In a moving acoustic medium, sound waves travel differently with and against the fluid flow. This well-established acoustic effect is backed by the intuition that the fluid velocity bias imparts momentum on the propagating acoustic waves, thus violating reciprocity. Based on this conception, fluid flow that is transverse to the wave direction of propagation will not break reciprocity. In this paper we contrast this common wisdom and theoretically show that the interplay between transverse mean flow and transverse structural gliding asymmetry can yield strong nonreciprocity and even, surprisingly, one-way acoustic waveguiding. To demonstrate that, we analyze a waveguide that comprises a few adjacent acoustic subdiffraction chains, i.e., linear chains of acoustic scatterers with monopolar or dipolar response. The structure is embedded within a medium with mean flow velocity that is transverse to the waveguide axis. We find the symmetry breaking conditions under which nonreciprocity is obtained, and we show how, under transverse mean flow, with Mach numbers as low as 0.02, one-way propagation of the acoustic wave is obtained on a sub-wavelength-thick acoustic waveguide. To demonstrate the phenomenon, we introduce two models. One in which the scatterers are assumed to be pointlike and the flow is homogeneous, and a second, in which we accommodate the issue of possible wakes created by the presence of scatterers inside the flow. The latter setup exhibits no direct interaction of the scatterers with the flow, by placing the waveguide in a quiescent medium adjacent to a flowing medium. Our results push forward the understanding of acoustic nonreciprocity, and may open another venue for the design of nonreciprocal acoustic wave devices for various applications.

We report, the use of laser-plasma-driven x rays to reveal the three-dimensional (3D) structure of a highly atomizing water spray. Soft x rays approximately 5 keV are generated by means of a laser-plasma accelerator. Transmission radiography measurements are performed at different angles, by rotating a multihole injector. Using computer tomography, the local liquid volume distribution and its spatial variation are retrieved in 3D, showing up to 55% liquid fraction at the nozzle outlet, which decreases to below 7% within only 1 mm. The resolution of the liquid volume fraction is 0.5% while the spatial resolution of the radiographic images is $11.5\mu \mathrm{m}$. The x-ray source used here provides successful measurements of liquid mass distribution over a relatively large volume and is very promising for the analysis of a variety of challenging transient spray systems, e.g., the injection of liquid synthetic and biofuels used for future clean-combustion applications.

Plasmons in two-dimensional electron systems (2DESs) feature ultrastrong confinement and are expected to efficiently mediate the interactions between light and charge carriers. Despite these expectations, the electromagnetic detectors exploiting 2D plasmon resonance have been so far inferior to their nonresonant counterparts. Here, we theoretically analyze the origin of these failures, and suggest a proper niche for 2D plasmonics in electromagnetic wave detection. We find that a confined 2DES supporting plasmon resonance has an upper limit of absorption cross section, which is identical to that of simple metallic dipole antenna. Small size of plasmonic resonators implies their weak dipole moments and impeded coupling to free-space radiation. Achieving the “dipole limit” of the absorption cross section in isolated 2DES is possible either at unrealistically long carrier momentum relaxation times, or at resonant frequencies below units of terahertz. We further show that amendment of even small metal contacts to 2DES promotes the coupling and reduces the fundamental mode frequency. The contacted resonators can still have deep-subwavelength size. They can be merged into compact arrays of detectors responding resonantly to multiple frequencies. Such arrays may find applications in multichannel wireless communications, hyper-spectral imaging, and energy harvesting.

This study examines Bragg’s band gap and its mechanical tuning in a stretch-buckled kirigami sheet with “zig-zag” distributed parallel cuts. When stretched beyond a critical threshold, the kirigami buckles out of plane and generates a three-dimensional periodic architecture. Our theoretical calculation, numerical simulation, and experiments confirm the transverse elastic wave-propagation band gaps and their correlation to stretching. This result opens an avenue of using kirigami as a simple and effective approach for creating and adapting periodicity for wave-propagation control.

A band-to-band tunneling FET (TFET) with an atomical two-dimensional (2D) channel is a potential candidate for the next-generation electronic device in view of its steep subthreshold swing and low power consumption. However, how to establish a precise physical model between the band property of 2D channel materials and the device performance of 2D TFETs is the key to accelerate their practical applications. Herein, through high-throughput first-principles calculations, we study the tunneling transport properties of 44 representative 2D materials with four kinds of crystal system. Particularly, we propose a well-defined and striking exponential scaling law $(\mathrm{SS}=A{\mathrm{e}}^{B{m}_r}+C)$ between the subthreshold swings (SSs) and reduced effective masses $(m_r)$ of 10-nm TFETs. According to the exponential model, 2D orthorhombic and trigonal crystal TFETs with the steep SS hold the reduced effective masses of more than 0.1 and $0.2m_0$, which could inhibit tunneling leakage current. These insights provide guidance for material screening in the construction of post-Moore 2D low-power transistors.

Unidirectional magnetoresistance (UMR) in a ferromagnetic bilayer due to the spin Hall effect (SHE) provides a facile means of probing in-plane magnetization to avoid complex magnetic tunnel junctions. However, the UMR signal is very weak and usually requires a lock-in amplifier for detection, even in a bilayer involving $\mathrm{Ta}$ or $\mathrm{Pt}$ with a large spin Hall angle (SHA). Here, we report a type of UMR, termed the anomalous UMR (AUMR), in a single $\mathrm{Co}\text{−}\mathrm{Fe}\text{−}\mathrm{B}$ layer without any adjacent SHE layers, where the UMR signal is about 10 times larger than that in $\mathrm{Ta}/\mathrm{Co}\text{−}\mathrm{Fe}\text{−}\mathrm{B}$ structures and can be detected by using conventional dc multimeters in the absence of lock-in amplifiers. We further demonstrate that the extracted AUMR, by excluding thermal contributions, shows reversal signs for the $\mathrm{Co}\text{−}\mathrm{Fe}\text{−}\mathrm{B}$ and $\mathrm{Ni}\text{−}\mathrm{Fe}$ single layers with opposite SHAs, indicating that the AUMR may originate from the self-generated spin accumulation interacting with magnetization through the giant-magnetoresistance-like mechanism. These results suggest that the AUMR contributes UMR signals larger than the interfacial spin Hall UMR in the $\mathrm{Co}\text{−}\mathrm{Fe}\text{−}\mathrm{B}$-based systems, providing a convenient and reliable approach to detect in-plane magnetization for two-terminal spintronic devices.

The ability to measure the passage of electrical current with high spatial and temporal resolution is vital for applications ranging from inspection of microscopic electronic circuits to biosensing. The ability to image such signals passively and remotely is of great importance, in order to measure without invasive disruption of the system under study or the signal itself. A recent approach to achieving this utilizes point defects in solid-state materials; in particular, nitrogen-vacancy centers in diamond. Acting as a high-density array of independent sensors, addressable opto-electronically and highly sensitive to factors including temperature and magnetic field, these are ideally suited to microscopic wide-field imaging. In this work, we demonstrate simultaneous spatially and temporally resolved recovery signals from a microscopic lithographically patterned circuit. Through application of a lock-in amplifier camera, we demonstrate micrometer-scale imaging resolution with a millimeter-scale field of view with simultaneous spatially resolved submillisecond (up to 3500 frames ${\mathrm{s}}^{-1}$) recovery of dc to kilohertz alternating and broadband pulsed-current electrical signals, without aliasing or undersampling. We demonstrate as examples of our method the recovery of synthetic signals replicating digital pulses in integrated circuits and signals that would be observed in a biological neuronal network in the brain.

We propose a scheme for the generation of optimal squeezed states for Ramsey interferometry. The scheme consists of an alternating series of one-axis twisting pulses and rotations, both of which are straightforward to implement experimentally. The resulting states show a metrological gain proportional to the Heisenberg limit. We demonstrate that the Heisenberg scaling is maintained even when placing constraints on the amplitude of the pulses implementing the one-axis twisting and when taking into account realistic losses due to photon scattering.

We show the physical realization and experimental demonstration of an exceptional point of sixth-order degeneracy in a triple-ladder (or three-way) microwave waveguide realized using three coupled microstrips on a grounded dielectric substrate. This three-way waveguide supports six Bloch eigenmodes, and all coalesce into a degenerate single eigenmode at a given frequency. The three-way waveguide is gainless, and this exceptional point is associated with a vanishing-group velocity and its multiple derivatives. Indeed, the $\omega -k$ dispersion diagram that we call the sixth-order degenerate band edge (6DBE) has six coalescing branches. We provide experimental verification of a sixth-order exceptional point by evaluating the degenerate wave number–frequency-dispersion diagram from the measurement of scattering parameters of a six-port unit cell. We also show the resonant behavior of a cavity made of the three-way waveguide with finite length. The unique properties of 6DBE can be exploited to design innovative high-Q resonators, oscillators, filters, and pulse-shaping devices.

Known coherent metasurfaces control the interference of waves of a given frequency with other coherent waves at the same frequency, either illuminating from a different direction or created as intermodulation products. In this paper, we introduce a class of metasurfaces that are modulated in time coherently with the illuminating radiation. Such modulation opens a possibility to control reflection, absorption, and transmission at multiple frequencies, including illuminations by two or more incoherent waves. In particular, we study dynamic resistive layers and show how to use them to design thin multifrequency perfect absorbers that overcome the bandwidth limit for static linear absorbers. Furthermore, we demonstrate possibilities of remote tuning of the absorption level. We hope that this work opens up alternative avenues in wave engineering using coherent modulation of metasurface parameters.

We investigate the physical origin of nonlinear damping due to mode coupling between several auto-oscillatory modes driven by spin-orbit torque in constricted $\mathrm{Pt}$/Py(permalloy) heterostructures by examining the dependence of the auto-oscillation on the temperature and the applied field orientation. We observe a transition in the nonlinear damping of the auto-oscillation modes extracted from the total oscillation power as a function of the drive current, which coincides with the onset of power redistribution among several modes and the crossover from linewidth narrowing to linewidth broadening in all individual modes. This indicates the activation of another relaxation process by nonlinear magnon-magnon scattering within the modes. We also find that both nonlinear damping and the threshold current in the mode-interaction damping regime at high drive current after transition are temperature independent, suggesting that the mode coupling occurs dominantly through a nonthermal magnon-scattering process via a dipole or exchange interaction rather than thermally excited magnon-mediated scattering. This finding presents a promising pathway toward overcoming the current limitations of efficiently controlling the interaction between two highly nonlinear magnetic oscillators to prevent mode crosstalk or intermode energy transfer and deepens the understanding of complex nonlinear spin dynamics in multimode spin-wave systems.

The current density in a superconductor with turnarounds or constrictions is nonuniform due to a geometrical current-crowding effect. This effect reduces the critical current in the superconducting structure compared to a straight segment and is of importance when designing superconducting devices. We investigate the current-crowding effect in numerical simulations within the generalized time-dependent Ginzburg-Landau (GTDGL) model. The results are validated experimentally by measuring the magnetic field dependence of the critical current in superconducting-nanowire structures, similar to those employed in single-photon detector devices. Comparing the results with London theory, we conclude that the reduction in critical current is significantly smaller in the GTDGL model. This difference is attributed to the current redistribution effect, which reduces the current density at weak points of the superconductor and counteracts the current-crowding effect. We numerically investigate the effect of the fill factor on the critical current in a meander and conclude that the reduction of the critical current is low enough to justify fill factors higher than 33% for applications where the detection efficiency is critical. Finally, we propose a meander design that can combine a high fill factor and low current crowding.

We report contactless measurements of the viscoelastic rheological properties of soft gels. The experiments are performed using a colloidal-probe atomic force microscope in a liquid environment and in dynamic mode. The mechanical response is measured as a function of the liquid gap thickness for different oscillation frequencies. Our measurements reveal an elastohydrodynamic coupling between the flow induced by the probe oscillation and the viscoelastic deformation of the gels. The data are quantitatively described by a viscoelastic lubrication model. The frequency-dependent storage and loss moduli of the polydimethylsiloxane gels are extracted from fits of the data to the model and are in good agreement with the Chasset-Thirion law. Our results demonstrate that contactless colloidal-probe methods are powerful tools that can be used for probing soft interfaces finely over a wide range of frequencies.

Today’s quantum processors composed of fifty or more qubits have allowed us to enter a computational era where the output results are not easily simulatable on the world’s biggest supercomputers. What we have not seen yet, however, is whether or not the complexity achievable with such quantum processors would ever be useful for any practical applications. In the quantum neural network approach, there has been an expectation that the Hilbert space can serve as a computational resource by providing a large feature space; however, it is not understood, nor illustrated in what way the Hilbert space provides that computational power. In this work we introduce a resource-efficient quantum neural network model with a focus on the role of the feature space the quantum processors can give. Then we apply it for classification tasks, showing a quantum advantage in terms of the physical resources needed to realize the neural network. To facilitate the Hilbert space as a large feature space, we utilize scale-free networks that can be generated in a discrete-time crystal model. The virtue of this approach is in both its theoretical simplicity and its practical applicability: our quantum neural network model is simple enough to identify the role of the Hilbert space as the feature space, and, furthermore, can be implemented with current technology. Our model does not require optimization of the quantum processor, and hence once the quantum processor has been set, it can be used for other different classification problems without any changes in the quantum processor itself.

We report on the direct current (dc) current-voltage ($I$-$V$) characteristics of few-layer muscovite mica (MuM) flakes exfoliated and transferred onto $\mathrm{Si}\mathrm{O}$${}_2$/$\mathrm{Si}$ substrate, under different substrate dc bias voltages. Contrary to usual observations in conventional two-dimensional systems, we observe an increase in the in-plane electrical conductivity with a reducing thickness of MuM flakes. At a given voltage, the electrical conductivity of approximately five-layered MuM flake ($T_3$) is 3 orders of magnitude larger than that in approximately ten-layered MuM flake ($T_2$). The $I$-$V$ characteristics are used to analyze the mechanism of conduction. The model-based analysis reveals the hopping-conduction mechanism to be dominant as compared to the Poole-Frenkel effect. The thickness-dependent work function is measured using Kelvin probe force microscopy for a MuM flake on $\mathrm{Si}$ substrate. Assuming that the measured work function is correlated with the Fermi level, we report an upward movement of the Fermi level, toward the conduction band with the reducing thickness of MuM flakes, indicating an increase in the conduction-band carrier density. The observed increase in conductivity in $T_3$ when compared to $T_2$ may be attributed to surface doping due to the increased contribution from ${\mathrm{K}}^{+}$ ions and lattice relaxation. Our results show that there is a possibility of using few-layer mica as a wide-band-gap semiconductor and that it can open up different avenues for two-dimensional electronic devices.