A noteworthy challenge in actual wireless power transfer (WPT) applications is to achieve stable power transfer efficiency at varying distances. It was recently proposed and demonstrated that maximum transferred efficiency was ensured when a two-coil WPT system was always tuned at the purely real eigenfrequency. However, this condition prevents optimal operation with a fixed operating frequency. To address the issue, we introduce the concept of high-order (higher than second-order) parity-time (PT) symmetry to bypass this difficulty and construct straightforward physical pictures to obtain the mechanism of stable and efficient power transfer in a three-coil WPT system. Before the PT phase transition, there is a coupling-independent entirely real eigenfrequency, despite three branched eigenmodes existing. It means that this WPT system, with high-order PT symmetry, could be highly efficient without frequency tracking at varying distances. Additionally, the nonideal high-order PT model, with asymmetric coupling for practical WPT applications, is discussed. It is demonstrated experimentally that the efficient stability of the high-order WPT system is significantly superior to that of the second-order system at a fixed frequency, which can be explained by the fact that the eigenfrequency of the nonideal high-order model used is less sensitive to the coupling strength than that of the second-order model. Furthermore, the idle power loss of this WPT system is less than that of the other at the resonant frequency (in an idle state without receiver terminals), reducing power consumption at the transmitters and benefiting wireless charging intermittently. As an example, the high-order PT symmetric model is applied to the WPT system with miniaturized receivers, showing stable transfer efficiency with a wide range of axial transfer distances and lateral misalignment. Our work provides an alternative application for the study of high-order PT physics in designing a properly multiple-coil WPT system in the long term.

Experimental studies of the anomalous Hall effect are performed for thin-film $\mathrm{Ta}/\mathrm{Tb}\text{−}\mathrm{Fe}\text{−}\mathrm{Co}$ over a wide range of temperatures and magnetic fields up to 3 T. Far from the compensation temperature ($T_{M}$ = 277 K), the field dependence has a conventional shape of a single hysteresis loop; just below the compensation point, the dependence is anomalous with the shape of a triple hysteresis. To understand this behavior, we experimentally reveal the magnetic phase diagram and theoretically analyze it in terms of spin-flop-like phase transitions. In this case, we observe the dominance of the $\mathrm{Fe}\text{−}\mathrm{Co}$ sublattice, which is a subject of the strong interaction with the $\mathrm{Ta}$ layer. This $\mathrm{Fe}\text{−}\mathrm{Co}$ anisotropy enhancement is expressed in the appearance of abnormal wing-shaped hysteresis loops near the compensation point and, in the unusual phase diagram, where the first-order phase transition line deviates towards low temperatures. This effect can be useful for the design of ultrafast ferrimagnetic devices with desired switching parameters.

The interface between a magnetic material and a heavy metal that has a large spin-orbit interaction is at the root of various spin-related phenomena. In this paper, we address the peculiar spin-dependent transport at a $\mathrm{Pt}/{\mathrm{Cr}}_2{\mathrm{O}}_3$ interface by exploring the origin of the nonlinear anomalous Hall effect (AHE) in $\mathrm{Pt}/{\mathrm{Cr}}_2{\mathrm{O}}_3$ bilayers. X-ray magnetic circular dichroism (XMCD) measurements show no appreciable magnetic moment at the interface originating from $\mathrm{Cr}$ 3d and $\mathrm{Pt}$ 5d orbitals, which could be associated with the AHE response. A possible interfacial magnetic moment M at the $\mathrm{Pt}/{\mathrm{Cr}}_2{\mathrm{O}}_3$ interface, assumed from the detection limit of the XMCD measurements, yields an anomalous Hall conductivity ($\sigma \mathrm{AHE}$) per unit net magnetic moment (M), −${\sigma }_{\mathrm{AHE}}$/M, of 0.57 ${\mathrm{V}}^{-1}$, which is extraordinary large compared with that for general magnetic materials. Together with first-principles calculations, the results suggest the possibility of an intrinsic AHE in the $\mathrm{Pt}/{\mathrm{Cr}}_2{\mathrm{O}}_3$ interface that does not rely on the net magnetic moment.

Various recent experiments have proven the theoretical prediction that domain walls in planar magnetic structures can channel spin waves as magnonic waveguides, establishing a platform for building magnonic devices. Recently, three-dimensional nanomagnetism has been boosted and become a significant branch of magnetism, because three-dimensional magnetic structures expose much emerging physics hidden behind planar ones and will provide broader room for device engineering. Skyrmions and antiskyrmions, as natural three-dimensional magnetic configurations, are not considered yet in the context of spin-wave channeling and steering. Here, we show that skyrmion tubes can act as nonplanar magnonic waveguides, if excited suitably. An isolated skyrmion tube in a magnetic nanoprism induces spatially separate internal and edge channels of spin waves; the internal channel has a narrower energy gap, compared with that of the edge channel and, accordingly, can transmit signals at lower frequencies. Additionally, we verify that those spin-wave beams along the magnetic nanoprism are restricted to the regions of potential wells. Transmission of spin-wave signals in such waveguides results from the coherent propagation of locally driven eigenmodes of skyrmions, i.e., the breathing and rotational modes. Finally, we find that spin waves along the internal channels are less susceptible to the magnetic field than those along the edge channels. Our work will open up an arena for spin-wave manipulation and help to bridge between skyrmionics and magnonics.

Here, we report two kinds of absolute optical instruments that can make stigmatic images for geometric optics in two-dimensional space. One is called the duplex Mikaelian lens, which is made by splicing two half-Mikaelian lenses with different periods. The other is an exponential conformal transformer of a duplex Mikaelian lens with a rational number of the ratio of different periods of its two half-Mikaelian lenses, which we call a duplex Maxwell’s fish-eye lens. Duplex Mikaelian lenses have continuous translation symmetry with an arbitrary real number, while duplex Maxwell’s fish-eye lenses have continuous rotation symmetry from zero to 2π. Hence, each duplex Maxwell’s fish-eye lens corresponds to a duplex Mikaelian lens. We further demonstrate the caustic effect of geometric optics in duplex Mikaelian lenses and duplex Maxwell’s fish-eye lenses. In addition, we investigate the Talbot effect of wave optics in the duplex Mikaelian lens based on numeric calculations. Our findings, based on splicing and exponential conformal mapping, enlarge the family of absolute optical instruments.

In Nature, biological materials such as nacre, bone, and dentin display an enhanced mechanical strength due to their structure characterized by hard inclusions embedded in a soft matrix. This structure has inspired the design of artificial materials with optimized properties. Thus, for given the mechanical properties of matrix and inclusions, it is fundamental to understand how the global observables, essentially strength, and ultimate strain are determined by the geometrical parameters of the inclusions. In this paper, we address this question by extending the two-dimensional random fuse model, which has been widely used to extract statistical properties of fracture processes, to the case of staggered stiff inclusions. We thus investigate numerically how emergent mechanical properties can be optimized by tuning geometrical dimensions and the arrangement of the inclusions. To do this, we adopt an optimization procedure based on an evolutionary algorithm to efficiently explore the parameter space and to determine the most favorable geometrical features of the inclusions for improved strength or ductility, or both. Various lattice sizes and volume fractions are considered. Depending on inclusion sizes and aspect ratios, composite strength or ultimate strain can be maximized, with the Pareto front for simultaneous optimization of the two being interpolated by a simple power law. Characteristic exponents for damage avalanche distributions are found to vary with respect to homogeneous structures, indicating increased fracture ductility simply due to optimized geometrical features. Our study indicates the possibility through structural optimization of creating staggered composites that allow significant advantages in terms of weight reduction and fuel consumption in automotive applications.

The recent demonstration of resonant tunneling transport in nitride semiconductors has led to an invigorated effort to harness this quantum transport regime for practical applications. In polar semiconductors, however, the interplay between fixed polarization charges and mobile free carriers leads to asymmetric transport characteristics. Here, we investigate the possibility of using degenerately doped contact layers to screen the built-in polarization fields and recover symmetric resonant injection. Thanks to a high doping density, negative differential conductance is observed under both bias polarities of $\mathrm{Ga}\mathrm{N}$/$\mathrm{Al}\mathrm{N}$ resonant tunneling diodes (RTDs). Moreover, our analytical model reveals a lower bound for the minimum resonant-tunneling voltage achieved via uniform doping, owing to the dopant solubility limit. Charge storage dynamics is also studied by impedance measurements, showing that at close-to-equilibrium conditions, polar RTDs behave effectively as parallel-plate capacitors. These mechanisms are completely reproduced by our analytical model, providing a theoretical framework useful in the design and analysis of polar resonant-tunneling devices.

Acoustic analogue computation and signal processing are of great significance; however, it is challenging to realize acoustic computing devices because of their limitations of single function and complex structure. Here, an acoustic multifunctional device, which can gate or amplify acoustic waves without resorting to altering the frequency and structure using a passive acoustic parity-time (PT)-symmetric metamaterial, is realized theoretically and experimentally. The metamaterial is constructed by five lossless-loss periodically distributed media, which are modulated to achieve passive PT symmetry. At the coherent perfect absorber emitter point in the broken PT-symmetric phase, the logic gates (and, or, xor, and not) and small signal amplifier are realized in a single system by adjusting the phase and amplitude differences between two incoming beams, respectively. This work provides a route for the connection between PT symmetry and the acoustic metamaterial, which has potential applications in acoustic modulation and acoustic multifunctional devices.

We study, in the absence of a magnetic field, the stability of skyrmions in a disk-shaped ferromagnetic multilayer in the presence of locally pinned spins at the edge. This particular boundary condition suppresses the formation of helical states yet enhances the stabilization of states with rotational symmetry, such as Néel skyrmions. Numerical simulation shows that the size of the disk is a viable parameter to control the creation and annihilation of skyrmionic states. We also find that the demagnetization field renormalizes the Dzyaloshinskii-Moriya interaction. This study opens an alternative avenue to the generation, stabilization, and control of magnetic skyrmions in field-free heterostructures.

We propose simple protocols for performing quantum noise spectroscopy based on the method of transfer tensor maps (TTMs) [Cerrillo and Cao, Phys. Rev. Lett. 112, 110401 (2014)]. The TTM approach is a systematic way to deduce the memory kernel of a time-nonlocal quantum master equation via quantum process tomography. With access to the memory kernel, it is possible to (1) assess the non-Markovianity of a quantum process, (2) reconstruct the noise spectral density beyond pure-dephasing models, and (3) investigate collective decoherence in multiqubit devices. We illustrate the usefulness of TTM spectroscopy on the IBM Quantum Experience platform, and demonstrate that the qubits in the IBM device are subject to mild non-Markovian dissipation with spatial correlations.

Intense ultrafast laser interaction with solid-density plasma can lead to relativistic transient electron dynamics that are favorable for the generation of high-field terahertz (THz) pulses. We investigate the temporal characteristics of such THz pulses. Single-shot electro-optic measurements enable us to record the complete temporal profiles of the THz pulses emanating from planar $\mathrm{Cu}$ and aligned $\mathrm{Cu}$-nanorod-array plasma. The temporal properties of the THz pulses exhibit several transients. Fully relativistic two-dimensional particle-in-cell simulations corroborate experimental observations and reveal that these features originate from electron microbunch emission from the target. Our results demonstrate that target nanostructuring provides a route to control such ultrafast electron dynamics and hence THz-pulse properties in the time domain.

Attosecond pulses in the soft x-ray (SXR) to water-window energy region offer the tools for creating and studying target-specific localized inner-shell electrons or holes in materials, enabling monitoring or controlling charge and energy flows in a dynamic system on attosecond timescales. Recently, a number of laboratories have reported generation of continuum harmonics in the hundred-electron-volt to kilovolt region with few-cycle long-wavelength mid-infrared lasers. These harmonics have the bandwidth to support pulses with durations of a few to a few-ten attoseconds. But harmonics generated in a gas medium have attochirps that cannot be fully compensated by materials over a broad spectral range; thus, realistically what are the typical shortest attosecond pulses that one can generate? To answer this question, it is essential that the temporal attosecond pulses be accurately characterized. By reanalyzing the soft x-ray attosecond metrology reported in three recent experiments [Li et al. 53-attosecond x-ray pulses reach the carbon K-edge, Nat. Commun. 8, 186 (2017); Gaumnitz et al. Streaking of 43-attosecond soft x-ray pulses generated by a passively CEP-stable mid-infrared driver, Opt. Exp. 25, 27506 (2017); Cousin et al. Attosecond Streaking in the Water Window: A New Regime of Attosecond Pulse Characterization, Phys. Rev. X 7, 041030 (2017)] using a newly developed broadband phase retrieval algorithm, we demonstrate that the generated attosecond pulses in the first two papers have durations of about 60 as, longer than what they have reported. Similarly, the duration from the third experiment is retrieved to be about half of the 322-as upper limit cited in that work. We also introduce the autocorrelation (AC) of the streaking spectrogram. By comparing the ACs from the experiments and from the retrieved SXR pulses, the accuracy of the retrieved results can be directly visualized. Our retrieval method is fast and accurate, and it shall provide a powerful tool for the metrology of the emerging few-ten-attosecond pulses.

Terahertz technology has attracted wide attention because of its potential applications in diverse fields, such as spectroscopy, imaging, and nondestructive evaluation, but the lack of functional devices, including polarization convertors, is still a major hurdle for these applications. In previous studies on metamaterial-based polarization convertors, only one single polarization conversion is generally achieved. This paper presents a transmissive dual-functional terahertz waveplate based on an all-dielectric metamaterial composed of periodic subwavelength pillars made of pure silicon that can achieve two different polarization conversions (i.e., a combination of quarter- and half-waveplates) at the same operation frequency and for x- and y-polarized incidences, respectively. Theoretical and numerical analyses are provided to illustrate the design principle. A sample is fabricated and characterized, and the experimental results show that the two polarization conversions are obtained at 1.01 THz, which is in good agreement with the corresponding simulations. Such a dual-functional terahertz waveplate will find applications in terahertz systems and the design strategy can be extended to other frequency ranges as well.

Characterization of spin-orbit torques (SOTs) in the perpendicular magnetic anisotropy (PMA) system is of great importance for fundamental studies and technological applications in spintronic devices. Here, we report a spin Hall magnetoresistance (SMR) based spin-torque ferromagnetic resonance (ST FMR) study of SOT efficiency in perpendicularly magnetized $\mathrm{W}$/${\mathrm{Co}}_{40}{\mathrm{Fe}}_{40}{\mathrm{B}}_{20}$/$\mathrm{Mg}\mathrm{O}$ structures. A full analysis of the ST FMR spectrum is developed for the PMA sample. A typical ST FMR spectrum of the PMA system shows two resonance modes, i.e., in-plane and out-of-plane magnetization precession modes. By performing modulation of the damping measurement for the in-plane mode, the dampinglike torque efficiency is determined to be −0.38, which is consistent with the reference value obtained in samples with in-plane magnetic anisotropy. The dampinglike torque efficiency of −0.31 is also obtained from the out-of-plane resonance mode, in which the significant contribution of the spin-pumping-induced inverse spin Hall effect is carefully considered. The obtained values of dampinglike torque efficiency by different means are consistent with each other. The present work provides useful insights to determine the dampinglike torque efficiency of the PMA sample via ST FMR measurements.

Terahertz (THz) signals, mainly generated by photonic or electronic approaches, are being sought for various applications, whereas the development of magnetic source might be a necessary step to harness the magnetic nature of electromagnetic radiation. We show that the relativistic effect on the current-driven domain-wall motion induces THz spin-wave emission in ferrimagnets. The required current density increases dramatically in materials with strong exchange interaction and rapidly exceeds ${10}^{12}\mathrm{A}{\mathrm{m}}^{-2}$, leading to the device breakdown and thus the lack of experimental evidence. By translating the collective magnetization oscillations into voltage signals, we propose a three-terminal device for the electrical detection of THz spin wave. Through material engineering, a wide frequency range from 264 GHz to 1.1 THz and uniform continuous signals with improved output power can be obtained. As a reverse effect, the spin wave generated in this system is able to move the ferrimagnetic domain wall. Our work provides guidelines for the experimental verification of THz spin wave, and could stimulate the design of THz spintronic oscillators for wideband applications as well as the all-magnon spintronic devices.

By performing comprehensive first-principles calculations, we study the hard magnetic properties of ${\mathrm{Ce}}_2$${\mathrm{Co}}_{17}$ and ${\mathrm{Ce}}_2$${\mathrm{Fe}}_{17}$ under $\mathrm{Zr}$ doping with varying $\mathrm{Co}$ concentrations, given the substantial experimental success of $\mathrm{Zr}$ in improving the magnetic anisotropy energy (MAE) in ${\mathrm{Ce}}_2$${\mathrm{Co}}_{17}$ and ${\mathrm{Sm}}_2$${\mathrm{Co}}_{17}$. We find that the magnetic properties of these alloys become comparable to state-of-the-art magnetic materials by doping with two elements, $\mathrm{Zr}$ and $\mathrm{Fe}$, with potential-energy products $B{H}_{max}$ of 40 MG Oe. The $\mathrm{Zr}$ substitution is particularly helpful for improving the MAE, leading to a magnetic hardness parameter $K_1/{\mu }_0{M}_s^2$ that significantly exceeds unity for a substantial range of $\mathrm{Co}$ concentrations (between 60% and 100% of the $3d$-element concentration). The calculated MAE exhibits a strong dependence on the $\mathrm{Co}$ concentration, indicative of a likely valence fluctuation with $\mathrm{Co}$ alloying, and shows a maximum value of 7.78 ${\mathrm{MJ}/\mathrm{m}}^3$ for 60% $\mathrm{Co}$ doping. Thus, upon experimental verification, these alloys may become competitors to the better-known permanent-magnet materials ${\mathrm{Nd}}_2$${\mathrm{Fe}}_{14}$$\mathrm{B}$ and ${\mathrm{Sm}\mathrm{Co}}_5$.

We experimentally investigate the origin of two-magnon scattering (TMS) in heavy-metal (HM)/ferromagnet (FM)/oxide heterostructures (FM = $\mathrm{Co}$, ${\mathrm{Ni}}_{81}{\mathrm{Fe}}_{19}$, or ${\mathrm{Fe}}_{60}{\mathrm{Co}}_{20}{\mathrm{B}}_{20}$) by varying the materials located above and below the FM layer. We show that strong TMS in HM/FM/oxide systems arises primarily at the HM/FM interface and increases with the strength of the interfacial spin-orbit coupling and magnetic roughness at this interface. TMS at the FM/oxide interface is relatively weak, even in systems where spin-orbit coupling at this interface generates strong interfacial magnetic anisotropy. We also suggest that the spin-current-induced excitation of nonuniform short-wavelength magnons at the HM/FM interface may function as a mechanism of spin memory loss for the spin-orbit torque exerted on the uniform mode.

Heat transfer from irradiated metallic nanoparticles is relevant to a broad array of applications ranging from water desalination to photoacoustics. The efficiency of such processes depends on the ability of these nanoparticles to absorb the pulsed illuminating light and to quickly transfer energy to the environment. Here we show that compared with homogeneous gold nanoparticles having the same size, gold-silica core-shell nanoparticles enable heat transfer to liquid water that is faster. We reach this conclusion by considering both analytical and numerical calculations. The key factor explaining enhanced heat transfer is the direct interfacial coupling between metal electrons and silica phonons. We discuss how to achieve fast heating of water in the vicinity of the particle and show that optimal conditions involve nanoparticles with thin silica shells irradiated by ultrafast laser pulses. Our findings should serve as guides for the optimization of thermoplasmonic applications of core-shell nanoparticles.