For semiconductors, adding a degree of freedom beyond charge, e.g., spin and valley, will lead to alternative physics and device applications. Here, we demonstrate that another electronic degree of freedom, the chirality of Weyl node, can be used in the Weyl semiconductor tellurium, a unique system harnessing intriguing Weyl physics and high tunability of the semiconductor. By constructing a field-effect device based on quasi-two-dimensional tellurium flakes, the Fermi level can be effectively tuned from the top of valence bands, where the Weyl nodes locate at, into the bandgap via electrostatic gating. In addition to a significant reduction of channel conductivity, a transition from chiral-anomaly-induced negative magnetoresistance to conventional positive magnetoresistance occurs at the same time, indicating complete suppression of the chirality-related topological transport. The simultaneous switch of both conducting and topological states is unprecedented in previous Weyl semimetals. Our findings pave the way for developing new-principle semiconductor devices with fascinating functionalities.

Ambipolar field-effect transistors (FETs) based on two-dimensional (2D) channel materials have enabled various applications such as in-memory computing, nonlinear logic, and frequency multipliers, taking advantage of the simultaneous transport of electrons and holes. From a device simulation perspective, it is much harder to handle ambipolar transport than unipolar electron or hole transport, mainly due to the splitting of quasi-Fermi-levels. While setting up a proper recombination model, the problem can be addressed by self-consistently solving the continuity equations, this brings about tremendous computational complexity and model uncertainty. In this work, we introduce the concept of quasi-Fermi-level phase space (QFLPS) that is spanned by the electron and hole quasi-Fermi potentials ($V_n$ and $V_p$), on which the drain-source current is written as a curve integral within the QFLPS without explicitly involving the spatial message. However, the curve requires the relation between $V_n$ and $V_p$ that is unknown a priori. A promising solution is to search the conditions where the integral is almost path independent, such that the drain current may be evaluated through the simplest path. We convert the problem into inspecting the conservative property of a field defined on QFLPS, the work done by which (i.e., the curve integral) gives the current. A detailed analysis for devices working under normal conditions predicts that the QFLPS can be divided into full-curl and null-curl regions, where within the latter the integral can be regarded as path independent. The exact criteria have been prescribed and the simplest linear path from source to drain is suggested as the path for fast calculation, leading to the so-called quasiequilibrium approximation (QEA). For extensive well-designed examples, the QEA results have been compared with the exact solutions derived from continuity equations. The concept of proper Fermi path has been proposed to connect the approximations and exact solutions. QFLPS is not only a theoretical basis for the fast evaluation of drain current, but it also provides a general platform for analyzing the device behaviors of ambipolar 2D FETs. Actually, it is applicable to unipolar FETs as well. Based on QEA’s current formula, typical applications of QFLPS are demonstrated, including recovering the various operational modes of 2D FETs as well as the logic inverter, and an ambipolar-2D-FET-based threshold inverter quantization circuit as the core component of a flash analog-to-digital converter is proposed, which achieves a linearithmic dependence of the chip size with the quantization levels compared with the quadratic dependence offered by the traditional CMOS scheme. The prospect of QFLPS in 2D FET-based circuit design is demonstrated.

Drift instabilities in bounded two-dimensional electron systems (2DESs) promise robust applications in terahertz technology. Until recent years, the severest obstacle for drift instability development was the Ohmic loss, but now the focus has shifted to viscous limitations. With this work, we aim to construct a firm foundation for further research in viscosity-limited plasmon instabilities. Namely, with the help of a recently developed perturbative approach, we show that highly unstable plasmonic modes in viscosity-free models are most subject to viscous damping in realistic 2DESs. We illustrate such a gain-viscosity competition in a toy model and describe an algorithm for maximizing the ratio of drift gain to viscous loss for a given setup. More importantly, we provide a tool for deliberate choice of a setup and in a concrete example we show how to design the device that would exhibit the lowest possible instability threshold current.

To prevent the severe effects of earthquake on built systems, structural engineering pursues attenuation of vibrations on structures. A recently surfaced means to structural vibration mitigation exploits the concept of metamaterials, i.e., of configurations able to control wave propagation in specific frequency ranges, termed band gaps. The current study harnesses the potency of a geometrically nonlinear unit-cell design, which can develop negative stiffness, and explores the vibration-attenuation capabilities of the resulting metamaterial device. An analytical approach is followed to calculate the expected attenuation zone, as well as for calculating the dependence on the amplitude of the input, a hallmark of the nonlinear behavior. For the purpose of validation of a proof-of-concept system, dynamic tests are performed on a scaled model assembled using LEGO${}^{\circledR }$ components. Besides showing that such a nonlinear system can be easily constructed, these tests illustrate the potential of this nonlinear design for vibration reduction within the targeted band-gap frequency zones and the protection that it can offer to a primary system. Finally, numerical analyses are used to verify the analytical calculations of the dispersion relation and are additionally compared to the experimental results, evaluating the incorporated modeling assumptions. The possibility to lower the band gap in the typical seismic engineering frequency range and to maintain a broadband attenuation at low frequency show that negative stiffness may enhance the performance of metamaterials for seismic protection.

Recently proposed spintronic devices use magnetic skyrmions as bits of information. The reliable detection of those chiral magnetic objects is an indispensable requirement. Yet, the high mobility of magnetic skyrmions leads to their stochastic motion at finite temperatures, which hinders the precise measurement of the topological numbers. Here, we demonstrate the successful training of artificial neural networks to reconstruct the skyrmion number in confined geometries from time-integrated, dimensionally reduced data. Our results prove the possibility of recovering the topological charge from a time-averaged measurement and hence smeared dynamic skyrmion ensemble, which is of immediate relevance to the interpretation of experimental results, skyrmion-based computing, and memory concepts.

We report on nanometer magnetic imaging of two-dimensional current flow in bilayer graphene devices at room temperature. By combining dynamical modulation of the source-drain current with ac quantum sensing of a nitrogen-vacancy center in the diamond probe tip, we acquire magnetic field and current density maps with excellent sensitivities of 4.6 nT and $20\mathrm{nA}/\mu \mathrm{m}$, respectively. The spatial resolution is 50–100 nm. We introduce a set of methods for increasing the technique’s dynamic range and for mitigating undesired back-action of magnetometry operation (scanning tip, laser and microwave pulses) on the electronic transport. Finally, we show that our imaging technique is able to resolve small variations in the current flow pattern in response to changes in the background potential. Our experiments demonstrate the feasibility for detecting and imaging subtle spatial features of nanoscale transport in two-dimensional materials and conductors.

This work proposes and numerically studies a skyrmionic-magnon-switch (SMS) device, where a Néel-type skyrmion acts as a programmable scattering center to route propagating spin waves. Our results show that the effective deflection of magnons by a skyrmion strongly depends on the ratio of magnon wavelength to skyrmion diameter, which is as large as 30° when the skyrmion diameter is comparable to the magnon wavelength. This offers the possibility to program the routing of magnons in a network of ferromagnetic wires, by controlling the perpendicular magnetic anisotropy of the ferromagnetic film via the voltage-controlled magnetic anisotropy effect, which, in turn, controls the skyrmion size, stability, and spin-wave deflection angle. The proposed SMS device allows for electrical routing of spin waves between branches of a magnonic circuit, a functionality that is fundamental to emerging magnonic logic and computing concepts. It is shown that on:off ratios as large as 90× are achievable in such an SMS device using realistic material parameters.

Quantum information theory sets the ultimate limits for any information-processing task. In rangefinding and LIDAR, the presence or absence of a target can be tested by detecting different states at the receiver. In this Paper, we use quantum hypothesis testing for an unknown coherent-state return signal in order to derive the limits of symmetric and asymmetric error probabilities of single-shot ranging experiments. We engineer a single measurement independent of the range, which in some cases saturates the quantum bound and for others is presumably the best measurement to approach it. This work bridges the gap between quantum information and quantum sensing and engineering and will contribute to devising better ranging sensors, as well as setting the path for finding practical limits for other quantum tasks.

${\mathrm{Bi}\mathrm{Fe}\mathrm{O}}_3$ is a rich room-temperature multiferroic material in which noncollinear antiferromagnetic spin cycloids can be deterministically controlled by an electric field through the magnetoelectric interaction, opening perspectives for low-power reconfigurable antiferromagnetic spintronics. Using a commercial scanning nitrogen-vacancy (N-V) magnetometer, we are able to image two different types of spin cycloids stabilized in strain-engineered ${\mathrm{Bi}\mathrm{Fe}\mathrm{O}}_3$ epitaxial thin films. We show that, in these samples harboring two ferroelectric variants, each ferroelectric domain is coupled to a single spin cycloid, giving rise to a zigzag magnetic pattern. These ferroelectric domains can be manipulated at the local scale by piezoresponse force microscopy, allowing the design of micron-sized single domains. Thanks to its coupled optical microscope and fast-imaging capabilities, the scanning N-V magnetometer enables a quick repositioning in such areas of interest. Finally, quantitative imaging on single ferroelectric domains provides insights into the physical parameters of each spin-cycloid type and their impact on the magnetic-stray-field measurements.

A high acoustic energy density is required in the acoustic resonator to increase the throughput of acoustophoresis devices. In this study, through both experiments and numerical simulations, we find that the energy density in bulk-wave-acoustophoresis devices can be enhanced by actuating the device from the side. Based on qualitative free-flow focusing experiments, side actuation shows clear superiority to bottom actuation under various input powers and flow rates. Quantitative measurements using confocal microparticle image velocimetry confirm an increase by a factor of 4 in energy density using side actuation. Numerical simulations reveal that side actuation leads to significant symmetry breaking, which accounts for strong acoustic fields in the channel, and the device energy-conversion efficiency using side actuation is also higher than that using bottom actuation for different device aspect ratios. Submicrometer particle focusing is performed using an acoustophoresis device with side actuation and more than 90% of 500-nm-diameter particles are focused under a total flow rate of $30\mu \mathrm{l}{\min }^{-1}$ at an input power of 235 mW, achieved without using a power amplifier.

Coupling a resonator to a superconducting qubit enables various operations on the qubit, including dispersive readout and unconditional reset. The speed of these operations is limited by the external decay rate of the resonator. However, increasing the decay rate also increases the rate of qubit decay via the resonator, limiting the qubit lifetime. Here, we demonstrate that the resonator-mediated qubit decay can be suppressed by utilizing the distributed-element, multimode nature of the resonator. We show that the suppression exceeds 2 orders of magnitude over a bandwidth of 600 MHz. We use this “intrinsic Purcell filter” to demonstrate a 40-ns readout with 99.1% fidelity and a 100-ns reset with residual excitation of less than 1.7%.

Specialized radio-frequency coils and sensors placed inside a magnetic resonance imaging (MRI) scanner considerably extend its functionality. However, since cable-connected in-bore devices have several disadvantages compared to the wireless ones, the latter are currently under active development. One of the promising concepts in wireless MRI coils is energy harvesting, which relies on converting the energy carried by the radio-frequency MRI field without the need for additional transmitters, similar to common wireless power-transfer solutions. In this paper, we propose a compact harvesting coil design based on the combination of loop and butterfly coils that allows energy harvesting of a circularly polarized field. By performing numerical simulations and experiments with commonly used Siemens Espree and Avanto $1.5$ tesla MRI scanners, we demonstrate that the proposed approach is safe, efficient, does not decrease the quality of MRI images, and allows doubling the harvested voltage compared to the setups with linearly polarized fields.

Degenerate parametric amplifiers (DPAs) exhibit the unique property of phase-sensitive gain and can be used to noiselessly amplify small signals or squeeze field fluctuations beneath the vacuum level. In the microwave domain, these amplifiers have been utilized to measure qubits in elementary quantum processors, search for dark matter, facilitate high-sensitivity spin resonance spectroscopy and have even been proposed as the building blocks for a measurement-based quantum computer. Until now, microwave DPAs have almost exclusively been made from nonlinear Josephson junctions, which exhibit high-order nonlinearities that limit their dynamic range and squeezing potential. In this work we investigate a microwave DPA that exploits a nonlinearity engineered from kinetic inductance. The device has a simple design and displays a dynamic range that is four orders of magnitude greater than state-of-the-art Josephson DPAs. We measure phase-sensitive gains up to $50\mathrm{dB}$ and determine that the device likely operates near the quantum noise limit. Additionally, we show that the higher-order nonlinearities that limit other microwave DPAs are almost nonexistent for this amplifier, which allows us to demonstrate its potential to perform squeezing by measuring the deamplification of coherent states by as much as 26 dB.

A series of III-V ternary and quarternary digital alloy avalanche photodiodes have recently been seen to exhibit very low excess noise. Using band inversion of an environment-dependent atomistic tight-binding description of short-period superlattices, we argue that a combination of increased effective mass, minigaps, and band split-off are primarily responsible for the observed superior performance. These properties significantly limit the ionization rate of one carrier type, either holes or electrons, making the avalanche multiplication process unipolar in nature. The unipolar behavior in turn reduces the stochasticity of the multiplication gain. The effects of band folding on carrier transport are studied using the nonequilibrium Green’s function method that accounts for quantum tunneling, and the Boltzmann transport equation model for scattering. It is shown here that carrier transport by intraband tunneling and optical phonon scattering are reduced in materials with low excess noise. Based on our calculations, we propose five simple inequalities that can be used to approximately evaluate the suitability of digital alloys for designing low-noise photodetectors. We evaluate the performance of multiple digital alloys using these criteria and demonstrate their validity.

Coherent control of collective spin excitations (magnons) with acoustic phonons is a key technology for future hybrid spintronics devices. Surface acoustic waves (SAWs), owing to their high frequency, short wavelength, and low radiation loss, could provide an appropriate platform for integrating the devices on a chip. However, the tiny coupling efficiency in previous experiments using traveling SAWs limits the availability of this prospect. We report a planar cavity magnomechanical system, where standing acoustic waves (i.e., a SAW cavity) enhance the spatial and spectral power density to implement magnified magnon-phonon coupling. Excitation of spin-wave resonance involves significant acoustic power absorption approaching $90\mathrm{\%}$, whereas the collective spin motion exerts hybridization force on the cavity dynamics, leading to coupling cooperativity exceeding unity at room temperature in such a chip-scale device. The cavity magnomechanical system paves the way to the coherent acoustic control of magnons and the development of alternative spin-acoustic technologies for classical and quantum applications.

Time-dependent interphase diffusion processes in multiphase heterogeneous media are ubiquitous phenomena in physics, chemistry and biology. Examples of heterogeneous media include composites, geological media, gels, foams, and cell aggregates. The recently developed concept of spreadability, $\mathcal{S}(t)$, provides a direct link between time-dependent diffusive transport and the microstructure of two-phase media across length scales [Torquato, S., Phys. Rev. E., 104 054102 (2021)]. To investigate the capacity of $\mathcal{S}(t)$ to probe microstructures of real heterogeneous media, we explicitly compute $\mathcal{S}(t)$ for well-known two-dimensional and three-dimensional idealized model structures that span across nonhyperuniform and hyperuniform classes. Among the former class, we study fully penetrable spheres and equilibrium hard spheres, and in the latter class, we examine sphere packings derived from “perfect glasses,” uniformly randomized lattices (URLs), disordered stealthy hyperuniform point processes, and Bravais lattices. Hyperuniform media are characterized by an anomalous suppression of volume fraction fluctuations at large length scales compared to that of any nonhyperuniform medium. We further confirm that the small-, intermediate-, and long-time behaviors of $\mathcal{S}(t)$ sensitively capture the small-, intermediate-, and large-scale characteristics of the models. In instances in which the spectral density ${\tilde{\chi }}_{{}_V}(\mathbf{k})$ has a power-law form $B|\mathbf{k}{|}^{\alpha }$ in the limit $|\mathbf{k}|\to 0$, the long-time spreadability provides a simple means to extract the value of the coefficients $\alpha$ and $B$ that is robust against noise in ${\tilde{\chi }}_{{}_V}(\mathbf{k})$ at small wave numbers. For typical nonhyperuniform media, the intermediate-time spreadability is slower for models with larger values of the coefficient $B={\tilde{\chi }}_{{}_V}(0)$. Interestingly, the excess spreadability $\mathcal{S}(\mathrm{\infty })-\mathcal{S}(t)$ for URL packings has nearly exponential decay at small to intermediate $t$, but transforms to a power-law decay at large $t$, and the time for this transition has a logarithmic divergence in the limit of vanishing lattice perturbation. Our study of the aforementioned models enables us to devise an algorithm that efficiently and accurately extracts large-scale behaviors from diffusion data alone. Lessons learned from such analyses of our models are used to determine accurately the large-scale structural characteristics of a sample Fontainebleau sandstone, which we show is nonhyperuniform. Our study demonstrates the practical utility of the diffusion spreadability to extract crucial microstructural information from real data across length scales and provides a basis for the inverse design of materials with desirable time-dependent diffusion properties.

We present general analytical criteria for the design of lossless reciprocal two-port systems, which exhibit prescribed scattering spectra $S(\omega )$ satisfying $S_{22}(\omega )=e^{i\phi }{S}_{11}(\omega )$, including symmetric ($S_{22}=S_{11}$) or “antimetric” ($S_{22}=-S_{11}$) responses, such as standard filters (Butterworth, Chebyshev, elliptic, etc.). We show that the non-normalized resonant (quasinormal) modes (QNMs) of all such two-port systems couple to the input and output ports with specific unitary ratios, whose relative signs determine the position of the scattering zeros on the real frequency axis. This allows us to obtain design criteria assigning values to the poles, background response, and QNM-to-port coupling coefficients. Filter devices can then be designed via a well-conditioned nonlinear optimization (or root-finding) problem using a numerical eigensolver. As an application, we design multiple microwave metasurfaces configured for polarization-preserving transmission, reflective polarization conversion, or diffractive “perfect anomalous reflection” to realize filters that precisely match standard bandpass or bandstop filters of various types, orders and bandwidths, with focus on the best-performing elliptic filters.

This paper is a contribution to the Physical Review Applied collection titled Photovoltaic Energy Conversion.

Exciton diffusion plays a decisive role in various organic optoelectronic applications, including lasing, photodiodes, light-emitting diodes, and solar cells. Understanding the role that exciton diffusion plays in organic solar cells is crucial to understanding the recent rise in power conversion efficiencies brought about by nonfullerene acceptor (NFA) molecules. Established methods for quantifying exciton diffusion lengths in organic semiconductors require specialized equipment designed for measuring high-resolution time-resolved photoluminescence (TRPL). In this paper we introduce an approach, named pulsed-photoluminescence quantum yield (PLQY), to determine the diffusion length of excitons in organic semiconductors without any temporal measurements. Using a Monte Carlo model, the dynamics within a thin-film semiconductor are simulated and the results are analyzed using both pulsed-PLQY and TRPL methods. It is found that pulsed-PLQY has a larger operational window and depends less on the excitation fluence than the TRPL approach. The simulated results are validated experimentally on a well-understood organic semiconductor, after which pulsed-PLQY is used to evaluate the diffusion length in a variety of technologically relevant materials. It is found that the diffusion lengths in NFAs are much larger than in the benchmark fullerene and that this increase is driven by an increase in diffusivity. This result helps explain the high charge generation yield in low-offset state-of-the-art NFA solar cells.

Benchmarking and characterizing quantum states and logic gates is essential in the development of devices for quantum computing. We introduce a Bayesian approach to self-consistent process tomography, called fast Bayesian tomography (FBT), and experimentally demonstrate its performance in characterizing a two-qubit gate set on a silicon-based spin qubit device. FBT is built on an adaptive self-consistent linearization that is robust to model approximation errors. Our method offers several advantages over other self-consistent tomographic methods. Most notably, FBT can leverage prior information from randomized benchmarking (or other characterization measurements), and can be performed in real time, providing continuously updated estimates of full process matrices while data are acquired.

The rotational erection system (RES) is an origami-based design method to generate a three-dimensional (3D) structure from a planar sheet. Its rotational and translational kinematics are fully encoded in the form of prescribed cuts and folds. Here we characterize the mechanical properties of a threefold symmetric RES by combining finite element analysis and a physical experiment. Whereas the crease and cut patterns imply that the system is not rigid foldable, we demonstrate that plate bending creates a physical route connecting the two energetically separated configurations. The elasticity of the RES allows the RES to morph into a 3D shape via a snapthrough transition. We show that the energy barrier for bistability is independent of the entire span of the structure, and depends solely on its aspect ratio. We also indicate that the standing RES has structural rigidity and resilience, owing to its unique self-locking mechanism, indicating its superior load-bearing performance in a range of applications. The present study clarifies the fundamental actuation mechanism of an origami-based deployable structure extended with chiral patterned cuts, which facilitates a way to use the optimally designed RES in a variety of human-made systems.