Acoustic radiation torque (ART) plays an important role in the subject of acoustophoresis, which could induce the spinning rotation of particles on their own axes. The partial wave series method (PWSM) and T matrix method (TMM) are employed to investigate the three-dimensional radiation torques of both spherical and nonspherical objects in a single Bessel beam over broader frequencies (from Rayleigh scattering to geometric-optics regimes), with emphasis on the parametric conditions for torque reversal and the corresponding physical mechanisms. For elastic objects, the dipole, quadrupole, and even several higher-scattering modes are dominant to induce the axial ARTs beyond the Rayleigh limit with a relatively large offset from the particle centroid to the beam axis in Bessel beams. The reversals appear with parametric conditions (including wave number, cone angle, and offset) for the extrema or null values of the corresponding cylindrical Bessel functions. The reversal of transverse ART from a rigid nonspherical particle is also investigated in an ordinary Bessel beam and the geometrical surface roughness is briefly studied for the effect on the radiation torques. This suggests the possibility of acoustic tweezers controlling the spinning motion of particles within and beyond the Rayleigh limit.

Spin-orbit torque in heavy metal/ferromagnet heterostructures with broken spatial inversion symmetry provides an efficient mechanism for manipulating magnetization using a charge current. We report the presence of a spin torque in a single ferromagnetic layer in both asymmetric $\mathrm{Mg}\mathrm{O}/{\mathrm{Fe}}_{0.8}{\mathrm{Mn}}_{0.2}$ and symmetric $\mathrm{Mg}\mathrm{O}/{\mathrm{Fe}}_{0.8}{\mathrm{Mn}}_{0.2}/\mathrm{Mg}\mathrm{O}$ structures, which manifests itself in the form of an effective field transverse to the charge current. The current to effective field conversion efficiency, which is characterized using both the nonlinear magnetoresistance and second-order planar Hall effect methods, is comparable to the efficiency in typical heavy metal/ferromagnet bilayers. We argue that the torque is caused by spin rotation in the vicinity of the surface via impurity scattering in the presence of a strong spin-orbit coupling. Instead of cancelling each other, the torques from the top and bottom surfaces simply add up, leading to a fairly large net torque, which is readily observed experimentally.

Scanning probe microscopy is often extended beyond simple topographic imaging to study electrical forces and sample properties, with the most widely used experiment being frequency-modulated Kelvin probe force microscopy. The equations commonly used to interpret this frequency-modulated experiment, however, rely on two hidden assumptions. The first assumption is that the tip charge oscillates in phase with the cantilever motion to keep the tip voltage constant. The second assumption is that any changes in the tip-sample interaction happen slowly. Starting from an electromechanical model of the cantilever-sample interaction, we use Lagrangian mechanics to derive coupled equations of motion for the cantilever position and charge. We solve these equations analytically using perturbation theory, and, for verification, numerically. This general approach rigorously describes scanned probe experiments even in the case when the usual assumptions of fast tip charging and slowly changing samples properties are violated. We develop a Magnus-expansion approximation to illustrate how abrupt changes in the tip-sample interaction cause abrupt changes in the cantilever amplitude and phase. We show that feedback-free time-resolved electric force microscopy cannot uniquely determine subcycle photocapacitance dynamics. We then use first-order perturbation theory to relate cantilever frequency shift and dissipation to the sample impedance even when the tip charge oscillates out of phase with the cantilever motion. Analogous to the treatment of impedance spectroscopy in electrochemistry, we apply this approximation to determine the cantilever frequency shift and dissipation for an arbitrary sample impedance in both local dielectric spectroscopy and broadband local dielectric spectroscopy experiments. The general approaches that we develop provide a path forward for rigorously modeling the coupled motion of the cantilever position and charge in the wide range of electrical scanned probe microscopy experiments where the hidden assumptions of the conventional equations are violated or inapplicable.

The impact of spin-transfer torque (STT) on the write-error rate of a voltage-torque-based magnetoresistive random-access memory is theoretically analyzed by use of the macrospin model. During the voltage pulse the STT assists or suppresses the precessional motion of the magnetization depending on the initial magnetization direction. The characteristic value of the current density is derived by our balancing the STT and the external-field torque, which is about $5\times {10}^{11}\mathrm{A}/{\mathrm{m}}^2$. The results show that the write-error rate is insensitive to the STT below a current density of ${10}^{10}\mathrm{A}/{\mathrm{m}}^2$.

A model of superconducting computer memory exploiting the orthogonal spin transfer (OST) in a pseudo-spin-valve (PSV) that is controlled by a three-terminal Josephson superconducting-ferromagnetic transistor (SFT) is developed. The building blocks of the memory are hybrid PSV and SFT structures. The memory model is formulated in terms of the equation-defined PSV and SFT devices integrated into the PSV-SFT-memory-cell (MC) circuit. Logical units “0” and “1” are associated with the two PSV states characterized by two different resistance values. Elementary logical operations comprising the read-write processes occur when a word pulse applied to the SFT’s injector coincides with the respective bit pulse acting on the MC. Physically, a word pulse switches the SFT to a resistive state, causing PSV switching between the logical “0” and “1” states. Thus, the whole switching dynamics of the MC depends on the nonequilibrium and nonstationary properties of the PSV and SFT. Modeling of the single MC as well as larger MC-based circuits comprising 12 and 30 elements, respectively, suggests that such memory cells can undergo ultrafast switching (subnanosecond) and have low energy consumption per operation (sub-100 fJ). The model suggested allows the study of the influence of noise, the punch-through effect, cross talk, parasitic effects, etc. The results obtained suggest that the hybrid PSV-SFT structures are well suited to superconducting computing circuits as they are built from magnetic and nonmagnetic transition metals and therefore have low impedances (1–30 Ω).

We study the structural, optical, and transport properties of sidewall quantum wires (QWRs) on $\mathrm{Ga}\mathrm{As}$(001) substrates. QWRs are grown by MBE on $\mathrm{Ga}\mathrm{As}$(001) substrates prepatterned with shallow ridges. They form as a consequence of material accumulation on the sidewalls of the ridges during the overgrowth of a quantum well (QW) on the patterned surface. The QWRs are approximately 200 nm wide and have emission energies red shifted by 27 meV with respect to the surrounding QW. Spatially resolved spectroscopic PL studies indicate that the QW thickness reduces around the QWRs, thus creating a 4-meV energy barrier for the transfer of carriers from the QW to the QWR. We show that the QWRs act as efficient channels for the transport of optically excited electrons and holes over tens of $\mu \mathrm{m}$ by a high-frequency surface acoustic wave (SAW). These results demonstrate the feasibility of efficient ambipolar transport in QWRs with submicrometer dimensions, photolithographically defined on $\mathrm{Ga}\mathrm{As}$ substrates.

We demonstrate a metamaterial superlens: a planar array of discrete subwavelength metamolecules with individual scattering characteristics tailored to vary spatially to create subdiffraction superoscillatory focus of, in principle, arbitrary shape and size. Metamaterial free-space lenses with previously unattainable effective numerical apertures – as high as 1.52 – and foci as small as 0.33λ in size are demonstrated. Super-resolution imaging with such lenses is experimentally verified breaking the conventional diffraction limit of resolution and exhibiting resolution close to the size of the focus. Our approach will enable far-field label-free super-resolution nonalgorithmic microscopies at harmless levels of intensity, including imaging inside cells, nanostructures, and silicon chips, without impregnating them with fluorescent materials.

We propose a spin valve based on a blue-phosphorus monolayer sandwiched between two half-metallic two-dimensional $\mathrm{Cr}\mathrm{N}$ layers. We use density-functional theory combined with Boltzmann transport theory to investigate both the structural and the magnetic stability of the $\mathrm{Cr}\mathrm{N}$/$\mathrm{P}$/$\mathrm{Cr}\mathrm{N}$ heterostructure and to study its spin-dependent transport properties. Among the different possible layer stackings considered, only one is shown to be thermodynamically stable, corresponding to the AA stacking. In this geometry, the critical temperature of magnetic ordering is estimated to be around 150 K. The electronic structure of $\mathrm{Cr}\mathrm{N}$/$\mathrm{P}$/$\mathrm{Cr}\mathrm{N}$ is strongly dependent on the mutual orientation of the magnetic moments in individual $\mathrm{Cr}\mathrm{N}$ layers. If the alignment is parallel, only one spin channel predominantly contributes to the electronic bands in the vicinity of the Fermi energy. In the case of an antiparallel alignment, both spin channels contribute to the electronic states. The alteration of magnetic moments affects electronic transport, causing magnetoresistance of up to 12% at moderate dopings.

Dielectric laser acceleration (DLA) represents a promising approach to building miniature particle accelerators on a chip. However, similar to conventional rf accelerators, an automatic and reconfigurable control mechanism is needed to scale DLA technology towards high-energy gains and practical applications. We propose a system providing control of the laser coupling to DLA using integrated optics and introduce a component for power distribution using a reconfigurable mesh of Mach-Zehnder interferometers. Using numerical simulations, we show how such a mesh may be sequentially and efficiently tuned to optimize power distribution in the circuit and find that this strategy has favorable scaling properties with respect to size of the mesh.

Increasing dampinglike spin-orbit torque (SOT) is of fundamental importance for enabling new research into spintronics phenomena and also technologically urgent for advancing low-power spin-torque memory, logic, and oscillator devices. Here, we demonstrate that enhancing interfacial scattering by inserting ultrathin layers within spin Hall metals with intrinsic or side-jump mechanisms can significantly enhance the spin Hall ratio. The dampinglike SOT is enhanced by a factor of 2 via submonolayer $\mathrm{Hf}$ insertion, as evidenced by both harmonic response measurements and current-induced switching of in-plane magnetized magnetic memory devices with the record low critical switching current of approximately 73 µA (switching current density of approximately $3.6\times {10}^6\mathrm{A}/{\mathrm{cm}}^2$). This work demonstrates a very effective strategy for maximizing dampinglike SOT for low-power spin-torque devices.

Bubbles have a rich history as transducers in particle-physics experiments. In a solid-state analogue, we use bubble domains in nanomagnetic films to measure magnetic nanoparticles. This technique can determine the magnetic orientation of a single nanoparticle in a fraction of a second and generate a full hysteresis loop in a few seconds. We achieve this high throughput by tuning the nanomagnetic properties of the films, including the Dzyaloshinskii-Moriya interaction, in an application of topological protection from the skyrmion state to a nanoparticle sensor. We develop the technique on nickel-iron nanorods and iron-oxide nanoparticles, which delineate a wide range of properties and applications. Bubble magnetometry enables precise statistical analysis of the magnetic hysteresis of dispersed nanoparticles, and direct measurement of a transition from superparamagnetic behavior as single nanoparticles to collective behavior in nanoscale agglomerates. These results demonstrate a practical capability for measuring the heterogeneity and interaction of magnetic nanoparticles.

In this paper, we investigate the boundary-layer profiles that form over a sharp, hollow cylinder in supersonic air and ${\mathrm{N}}_2$ flows with a krypton tagging velocimetry (KTV) single-laser scheme. The supersonic flows are generated by the passage of the primary shock wave over the model in the Stevens shock tube. The experiments are performed in two gas mixtures doped with $\mathrm{Kr}$: 99% ${\mathrm{N}}_2/1$% $\mathrm{Kr}$, to model ${\mathrm{N}}_2$, and 75% ${\mathrm{N}}_2/20$% ${\mathrm{O}}_2/5$% $\mathrm{Kr}$, to model air. The experimental setup allows us to vary the pressure and Reynolds number from 3–25 kPa and $1.5\times {10}^5$ to $1.5\times {10}^6{\mathrm{m}}^{-1}$, respectively, while the Mach number is kept fixed at 1.7. The static temperature and pressure (but not the velocity) are representative of typical large-scale high-enthalpy hypersonic impulse facilities. The KTV data points over the hollow cylinder are mapped to corresponding wall-normal locations above a flat plate, which enables comparison with the similarity solution for compressible boundary-layer flow. Agreement between the similarity solution and experimental results is excellent. Relative to previous two-laser KTV schemes, the single-laser approach used in this work has the advantage of being simpler and more cost effective, but it has a higher laser-energy requirement, 10 mJ/pulse in these experiments. Single-laser KTV is implemented by increasing the energy of the write-laser pulse to a sufficient level such that the $\mathrm{Kr}$ becomes partially ionized via a ($2+1$) resonance-enhanced, multiphoton ionization (REMPI) process with an excitation wavelength of 212.6 nm. The write step records the fluorescence that results primarily from the spontaneous emission from the two-photon excitation. After a prescribed delay, the read step records the fluorescence that results from the transitions that follow the recombination process. The signal-to-noise ratio (SNR) is sufficient to extract velocity profiles from single-shot, shock-tube experiments. Two-photon absorption cross-section calculations and emission spectra are presented to justify the chosen excitation wavelength and support our understanding of the $\mathrm{Kr}$ excitation and emission scheme.

Spectrum is a basic dimension of waves ranging from electromagnetic to acoustic waves where information can be encoded and multiplexed. The manipulation of the sound spectrum is desirable in applications of acoustic communication and voice encryption, which, however, is challenging to realize. Here, based on temporally modulated waveguides, we create effective gauge fields to generate frequency domain Bloch oscillations (FBOs) to control the spectrum of sound. The modulation can induce mode transitions in the waveguide band and form a discrete frequency lattice where the wave vector mismatch during transitions acts as an effective electric field that drives FBOs. Furthermore, we find the modulation phase accompanying transitions serves as an effective gauge potential that can control the initial oscillation phase. We report that multiple FBOs with judiciously designed oscillation phases can be further cascaded to realize acoustic spectrum reconstruction, unidirectional transduction, and bandwidth engineering. This study reveals the significance of gauge fields in FBOs and functionalizes its cascaded configurations for advanced control of the sound spectrum. This paradigm may find versatile applications in acoustic secure communication, information encryption, and processing.

Acoustic bianisotropic materials couple pressure and local particle velocity fields to simultaneously excite monopole and dipole scattering, which results in asymmetric wave transmission and reflection of airborne sound. We systematically realize an arbitrarily given bianisotropic coupling between the pressure and velocity fields for asymmetric wave propagation by an acoustic grating with inversion symmetry breaking. This acoustic bianisotropic grating is designed by optimizing the unit cells with a finite element method to achieve the desired scattering wavevectors determined by the bianisotropic induced asymmetric wave propagation. The symmetry and Bloch wavevectors in the reciprocal space resulting from the grating are analyzed, which match with the desired scattering wavevectors. The designed structures are fabricated for the experimental demonstration of the bianisotropic properties. The measured results match with the desired asymmetric wave scattering fields.

We exploit the nonlinearity arising from the spin-photon interaction in an $\mathrm{In}\mathrm{As}$ quantum dot to demonstrate phase shifts of scattered light pulses at the single-photon level. Photon phase shifts of close to ${90}^{\circ }$ are achieved using a charged quantum dot in a micropillar cavity. We also demonstrate a photon phase switch by using a spin-pumping mechanism through Raman transitions in an in-plane magnetic field. The experimental findings are supported by a theoretical model that explores the dynamics of the system. Our results demonstrate the potential of quantum-dot-induced nonlinearities for quantum information processing.

Radio-frequency (rf) reflectometry is implemented in hybrid semiconductor-superconductor nanowire systems designed to probe Majorana zero modes. Two approaches are presented. In the first, hybrid nanowire-based devices are part of a resonant circuit, allowing conductance to be measured as a function of several gate voltages approximately $40$ times faster than using conventional low-frequency lock-in methods. In the second, nanowire devices are capacitively coupled to a nearby rf single-electron transistor made from a separate nanowire, allowing rf detection of charge, including charge-only measurement of the crossover from $2e$ interisland charge transitions at zero magnetic field to $1e$ transitions at axial magnetic fields above 0.6 T, where a topological state is expected. Single-electron sensing yields a signal-to-noise ratio exceeding 3 and a visibility of 99.8% for a measurement time of $1\mu \mathrm{s}$.

We study optical and magneto-optical (MO) properties of epitaxially grown modulated $\mathrm{Fe}$-$\mathrm{Pt}$ multilayer metamaterials by means of generalized MO ellipsometry (GME). Large Kerr rotation, ellipticity, and MO coupling parameters in the visible and near-infrared regions are obtained for modulated metamaterials, in which the stacking sequences follow an inverse Fibonacci series. In contrast, periodic multilayers show small MO parameters, although they otherwise exhibit magnetic and optical properties that are very similar to those for the modulated metamaterials. The present study demonstrates that specific thickness modulation can have a significant materials design potential for one-dimensional magnetic metamaterials.

The prediction of ultralow magnetic damping in ${\mathrm{Co}}_2\mathrm{Mn}Z$ Heusler half-metal thin-film magnets is explored in this study and the damping response is shown to be linked to the underlying electronic properties. By substitution of the Z elements in high-crystalline-quality films (${\mathrm{Co}}_2\mathrm{Mn}Z$ with Z = $\mathrm{Si}$, $\mathrm{Ge}$, $\mathrm{Sn}$, $\mathrm{Al}$, $\mathrm{Ga}$, $\mathrm{Sb}$), electronic properties such as the minority-spin band gap, Fermi-energy position in the band gap, and spin polarization can be tuned and the consequences for magnetization dynamics analyzed. The experimental results allow us to directly explore the interplay of spin polarization, spin gap, Fermi-energy position, and the magnetic damping obtained in these films, together with predictions from ab initio calculations. The ultralow magnetic damping coefficients measured in the range from 4.1 × ${10}^{-4}$ to 9 × ${10}^{-4}$ for ${\mathrm{Co}}_2\mathrm{Mn}\mathrm{Si}$, ${\mathrm{Co}}_2\mathrm{Mn}\mathrm{Ge}$, ${\mathrm{Co}}_2\mathrm{Mn}\mathrm{Sn}$, and ${\mathrm{Co}}_2\mathrm{Mn}\mathrm{Sb}$ are the lowest values obtained on a conductive layer and offer a clear experimental demonstration of theoretical predictions on half-metal magnetic Heusler compounds and a pathway for future materials design.

$\mathrm{L}{1}_0$ $\mathrm{Mn}\mathrm{Al}$ permanent-magnet alloys are promising candidates to plug the gap in performance and applications between rare-earth permanent magnets and ferrites; doping with a third atom is essential to achieve decent magnetic properties and good $\mathrm{L}{1}_0$-phase stability. In this work, the influence of substitutional atoms on intrinsic magnetic properties in $\mathrm{L}{1}_0$ $\mathrm{Mn}\mathrm{Al}$ alloys is investigated by combined experimental and theoretical studies. A highly pure $\mathrm{L}{1}_0$ phase is synthesized in ${\mathrm{Mn}}_{55}{\mathrm{Al}}_{45-x}{M}_x$ ($M$ = $\mathrm{Co}$, $\mathrm{Cu}$, $\mathrm{Ga}$) alloys by regulation of the heat treatment, on the basis of ternary phase diagrams. It is demonstrated by experimental measurements and calculations that $\mathrm{Cu}$ and $\mathrm{Co}$ tend to enter 1a (0, 0, 0) sites, where they degrade the intrinsic magnetic properties of the alloys, and $\mathrm{Ga}$ prefers the 1d (1/2, 1/2, 1/2) sites, where it improves them. The excess valence electrons of $\mathrm{Cu}$ or $\mathrm{Co}$ compared with $\mathrm{Mn}$ lead to stronger bonding with $\mathrm{Al}$, resulting in the preference of occupying 1a (0, 0, 0) sites. The similar valence-electron structure of $\mathrm{Ga}$ compared with $\mathrm{Al}$ makes $\mathrm{Ga}$ tend to occupy 1d (1/2, 1/2, 1/2) sites. On the basis of this understanding, we propose a strategy of replacing the antiferromagnetic $\mathrm{Mn}$ atoms by 3d atoms with fewer valence electrons than $\mathrm{Mn}$, which may effectively enhance the intrinsic magnetic properties. Moreover, the stability of the alloyed $\mathrm{L}{1}_0$ phase is investigated; $\mathrm{Co}$ is found to degrade the stability of the phase, but the addition of $\mathrm{Cu}$ or $\mathrm{Ga}$ can stabilize it. These results provide guidance for further performance optimization and composition design of $\mathrm{Mn}$-based rare-earth permanent magnets.

One of the most promising metasurface architectures for the microwave and terahertz frequency ranges consists of three patterned metallic layers separated by dielectrics. Such metasurfaces are well suited to planar fabrication techniques and their synthesis is facilitated by modeling them as impedance sheets separated by transmission lines. We show that this model can be significantly inaccurate in some cases, due to near-field coupling between metallic layers. This problem is particularly severe for higher frequency designs, where fabrication tolerances prevent the patterns from being highly subwavelength in size. Since the near-field coupling is difficult to describe analytically, correcting for it in a design typically requires numerical optimization. We propose an extension of the widely used equivalent-circuit model to incorporate near-field coupling and show that the extended model can predict the scattering parameters of a metasurface accurately. Based on our extended model, we introduce an improved metasurface synthesis algorithm that gives physical insight to the problem and efficiently compensates for the perturbations induced by near-field coupling. Using the proposed algorithm, a Huygens metasurface for beam refraction is synthesized showing a performance close to the theoretical efficiency limit despite the presence of strong near-field coupling.