple frequency ranges. The objective functions within single targeted frequency ranges are solved by the adoption of the genetic algorithm method. The weighted sum method is used to gain an optimal solution for multi-frequency range optimization. The metamaterial beams with optimal resonator connection methods and mass distributions demonstrate greatly enhanced vibration attenuation at frequencies of interest compared with other beams. The work is expected to provide the necessary theoretical basis and incentive for future researchers working on the design of metamaterials with extended, tuned, and optimized stop bands.

Mobility spectrum analysis (MSA) is a process of recovering the spectrum of carrier mobilities in an electronic material from a magnetic-field-dependent Hall measurement. This paper first presents a set of useful general properties of MSA that are true for all MSA methods. One such property is that Hall data can be classified as either “physical,” with at least one spectrum of carrier mobilities that exactly fits the data, or “unphysical,” where no mobility spectra exactly fit the data and the analysis must find approximate solutions. Almost all experimental Hall data collected at seven or more magnetic fields are slightly unphysical. The closest approximate solution for such unphysical data is always a set of discrete-carrier mobilities. A new method for multicarrier MSA is presented to display the likely properties of the discrete carriers as two-dimensional spectra vs both mobility and carrier density instead of one-dimensional spectra vs only mobility, as in previous methods. Measurement uncertainties are included as a fundamental part of the analysis so that the user can easily view the resultant uncertainty in the derived carrier properties. A separate method is also presented for reliably determining a smooth, high-entropy spectrum of carrier mobilities, which is constructed as the maximum-entropy linear combination of discrete-carrier solutions to the Hall data. An example using calculated Hall data corresponding to acoustic phonon scattering shows that the technique could potentially be used to measure the continuous mobility distributions that result from different scattering mechanisms.

A Weyl semimetal can be considered as a system composed of two Weyl cones breaking time-reversal symmetry, which exhibits strong anisotropic hyperbolic characteristics. The off diagonal components of the permittivity tensor of a Weyl semimetal produce the Faraday and Kerr effects. In this paper, we investigate the Goos–Hanchen (GH) shifts when a light is reflected from the interface of air and a Weyl semimetal. It is demonstrated that the angular GH shift is still tiny; however, the spatial GH shift can be huge and is very sensitive to change in the incident angle. The spatial GH shift can change from positive to negative with an increase in the incident angle. The spatial GH shifts vary at different distances of two Weyl cones and thicknesses of a Weyl semimetal film. Furthermore, the tunable GH shift can be realized by changing the tilting parameter and Fermi energy. These adjustable and giant GH shifts may find practical applications in optical shift devices and optical sensors.

In this paper, we investigate the acoustic flatbands (FBs) in phononic crystal (PnC) defect lattices. The defects are introduced into a PnC composed of periodic rigid rods in the air background. Since the acoustic energy is highly confined inside the PnC defects, the interaction between the defects can be described by the tight-binding model. We construct the PnC defects in two bipartite lattices, namely, the stub and Lieb lattices. The acoustic FBs can be observed for both of the lattices. Moreover, the acoustic FBs are protected by the chiral symmetry. That is, the FBs can be preserved even though the hopping strengths between the neighboring defects are perturbed. The proposed PnC defect lattices provide a feasible platform for the study of acoustic FB systems and topological insulators.

It is well known that Weyl semimetals (WSMs) are classified into two types of type-I and type-II depending on whether or not they have electron and hole pockets. Also, these WSMs have peculiar transport properties such as negative longitudinal magnetoresistance and planar Hall effect because of a chiral anomaly. In this paper, however, we show that the chiral anomaly can cause positive longitudinal magnetoresistance in type-II WSMs. Here, we investigate longitudinal and transverse magnetoconductivities of time reversal symmetry broken type-I and type-II WSMs using a tight-binding model. The model allows us to describe both types of type-I and type-II WSMs by tuning parameters, and it has two Weyl points that are separated along the k x-direction. The numerical calculations of these conductivities are performed using the Boltzmann equation including the Berry curvature. It is found that longitudinal magnetoconductivities in the x-direction can have both positive and negative values depending on the magnitude of the inclination of a Weyl cone. This is because the zeroth Landau energy-level becomes either a hole-like one or an electron-like one depending on the magnitude of the inclination of the Weyl cone in type-II WSMs. These results imply that we can make a high MR-ratio device using type-II WSMs by tuning the inclination of their cones if it is possible to change their energy bands by the application of electric field and so on.

We investigate a quadruple quantum dot setup that can be employed to sense the temperature of an electrically isolated remote target reservoir. Such a setup was conceived earlier by S\'anchez et. al. (New Journal of Physics, 19, 113040) as non-local thermodynamic engine and relies on the electrostatic interaction between Coulomb-coupled quantum dots. The conjugation of Coulomb-coupling and energy-filtering results in an overall change in conductance with remote reservoir temperature. The performance of the thermometer is then theoretically investigated using density matrix formulation, and it is demonstrated that the quadruple quantum dot design ensures a superior temperature sensitivity and noise robustness compared to a simple thermometer consisting of two Coulomb-coupled quantum dots. In the end, we investigate the regime of operation and comment on the ground state configuration for optimal performance of the thermometer. The setup investigated in this paper can be employed to construct highly efficient non-local cryogenic thermometers., Comment: 18 Pages, 11 Figures

This Perspective reviews the fundamental physics of space–charge interactions that are important in various media: vacuum gap, air gap, liquids, and solids including quantum materials. It outlines the critical and recent developments since a previous review paper on diode physics [Zhang et al. Appl. Phys. Rev. 4, 011304 (2017)] with particular emphasis on various theoretical aspects of the space–charge limited current (SCLC) model: physics at the nano-scale, time-dependent, and transient behaviors; higher-dimensional models; and transitions between electron emission mechanisms and material properties. While many studies focus on steady-state SCLC, the increasing importance of fast-rise time electric pulses, high frequency microwave and terahertz sources, and ultrafast lasers has motivated theoretical investigations in time-dependent SCLC. We particularly focus on recent studies in discrete particle effects, temporal phenomena, time-dependent photoemission to SCLC, and AC beam loading. Due to the reduction in the physical size and complicated geometries, we report recent studies in multi-dimensional SCLC, including finite particle effects, protrusive SCLC, novel techniques for exotic geometries, and fractional models. Due to the importance of using SCLC models in determining the mobility of organic materials, this paper shows the transition of the SCLC model between classical bulk solids and recent two-dimensional (2D) Dirac materials. Next, we describe some selected applications of SCLC in nanodiodes, including nanoscale vacuum-channel transistors, microplasma transistors, thermionic energy converters, and multipactor. Finally, we conclude by highlighting future directions in theoretical modeling and applications of SCLC., Peer-reviewed (ritrýnd grein)

The design concept of microwave-assisted magnetic recording (MAMR) using the flux control (FC) effect has been proposed as a technology for hard disk drives (HDDs). In this type of MAMR, the magnetization of an in-gap device (FC device) is reversed against the gap field by spin-transfer torque, enhancing the amplitude and gradient of the recording field. In this paper, we study the magnetization dynamics of an FC device fabricated in the write gap of an HDD write head. The operation of the FC device is analyzed by measuring the temporal resistance change in the sub-nanosecond region. Reversal of the FC device becomes faster as the bias current is increased and can be completed by 0.5 ns after the transition of the write current. The experimental results are reproduced by micromagnetic simulations using a head model, confirming that the simulations correctly describe the magnetization dynamics of the actual device. The simulations show that the recording field gain by the FC device appears with little delay after the rise of the recording field and that the FC device operates effectively even at a fast write rate of approximately 3 Gbit/s. Furthermore, we demonstrate the effectiveness of boosting the bias current, which can realize both fast and reliable operation of the FC device. These results indicate that the FC device operates as designed and that MAMR using the FC effect is promising for extending the recording density of HDDs.

Vortex electromagnetic waves carrying orbital angular momentum (OAM) have been widely discussed for potential applications in wireless communications. Belonging to the Laguerre–Gaussian beams family, such type of waves present a hollow conical shape and divergence characteristics along with a directional radiation. In this paper, an innovative method to produce omnidirectional OAM beams based on spatial transformation is proposed at microwave frequencies. As a proof-of-concept demonstration, a lens with omnidirectional radiation in the horizontal plane is designed and simulated with an incident vortex beam carrying the OAM mode l = +2. The designed lens can be potentially implemented with an all-dielectric medium showing a gradient permittivity distribution. Furthermore, the proposed lens presents good performances over a wide operational bandwidth spanning from 8 to 17 GHz. By converting the directional beam to an omnidirectional one, the proposed method opens the door to the potential development of microwave vortex antenna systems.

There has been growing clinical evidence of the value of targeted alpha therapy for treatment of several cancers. The work has been slowed by the lack of availability of the key alpha emitting isotopes, especially Ac-225. Until this time, most of the supply has been from three Th-229 generators that are milked to produce hundreds of mCi of Ac-225 every month. There has been a growing effort to produce new sources of Ac-225 from several different accelerator-based routes. It can be produced with medical-isotope cyclotrons with a proton energy of at least 16 MeV using the reaction Ra-226(p,2n)Ac-225. It can also be produced by using high-energy protons (150 to 800 MeV) for spallation of a thorium target. Significant experimental work has been applied to both processes. It can also be produced by the photonuclear reaction, Ra-226({\gamma},n)Ra-225. The Ra-225 decays via beta decay to Ac-225 with a half life of 14.9 days. The photons are produced by an intense beam of electrons with an energy about 25 to 30 MeV. This paper will provide a technical description of radium targets and a target chamber that would be capable of producing a yield of four curies of Ra-225 from a 10-day irradiation of one gram of radium segmented into two to four separate encapsulated targets, at a beam power of 20 kW. These targets could be milked at least three times, yielding nearly four curies of Ac-225. There is also a description of a method to reduce production of Ac-227 to values less than a few parts per million of the yield of Ac-225. The Monte Carlo code Fluka has been used to model the yields of Ra-225 and support the design concept to reduce the production of Ac-227. It has also been used to model the experimental results by Maslov et al. [https://doi.org/10.1134/S1066362206020184] to provide reasonable confidence in the cross-section value used by the code., Comment: 37 pages, 19 figures, 6 tables Submitted to Journal of Applied Physics

We have developed a new ultra-low field frequency-swept (FS) electrically detected magnetic resonance (EDMR) spectrometer to perform sensitive EDMR measurements of 4H-silicon carbide (SiC) metal–oxide–semiconductor field-effect transistors at sub-millitesla (mT) magnetic fields. The new spectrometer design enables the detection of so-called ultra-strong coupling effects such as multiple-photon transitions and Bloch–Siegert shifts. In this paper, we present a new spectrometer design and discuss ultra-low field FS-EDMR sensitivity to both multiphoton transitions and Bloch–Siegert shifts of the FS-EDMR response. FS-EDMR effectively eliminates the interference of the sub-mT EDMR response from a near-zero field magnetoresistance (NZFMR) phenomenon that pervades the sub-mT regime in a magnetic field-swept EDMR scheme. We discuss an automatic power leveling scheme, which enables frequency sweeping. We also present results illustrating the Bloch–Siegert shift of the FS-EDMR response. Finally, we study the two-photon transition line shape in the 4H-SiC transistor as a function of the static field, in which we observe a collapse of the two-photon linewidth with decreasing static field and compare our results to the theory of two-photon absorption in EDMR.

In the last two decades, significant progress has been made in combining ferroelectricity and magnetism in the same material. Usually, magnetism and ferroelectricity were thought of as independent phenomena. However, the discovery of spin-induced ferroelectricity with a strong coupling between magnetism and electric polarization is intriguing. Whether they interact non-trivially is determined by the symmetries of the crystal lattice and the appearing magnetic structure, which, in turn, depends on the nature of magnetic ions, their exchange, and anisotropic interactions, the presence of frustration, etc. Several structural families of compounds exhibiting magnetoelectric (ME) or multiferroic properties have already been reported. This paper gives an account of the richness of structure, magnetism, magnetoelectric and multiferroic properties of spinels. After presenting symmetry aspects of the crystal and magnetic structures of spinels related to these phenomena, we give several examples of magnetoelectric and multiferroic spinels explaining the relations between magnetism and electric polarization and discuss their future perspectives.

Higher-order topological insulators, which go beyond the conventional bulk-boundary correspondence, have become a hot research topic in recent years. In this paper, we propose a two dimensional second-order topological photonic crystal based on a C4 symmetric lattice. By tuning the distance between the dielectric cylinders, the topological phase transition of the structure is realized, one edge state and two types of corner states (one topological and one trivial corner state) are found. Our findings provide an evolution of bulk–corner–edge–corner–bulk in a hierarchy of dimensions, which enriches the study of corner states in photonic systems. Besides, we distinguish two corner states by discussing robustness and mode area, and we verify the topological corner state can be embedded in the bulk states, forming a bound state in continuum by simulation.

Machine learning is an extremely powerful tool for the modern theoretical chemist since it provides a method for bypassing costly algorithms for solving the Schrodinger equation. Already, it has proven able to infer molecular and atomic properties such as charges, enthalpies, dipoles, excited state energies, and others. Most of these machine learning algorithms proceed by inferring properties of individual atoms, even breaking down total molecular energy into individual atomic contributions. In this paper, we introduce a modified version of the Hierarchically Interacting Particle Neural Network (HIP-NN) capable of making predictions on the bonds between atoms rather than on the atoms themselves. We train the modified HIP-NN to infer bond orders for a large number of small organic molecules as computed via the Natural Bond Orbital package. We demonstrate that the trained model is extensible to molecules much larger than those in the training set by studying its performance on the COMP6 dataset. This method has applications in cheminformatics and force field parameterization and opens a promising future for machine learning models to predict other quantities that are defined between atoms such as density matrix elements, Hamiltonian parameters, and molecular reactivities.

A flexoelectric actuator represents a new approach for exciting Lamb waves in plate structures and, thus, can be potentially used for active structural health monitoring systems and microsensors. This paper analytically investigated the performance of a flexoelectric actuator for Lamb wave excitation. The constitutive equation for a thin plate flexoelectric structure was derived using the thermodynamics theory. A significant size effect could be demonstrated by the constitutive equation, which suggests a greatly enhanced curvature excitation performance at a small dimension. The interaction between the ideally bonded flexoelectric actuator with the plate could be simplified into two pairs of pin force and pin moment under the assumption of a weak coupling condition. The strain and the displacement responses to the pin-force and pin-moment pairs were obtained using the integral transform techniques and residue calculus. The pin-moment component mainly yielded an antisymmetric Lamb wave mode, while the pin force was reminiscent of its piezoelectric counterpart. The tuning of either a symmetric or an antisymmetric mode could be achieved by choosing an appropriate excitation frequency. The response under a five-cycle Hanning-window toneburst excitation signal was compared with the finite element analysis results and good agreement between them was found.