The interfacing of magnetic materials with nonmagnetic heavy metals with a large spin-orbit coupling, such as $\mathrm{Pt}$, results in an asymmetric exchange interaction at the interface due to the Dzyaloshinskii-Moriya interaction (DMI), which in turn leads to the formation of skyrmions and topological spin structures in perpendicularly magnetized multilayers. Here, we show that out-of-plane spin domains with lateral dimensions from 200 nm to 2 $\mu \mathrm{m}$ are stabilized in in-plane magnetized $\mathrm{Ta}$/$\mathrm{Co}$/$\mathrm{Pt}$ trilayers. We show that these spin textures are largely insensitive to the direction of the in-plane magnetization switched by either magnetic fields or electric fields applied across a ${\mathrm{Si}}_3{\mathrm{N}}_4$ gate dielectric. The results of micromagnetic simulations indicate that the DMI is required for the stabilization of such out-of-plane domains and that the presence of surface roughness helps to stabilize larger structures, in agreement with experimental results. We identify these spin structures as meronlike topological textures, characterized by a perpendicular spin texture in an uniformly in-plane magnetized system.

Plasmonic lattices consisting of nanoparticles in a homogeneous environment are well known for their support of so-called surface lattice resonances. They are associated with localized surface plasmons coupled to each other via photons propagating freely along the structure. Interestingly, such structures provide narrow resonances in plasmonic structures without their coupling to any external high-$Q$ resonators. In this paper, we explore modes in a stack of two identical plasmonic lattices. We demonstrate that such a structure is able to support a mode that is positioned strictly on a Rayleigh anomaly and does not shift with the variation of distance between two lattices over wide limits. Given the fact that the period is the most stably reproduced quantity in an experiment, such behavior can be used to simplify the fabrication of structures with resonances at desired energies. The application of this effect can be very useful in lasing, light emission from metasurfaces, optical filters, and many other optical devices.

We propose and experimentally demonstrate a generic framework, requiring neither active components nor complex designs, for undistorted asymmetric elastic-wave transmission in a thin plate using just a layer of lossless metasurface. The asymmetric transmission stems from the uneven diffraction of the +1st and −1st orders on opposite sides of the metasurface. Compared with previous loss-induced strategies, the present metasurface provides nearly total transmission on the transportation side, but total reflection on the opposite side, exhibiting a higher contrast ratio of the transmission. Moreover, we show that this strongly asymmetric behavior is robust with respect to the incidence angle, the frequency, and the effects of loss. The present work paves the way to compact rectification, high-resolution ultrasonography, and vibration and noise control in elastodynamics and acoustics.

Topological invariance has recently attracted rapidly growing attention, which can be characterized in a one-dimensional (1D) system by the Zak phase, a special kind of Berry phase. While acoustic systems are proven to be excellent platforms for exemplifying the diversity of topological properties, it remains challenging to downscale the device, since such interface states are usually attained at higher-frequency regimes. Here, we report both theoretically and experimentally the realization of interface states in the low-frequency band gap of a 1D chain of Helmholtz resonators. By numerically inspecting the Zak phase of the bulk band of the proposed system, we reveal the transition point of the band structure in a low-frequency range and elucidate the underlying mechanism of interface states at the lowest band gap in the spectrum. As a result, the lattice constant is reduced to the subwavelength scale, instead of being comparable to the wavelength. Our proposed mechanism is verified experimentally by measuring acoustic intensity for the interface state. The experimental results agree quite well with the theoretical predictions, showing the existence of interface states at the desired frequencies. We anticipate our mechanism to open up the possibility for the miniaturization and integration of acoustic functional devices and have far-reaching implications in diverse applications, such as enhanced sensing and biomedical imaging.

Schottky-barrier inhomogeneities are expected at the metal–transition-metal-dichalcogenide (TMDC) interface and this can impact device performance. However, it is difficult to account for the distribution of interface inhomogeneity as most techniques average over the spot area of the analytical tool (e.g., few hundred micrometers squared for photoelectron-based techniques), or the entire device measured for electrical current-voltage (I-V) measurements. Commonly used models to extract Schottky-barrier heights neglect or fail to account for such inhomogeneities, which can lead to the extraction of incorrect Schottky-barrier heights and Richardson constants that are orders of magnitude away from theoretically expected values. Here, we show that a Gaussian-modified thermionic emission model gives the best fit to experimental temperature-dependent current-voltage (I-V-T) data of van der Waals $\mathrm{Au}$/p-${\mathrm{Mo}\mathrm{S}}_2$ interfaces and allow the deconvolution of the Schottky-barrier heights of the defective regions from the pristine region. By the inclusion of a Gaussian-distributed Schottky-barrier height in the macroscopic I-V-T analysis, we demonstrate that interface inhomogeneities due to defects are deconvoluted and well correlated to the impact on the device behavior across a wide temperature range from a room temperature of 300 K down to 120 K. We verify the Gaussian thermionic model across two different types of p-${\mathrm{Mo}\mathrm{S}}_2$ (geological bulk crystals and synthetic flux-grown crystals), and finally compare the macroscopic Schottky-barrier heights with the results of a nanoscopic technique, ballistic hole emission microscopy (BHEM). The results obtained using BHEM are consistent with the pristine $\mathrm{Au}$/p-${\mathrm{Mo}\mathrm{S}}_2$ Schottky-barrier height extracted from the Gaussian-modified thermionic emission model over hundreds of nanometers. Our findings show that the inclusion of Schottky-barrier inhomogeneities in the analysis of I-V-T data is useful to elucidate the impact of defects (e.g., grain boundaries, metallic impurities, etc.) and hence their influence on device behavior. We also find that the effective Richardson constant, a material-specific constant typically treated as merely a fitting constant, is a useful parameter to check for the validity of the transport model.

We present a model for how frequency fluctuations comparable to the total cavity linewidth may arise in tunable and nonlinear microwave cavities and how these fluctuations affect the measurement of scattering-matrix elements. We apply this model to the specific case of a two-sided cavity and obtain closed-form expressions for the average scattering-matrix elements in several important cases. A key signature of our model is the subtle deformation of the trajectories swept out by scattering-matrix elements in the complex plane. Despite this signature, the fluctuating and nonfluctuating models are qualitatively similar enough to be mistaken for one another. In the case of tunable cavities, we show that if one fails to account for these fluctuations, then one finds damping rates that appear to depend on the tuning parameter, which is a common observation in such systems. In the case of a Kerr cavity, we show that there exists a fundamental lower bound to the scale of these frequency fluctuations in the steady state, imposed by quantum-mechanical uncertainty, which can appreciably affect the apparent damping rates of the cavity as the strength of the nonlinearity approaches the cavity linewidth. By using our model as a fitting function for experimental data, however, one can extract both the true damping rates of the cavity and the effective scale of these frequency fluctuations over the bandwidth of the scattering measurement. Lastly, we compare this method for observing frequency fluctuations to other methods, one of which we extend beyond the regime of small fluctuations, and we discuss how ringdown measurements can be performed accurately in the presence of frequency fluctuations.

In ideal fluids, acoustic waves are known as longitudinal sound waves, the particle velocity of which is parallel to the wave traveling direction. The emergence of metasurfaces offers extensive flexibility in acoustic wave manipulations. However, achieving a sound wave with transversely polarized particle velocity by means of metasurfaces is not yet achievable. Here, we demonstrate that a sound wave with polarized particle velocity perpendicular to the traveling direction can exist in an acoustic metawaveguide. The metawaveguide is made of a pair of spaced acoustic metasurfaces, each comprising alternatingly arranged membranes that resonate phase reversibly. Bearing transversely polarized particle velocity, the sound wave shows lateral dependence, frequency selectivity, and insensitivity to longitudinal irregularity, all distinct from that in a traditional waveguide. More interestingly, the transverse particle velocity is axis symmetric, and thus, acoustophoresis traps are formed in the center of the metawaveguide. The advent of a sound wave with transverse particle velocity may open another avenue for more sophisticated acoustic applications, such as resistance-free insulation, acoustic “circuits,” and wave-matter interaction related bioacoustics.

Intelligent metamaterials have aroused intensive research interest due to their adaptive responses to environmental changes. However, existing transformation-thermotics-based metamaterials have almost no intelligence because their parameters are crucially dependent on environmental parameters. Therefore, once the environment changes, these thermal metamaterials can no longer work, which limits practical applications. To solve the problem, we propose a mechanism for intelligent thermal metamaterials based on transformation-invariant metamaterials. As an application, we design intelligent thermal rotators, which can guide the direction of heat flux with different environmental parameters. Since the adaptive behavior is similar to chameleons, the present rotators are also called chameleonlike rotators. We further perform finite-element simulations and laboratory experiments to validate the scheme, and the chameleonlike behavior is clearly demonstrated. These results have potential applications for the implementation of adaptive and adjustable thermal metamaterials. Similar behaviors can also be expected in other fields like hydrodynamics.

Multimode bulk acoustic systems show promise for use in superconducting quantum computation. They can serve as a medium-term memory storage, with exceptional coherence times being demonstrated, and they exhibit a mode density that is physically highly compact. Here, we experimentally demonstrate accessing individual acoustic modes without being hindered by the uniform frequency spacing of the modes. We use sideband control where a low-frequency modulation is applied to the transmon qubit energy. The amplitude of the modulation defines the qubit-acoustic mode coupling and its frequency detunes the acoustic sidebands, therefore selectively enabling the switching on or off of the interaction, and it allows for full control of the individual modes.

Interfacial magnetism emerging from orbital hybridization is the key facilitator for practical nanoscale spintronic devices. Most devices require a capping layer, and it is widely assumed that capping-layer variations do not change the fundamental properties of magnetic films underneath. In a model $\mathrm{Co}\text{−}\mathrm{Fe}\text{−}\mathrm{B}/\mathrm{Mg}\mathrm{O}$ system, interfacial investigations, so far, have been focused on the hybridization of $\mathrm{Fe}$ d and $\mathrm{O}$ p orbitals. However, the role of the capping layer has largely been ignored by adopting a reductionist scenario of just oxidation or charge modification. Here, we report strong modifications of interfacial magnetism in $\mathrm{Co}\text{−}\mathrm{Fe}\text{−}\mathrm{B}/\mathrm{Mg}\mathrm{O}$ by systematically changing the $\mathrm{Ru}$ capping thickness using element-specific x-ray magnetic circular dichroism and x-ray absorption spectroscopy at $\mathrm{Fe}$ $L_{2,3}$, $\mathrm{Co}$ $L_{2,3}$, and $\mathrm{O}$ K edges, along with corresponding spin and orbital magnetic moment calculations and magnetometry measurements. We observe unusual spin-flip-like transitions due to capping layers and find direct evidence for systematic spin and orbital modifications, beyond just charge transformation, strikingly captured by oxygen x-ray absorption and dichroism spectra. Our result shows the importance of the capping layer and provides a complete picture of rich interfacial magnetism in the $\mathrm{Co}\text{−}\mathrm{Fe}\text{−}\mathrm{B}/\mathrm{Mg}\mathrm{O}$ system.

We present a polarization-insensitive air-spaced triple-band metamaterial perfect absorber (MMPA), consisting of a metamaterial layer and metallic ground plane operating at terahertz frequencies. Three near-unity absorption peaks can be individually determined by the geometry of the ring resonators within one unit cell, since the inter-unit-cell coupling is negligible. However, for sufficiently small interlayer spacing ($\stackrel{<}{\sim \phantom{{}_x}}20\mu \mathrm{m}$), coupling between the metamaterial layer and the ground plane is non-negligible. Therefore, near-field interactions must be taken into account for a full understanding of the electromagnetic response (EMR). Interference theory is often used to model the EMR of MMPAs analytically, in which interlayer coupling between the metamaterial and ground plane is usually neglected, resulting in a predicted blueshift of the absorption peaks in comparison to experiment. To account for near-field coupling, we incorporate correction terms into the analytical interference model by taking into account the effective interlayer capacitance and inductance. This results in good agreement between interference theory and experiment (and full-wave numerical simulations). Our findings demonstrate that interlayer coupling is an important design parameter for ultrathin MMPAs.

The reservoir computing paradigm posits that complex physical systems can be used to simplify pattern recognition tasks and nonlinear signal prediction. We show that random topological magnetic textures pinned by grain inhomogeneities demonstrate desirable dynamical responses for the implementation of reservoir computing as applied to ac current pulses. By harnessing the complex resistance or magnetization responses exhibited by random magnetic skyrmion textures to demonstrate simple pattern recognition, we explain how spintronics systems offer an advantage in the search for an ideal reservoir computer. The dynamical properties of compact skyrmion fabrics, coupled with their CMOS integrability operating on similar length and timescales, open the door for skyrmion-based reservoir computing applications.

Superconductor-semiconductor-nanowire hybrid structures can serve as versatile building blocks to realize Majorana circuits or superconducting qubits based on quantized levels such as Andreev qubits. For all these applications, it is essential that the superconductor-semiconductor interface is as clean as possible. Furthermore, the shape and dimensions of the superconducting electrodes need to be precisely controlled. We fabricated self-defined $\mathrm{In}\mathrm{As}$/$\mathrm{Al}$ core-shell nanowire junctions by a fully in-situ approach, which meet all these criteria. Transmission electron microscopy measurements confirm the sharp and clean interface between the nanowire and the in-situ deposited $\mathrm{Al}$ electrodes that are formed by means of shadow evaporation. Furthermore, we report on tunnel spectroscopy, gate, and magnetic field-dependent transport measurements. The achievable short junction lengths, the observed hard gap, and the magnetic field robustness make this hybrid structure very attractive for applications that rely on a precise control of the number of subgap states, like Andreev qubits or topological systems.

We investigate the effects of atomistic interfaces on the device performance of ${\mathrm{Hf}\mathrm{O}}_2$-based ferroelectric tunnel junctions (FTJs) using density-functional calculations. The atomistic structures of ${\mathrm{Hf}\mathrm{O}}_2$ FTJs with different interfacial conditions are constructed and their device performances, such as on:off current ratio are evaluated. We find that without the external effects such as dissimilar metal electrodes or composite layers inserted to make an asymmetric potential barrier, the intrinsic effects stemming from the interface dipoles can be tailored toward achieving high device performance. Especially, the atomistic asymmetry effect, which gives the same effect as the built-in electric field, can be exploited for a high on:off current ratio at low external bias voltages. We demonstrate that the asymmetrically terminated $\mathrm{Zr}$-doped ${\mathrm{Hf}\mathrm{O}}_2$ FTJ exhibits a current ratio of 12, which is higher than any previously reported values, theoretically or experimentally, for ${\mathrm{Hf}\mathrm{O}}_2$-based FTJs with symmetric metal electrodes.

We show that a space-time-varying medium can function as a front-end transceiver, i.e., an antenna-mixer-amplifier. Such a functionality is endowed by space-time surface waves associated with complex space-time wave vectors in a subluminal space-time medium. The proposed structure introduces pure frequency up and down conversions, with very weak undesired time harmonics. In contrast to other recently proposed space-time mixers, a large frequency up- to down-conversion ratio, associated with gain, is achievable. Furthermore, as the structure does not operate based on progressive energy transition between the space-time modulation and the incident wave, it possesses a subwavelength thickness (metasurface). Such a multifunctional, highly efficient, and compact medium is expected to find various applications in modern wireless telecommunication systems.

As a problem in acoustics, sound insulation finds wide applications in diverse situations. We design and experimentally implement an omnidirectional broadband acoustic ventilation barrier using a topology-optimization method. We propose a broadband acoustic barrier constructed of interlaced contraction structure with a central hollow channel, utilizing Bragg’s scattering mechanism happening at impedance discontinuities. Based on this barrier, a density-based topology-optimization method is employed for the inverse design to broaden the sound-insulation band nearly 90% while keeping a low transmission. The topology-optimization method systematically optimizes the scatterers distributed at intervals in our initial barrier in dimensions of size, shape, and orientation to possess many complicated features, which produces the maximum reflection at a desired frequency range. Experiments with acoustic waves of different incident angles are conducted to validate the optimized design, whose results are consistent with the simulations. The measured ventilation rate of the optimized barrier reaches 60.8% compared to initial barrier (49.2%) due to the reduction of fill rate of solid material and aerodynamic loss, demonstrating a high ventilation effect. Our design opens routes to design sound insulators that enable applications to be air permeable yet sound proofing simultaneously.

The presence of nondegenerate valley states in silicon can drastically affect electron dynamics in silicon-based heterostructures, leading to electron spin relaxation and spin-valley coupling. In the context of solid-state spin qubits, it is important to understand the interplay between spin and valley degrees of freedom to avoid or alleviate these decoherence mechanisms. Here we report the observation of relaxation from the excited valley state to the ground state in a $\mathrm{Si}/\mathrm{Si}$-$\mathrm{Ge}$ quantum dot at zero magnetic field. Valley-state readout is aided by a valley-dependent tunneling effect, which we attribute to valley-orbit coupling. We find a long intervalley relaxation time of 12.0 ± 0.3 ms, a value that is unmodified when a magnetic field is applied. Furthermore, we compare our findings with the spin relaxation time and find that the spin-valley “hot spot” relaxation is roughly 4 times slower than intervalley relaxation, consistent with established theoretical predictions. The precision of this technique, adapted from electron spin readout via energy-dependent tunneling, is an improvement over indirect valley relaxation measurements and could be a useful probe of valley physics in spin-qubit and valley-qubit implementations.