A simple way of implementing a scalable laser-driven nanophotonic electron accelerator on a chip is presented. The design requires only a single incident laser pulse and can be fabricated straightforwardly on commercial silicon-on-insulator wafers. We investigate the low-energy regime of tabletop electron microscopes where the silicon structures safely allow peak gradients of about 150 MeV/m. By means of a three-dimensional alternating-phase-focusing scheme, we obtain about half of the peak gradient as the average gradient with six-dimensional confinement and full-length scalability. The structures are completely designed within the device layer of the wafer and can be arranged in stages. We choose the stages as energy doublers and outline how errors in the handshake between the stages can be corrected by on-chip steerers. Since the electron pulse length in the attosecond realm is preserved, our chip is the ideal energy booster for ultrafast-electron-diffraction machines, opening the megaelectronvolt scale on tabletop setups.

Using a type-x spin-orbit-torque (SOT) switching scheme, in which the easy axis (EA) of the ferromagnetic (FM) layer and the charge-current flow direction are collinear, it is possible to realize a lower-power-consumption, higher-density, and better-performance SOT magnetoresistive random-access memory (SOT MRAM) compared with the conventional type-y design. Here, we systematically investigate type-x SOT switching properties through both macrospin and micromagnetic simulations. The out-of-plane external field and anisotropic field dependence of the switching-current density ($J_{\mathrm{sw}}$) is first examined in the ideal type-x configuration. Next, we study the FM-layer canting-angle ($\phi$) dependence of $J_{\mathrm{sw}}$ through macrospin simulations and experiments, which show the transformation of switching dynamics from type x to type y with increasing $\phi$. By further integrating fieldlike torque (FLT) into the simulated system, we find that a positive FLT can assist type-x SOT switching, while a negative one brings about complex dynamics. More crucially, with the existence of a sizable FLT, type-x switching mode results in a lower critical switching current than that of type y at a current pulse width less than about 10 ns, indicating the advantage of employing the type-x design for ultrafast switching using material systems with FLT. Our work provides a thorough examination of the type-x SOT scheme with various device and materials parameters, which can be informative for designing next-generation SOT MRAM.

Recently, the concept of perpendicular shape anisotropy spin-transfer-torque magnetic random-access memory (PSA STT MRAM) was proposed to enhance the thermal stability factor, $\mathrm{\Delta }$, of the storage layer through its shape anisotropy, thus enabling the downsizing scalability of conventional STT MRAM beyond sub-20-nm technological nodes. However, this is expected to negatively impact the writing current and switching time, calling for the search for the best compromise. Here, we report a micromagnetic study of the STT-driven magnetization reversal of sub-20-nm PSA STT MRAM cells, with a particular emphasis on the technologically relevant case of $\mathrm{\Delta }\simeq 80$, thus providing guidelines for their practical design. For an aspect ratio ($\eta$) of the storage layer of $\eta \le 1$, magnetization reversal obeys a macrospinlike mechanism, while for $\eta >1$ a noncoherent reversal is observed, which is characterized by buckling or nucleation of a transverse domain wall at the ferromagnet-insulator interface, which then propagates along the vertical axis. Decreasing the lateral size while maintaining a constant value of $\mathrm{\Delta }$ implies a larger height, which is found to lead to an increase of the switching voltage. In all cases, the inverse of the switching time scales linearly with the applied voltage.

We experimentally and theoretically study the dynamics of DNA driven by artificial electrokinetic noise back and forth between neighboring nanopits inside a nanofluidic device. The dynamics are consistent with a noise-assisted barrier-crossing process, exhibiting a rapid increase in the hopping rate with noise level beyond a threshold. A simple Arrhenius model with a fixed energy barrier of several $k_{B}T$ describes the hopping dynamics well at low noise levels but to accurately describe the dynamics over a wider range, we develop a numerical model that additionally accounts for the lateral extent of the free-energy landscape. The experimental and numerical methods reported here significantly expand the range of noise levels and time scales over which activated DNA hopping processes can be controlled and investigated.

Qubit coherence and gate fidelity are typically considered the two most useful metrics for characterizing a quantum processor. An equally useful metric is interqubit connectivity as it minimizes gate count and allows implementing algorithms efficiently with reduced error. However, interqubit connectivity in superconducting processors tends to be limited to nearest neighbor due to practical constraints in typical planar realizations. Here, we introduce a superconducting architecture that uses a ring resonator as a multiqubit coupling element to provide beyond nearest-neighbor connectivity without compromising on coupling uniformity or introducing fabrication complexities. We theoretically analyze the interqubit coupling as a function of frequency for a pair of qubits placed at different positions along the ring resonator and show that for carefully chosen operating frequency and angular spacing between the qubits, the variation of coupling can be minimized. For an operating frequency between the first two resonances of the ring resonator and a ${30}^{\circ }$ angular spacing for qubits, we compute interqubit coupling for a device capable of supporting up to 12 qubits with each qubit connected to nine other qubits. Using four qubits positioned strategically in the ring-resonator coupler, we experimentally verify the theoretical prediction for all possible angular spacings between the two qubits and demonstrate good agreement. Just like the standard bus resonator coupler, the coupling in the ring-resonator coupler is mediated via virtual photons since the operating frequency is far away from the resonant modes of the ring coupler. This ensures that any small internal loss in the ring resonator does not introduce decoherence during the coupling operation. We also compute extensions of this idea involving larger ring resonators and a multiring system and show the possibility of highly connected networks with larger number of qubits. Apart from being plug and play for existing superconducting architectures, our concept is scalable, adaptable to other platforms and has the potential to significantly accelerate progress in quantum computing, annealing, simulations, and error correction.

Optical emission of plasma induced by a pulsed laser ablating the surface of a randomly packed granular (RPG) material in air is investigated experimentally, taking sieved copper microspheres with discrete diameters, d, ranging from 49 to 390 µm as examples. We find a steplike phenomenon (also called a critical-like phenomenon) in the dependence of plasma emission on the grain size: when d is less than a critical size, only weak emissions are detected; however, when d exceeds the critical size, the emissions abruptly become at least 5 times stronger. Such a phenomenon can be explained by considering the RPG material as a non-Newtonian fluid with a yield stress and an effective viscosity. Specifically, in the range of the grain size above (below) the critical value, the RPG target has a yield stress larger (smaller) than the shock pressure imparted by the energetic processes of plasma generation and expansion, and thus, behaves like an elastic solid (a viscous fluid) to assist (impede) the formation of the plasma with high temperature and high density as an optical emission source for spectrochemical analysis. This work not only has significance in assessing a lower size limit for direct multielement analysis of RPG material using laser-induced breakdown spectroscopy (LIBS), but also shows that the mechanical characteristics of RPG material may be probed using LIBS.

Using first-principles calculations, we investigate the properties of the ${\mathrm{Co}\mathrm{S}}_2$ monolayer pentagonal structure as a possible anode material for $\mathrm{Li}$- and $\mathrm{Na}$-ion batteries. The geometrical optimization reveals that the metal atom prefers to be adsorbed on the hollow site, and that the diffusion barrier of $\mathrm{Li}$ and $\mathrm{Na}$ can be as low as 0.22 eV, which would allow a relatively fast diffusion on the surface. Moreover, our calculations demonstrate that the ${\mathrm{Co}\mathrm{S}}_2$ monolayer has a theoretical specific capacity of 653.31 and $326.77\mathrm{mAh}{\mathrm{g}}^{-1}$ for $\mathrm{Li}$ and $\mathrm{Na}$, respectively, which in the case of $\mathrm{Li}$ is larger than the capacity of other two-dimensional materials used as an anode material. Overall, the ${\mathrm{Co}\mathrm{S}}_2$ monolayer in this structure shows excellent electrochemical properties, making it a promising candidate for use as the anode material in metal-ion batteries.

Elastic metamaterials possess band gaps, or frequency ranges that are forbidden to wave propagation. Existing solutions for impeding three-dimensional (3D) wave propagation largely rest on high-volume fractions of mass inclusions that induce and tailor negative effective density-based local resonances. This study introduces a class of elastic metamaterials that achieve low-frequency band gaps with a volume fraction as low as 3% (mass density as low as $0.034\mathrm{g}/{\mathrm{cm}}^3$). The working of the proposed design hinges on a 3D trampolinelike mode behavior that gives rise to wide, omnidirectional, and low-frequency band gaps for elastic waves despite very low-mass densities. Such a 3D trampoline effect is derived from a network of overhanging nodal microarchitectures that act as locally resonating elements, which give rise to band gaps at low frequencies. The dynamic effective properties of the metamaterial are numerically examined, which reveal that the band gap associated with the trampoline effect is resulted from a negative effective modulus coupled with a near-zero yet positive effective density. The experimental characterization is then made possible by fabricating the metamaterial via a light-based printing system that is capable of realizing microarchitectures with overhanging microfeatures. This design strategy could be useful to applications where simultaneous light weight and vibration control is desired.

Analysis of magnetic susceptibility of nonellipsoidal samples is a long-standing problem in experimental studies of magnetism and superconductivity. Here the quantitative description of the Meissner-London response (no Abrikosov vortices) of right circular cylinders in an axial magnetic field is given. Three-dimensional adaptive finite-element modeling is used to calculate the total magnetic moment ($m$) in a wide range of London penetration depth ($\lambda$) to sample size ($a$) ratios, $\lambda /a$. By fitting numerical solutions, the closed-form empirical magnetic susceptibility is obtained involving only sample dimensions and $\lambda$, thus providing a recipe for determining the London penetration depth from the precision measurements of the magnetic susceptibility. Detailed protocols of the experimental data analysis using the developed approach are given. The results can be readily extended to the most frequently used cuboid-shaped samples.

In this work, we study the current-phase relation ($\mathrm{C}\mathrm{\Phi }\mathrm{R}$) of lithographically fabricated molybdenum germanium (${\mathrm{Mo}}_{79}{\mathrm{Ge}}_{21}$) nanobridges that is intimately linked with the nanobridge’s kinetic inductance. We do this by imbedding the nanobridges in a superconducting quantum interference device (SQUID). We observe that, for temperatures far below $T_c$, the $\mathrm{C}\mathrm{\Phi }\mathrm{R}$ is linear, as long as the condensate is not weakened by the presence of a supercurrent. We demonstrate lithographic control over the nanobridge kinetic inductance, which scales with the nanobridge aspect ratio. This allows the $I_c(B)$ characteristic of the SQUID to be tuned. The SQUID properties that can be controlled in this way include the SQUID’s sensitivity and the positions of the critical-current maxima. These observations can be of use for the design and operation of future superconducting devices, such as magnetic memories or flux qubits.

Quantum secure direct communication (QSDC) realizes the transmission of secret messages directly in a quantum channel. A continuous-variable- (CV) based quantum-communication system allows high-speed, large-capacity information transmission in optical telecommunication systems. In this work, we propose a QSDC protocol based on Gaussian mapping. As for Gaussian modulation, the designed Gaussian-mapping scheme effectively solves the problem, which results from the nonuniformity of the secret-message bitstream, and is applicable to different modulation variances. The system performance analysis shows that the proposed scheme can realize inerrant Gaussian modulation of secret messages, and secure transmission of messages under information theory. Moreover, our work hopes to stimulate discussion on experimental realization and practical advance of CV-QSDC protocol.

Antiferromagnetism within the two-dimensional (2D) family offers a platform for spintronics. The emergent 2D semiconductor ${\mathrm{Cr}\mathrm{PS}}_4$ is proved to be composed of ferromagnetic layers with antiferromagnetic coupling along the stacking direction in the experiment. By using the first-principles quantum-transport simulation, we evaluate the spin-resolved transport in the magnetic tunnel junction built by the 2D ${\mathrm{Cr}\mathrm{PS}}_4$ tunnel barrier with large thickness ranges. We find the magnetoresistance generally increases with the number of tunnel layers from 140% (three layers) to a surprising 370 000% (10 layers). An odd-even oscillation magnetoresistance behavior exists in few layers due to the electrode option. Our results will inspire further experimental verification and provide vital insights for 2D spintronics design.

Quantum-cascade-laser- (QCL) pumped molecular lasers (QPMLs) have recently been introduced as a source of powerful ($>1$ mW) tunable ($>1$ THz) narrow-band ($<10$ kHz) continuous-wave terahertz radiation. The performance of these lasers depends critically on molecular collision physics, pump saturation, and on the design of the laser cavity. Using a validated three-level model that captures the essential collision and saturation behaviors of the QPML gas nitrous oxide (${\mathrm{N}}_2\mathrm{O}$), we explore how the threshold pump power and output terahertz power depend on the pump power and gas pressure, as well as on the diameter, length, and output-coupler transmissivity of a cylindrical cavity. The analysis indicates that maximum power occurs as pump saturation is minimized in a manner that depends much more sensitively on pressure than on cell diameter, length, or transmissivity. A near-optimal compact laser cavity can produce tens of milliwatts of power tunable over frequencies above 1 THz when pumped by a multiwatt QCL.

The hybridization of magnons and phonons is playing a critical role in the emerging field of hybrid magnonics because it combines the high tunability of magnetism with the long lifetime of mechanics for comprehensive coherent information processing. Recently there has been increasing interest in thin-film bulk acoustic waves because of their long lifetimes at high frequencies. However, the unique multimode nature of such phonon modes has not been exploited as an important resource for coherent information processing. In this Letter we study the simultaneous hybridization of multiple high-overtone bulk acoustic resonances with a magnon and a microwave mode. The demonstrated multimode hybridization allows us to observe coherent pulse echoes, opening opportunities for both fundamental studies and practical applications of hybrid magnonics.

Rydberg electromagnetically induced transparency (EIT) enables the realization of atom-based Systeme-International-traceable microwave sensing, imaging, and communication devices by exploiting the strong microwave electric dipole coupling of highly excited Rydberg states. Essential to the development of robust devices is a careful characterization of the sensor performance and systematic uncertainties. In this work, we present a comparison of microwave-induced EIT splitting in a cesium atomic vapor for four possible Rydberg couplings, $65S_{1/2}\to 65P_{1/2}$, $66S_{1/2}\to 66P_{3/2}$, $79D_{5/2}\to 81P_{3/2}$, and $62D_{5/2}\to 60F_{7/2}$, at microwave transition frequencies around 13 GHz. Our work highlights the impact of multiphoton couplings on neighboring Rydberg states in breaking both the symmetry and linearity of the observed splitting, with excellent agreement between experimental observations and a theoretical model accounting for multiphoton couplings. We identify an optimal angular-state choice for robust microwave measurements, as well as demonstrating an alternative regime in which microwave polarization can be measured.

The current due to cold tunneling of electrons from a metallic surface exposed to high electric fields, regularly named “dark current,” is commonly described in modern literature using an analytical approximate solution provided by Murphy and Good [Phys. Rev. 102, 1464 (1956)]. This expression, which corrects earlier work by Fowler and Nordheim, is a Fowler-Nordheim-type equation: $I\sim E^2\exp (-a/E)$, where $I$ is the dark current, $E$ is the local electric field, and $a$ is a system-specific constant. In this paper, a numerical approximation, rather than the analytical one given by Murphy and Good, is presented. This approximation is accurate over a wide range of fields, and is used to derive the effective field enhancement factor $\beta$. On the basis of this approximation, and considering local field and current fluctuations, two alternative methods for $\beta$ estimation are presented. These methods allow instantaneous field-specific estimation of $\beta$, rather than the average estimate derived with current methods. The applicability of fluctuation-based methods is demonstrated by numerical simulation in a variety of conditions. The methods are applied to estimate $\beta$ using fluctuation analysis in experimental measurements that were not dedicated for this purpose. An open-source code for the implementation of fluctuation-derived $\beta$ estimation is provided, with an analysis of possible future experimental opportunities using dedicated experiments.

Ultrasound focusing, with strong penetrability and superior biocompatibility, has attracted substantial interest in biomedical science. Conventional focusing techniques generally rely on an active transducer array or curved lens, and acoustic metamaterials have recently emerged as an alternative approach. However, the low operating frequency of an ultrasound focusing metasurface below 1 MHz and the narrow bandwidth hinder its broad applications in practice. Here, we experimentally realize broadband ultrasound focusing at megahertz frequencies based on a compact and passive metasurface. Three-dimensional spot focusing, ranging from 0.5 to 1.4 MHz, with high spatial resolution (Full width at half maximum of 0.63λ and full length at half maximum of 2.75λ at 1 MHz) and tunable focal length (18.5–71 mm), is demonstrated. The focusing pattern is further enriched by realizing a broadband ultrasound scalpel, which effectively converges sound energy within a sharp “needle” area. Additionally, accompanied by an enhancement of ultrasound intensity, a prominent temperature rise appears in the focal region, the dependence of which on the ultrasound frequency is investigated and the optimized operating frequency is revealed. Broadband and subwavelength ultrasound spot focusing and a scalpel at megahertz frequencies would promote applications in particle manipulation, nondestructive detection, diagnostic sonography, and ultrasound therapy.