Experiments on magnetic tunnel junctions (MTJs) show that the use of a (001)-oriented spinel ${\mathrm{Mg}\mathrm{Al}}_2{\mathrm{O}}_4$ barrier improves the robustness of the tunneling magnetoresistance (TMR) ratio against bias voltage [Sukegawa et al., Appl. Phys. Lett. 96, 212505 (2010)]; however, the maximum TMR ratio is very small compared with that of the $\mathrm{Mg}\mathrm{O}$-based MTJ. To overcome this problem, we propose a MTJ with a trilayered tunnel-barrier junction, $\mathrm{Fe}$/$\mathrm{Mg}\mathrm{O}$/${\mathrm{Mg}\mathrm{Al}}_2{\mathrm{O}}_4$/$\mathrm{Mg}\mathrm{O}$/$\mathrm{Fe}$, from first-principles calculations. The presence of the $\mathrm{Mg}\mathrm{O}$ interlayer between $\mathrm{Fe}$ and ${\mathrm{Mg}\mathrm{Al}}_2{\mathrm{O}}_4$ has the effect of enhancing the TMR ratio to more than $1000\mathrm{\%}$ at zero bias. The large TMR is maintained under a bias voltage. The results indicate the potential of a hybrid-type tunnel barrier that combines the advantages of MTJs containing a single $\mathrm{Mg}\mathrm{O}$ barrier (high TMR) and a single ${\mathrm{Mg}\mathrm{Al}}_2{\mathrm{O}}_4$ barrier (robustness to bias voltages). The $\mathrm{Mg}\mathrm{O}$ interlayer is found to play a key role in suppressing the transmittance of the minority-spin channel, and thus, the tunneling conductance of antiparallel magnetization is significantly reduced.

Temporal, spectral, and sample-to-sample fluctuations in coherence properties of qubits form an outstanding challenge for the development of upscaled fault-tolerant quantum computers. A ubiquitous source for these fluctuations in superconducting qubits is a set of atomic scale defects with a two-level structure. Here we propose a way to mitigate these fluctuations and stabilize the qubit performance. We show that frequency modulation of a qubit or, alternatively, of the two-level defects, leads to averaging of the qubit relaxation rate over a wide interval of frequencies.

Relying on both electromechanical and micromagnetic simulations, we propose a method to control the trajectory of current-driven skyrmions using an electric field in hybrid piezoelectric-magnetic systems. By applying a voltage between two lateral electrodes, a transverse strain gradient is created, as a result of the nonuniform electric field profile in the piezoelectric material. Due to magnetoelastic coupling, this transverse gradient leads to a lateral force on the skyrmions that can be used to suppress the skyrmion Hall angle for any given current density, if a proper voltage is applied. We show that this method works under realistic conditions, such as the presence of disorder in the ferromagnet, and that skyrmion trajectories can be controlled with moderate voltages. Moreover, our method allows the maximum current density that can be injected before the skyrmion is annihilated at the nanostrip edge to be increased, which leads to an increase in the maximum achievable velocities.

We demonstrate size-selective optical trapping and transport for nanoparticles near an optical nanofiber taper. Using a two-wavelength counterpropagating mode configuration, we show that 100-nm-diameter and 150-nm-diameter gold nanospheres (GNSs) are trapped by the evanescent field in the taper region at different optical powers. Conversely, when one nanoparticle species is trapped the other may be transported, leading to a sievelike effect. Our results show that sophisticated optical manipulation can be achieved in a passive configuration by taking advantage of mode behavior in nanophotonic devices.

The characterization of sound fields over space is fundamental in acoustics. Yet, the volumetric acquisition of sound fields using conventional electromechanical transducers is challenging due to the high sampling requirements needed in a three-dimensional domain. In this study, we introduce an acousto-optic sensing method to remotely sample acoustic fields in a volume using a sparse set of noninvasive optical measurements. Optical methods that use light as a sensing element have drawn significant attention recently, since they allow for remote, noninvasive sampling of acoustic fields with fine spatial resolution. The practical deployment of optical sensing in acoustics, however, has been hindered by the limited acquisition of data available, and the unsuitability of existing methods to reconstruct sound fields in scarce-data regimes. The proposed methodology relies on projecting the measured data on a basis that satisfies the wave equation, thus alleviating the measurement requirements associated with traditional reconstruction methods. The proposed approach leads to accuracy improvements of 20 dB relative to current methods, and a 90% reduction of measured data. We demonstrate the approach experimentally by reconstructing in situ the three-dimensional sound field inside a room from remote measurements of the acousto-optic interaction. These results are key to advancing the use of acousto-optical sensing methods, which are currently limited to simplified domains. The work presented in this paper brings about the possibility to solve so far intractable problems, such as the volumetric and remote acquisition of complex three-dimensional acoustic fields.

Quantum Hall physics has been theoretically predicted in four dimensions and higher. In hypothetical 2n dimensions, the topological characters of both the bulk and the boundary are manifested as quantized nonlinear transport coefficients that respectively connect to the $n\mathrm{th}$ Chern number of the bulk gap projection and to the $n\mathrm{th}$ winding number of the Weyl spectral singularities on the ($2n-1$)-dimensional boundaries. Here, we introduce the concept of phason engineering in metamaterials and use it as a vehicle to access and apply the quantum Hall physics in arbitrary dimensions. Using these specialized design principles, we fabricate a reconfigurable two-dimensional aperiodic acoustic crystal with a phason living on a 2-torus, giving us access to the four-dimensional quantum Hall physics. Also, we supply a direct experimental confirmation that the topological boundary spectrum assembles in a Weyl singularity when mapped as a function of the quasimomenta. We also demonstrate topological wave steering enabled by the Weyl physics of the three-dimensional boundaries.

Squeezed light enables measurements with sensitivity beyond the quantum noise limit (QNL) for optical techniques such as spectroscopy, gravitational wave detection, magnetometry, and imaging. Precision of a measurement—as quantified by the variance of repeated estimates—has also been enhanced beyond the QNL using squeezed light. However, sub-QNL sensitivity is not sufficient to achieve sub-QNL precision. Furthermore, demonstrations of sub-QNL precision in estimating transmission have been limited to picowatts of probe power. Here we demonstrate simultaneous enhancement of precision and sensitivity to beyond the QNL for estimating modulated transmission with a squeezed amplitude probe of $0.2$ mW average ($25$ W peak) power, which is 8 orders of magnitude above the power limitations of previous sub-QNL precision measurements of transmission. Our approach enables measurements that compete with the optical powers of current classical techniques, but have both improved precision and sensitivity beyond the classical limit.

Electronic devices are extremely sensitive to defects in their constituent semiconductors, but locating electronic point defects in bulk semiconductors has previously been impossible. Here we apply scanning transmission electron microscopy (STEM) electron-beam-induced current (EBIC) imaging to map electronic defects in a $\mathrm{GaAs}$ nanowire Schottky diode. Imaging with a nondamaging 80 or 200 kV STEM acceleration potential reveals a minority-carrier diffusion length that decreases near the surface of the hexagonal nanowire, thereby demonstrating that the device’s charge collection efficiency (CCE) is limited by surface defects. Imaging with a 300 keV STEM beam introduces vacancy-interstitial (or Frenkel) defects in the $\mathrm{GaAs}$ that increase carrier recombination and reduce the CCE of the diode. We create, locate, and characterize a single insertion event, determining that a defect inserted 7 nm from the Schottky interface broadly reduces the CCE by $10\mathrm{\%}$ across the entire nanowire device. Variable-energy STEM EBIC imaging thus allows both benign mapping and pinpoint modification of a device’s electron-hole-recombination landscape, enabling controlled experiments that illuminate the impact of both extended (one- and two-dimensional) and point (zero-dimensional) defects on semiconductor device performance.

Wavelength-thick multifunctional metagratings for waterborne sound are highly desirable in various application scenarios, such as human health monitoring and ocean exploration. In this article, we present a class of metagratings with which distinct and switchable wave manipulation functionalities, such as extraordinary transmission, total reflection, and abnormal reflection can be achieved through a single acoustic grating. Simultaneous and high-efficiency control over both reflected and transmitted waves is achieved through a systematic design approach in which wave diffraction theory, effective medium theory, and intelligent optimization algorithms are concurrently utilized. Switching between different functionalities can be achieved simply by changing the operation frequency, or by changing the incident angles. Robust and perfect wave-front manipulation effects are obtained over a wide range of incident angles and operation frequencies. Our work not only provides a design paradigm of planar acoustic devices for multiple manipulation purposes, but also presents a feasible solution for the development of integrated acoustic devices for distinct underwater applications.

We propose a fast mixed-state control scheme to transfer the quantum state along designable trajectories in Hilbert space, which is robust to multiple decoherence noises. Starting with the dynamical invariants of open quantum systems, we present the shortcuts to adiabaticity (STA) of open quantum systems at first, then apply the STA to speed up the adiabatic steady process. Our scheme drives open systems from a initial steady state to a target steady state by a controlled Liouvillian that possesses the same form as the reference (original) one that is accessible in present-day experiments. An experimental design with currently available parameters for the nitrogen-vacancy (N-$V$) center in diamond is suggested and discussed.

A transducer of single photons between microwave and optical frequencies can be used to realize quantum communication over optical fiber links between distant superconducting quantum computers. A promising scalable approach to constructing such a transducer is to use ensembles of quantum emitters interacting simultaneously with electromagnetic fields at optical and microwave frequencies. However, inhomogeneous broadening in the transition frequencies of the emitters can be detrimental to this collective action. In this paper, we utilize a gradient-based optimization strategy to design the temporal shape of the laser field driving the transduction system to mitigate the effects of inhomogeneous broadening. We study the improvement of transduction efficiencies as a function of inhomogeneous broadening in different single-emitter cooperativity regimes and correlate it with a restoration of super-radiance effects in the emitter ensembles. Furthermore, to assess the optimality of our pulse designs, we provide certifiable bounds on the design problem and compare them to the achieved performance.

Carrier transport in type-II superlattice photodetectors is investigated by means of a rigorous nonequilibrium Green’s function model based on a physics-based Büttiker-probe formalism. Intraband scattering self-energies (carrier-phonon interactions) are computed in the self-consistent Born approximation, while interband self-energies (Shockley-Read-Hall and optical transitions) are included in terms of semiclassical generation-recombination rates, neglecting interband renormalization effects. Current conservation is achieved with an efficient Newton-Raphson algorithm. While carrier transport in infrared detectors is usually understood in terms of quantities (e.g., mobilities and quasi-Fermi-levels) that are admittedly not germane to nonequilibrium Green’s function theory, the proposed model provides a quantum-kinetic description of tunneling, miniband transport, hopping, and carrier extraction within a drift-diffusion-friendly framework. The connection with semiclassical theories allows exploration of the possibilities offered by Poisson-Schrödinger or localization landscape drift-diffusion approaches.

This work presents an investigation of carrier transport in $\mathrm{Ga}\mathrm{N}$-based light-emitting diodes in the subthreshold forward-bias regime where tunneling processes are relevant. A quantum kinetic theory of trap-assisted tunneling is developed within the framework of the nonequilibrium Green’s function formalism. Based on fully nonlocal scattering self-energies computed in the self-consistent Born approximation and a multiband description of the electronic structure, the model provides access to spectral quantities, such as the local density of states and the current density, which are essential to understand the nature of the tunneling process. The quantum nonradiative recombination rates can be reproduced by the conventional Shockley-Read-Hall theory, provided that the classical charge is replaced with the correct quantum charge, which means that trap-assisted tunneling can be described with drift-diffusion solvers complemented with appropriate quantum corrections for the calculation of the local density of states. The subthreshold $I$-$V$ characteristics and ideality factors predicted by the quantum kinetic model are in agreement with measurements.

Two-dimensional (2D) semiconductors have attracted tremendous interest as natural passivation and atomically thin channels could facilitate continued transistor scaling. However, air-stable 2D semiconductors with high performance are quite elusive. Recently, an extremely-air-stable ${\mathrm{Mo}\mathrm{Si}}_2{\mathrm{N}}_4$ monolayer was successfully fabricated [Hong et al., Science 369, 670 (2020)]. To further reveal its potential application in sub-5-nm metal-oxide-semiconductor field-effect transistors (MOSFETs), there is an urgent need to develop integrated circuits. Here, we report first-principles quantum-transport simulations on the performance limits of n- and p-type sub-5-nm monolayer (ML) ${\mathrm{Mo}\mathrm{Si}}_2{\mathrm{N}}_4$ MOSFETs. We find that the on-state current in the ${\mathrm{Mo}\mathrm{Si}}_2{\mathrm{N}}_4$ MOSFETs can be effectively manipulated by the length of gate and underlap, as well as the doping concentration. Very strikingly, we also find that for the n-type devices the optimized on-state currents can reach up to 1390 and 1025 µA/µm for high-performance and low-power (LP) applications, respectively, both of which satisfy the International Technology Roadmap for Semiconductors (ITRS) requirements. The optimized on-state current can meet the LP application (348 µA/µm) for p-type devices. Finally, we find that the ${\mathrm{Mo}\mathrm{Si}}_2{\mathrm{N}}_4$ MOSFETs have an ultralow subthreshold swing and power-delay product, which have the potential to realize high-speed and low-power consumption devices. Our results show that ${\mathrm{Mo}\mathrm{Si}}_2{\mathrm{N}}_4$ is an ideal 2D channel material for future competitive ultrascaled devices.

Super-resolution acoustic imaging with state-of-the-art spatial resolution (λ/50), with λ being the wavelength, is showcased with a holey-structured metalens. However, the imaging mechanism under unity transmission based on Fabry-Perot resonances means the metalens fundamentally suffers from narrow bandwidth and limited deep-subwavelength contrast, and therefore further advancement of deep-subwavelength imaging has been stalled. Here we break the barriers for deep-subwavelength acoustic imaging comprehensively in spatial resolution, resolving contrast, and working bandwidth, by exploiting field enhancement inside the metalens. A microscopic model is established to theoretically reveal the underlying physics for escalated deep-subwavelength acoustic imaging. For a proof-of-concept, the imaging performance of the proposed method is numerically proven and experimentally demonstrated. Specifically, a breakthrough resolution below λ/100 is achieved while resolving contrast is improved by at least 6.5 times and working bandwidth is broadened to approximately 25% of the operating frequency. Furthermore, pulsed acoustic imaging on the deep-subwavelength scale is showcased, which is an important step towards the practical application of the ultrahigh-resolution acoustic imaging technique. We believe the work presented here may greatly benefit a variety of fields in acoustics, such as visualizing subcutaneous structures in medical diagnosis and characterizing subsurface flaws in industrial nondestructive evaluation.

The Fano resonance is a widespread wave-scattering phenomenon associated with an ultrasharp line shape, which just serves a narrow working frequency range around the interference frequency, rendering the realization of Fano-based applications extremely challenging. Here, we present and experimentally verify a mechanism of acoustic consecutive Fano resonances (ACFRs) with a symmetric profile for broadband sound attenuation, and extend to a practical implementation of a tunable low-frequency double-helix metasilencer. Based on the ACFRs’ dependence on material parameters in the bilayer metamaterial model, we employ an inverse design using Bayesian machine learning to search the optimal broadband insulating performance with a rapid convergence speed (15 iterations). For practical requirement, we extend the ACFRs’ prototype to a continuously tunable double-helix metastructure for broadband low-frequency sound attenuation. This broadband effect can be interpreted by the dual-band single-negativity property. A good agreement between numerical simulation and experiment evidences the effectiveness of the proposed metasilencer with tunable sound attenuation (>90%) in 425–865 Hz and high ventilation (>80%) at various double-helix combinations. Our proposed ACFRs’ mechanism and its associated metastructure would open routes to promising acoustic metamaterial-based applications, such as filtering, switching, and sensing, and beyond.