The pyroelectric response of materials is a promising phenomenon that is used in many applications. Here, we report first-principles lattice dynamics and ab initio molecular dynamics calculations to investigate the mechanisms of the pyroelectric effect in the $Pca{2}_1$ orthorhombic phase of ${\mathrm{Hf}\mathrm{O}}_2$; this is an emerging pyroelectric material with promising technological applications. With first-principles lattice dynamics calculations, we demonstrate a large pyroelectric response that is intrinsic to the pure $Pca{2}_1$ orthorhombic phase. Unlike most conventional ferroelectric pyroelectrics, such large pyroelectricity arises unexpectedly from the secondary effect (induced by the negative longitudinal piezoelectric effect) that has not received due attention. Ab initio molecular dynamics simulations further demonstrate a giant pyroelectric response that is associated with an orthorhombic (polar) to tetragonal (nonpolar) structural phase transition. Upon the incorporation of $\mathrm{Si}$ dopants with increasing dopant concentrations, the phase transition persists, but occurs at reduced temperatures with enhanced pyroelectric responses. The present work highlights the importance of the secondary pyroelectric effect responsible for the pyroelectricity in ${\mathrm{Hf}\mathrm{O}}_2$ and its promising potential for device applications.

A method for noncontact particle manipulation is realized here by employing parametric excitation (PE) on levitated particles in acoustic traps. PE enables one to oscillate a particle at one of its natural frequencies in the direction associated with the vibration mode, at a controllable amplitude, while the acoustic traps remain fixed in space. Moreover, the method allows one to select and oscillate a specific particle, while multiple particles are being levitated, by taking advantage of the relatively narrow bandwidth of PE. The underlying principle that allows levitation and PE is based on acoustic radiation forces, which are generated due to nonlinear phenomena in strong ultrasound wave fields. To utilize the method, the levitated particle dynamics within the acoustic trap should be known. Therefore, a lumped parameter model that describes the particle's nonlinear dynamics is derived and its parameters are estimated from measured data. The dynamics of a levitated rigid sphere, when subjected to PE, is studied analytically, numerically, and experimentally. The analytical solution is approximated by the method of multiple scales, and is in a good agreement with the experimental results and numerical simulations. Finally, it is experimentally shown that, if a group of particles is levitated, a specific selected particle can be excited, while the others remain mostly still. It is further shown that the particle's oscillation amplitude can be made so large that it will be ejected from the acoustic trap.

Magnetic imaging techniques visualize magnetization states in detail, including magnetization directions and magnetic domains. Generally, such imaging methods determine only the relative magnetization, while the absolute value of magnetization remains unknown. Measuring absolute magnetization requires the separate measurement of a reference material, but for samples with just a few ferromagnetic layers, these techniques can no longer be applied. We show a simple magneto-optical imaging method for obtaining the absolute value of magnetization using the parallel stripe domains formed by an electric current in thin films. The parallel stripes in the films consist of repeating up- and down-magnetized domains with a certain periodicity. In the absence of an external magnetic field, the normalized areas of up and down domains are equal. However, application of an external perpendicular magnetic field can change these normalized areas of each domain because the magnetic field prefers only one domain depending on the direction. Measuring these normalized areas and the stripe periodicity using conventional imaging techniques is sufficient to determine the absolute value of magnetization.

Creating complex spatial objects from a flat sheet of material using origami folding techniques has attracted attention in science and engineering. We design, model, and fabricate reconfigurable origami-inspired metamaterials whose unit cells can be easily tailored into numerous specific shapes. Via combinations of different shapes of the unit cell in the metamaterial panel, we numerically and experimentally demonstrate various specific sound wave manipulations, including acoustic focusing, beam splitting, sound localization, and one-way transmission. Our design features simple configuration, tunable structure, multiple functionalities, and is light weight. This finding provides an alternative route for controllable sound manipulation, paving the way toward a reconfigurable functional metadevice based on origami-inspired metamaterials.

Anticounterfeiting is an effective method to decrease the harmful effects of counterfeit goods. Optical metasurfaces can tailor light's amplitude, phase, and polarization at subwavelength resolution, enabling compact optical devices with unusual functionalities that can outperform conventional bulky components. We propose and experimentally demonstrate a multichannel metasurface device that can reconstruct helicity multiplexed holographic images and hide a grayscale image in the polarization profile of a light beam, which can be used for high-level anticounterfeiting. A dielectric metasurface consisting of nanopillars with spatially variant orientations is used to realize distinct functionalities in multiple channels. The two holographic images can be switched by changing the helicity of the incident light, while the encoded image in the polarization profile of the light beam can be revealed by a polarizer. Our work demonstrates the feasibility of the multifunctional device based on the independent control of phase and polarization in different channels, which may open an alternative window for compact optical devices with high density of functionalities for anticounterfeiting.

$\mathrm{Cr}\mathrm{As}$ is a well-known superconductor, which shows a pronounced volume contraction around a Néel temperature ($T_N$) of 260 K. However, it has not been considered as a negative thermal expansion (NTE) material for industrial applications partially because the temperature window of its phase transition is limited and does not cover room temperature. We prepare some $\mathrm{Cr}\mathrm{As}$-based compounds (${\mathrm{Cr}}_{0.98}{\mathrm{Ni}}_{0.02}\mathrm{As}$, $\mathrm{Cr}\mathrm{As}$, and ${\mathrm{Cr}\mathrm{As}}_{0.91}{\mathrm{S}}_{0.09}$) and investigate their linear NTE properties. By chemical modification, a giant NTE can be realized in this system with a broad temperature window of 218 K from 122 to 340 K. Typically, a linear NTE coefficient of $-28.3\times {10}^{-6}{\mathrm{K}}^{-1}$ in a broad NTE operation-temperature window of 98 K (from 242 to 340 K) is obtained in the ${\mathrm{Cr}\mathrm{As}}_{0.91}{\mathrm{S}}_{0.09}$ compound. In the region between 275 and 315 K around room temperature, the linear NTE coefficient ($-51.2\times {10}^{-6}{\mathrm{K}}^{-1}$) remains nearly independent of temperature. In addition, the $\mathrm{Cr}\mathrm{As}$-based compounds show large thermal and electrical conductivities and their dimensions are insensitive to the external magnetic field. The aforementioned excellent properties suggest that the $\mathrm{Cr}\mathrm{As}$-based compounds can serve as an alternative type of NTE materials and have promising applications, especially in the magnetic field environment.

In this work, we present a numerical analysis comparing the field-effect passivation provided by fixed dielectric charges on planar and textured undiffused silicon surfaces. We focus particularly on the ratio of effective surface recombination velocities ($S_{\mathrm{eff}}$) of planar and textured surfaces ($\sigma =S_{\mathrm{eff},\mathrm{textured}}/S_{\mathrm{eff},\mathrm{planar}}$) utilizing the same fixed dielectric charge density ($Q_f$) under the same injection conditions. To ensure a difference between the space-charge region profiles of textured and planar surfaces, we specifically analyze structures with large aspect ratios and submicron feature sizes. A wide range of $Q_f$ ($1\times {10}^7$ to $1\times {10}^{13}{\mathrm{cm}}^{-2}$) having both negative and positive polarities is covered in the analysis. We show that $\sigma$ approximately follows the surface area enhancement for very small and large $Q_f$, which induce weak band-bending and strong accumulation or inversion conditions, respectively. For moderately large $Q_f$, on the other hand, $\sigma$ shows a stronger deviation from the surface area enhancement. To elucidate the underlying physical mechanisms leading to the nonmonotonous relation between $\sigma$ and $Q_f$, we analyze the electrostatics of opposing charged surfaces. We show that an enhanced field-effect passivation is not realized along a large portion of a textured surface having a large $Q_f$ due to the significant carrier localization near the surface.

Deep-frequency-modulation interferometry combines optical minimalism with multifringe readout. However, precision is key for applications such as optical gradiometers for satellite geodesy or as dimensional sensors for ground-based gravity experiments. We present a single-component interferometer smaller than a cubic inch. Two of these are compared to each other to demonstrate tilt and displacement measurements with a precision of less than 20 $\mathrm{nrad}/\sqrt{\mathrm{Hz}}$ and 1 $\mathrm{pm}/\sqrt{\mathrm{Hz}}$ at frequencies below 1 Hz.

We present results for a $\mathrm{Au}$-$\mathrm{Ge}$ alloy that is useful as a resistance-based thermometer from room temperature down to at least 0.2 K. Over a wide range, the electrical resistivity of the alloy shows a logarithmic temperature dependence, which simultaneously retains the sensitivity required for practical thermometry while also maintaining a relatively modest and easily measurable value of resistivity. We characterize the sensitivity of the alloy as a possible thermometer and show that it compares favorably with commercially available temperature sensors. We experimentally identify that the characteristic logarithmic temperature dependence of the alloy stems from Kondo-like behavior induced by the specific heat treatment it undergoes.

This work presents theoretical investigations on the spontaneous-radiation loss of two-dimensional van der Waals heterojunction photovoltaic cells (2D-PVcs) combined with an interference-based light-trapping structure. We find that 2D-PVcs possess much lower spontaneous radiation loss, since the solid angle of the spontaneous radiation is a magnitude smaller than that in traditional PVcs. The efficiency limit of a 2D-PVc can be $10\mathrm{\%}$ larger than that of a traditional PVc. Furthermore, we demonstrate that the spontaneous-radiation loss can be further reduced by introducing only a weak light-concentrating system, approaching the efficiency limit of traditional PVcs under perfect focus. In a trapped structure, the smaller the light absorption coefficients of the photovoltaic cells, the higher are the efficiency limits that are achieved. Finally, the spontaneous-radiation loss increases as the battery thickness increases, indicating that the 2D-PVc has the lowest spontaneous emission loss because of its extremely low thickness.

Spin-orbit coupling (SOC) in nonmagnetic heavy metals (HMs) gives rise to the spin Hall effect (SHE), which is widely used to switch perpendicularly magnetized structures with an assistive magnetic field. In a similar manner, the anomalous Hall effect (AHE) arising from bulk SOC in ferromagnets (FMs) can also induce spin polarization. We theoretically show how both SHE and AHE can be used to realize field-free switching of perpendicular magnetization in FM1(fixed)/HM/FM2(free) trilayer structures. We calculate the resulting spin torque due to SHE (in the HM) and/or AHE (in the fixed FM1) analytically and numerically, and perform macrospin switching simulation without an assistive magnetic field. While it is understandable that AHE can achieve field-free switching due to the out-of-plane polarization it generates, rather surprisingly, field-free switching of perpendicular magnetization can also occur in the sole presence of SHE when the fixed FM1 has an out-of-plane magnetization. In addition, the AHE and SHE can be combined constructively for magnetization switching by having spin Hall and anomalous Hall angles of opposite signs.

Typical acoustic refractive metasurfaces governed by the generalized Snell law require several types of subwavelength subunits to provide an extra phase gradient along the surface. However, current design strategies have several drawbacks; for instance, the complex subwavelength subunits result in inevitable viscous loss, and ignoring the coupling between adjacent subunits leads to low efficiency in wavefront manipulations, especially for large angles. To overcome these limitations, we propose a metasurface or metagrating composed of only one straight channel and several surface-etched grooves per period. By harnessing the nonlocal coupling between the channel and the grooves, and the evanescent modes inside them, superior feasible acoustic transmission manipulations can be achieved. Nearly perfect acoustic bending with transmission efficiency up to $95\mathrm{\%}$ is obtained in theory and experiments for an extremely large angle of ${81}^{\circ }$. Our results extend the realm of acoustic metasurfaces and offer alternative opportunities for the development of highly efficient acoustic devices for wavefront manipulations.

We describe an electrothermal model for the turn-on dynamics of superconducting nanowire single-photon detectors (SNSPDs). By extracting a scaling law from a well-known electrothermal model of SNSPDs, we show that the rise time of the readout signal encodes the photon number as well as the length of the nanowire with scaling $t_{\mathrm{rise}}\propto \sqrt{\ell /n}$. We show that these results hold regardless of the exact form of the thermal effects. This explains how SNSPDs have an inherent photon-number-resolving capability. We experimentally verify the photon-number dependence by collecting waveforms for different photon numbers, rescaling them according to our predicted relation, and performing statistical analysis that shows that there is no statistical significance between the rescaled curves. Additionally, we use our predicted dependence of the rise time on the detector length to provide further insight into previous theoretical work by other authors. By assuming a specific thermal model, we predict that rise time will scale with the bias current, $t_{\mathrm{rise}}\propto \sqrt{1/I_b}$. We fit this model to experimental data and find that $t_{\mathrm{rise}}\propto 1/(n^{0.52\pm 0.03}{I}_b^{0.63\pm 0.02})$, which suggests that further work is needed to better understand the bias-current dependence. This work gives insights into the nonequilibrium dynamics of thin superconducting films exposed to electromagnetic radiation.

We study the reconstruction-algorithm effect on positive and negative ghost imaging (GI). By introducing three GI algorithms (logarithmic GI, exponential GI, and trigonometric GI), we perform numerical simulations and experiments with different GI algorithms, which demonstrate that a positive or negative ghost image can be recovered by modulating the monotonicity of the object signal function. Our work broadens the GI reconstruction algorithm and deepens the understanding of positive and negative GI principle.

Plasmonic metamaterials can be used to enhance midinfrared sensors and detectors for immunosensing applications. The major challenge with them is to integrate these technologies in low-cost, compact, and promising platforms. As an emerging technique, we utilize toroidal resonant plasmonic metamaterials to develop an ultrasensitive label-free analytical platform for the detection of specific antibiotics in the midinfrared spectra. Taking advantage of the unique sensitivity of robustly squeezed electromagnetic fields in the toroidal plasmonic meta-atoms, we demonstrate that our proposed approach based on toroidal metastructures provides recognition of extremely-low-weight (approximately 0.6 KDa) biological targets at attomolar densities. We envisage that this understanding extends the capabilities of plasmonic metamaterials to analyze the presence of specific biological molecules and organisms with ultrahigh precision.

Random numbers are a fundamental ingredient in fields such as simulation, modeling, and cryptography. Good random numbers should be independent and uniformly distributed. Moreover, for cryptographic applications, they should also be unpredictable. A fundamental feature of quantum theory is that certain measurement outcomes are intrinsically random and unpredictable. These can be harnessed to provide unconditionally secure random numbers. We demonstrate a real-time self-testing source-independent quantum random-number generator (SI QRNG) that uses squeezed light as a source. We generate secure random numbers by measuring the quadratures of the electromagnetic field without making any assumptions about the source other than an energy bound; only the detection device is trusted. We use homodyne detection to measure alternately the $\hat{Q}$ and $\hat{P}$ conjugate quadratures of our source. $\hat{P}$ measurements allow us to estimate a bound on any classical or quantum side information that a malicious eavesdropper may obtain. This bound gives the minimum number of secure bits we can extract from the $\hat{Q}$ measurement. We discuss the performance of different estimators for this bound. We operate this QRNG with a squeezed-state source and compare its performance with a thermal-state source. This is a demonstration of a QRNG using a squeezed state, as well as an implementation of real-time quadrature switching for a SI QRNG.

We present the first experiments of dissolution-driven convection of carbon dioxide (${\mathrm{CO}}_2$) in a confined brine-saturated porous medium at high pressures. We designed an alternative cell that allows for both visual and quantitative analyses, and address the effects of free-phase ${\mathrm{CO}}_2$ and brine composition on convective dissolution. The visual examination of the gas volume combined with the measurement of pressure, which both evolve with dissolution, enables us to yield insights into the dynamics of convection in conditions that more closely reflect the geologic conditions. We find and analyze different dissolution events, including diffusive, early, and late convection and shutdown regimes. Our experiments reveal that in intermediate regimes, a so-called “quasisteady” state actually never happens. Dissolution flux continuously decreases in this regime, which is due to a negative feedback loop: the rapid reduction of pressure following convective dissolution (allowed by gas expansion), in turn, decreases the solubility of ${\mathrm{CO}}_2$ at the gas-brine interface and thus the instability strength. We introduce an alternative scaling factor that not only compensates the flux reduction but also the nonlinearities that arise from different salt types. We present robust scaling relations for the compensated flux and for the transition times between consecutive regimes in systems with $\mathrm{Na}\mathrm{Cl}$ ($\mathrm{Ra}\sim 3271–4841$) and $\mathrm{Na}\mathrm{Cl}+{\mathrm{Ca}\mathrm{Cl}}_2$ mixtures ($\mathrm{Ra}\sim 2919–4283$). We also find that $\mathrm{Na}\mathrm{Cl}+{\mathrm{Ca}\mathrm{Cl}}_2$ mixtures enjoy a longer intermediate period before the shutdown of dissolution, but with a lower dissolution flux, as compared to $\mathrm{Na}\mathrm{Cl}$ brines. The results provide an alternative perspective into how the presence of two separate phases in a closed system as well as different salt types may affect the predictive powers of our experiments and models for both the short- and long-term dynamics of convective dissolution in porous media.

We study the propagation of surface spin waves in two waveguides coupled through the dipole-dipole interaction. Essential for the observations made here is the magnetoelectric coupling between the spin waves and the effective ferroelectric polarization. This allows an external electric field to act on the spin waves and to modify the band gaps of magnonic excitations in individual layers. By on-off switching of the electric field and/or varying its strength or direction with respect to the equilibrium magnetization, it is possible to permit or prevent the propagation of spin waves in a selected waveguide. We propose an experimentally feasible nanoscale device operating as a high-fidelity surface-wave magnonic gate.

In this work, we experimentally report the acoustic realization of the two-dimensional Su-Schrieffer-Heeger model in a simple network of air channels. We analytically study the steady-state dynamics of the system using a set of discrete equations for the acoustic pressure, leading to the two-dimensional Su-Schrieffer-Heeger Hamiltonian matrix without use of the tight-binding approximation. By building an acoustic network operating in the audible regime, we experimentally demonstrate the existence of a topological band gap. Moreover, within this band gap we observe the associated edge waves even though the system is open to free space. Our results not only experimentally demonstrate topological edge waves in a zero-Berry-curvature system but also provide a flexible platform for the study of topological properties of sound waves.

Short-term wind-speed and wind-power characteristics at an offshore wind farm located in Ibaraki Prefecture (Japan) are statistically evaluated. Three models of a wind turbine’s power conversion are derived from wind-turbine measurements, for which time-domain simulation and statistical analyses of their power outputs are conducted. We show that the power fluctuations are highly affected by the deterministic electromechanical conversion system and are not solely stochastically described. We also show that aggregated output fluctuations of wind turbines are not mitigated on a second scale for variations less than about 5–6 standard deviations. This implies that large output variations of wind farms are fed into the power grid, which possibly cause both frequency and voltage fluctuations.