Silicon nitride ($\mathrm{Si}\mathrm{N}$) drumhead resonators offer a promising platform for thermal sensing owing to their high mechanical quality factor and the high temperature sensitivity of their resonance frequency. As such, gaining an understanding of heat transport in $\mathrm{Si}\mathrm{N}$ resonators as well as their noise limitations are of interest, both of which are goals of the present work. We first present measurements of radiative heat transport in $\mathrm{Si}\mathrm{N}$ membranes, which we use for benchmarking two recently proposed theoretical models. We measure the characteristic thermal response time of square $\mathrm{Si}\mathrm{N}$ membranes with a thickness of 90 ± 1.7 nm and side lengths from 1.5 to 12 mm. A clear transition between radiation- and conduction-dominated heat transport is measured, in close correspondence with theory. In the second portion of this work, we use our experimentally validated heat transport model to provide a closed-form expression for thermal fluctuation-induced frequency noise in $\mathrm{Si}\mathrm{N}$ membrane resonators. We find that, for large-area $\mathrm{Si}\mathrm{N}$ membranes, thermal fluctuations can be greater than thermomechanical contributions to frequency noise. For the specific case of thermal radiation sensing applications, we also derive the noise-equivalent power resulting from thermal fluctuation-induced frequency noise, and we show in which conditions it reduces to the classical detectivity limit of thermal radiation sensors. Our work therefore provides a path towards achieving thermal radiation sensors operating at the unattained fundamental detectivity limit of bolometric sensing. We also identify questions that remain when attempting to push the limits of radiation sensing; in particular, the effect of thermal fluctuation noise in closed-loop frequency tracking schemes remains to be clarified.

Vacuum-packed particle (VPP) systems have been recently used in the development of smart structures due to their ability to actively change material stiffness through the mechanism of granular jamming. By altering the strength of the vacuum pressure applied to the particles, the material can be proportionally and reversibly transitioned from a liquidlike low-stiffness state to a stiff state. The ability to control material stiffness in this way opens up different possibilities for the design of morphing and smart structures by allowing them to soften during deformation to reduce actuation energy requirements and to then stiffen once the desired shape has been achieved to provide a zero-energy holding mechanism. The following research describes two useful models that predict the mechanical response of VPP beams under flexural loading. Firstly, four-point bending tests with digital image correlation strain mapping are performed in order to measure the axial and transverse strains in a bending beam made from vacuum-packed particles. The test results show a nonlinear mechanical response, including a change in beam thickness with deformation, that motivated an analytical model of the structure incorporating a nonlinear material stress model based on the Mohr-Coulomb failure envelope. In addition, finite-element simulations are implemented using a Johnson-Cook model extension to predict the response under loading of three-dimensional beam elements at different vacuum pressure levels. Lastly, the models are compared to the experimental results, indicating good agreement. Both methods are shown to be useful for predicting the variation of stiffness of vacuum-packed particle beams.

The anisotropic nature of the electrical conductance in low-symmetry two-dimensional materials is of great interest for emerging devices, as the existence of anisotropy potentially offers an additional degree of freedom. The comprehensive modulation of the conductance anisotropy in electronic devices is highly desired. However, so far, only tuning of the magnitude has been reported. Here, we report the comprehensive electrical modulation of conductance anisotropy in low-symmetry ${\mathrm{Re}\mathrm{S}}_2$ transistors. Device design ensures two comparable processes, i.e., thermionic emission and carrier drift, involved in device transport, which have opposite in-plane anisotropic distributions of conductance. Application of a gate voltage could switch over the dominance between these two processes, therefore, reversing the polarization directions of the maximum conductance. Systematic investigations also show the strong geometrical and temperature dependences of gate modulation. Conductance anisotropy can be tuned from a fourfold difference, with the maximum polarization in the direction of the light effective mass, to a sevenfold difference polarized in the perpendicular direction, i.e., the direction of the heavy effective mass. The demonstrated modulation facilitates the conductance anisotropy as an additional degree of freedom for exploring functionalities for future electronics in low-symmetry semiconductors.

We study the behavior of hydrated graphite oxide (GO) at high temperatures using thermally-accelerated molecular dynamics simulations based on ab initio density-functional theory. Our results suggest that GO, a viable candidate for water treatment and desalination membranes, is more heat resilient than currently used organic materials. The system we consider to represent important aspects of thermal processes in highly disordered GO is a hydrated GO bilayer in a vacuum. Our study provides microscopic insight into reactions involving water and functional epoxy-$\mathrm{O}$ and $\mathrm{OH}$ groups bonded to graphene layers, and also describes the swelling of the structure by water vapor pressure at elevated temperatures. We find the system withstands simulation temperatures up to approximately $2500\mathrm{K}$ before the graphitic layers start decomposing, implying the possibility of cleaning biofouling residue from a GO-based membrane by heating it in an inert-gas atmosphere.

Unlike their electromagnetic and acoustic counterparts, elastic waves involve different wave modes. The interplay and the coupling among them increase the complexity of the problem while also offering a larger space for wave manipulation. Elastic bulk wave conversion in an elastic metamaterial has recently shown great promise in medical ultrasound and nondestructive testing. Unlike the transmission-type conversion, however, reflective elastic mode conversion has been explored less in terms of analysis and design, despite the enormous possibilities that it might offer for energy trapping and dissipation. In this work, we develop a theoretical framework for constructing elastic anisotropic metasurfaces that can enable reflective longitudinal-to-transverse (L-to-T) and transverse-to-longitudinal (T-to-L) wave conversions. We capitalize on the mechanism of multiple reflective mode conversion to achieve broadband, subwavelength, and near perfect sound absorption in the underwater environment. The reflective scattering properties of the metasurfaces are systematically exploited for incident longitudinal or transverse waves. The conversion mechanism is rooted in reflective Fabry-Perot (FP) resonance, whose occurrence conditions and features are predicted for prescribed effective parameters of the metasurface. We then establish an inverse-design framework for conceiving an underwater coating system formed by a viscoelastic rubber layer and the metasurface. A series of metasurfaces allowing for customized mode conversions are realized for delivering broadband low-frequency and high-efficiency underwater sound absorption. Specifically, an ultrathin rubber-metasurface layer in which the metasurface with a thickness of approximately λ/70 can lead to nearly 100% sound absorption. Furthermore, we demonstrate that a persistently high absorption (over 80%) can be obtained in a rather robust manner within a wide range of wave incidence angle from −60° to 60°. More importantly, high-efficiency acoustic absorption exceeding 75% can be readily achieved through multiple mode conversions within the ultrabroadband range featuring a relative bandwidth of 119%. We reveal the combined FP resonance mechanism of underwater sound absorption, i.e., the FP resonance of the metaconverter, which determines the L-to-T and T-to-L conversion ratio, and the FP resonance of the rubber-metasurface layers, which enhances the wave attenuation inside the rubber. The proposed reflective multiple mode-conversion mechanism and metasurface design methodology open a route towards a class of elastic-wave-based devices with promising potential for underwater applications.

The demands on adaptive optics are increasing to achieve high performance and meet the requirements for accuracy and reliability with these systems. At present, common technologies for actuator arrays are highly developed and designed in complex arrangements leading to bulky devices. In this work, we propose an approach for designing modular actuation arrays by means of kirigami metasheets realigned in parallel and leveled in plane. The study focuses on the numerical analysis of different cut patterns to couple lift-off motions within a thin, scalable sheet. A design of a modular actuation array is explained and illustrated in the context of a deformable mirror. The optimal lift-off positions within the array are calculated for this example by a proposed positioning algorithm, which makes use of common methods in adaptive optics such as Zernike polynomial fitting. The algorithm is validated by several simulations calculating the membrane deflection, and the overall concept is discussed.

Domain-wall nanoelectronics has attracted extensive attention since the discovery of conductive domain walls, where domain-wall $p$-$n$ junctions are indispensable for device applications. Here, we report on the fabrication of a domain-wall $p$-$n$ junction based on $n$-type and $p$-type conductive domain walls in an $x$-cut lithium niobate thin film on an insulator using a lateral-electric-field poling technique. The electron and hole mobilities in the $n$-type and $p$-type domain walls are measured to be 337 and $93{\mathrm{cm}}^2{\mathrm{V}}^{-1}{\mathrm{s}}^{-1}$, respectively, more than 400 times higher than those in the parent bulk crystal. The successful fabrication of a domain-wall $p$-$n$ junction with large electron and hole mobilities in the domain walls makes lithium niobate on an insulator a promising platform for domain-wall nanoelectronics and optoelectronics.

Hybrid magnonic systems host a variety of characteristic phenomena such as the magnetically induced transparency (MIT) and Purcell effect, which are considered useful for future coherent quantum-information processing. In this work, we experimentally demonstrate a tunable MIT effect in the ${\mathrm{Y}}_3{\mathrm{Fe}}_5{\mathrm{O}}_{12}(\mathrm{YIG})/\mathrm{Permalloy}(\mathrm{Py})$ magnon-magnon coupled system via changing the magnetic field orientations. By probing the magneto-optic effects of Py and YIG thin films, we identify clear features of MIT spectra induced by the mode hybridization between the uniform mode of Py and the perpendicular standing spin-wave modes of YIG. By changing the external magnetic field orientations, we observe a tunable coupling strength between the YIG’s spin-wave modes and the Py’s uniform mode, upon the application of an out-of-plane magnetic field. This observation is theoretically interpreted by a geometrical consideration of the Py and YIG magnetization under the oblique magnetic field even at a constant interfacial exchange coupling. Our findings show high promise for investigating tunable coherent phenomena with hybrid magnonic platforms.

Most microwave readout architectures in quantum computing or sensing rely on a semiconductor amplifier at 4 K, typically a high-electron mobility transistor (HEMT). Despite its remarkable noise performance, a conventional HEMT dissipates several milliwatts of power, posing a practical challenge to scale up the number of qubits or sensors addressed in these architectures. As an alternative, we present an amplification chain consisting of a kinetic inductance traveling-wave parametric amplifier (KITWPA) placed at 4 K, followed by a HEMT placed at 70 K, and demonstrate a chain-added noise $T_{\mathrm{\Sigma }}=6.3\pm 0.5$ K between 3.5 and 5.5 GHz. While, in principle, any parametric amplifier can be quantum limited even at 4 K, in practice we find the performance of the KITWPA to be limited by the temperature of its inputs and by an excess of noise $T_{\mathrm{ex}}=1.9$ K. The dissipation of the rf pump of the KITWPA constitutes the main power load at 4 K and is about 1% that of a HEMT. These combined noise and power dissipation values pave the way for the use of the KITWPA as a replacement for semiconductor amplifiers.

We construct an analytical formulation for nonlinear photoelectron emission in a dc-biased metallic nanovacuum gap triggered by a laser field, by exactly solving the one-dimensional time-dependent Schrödinger equation. We theoretically investigate the photoelectron energy spectra and emission current from left- and right-side surfaces of the asymmetric nanojunction with various dc biases, laser fields, and gap distances. The underlying photoemission mechanisms transitioning form multiphoton over-barrier emission to photon-assisted field tunneling, and the spatiotemporal dynamics of electron transport inside the gap are analyzed in detail. Our calculation shows applying a dc field could greatly reduce the interference oscillation in the transmission current in the nanogap, due to the shift of dominant emission away from the multiphoton over-barrier regime. Our results demonstrate that, besides the dc bias, varying the gap spacing could strongly influence the rectification on the photoelectron emission in a dc-biased metal-vacuum-metal gap. Our study provides useful guideline to the design of ultrafast nanogap-based signal rectification devices, such as photoelectron emitters and photodetectors, by choosing an optimal combination of dc bias, gap spacing, and material properties.

Sound propagating inside a room is multiply scattered by the boundaries and other obstacles, forming a complex reverberating sound field (RSF). Such RSFs are not only spatially disordered but also temporally scrambled. Although the spatial control of steady-state RSFs at single frequencies is successfully achieved by extending the idea of wavefront shaping to acoustics, time-coherent polychromatic control of RSFs has remained out of reach. Here, we report spatiotemporal acoustic wavefield shaping by adaptively reshaping acoustic impulse responses at arbitrary positions and different instants in a reverberating room using spatial sound modulators—a reconfigurable and programmable membrane-type acoustic metasurface capable of encoding desirable phase factors over a finite bandwidth. We further analyze the mechanism and performance of spatiotemporal wavefield shaping with a simple numerical model derived from multiple scattering in the time domain. Our results expand the capabilities of RSF control to the temporal domain and demonstrate the possibilities for transient engineering of polychromatic sound in acoustic communication, imaging, and holography.

Thermal convection is one of the three basic mechanisms of heat transfer, which profoundly influences the natural environment, social production, and our daily life. However, the high complexity of the governing equation, which describes the coupling of heat and mass transfer, makes it difficult to manipulate thermal convection at will in both theory and experiment. Here, we consider the heat transfer in Hele-Shaw cells, a widely used model of Stokes flow between two parallel plates with a small gap, and apply the scattering-cancellation technology to construct convective thermal materials with bilayer structures and homogeneous isotropic materials. By tailoring thermal conductivities and viscosities, we demonstrate cloaking devices that can hide obstacles from heat and fluid motion simultaneously, and verify their robustness under various thermal-convection environments by numerical simulations. Our results show that about 80% of the temperature and pressure disturbances in the background caused by obstacles can be eliminated by the cloak. The developed approach can be extended to control other convection-diffusion systems or even other multiphysics processes, and the results pave a promising path for designing various metadevices such as concentrators or sensors.

In a typical quantum annealing protocol, the system starts with a transverse field Hamiltonian that is gradually turned off and replaced by a longitudinal Ising Hamiltonian. The ground state of the Ising Hamiltonian encodes the solution to the computational problem of interest, and the state overlap with this ground state gives the success probability of the annealing protocol. The form of the annealing schedule can have a significant impact on the ground-state overlap at the end of the anneal, so precise control over these annealing schedules can be a powerful tool for increasing success probabilities of annealing protocols. Here we show how superconducting circuits, in particular capacitively shunted flux qubits, can be used to construct quantum annealing systems by providing tools for mapping circuit flux biases to Pauli coefficients. We use this mapping to find customized annealing schedules: appropriate circuit control biases that yield a desired annealing schedule, while accounting for the physical limitations of the circuitry. We then provide examples and proposals that utilize this capability to improve quantum annealing performance.

Quantum sensors based on spin defect ensembles have seen rapid development in recent years, with a wide array of target applications. Historically, these sensors have used optical methods to prepare or read out quantum states. However, these methods are limited to optically polarizable spin defects, and the spin ensemble size is typically limited by the available optical power or acceptable optical heat load. We demonstrate a solid-state sensor employing a nonoptical state preparation technique, which harnesses thermal population imbalances induced by the defects’ zero-field splitting. Readout is performed using the recently demonstrated microwave cavity readout technique, resulting in a sensor architecture that is entirely nonoptical and broadly applicable to all solid-state paramagnetic defects with a zero-field splitting. The implementation in this work uses ${\mathrm{Cr}}^{3+}$ defects in a sapphire (${\mathrm{Al}}_2{\mathrm{O}}_3$) crystal and a simple microwave architecture where the host crystal also serves as the high quality-factor resonator. This approach yields a near-unity filling factor and high single-spin-photon coupling, producing a broadband magnetometer with a minimum sensitivity of $9.7\mathrm{pT}/\sqrt{\mathrm{Hz}}$ near 5 kHz.

Wideband absorption is a popular topic in microwave engineering to protect sensitive devices against broadband sources. However, the Bode-Fano criterion defines the trade-off between bandwidth and efficiency for all passive linear time-invariant systems. In this paper, we propose a broadband absorber beyond the Bode-Fano limit by creating an energy trap using time-modulated switches or diodes. This work starts with an ideal-circuit model to prove the concept, followed by two electromagnetic realizations—a frequency-selective-surface approach for general bandwidth broadening and a low-profile printed-circuit-board design. The prototype of the latter is built and measured, demonstrating a Bode-Fano integral larger than 1. This approach paves the way to many practical ultrawideband-absorber designs.

We demonstrate the two-color cooling and trapping of alkaline-earth atoms in a grating magneto-optical trap (GMOT). The trap is formed by a single incident laser beam together with four secondary beams that are generated via diffraction from a nanostructured wafer. A grating structure for a GMOT operating with strontium atoms is optimized and fabricated. We trap ${10}^6{}^{88}\mathrm{Sr}$ atoms on the ${}^1{S}_0{\to }^1{P}_1$ transition at $461\mathrm{nm}$ and transfer 25% of these atoms to the second cooling stage on the narrower ${}^1{S}_0{\to }^3{P}_1$ intercombination transition at $689\mathrm{nm}$, preparing a sample of $2.5\times {10}^5$ atoms at $5\mu \mathrm{K}$. These results demonstrate the applicability of the GMOT technology in conjunction with two widely differing wavelengths and enable the continued miniaturization of alkaline-earth-based quantum technologies like optical atomic clocks.

Ferroelectric tunnel junctions (FTJs) are very promising candidates for nonvolatile memory devices and a large tunneling electroresistance (TER) ratio is essential for their high performance. This work intends to achieve large TER ratio by interfacial doping in FTJs by taking ${\mathrm{Pt}/\mathrm{Ba}\mathrm{Ti}\mathrm{O}}_3/\mathrm{Pt}$ tunnel junctions as an example. By introducing Na (or $\mathrm{Li}$) substitutions for $\mathrm{Ti}$ atoms at the right interface, the resultant strong Coulomb repulsion from the negatively charged ${\mathrm{Na}\mathrm{O}}_2$ interface pushes the electrons to higher energy in an increasing manner from left to right in the whole ${\mathrm{Ba}\mathrm{Ti}\mathrm{O}}_3$ barrier, which leads to rapidly increasing potential energy profile and partial metallization close to the right interface in the left polarization state. However, in the right polarization state, since the right ferroelectric polarization produces a decreasing potential energy profile from left to right, although the ${\mathrm{Na}\mathrm{O}}_2$ interface also pushes the electrons to much higher energy and the slope of the potential energy profile changes from negative to positive, the final slope of the potential energy profile is much less steeper and the Fermi level is always inside the band gap, leading to a completely insulating state. The substantially different distributions of the electrostatic potential energy profile in the two polarization states lead to great differences in the transport properties. Based on density-functional-theory calculations, a TER ratio up to ${10}^5$% is achieved. The results indicate that a negatively charged interface based on interfacial substitution is a promising method for obtaining a large TER ratio in FTJs, and thus will have implications for the further understanding and design of high-performance FTJs.