A phononic crystal formed in a suspended membrane provides full confinement of hypersonic waves and thus realizes a range of chip-scale manipulations. In this paper, we demonstrate mode-resolved real-space characterization of mechanical-vibration properties in cavities and waveguide systems. Multiple resonant modes in various designed cavities are independently characterized, and wavelength-scale high-$Q$ resonances up to $Q=4200$ under atmospheric conditions are confirmed. This also reveals that the waveguide allows us to observe mode-resolved wave transmission and thereby drive evanescently coupled cavities. The methods offer a significant tool with which to build compact and low-power microwave phononic circuitry for applications to signal processing and hybrid quantum systems.

We report on an experimental study of current-induced switching in perpendicular magnetic random-access memory (MRAM) cells with variable resistance-area products (RAs). Our results show that in addition to spin-transfer torque (STT), current-induced self-heating and voltage-controlled magnetic anisotropy also contribute to switching and can explain the RA dependencies of the switching-current density and STT efficiency. Our findings suggest that thermal optimization of perpendicular MRAM cells can result in significant reduction of switching currents.

Identically sized $\mathrm{Co}$ quantum dots (QDs) are constructed on monolayer tungsten disulfide (${\mathrm{WS}}_2$), forming coupled heterostructures. Topographical images investigated by means of in situ scanning tunneling microscopy (STM) show a bias-dependent feature. First-principles-calculated binding energies combined with the simulated STM images identify that the $\mathrm{Co}$ QDs possess a unique magic number, with a tetrahedral ${\mathrm{Co}}_4$ configuration. Numerical differential conductance measured by means of scanning tunneling spectroscopy indicates a p-type doping and an Ohmic contact property for the ${\mathrm{Co}}_4/{\mathrm{WS}}_2$ system. The mechanism of the transport and conduction properties of the coupled heterostructures is further revealed by analyzing the work functions and interfacial interaction. Our findings offer some references for the controlled fabrication of identically sized zero-dimensional and two-dimensional heterostructures, and we propose a feasible strategy for Ohmic interface contact in nanodevices.

Transformation theory is a powerful tool for material design to control waves in electromagnetics, acoustics, and elastic mechanics. Deformable materials are demonstrated to guide the propagation of ultrasonic waves freely. Toothed whales, such as dolphins and porpoises, may manipulate directional beams by simply deforming their foreheads. However, existing materials face great challenges in realizing their bioacoustic function. Here, we propose a bioinspired semianalytical conformal acoustics that can predict the evolution of acoustic functions strictly. Based on this method, we design a series of acoustic steering and collimation models. The sound-speed distribution functions can be determined by conformal mappings, and the acoustic-beam steering and expansion are numerically confirmed. We further experimentally fabricate acoustic steering and collimation devices by using metamaterials. The experimental results show good agreement with theoretical predictions. The proposed bioinspired conformal acoustics may bridge the gap between an animal’s biosonar and artificial materials, which show potential application value in underwater acoustics, medical ultrasonography, and other related applications.

In our original paper [Phys. Rev. Applied 6, 014017 (2016)], we considered electric power generation from Earth’s rotation through its own magnetic field. Jeener claims to prove that no such effect is possible [Phys. Rev. Applied 13, 028001 (2020)]. But this conclusion results from a failure to recognize a distinction between the corotation with Earth of the nonaxially symmetric components of Earth’s magnetic field and the nonrotation with Earth of the axially symmetric components. We use Earth’s geomagnetic field to demonstrate the important consequences of this distinction. Because the distinction has considerable empirical support, Jeener’s argument is likely incorrect.

In a 2016 article [Phys. Rev. Applied $\mathbf{6}$, 014017 (2016)], Chyba and Hand proposed a new scheme to generate electric power continuously at the expense of the Earth’s kinetic energy of rotation, by using an appropriately shaped cylindrical shell of a well-chosen conducting ferrite, rigidly attached to Earth. A recent experimental test [Phys. Rev. Applied $\mathbf{10}$, 054023 (2018)] gave a null result. In the first part of the present refutation, I use today’s standard electromagnetism and essentially the same model as Chyba and Hand to show in a very simple way that no device of the proposed type can produce continuous electric power, whatever its configuration or size. In the second part, I show that the prediction of nonzero continuous power by Chyba and Hand results from a confusion of frames of reference at a critical step of their derivation. When the confusion is clarified, the prediction becomes exactly zero. In the third part, I comment about the frequent invocation by Chyba and Hand of controversial legacy notions such as the existence of an intrinsic velocity of quasistatic magnetic fields.

Imaging ultrafast dynamic events is a long-standing scientific goal. It is difficult for conventional electronic imaging sensors based on charge-coupled devices (CCDs) or complementary metal oxide semiconductors (CMOS) to capture dynamic processes occurring on the nanosecond or even shorter timescales due to the limitations of on-chip storage and electronic read-out speed. Here, we develop a dynamic imaging technique with a very simple compact configuration, called ultrafast electro-optical deflection imaging (UEODI), which has a temporal resolution of up to 20 ps or an imaging speed of $5\times {10}^{10}$ frames per second (fps), and therefore, it enables the observation of nanosecond and even picosecond dynamic events. UEODI operates in a single-shot receive-only mode, and thus, it is highly beneficial for imaging nonrepetitive (or irreversible) dynamic events and a variety of luminescent objects. Moreover, when combined with the time-of-flight (TOF) method, UEODI can detect three-dimensional (3D) objects. Using UEODI, we visualize a molecular photoluminescent process and measure a 3D ladder structure. Considering the capabilities of UEODI, it will have very importation application prospects in both basic research and applied science, including biomedical imaging.

The gain-bandwidth product is a fundamental figure of merit that restricts the operation of optical amplifiers. Here, we introduce a design paradigm based on exceptional points, which relaxes this limitation and allows for the building of a new generation of optical amplifiers that exhibits a better gain-bandwidth scaling. Additionally, our results can be extended to other physical systems such as acoustics and microwaves.

Many efforts have been dedicated to enhancing near-field radiative heat transfer by designing different kinds of geometric shapes or introducing new materials. Besides just improving the heat-transfer rate, active and convenient thermal management is also important in micro- and nanothermal systems. We show that the introduction of an intermediate modulator, based on the $\mathrm{graphene}/h$-BN/graphene heterostructure, can enhance and manipulate thermal radiative heat transfer without changing the distance or other parameters of the emitter and absorber. Such three-body systems can increase the radiatively exchanged power several times over corresponding two-body counterparts. The introduced modulator can be viewed as a midrepeater to effectively enhance photon tunneling through evanescent modes. Furthermore, the heat-transfer rate can be modulated over a large range and can even be lower than that in two-body systems through the application of bias voltages. The mechanism can be explained by the change of energy-transmission coefficients between bodies through adjustment of the optical properties of graphene. The presented scheme may open a promising avenue for the active control of near-field heat transfer at the micro- and nanoscales.

We propose an experimental method in microsphere-assisted interference microscopy that makes it possible to reconstruct surface topographies as much with high quantitative depth accuracy as when the transversal feature sizes are smaller than the classical diffraction limit. The full-field super-resolution interference microscope consists not only of one glass microsphere in the object arm but also of a second similar microsphere in the reference arm. By compensating the sphere aberrations in the two interference arms, the spatial resolution appears to be considerably increased. The increase in the three-dimensional spatial resolution allows for the topography reconstruction of 300-nm-width grating lines and 200-nm-diameter transparent nanopillars.

We characterize the charge distribution in ion-gated turbostratic multilayer graphene employing broadband infrared transmittance spectroscopy. The experimental results evince the nonlinear screening of graphene and are in agreement with a theoretical model that accounts for the electrostatic coupling between the layers and the quantum capacitance of the graphene. We find that descriptions of charge distributions in these systems in the high-density regimes accessed experimentally via the formation of an electric double layer must include the capacitance of the multilayer graphene channel, which varies with charge and thickness. Specifically, the graphene-channel capacitance increases with thickness but tends to saturate after three layers, underscoring graphene’s qualities for ultrathin charge-storage applications. Through accurate determination of the layer-resolved carrier density in multilayer graphene, this sensitive technique may prove useful for the study of charge distributions in other two-dimensional multilayer devices.

The exact measurement of spin-orbit torque (SOT) in magnetic heterostructures is the basis for a solid understanding of its physical mechanism. Although different SOT terms can be properly separated from harmonic Hall voltage analysis, owing to different symmetries and magnetic-field dependences, this method may be invalid for a ferromagnetic layer with a small Hall coefficient. Here, we develop a method to determine the SOT from harmonic longitudinal voltage measurements. Starting from a standard total-energy expression of a magnetic system, the equations to compute the corresponding SOT effective fields are derived and then applied to an in-plane magnetized $\mathrm{Pt}/\mathrm{Ni}\text{−}\mathrm{Fe}/\mathrm{Mg}\mathrm{O}$ system. The effective SOT fields and spin Hall angle obtained for the system are in accordance with previous results measured by other methods. Our results illustrate the applicability of harmonic longitudinal voltage measurements for studying current-induced torques in magnetic heterostructures.

Lifting the valley degeneracy of monolayer transition-metal dichalcogenides (TMDs) can substantially break the balance of carriers in the $K$ and $K^{\mathrm{\prime }}$ valleys. This opens a promising way to utilize magnetic materials to break time-reversal symmetry to achieve valley splitting. In the work presented in this paper, we find that the magnitude of the valley splitting in TMD-based van der Waals (vdW) heterostructures is correlated with the strength of the magnetic proximity effect, which is positively related to the interlayer charge transfer and Coulomb interaction. As a result, for the same stacking, large valley splittings can be obtained only when the vdW heterostructure has a type-III, instead of a type-I or type-II, band alignment. We demonstrate this finding in $\mathrm{TMD}/\mathrm{Ni}{Y}_2$ ($Y=\mathrm{Cl},\mathrm{Br},\mathrm{I}$) vdW heterostructures in detail based on first-principles calculations, and predict several heterostructures with large valley splittings. This discovery can help to distinguish whether a magnetic material can enable TMDs to produce large valley splittings, and guide experiments for technological applications.

Individual disk-shaped $\mathrm{Co}\text{−}\mathrm{Fe}\text{−}\mathrm{B}$ nanodots are driven into a superparamagnetic state by a spin-transfer torque, and their time-dependent magnetoresistance fluctuations are measured as a function of current. A thin layer of oxidation at the edges has a dramatic effect on the magnetization dynamics. A combination of experimental results and atomistic spin simulations shows that pinning to oxide grains can reduce the likelihood that fluctuations lead to reversal, and can even change the easy-axis direction. Exchange-bias loop shifts and training effects are observed even at room temperature after brief exposure to small fields. The results have implications for studies of core-shell nanoparticles and small magnetic tunnel junctions and spin-torque oscillators.

The ability to manipulate acoustic fields beyond the diffraction limit offers possibilities for many applications, including ultrasound imaging and nondestructive testing. We numerically and experimentally report a simple method for subwavelength ultrasound focusing and superresolution imaging (i.e., images produced with a subwavelength spatial resolution) using a core-shell shaped lens. By changing the mechanical properties and size conditions of the lenses, we can significantly enhance the subwavelength properties of the focusing beams. These properties have a major effect on the image quality of ultrasonic systems. Using a carbon steel core and a Rexolite shell of total diameter of 12.8 mm positioned at a 2 MHz ($\lambda =0.74$ mm) plane wave incident field, we show that a beam with intensity gain of 20 dB, full width at half maximum (FWHM) of 0.44 mm ($0.6\lambda$), and full length at half maximum (FLHM) of 2.74 mm ($3.7\lambda$) can be achieved. Moreover, we experimentally demonstrate that a superresolution imaging system using the core-shell lens can scan objects with subdiffraction information. The results show that the superresolution method can double the spatial resolution and the focal depth can be increased using an appropriate size of lens. The proposed design system can be easily applied to assisted superresolution acoustic microscopy devices.

Superconducting-nanowire single-photon detectors promise efficient (approximately $100\mathrm{\%}$) and fast (approximately ${10}^9$ counts/s) detection of light at the single-photon level. They constitute one of the building blocks to realize integrated quantum optical circuits in a waveguide architecture. The optical response of single-photon detectors, however, is limited to measure only the presence of photons. It lacks the capability to resolve the spectrum of a possible broadband illumination. In this work, we propose the optical design for a superconducting-nanowire single-photon spectrometer in an integrated optical platform. We use a cascade of cavities with different resonance wavelengths side-coupled to a photonic crystal bus waveguide. This allows us to demultiplex different wavelengths into different spatial regions, where individual superconducting nanowires that measure the presence of single photons are placed next to these cavities. We use temporal coupled-mode theory to derive the optimal conditions to achieve a high absorption efficiency in the nanowire with fine spectral resolution. It is shown that the use of a mirror at the end of the cascaded system that terminates the photonic crystal bus waveguide increases the absorption efficiency up to unity, in principle, in the absence of loss. The expected response is demonstrated by full-wave simulations for both two-dimensional and three-dimensional structures. Absorption efficiencies of about $80\mathrm{\%}$ are achieved both in two-dimensional structures for four cascaded cavities and in three-dimensional structures for two cascaded cavities. The spectral resolution achieved is about $1\mathrm{nm}$. We expect that the proposed setup, both analytically studied and numerically demonstrated in this work, will offer a great impetus for future quantum nanophotonic on-chip technologies.

We study the nature of an environment-induced exceptional point in a non-Hermitian pair of coupled mechanical oscillators. The mechanical oscillators are a pair of pillars carved out of a single isotropic elastodynamic medium made of aluminum and consist of carefully controlled differential losses. The interoscillator coupling originates exclusively from background modes associated with the “environment,” that portion of the structure which, if perfectly rigid, would support the oscillators without coupling. We describe the effective interaction in terms of a coupled-mode framework in which only one nearby environmental mode can qualitatively reproduce changes to the exceptional-point characteristics. Our experimental and numerical demonstrations illustrate strategic directions utilizing environmental mode control for the implementation of exceptional-point degeneracies. Potential applications include a new type of noninvasive differential atomic force microscopy and hypersensitive sensors for the structural integrity of surfaces.

Antiferromagnets are outstanding candidates for the next generation of spintronic applications, with great potential for downscaling and decreasing power consumption. Recently, the manipulation of bulk properties of antiferromagnets has been realized by several different approaches. However, the interfacial spin order of antiferromagnets is an important integral part of spintronic devices, and thus, the successful control of interfacial antiferromagnetic spins is urgently desired. Here, we report the high controllability of interfacial spins in antiferromagnetic–ferromagnetic–heavy-metal heterostructure devices using spin-orbit torque (SOT) assisted by perpendicular or longitudinal magnetic fields. Switching of the interfacial spins from one direction to another through multiple intermediate states is demonstrated. The field-free SOT-induced reorientation of antiferromagnetic interfacial spins is also observed, which we attribute to the effective built-in out-of-plane field due to unequal upward and downward interfacial spin populations. Our work provides a precise way to modulate the interfacial spins at an antiferromagnet/ferromagnet interface via SOT, which will greatly promote innovative designs for next-generation spintronic devices.