The 100 most-cited articles in Applied Physics Letters, 2021

Silicon photonics is advancing rapidly in performance and capability with multiple fabrication facilities and foundries having advanced passive and active devices, including modulators, photodetectors, and lasers. Integration of photonics with electronics has been key to increasing the speed and aggregate bandwidth of silicon photonics based assemblies, with multiple approaches to achieving transceivers with capacities of 1.6 Tbps and higher. Progress in electronics has been rapid as well, with state-of-the-art chips including switches having many tens of billions of transistors. However, the electronic system performance is often limited by the input/output (I/O) and the power required to drive connections at a speed of tens of Gbps. Fortunately, the convergence of progress in silicon photonics and electronics means that co-packaged silicon photonics and electronics enable the continued progress of both fields and propel further innovation in both.

Two decades after their demonstration, superconducting nanowire single-photon detectors (SNSPDs) have become indispensable tools for quantum photonics as well as for many other photon-starved applications. This invention has not only led to a burgeoning academic field with a wide range of applications but also triggered industrial efforts. Current state-of-the-art SNSPDs combine near-unity detection efficiency over a wide spectral range, low dark counts, short dead times, and picosecond time resolution. The present perspective discusses important milestones and progress of SNSPDs research, emerging applications, and future challenges and gives an outlook on technological developments required to bring SNSPDs to the next level: a photon-counting, fast time-tagging imaging, and multi-pixel technology that is also compatible with quantum photonic integrated circuits.

Spintronics, that is, the utilization of electron spin to enrich the functionality of microelectronics, has led to the inception of numerous novel devices, particularly magnetic random-access memory (MRAM). Over the last decade, significant effort has been devoted to magnetization manipulation using spin-orbit torque (SOT), which shows great promise for ultrafast and energy-efficient MRAM. In this Perspective, we summarize the latest progress in the study of SOT and highlight some of the technical challenges facing the development of practical SOT devices. After introducing the basic concepts of SOT and its relevance for magnetization switching, we will focus on several methods to realize deterministic SOT switching in the absence of an external field, which is a requirement for practical SOT devices. Additionally, we summarize the materials used in SOT devices. The final section is devoted to the most important recent advances in the application of SOT devices, including SOT-MRAM, spin logic, spin Hall nano-oscillators, and neuromorphic devices.

A two-dimensional (2D) MoSi2N4 monolayer is an emerging class of air-stable 2D semiconductors possessing exceptional electrical and mechanical properties. Despite intensive recent research effort devoted to uncover the material properties of MoSi2N4, the physics of electrical contacts to MoSi2N4 remains largely unexplored thus far. In this work, we study van der Waals heterostructures composed of MoSi2N4 contacted by graphene and NbS2 monolayers using first-principles density functional theory calculations. We show that the MoSi2N4/NbS2 contact exhibits an ultralow Schottky barrier height (SBH), which is beneficial for nanoelectronics applications. For the MoSi2N4/graphene contact, the SBH can be modulated via the interlayer distance or via external electric fields, thus opening up an opportunity for reconfigurable and tunable nanoelectronic devices. Our findings provide insights into the physics of 2D electrical contacts to MoSi2N4 and shall offer a critical first step toward the design of high-performance electrical contacts to MoSi2N4-based 2D nanodevices.

We report on the demonstration of ferroelectricity in ScxAl1-xN grown by molecular beam epitaxy on GaN templates. Distinct polarization switching is unambiguously observed for ScxAl1-xN films with Sc contents in the range of 0.14–0.36. Sc0.20Al0.80N, which is nearly lattice-matched with GaN, exhibiting a coercive field of ∼ 4.2 MV/cm at 10 kHz and a remnant polarization of ∼135 μC/cm2. After electrical poling, Sc0.20Al0.80N presents a polarization retention time beyond 105 s. No obvious fatigue behavior can be found with up to 3 × 105 switching cycles. The work reported here is more than a technical achievement. The realization of ferroelectric single-crystalline III–V semiconductors by molecular beam epitaxy promises a thickness scaling into the nanometer regime and makes it possible to integrate high-performance ferroelectric functionality with well-established semiconductor platforms for a broad range of electronic, optoelectronic, and photonic device applications.

In this work, we demonstrate a high-performance ultraviolet phototransistor (UVPT) based on the AlGaN/GaN high-electron mobility transistor (HEMT) configuration. When the device is biased at off state, the peak photoresponsivity (R) of 3.6 × 107 A/W under 265 nm illumination and 1.0 × 106 A/W under 365 nm illumination can be obtained. Those two R values are one of the highest among the reported UVPTs at the same detection wavelength under off-state conditions. In addition, we investigate the gate-bias (VGS) dependent photoresponse of the fabricated device with the assistance of band structure analysis. It was found that a more negative VGS can significantly reduce the rise/decay time for 265 nm detection, especially under weak illumination. This can be attributed to a largely enhanced electric field in the absorptive AlGaN barrier that pushes the photo-generated carriers rapidly into the GaN channel. In contrast, the VGS has little impact on the switching time for 365 nm photodetection, since the GaN channel has a larger absorption depth and the entire UVPT simply acts as a…

(Si)GeSn semiconductors are finally coming of age after a long gestation period. The demonstration of device-quality epi-layers and quantum-engineered heterostructures has meant that tunable all-group IV Si-integrated infrared photonics is now a real possibility. Notwithstanding the recent exciting developments in (Si)GeSn materials and devices, this family of semiconductors is still facing serious limitations that need to be addressed to enable reliable and scalable applications. The main outstanding challenges include the difficulty to grow high-crystalline quality layers and heterostructures at the desired content and lattice strain, preserve the material integrity during growth and throughout device processing steps, and control doping and defect density. Other challenges are related to the lack of optimized device designs and predictive theoretical models to evaluate and simulate the fundamental properties and performance of (Si)GeSn layers and heterostructures. This Perspective highlights key strategies to circumvent these hurdles and hopefully bring this material system to…

In this work, we demonstrate the use of a Rydberg atom-based sensor for determining the angle of arrival of an incident radio frequency (RF) wave or signal. The technique uses electromagnetically induced transparency in Rydberg atomic vapor in conjunction with a heterodyne Rydberg atom-based mixer. The Rydberg atom mixer measures the phase of the incident RF wave at two different locations inside an atomic vapor cell. The phase difference at these two locations is related to the direction of arrival of the incident RF wave. To demonstrate this approach, we measure phase differences of an incident 19.18 GHz wave at two locations inside a vapor cell filled with cesium atoms for various incident angles. Comparisons of these measurements with both the full-wave simulation and the plane wave theoretical model show that these atom-based sub-wavelength phase measurements can be used to determine the angle of arrival of an RF field.

Solar-driven vapor generation is a sustainable and environmentally friendly method for water purification. Despite recent progress on photothermal steam generation, the rate of vapor generation remains low. Here, we enhance the vapor generation rate by combining solar-driven thermal and wind effects on a femtosecond-laser-textured superhydrophilic copper foam surface. Significant solar power can be absorbed and transformed into heat on the treated surface. This solar power can also be converted into electric power to generate wind to further accelerate steam generation. The upper superhydrophilic foam surface facilitates the continuous supply of water. A pre-wetted polyurethane sponge minimizes heat loss by preventing direct contact between the heated foam and bulk water. The as-prepared evaporator achieved a water evaporation rate of ∼7.6 kg m−2 h−1 under one sun irradiation (1 kW m−2) at a wind speed of 3 m s−1. This is a promising technology for enhancing water evaporation rates in seawater desalination and wastewater treatment applications.

In this Letter, we report a high-performance NiO/β-Ga2O3 pn heterojunction diode with an optimized interface by annealing. The electrical characteristics of the pn diode without annealing (PND) and with annealing (APND) are studied systematically. The APND device has a lower specific on-resistance of 4.1 mΩ cm2, compared to that of the PND, 5.4 mΩ cm2. Moreover, for the APND, a high breakdown voltage of 1630 V with lower leakage current is achieved, which is 730 V higher than that of the PND. The enhanced electrical performance of the APND leads to a record high power figure of merit of 0.65 GW/cm2 in Ga2O3-based pn diodes, which is among the best reported results in Ga2O3 power devices. In addition, the interface trap density of the diode decreases from 1.04 × 1012 to 1.33 × 1011 eV−1 cm−2 after annealing, contributing to much lower hysteresis. Simultaneously, the ideality factor n for the APND is steady at elevated temperatures due to the stable interface. The results of C−V characteristics reveal the bulk defects inside the nickel oxide film grown by sputtering, which are…

We report a lightweight tunable acoustic metamaterial with deep subwavelength thickness (e.g., λ/300) and strong load-bearing capability for underwater low-frequency and ultra-broadband acoustic perfect absorption. The metamaterial is constructed by introducing a rubber coating and an embedded metallic neck into a metallic hexagonal honeycomb Helmholtz resonator. Physically, the quasi-Helmholtz resonance triggered by the rubber coating together with the anti-phase cancellation caused by the embedded neck leads to superior sound absorption. Theoretical predictions of the metamaterial performance agree well with finite element simulation results. With fixed external morphology (e.g., honeycomb-cored sandwich panel) and fixed overall thickness (e.g., 50 mm), key internal geometrical parameters of the proposed metamaterial can be tailored to achieve tunable perfect absorption from, e.g., 100 Hz to 300 Hz. Further, combining such tunable quasi-Helmholtz resonance leads to ultra-broadband quasi-perfect absorption from, e.g., 306 Hz to 921 Hz. This work contributes to designing underwater…

In this Letter, high-performance β-Ga2O3 vertical heterojunction barrier Schottky (HJBS) diodes have been demonstrated together with the investigation of reverse leakage mechanisms. In HJBS configurations, NiO/β-Ga2O3 p-n heterojunctions and p-NiO field limiting rings (FLRs) are implemented by using a reactive sputtering technique at room temperature without intentional etching damages. Determined from the temperature-dependent current-voltage characteristics, the reverse leakage mechanism of the HJBS diode is identified to be Poole-Frenkel emission through localized trap sates with an energy level of EC-0.72 eV. With an uniform FLR width/spacing of 2 μm in HJBS, a maximum breakdown voltage (BV) of 1.89 kV and a specific on-resistance (Ron,sp) of 7.7 mΩ·cm2 are achieved, yielding a high Baliga's figure-of-merit (FOM, BV2/Ron,sp) of 0.46 GW/cm2. The electric field simulation and statistical experimental facts indicate that the electric field crowding effect at device edges is greatly suppressed by the shrinkage of p-NiO FLR spacing, and the capability of sustaining high BV is…

Halide perovskite exhibits remarkable photovoltaic (PV) performance so far, however, its toxicity and instability hinder the commercialization. Researchers are devoted to explore nontoxic and stable solar cell alternatives with high efficiency. Considering the octahedra network and the high symmetry crystalline structure of prototype Mg3NF3 (Pm-3m), which is similar to the halide perovskites and tends to create high p-s band edge states transition probability associated closely with the excellent PV performance. We extensively screened the potential PV materials from 64 compounds in A3MX3 (A = Mg, Ca, Sr, Ba; M = N, P, As, Sb; X = F, Cl, Br, I) structure based on the bandgap, theoretical efficiency, band edge states transition, and thermal stability. Three Pb-free compounds, i.e., Ba3PI3, Ba3AsI3, and Ba3SbI3, are found to be ideal stable PV materials with the efficiency (25.9%) comparable to that of halide perovskites. The optical absorption of the best PV candidate, i.e., Ba3SbI3, has been improved further by low energy Cu ion implantation, and the occurrence of the threshold…

Temporal metamaterials are artificial materials whose electromagnetic properties change over time. In analogy with spatial media and metamaterials, where their properties change smoothly or abruptly over space, temporal metamaterials can exhibit a smooth variation over time, realizing a temporal non-homogeneous medium, or a stepwise transition, and the temporal version of dielectric slabs or multilayer structures. In this Letter, we focus our attention on temporal multilayer structures, and we propose the synthesis of higher-order transfer functions by modeling the wave propagation through a generalized temporal multilayer structure, consisting of a cascade over time of different media. The tailoring of the scattering response of the temporal structure as a function of frequency is presented, deriving the corresponding scattering coefficients for a properly designed set of medium properties, i.e., permittivity and permeability, and application time, in analogy with what is typically done in optical and electromagnetic spatial multilayered structures. This allows us to design novel…

Semiconducting half-Heusler compounds with the valence electron count of 18 have been identified as a class of promising high-temperature thermoelectric materials. Recently, nominal 19-electron half-Heusler compounds, traditionally regarded as metals, have gained reacquaintance and popularity due to their unexpected high thermoelectric performance and fascinating defective structure. In this Perspective, we summarize the current progress of 18-electron half-Heusler thermoelectric materials and focus on the discovery and challenge of the cation-deficient 19-electron half-Heusler compounds with the vacancy-related short-range order. Further outlook on the discovery of promising half-Heusler thermoelectrics and the insightful understanding of the defect-tailored thermoelectric properties are offered.

This manuscript reports the temperature dependence of ferroelectric switching in Al0.84Sc0.16N, Al0.93B0.07N, and AlN thin films. Polarization reversal is demonstrated in all compositions and is strongly temperature dependent. Between room temperature and 300 °C, the coercive field drops by almost 50% in all samples, while there was very small temperature dependence of the remanent polarization value. Over this same temperature range, the relative permittivity increased between 5% and 10%. Polarization reversal was confirmed by piezoelectric coefficient analysis and chemical etching. Applying intrinsic/homogeneous switching models produces nonphysical fits, while models based on thermal activation suggest that switching is regulated by a distribution of pinning sites or nucleation barriers with an average activation energy near 28 meV.

The search for an ideal single-photon source has generated significant interest in discovering emitters in materials as well as developing new manipulation techniques to gain better control over the emitters' properties. Quantum emitters in atomically thin two-dimensional (2D) materials have proven to be very attractive with high brightness, operation under ambient conditions, and the ability to be integrated with a wide range of electronic and photonic platforms. This Perspective highlights some of the recent advances in quantum light generation from 2D materials, focusing on hexagonal boron nitride and transition metal dichalcogenides. Efforts in engineering and deterministically creating arrays of quantum emitters in 2D materials, their electrical excitation, and their integration with photonic devices are discussed. Finally, we address some of the challenges the field is facing and the near-term efforts to tackle them. We provide an outlook toward efficient and scalable quantum light generation from 2D materials to controllable and addressable on-chip quantum sources.

The transverse thermoelectric effect refers to the conversion of a temperature gradient into a transverse charge current, or vice versa, which appears in a conductor under a magnetic field or in a magnetic material with spontaneous magnetization. Among such phenomena, the anomalous Nernst effect in magnetic materials has been receiving increasing attention from the viewpoints of fundamental physics and thermoelectric applications owing to the rapid development of spin caloritronics and topological materials science. In this research trend, a conceptually different transverse thermoelectric conversion phenomenon appearing in thermoelectric/magnetic hybrid materials has been demonstrated, enabling the generation of a large transverse thermopower. Here, we review the recent progress in fundamental and applied studies on the transverse thermoelectric generation using magnetic materials. We anticipate that this perspective will further stimulate research activities on the transverse thermoelectric generation and lead to the development of next-generation thermal energy harvesting and…

In this paper, we show that high-performance β-Ga2O3 hetero-junction barrier Schottky (HJBS) diodes with various β-Ga2O3 periodic fin widths of 1.5/3/5 μm are demonstrated with the incorporation of p-type NiOx. The β-Ga2O3 HJBS diode achieves a low specific on-resistance (Ron,sp) of 1.94 mΩ cm2 with a breakdown voltage of 1.34 kV at a β-Ga2O3 periodic fin width of 3 μm, translating to a direct-current Baliga's power figure of merit (PFOM) of 0.93 GW/cm2. In addition, we find that by shrinking the β-Ga2O3 width, the reverse leakage current is minimized due to the enhanced sidewall depletion effect from p-type NiOx. β-Ga2O3 HJBS diodes with p-type NiOx turn out to be an effective route for Ga2O3 power device technology by considering the high PFOM while maintaining a suppressed reverse leakage current.

In this Letter, we report a back-end-of-line (BEOL), complementary metal–oxide–semiconductor (CMOS)-compatible Al0.64Sc0.36N-based ferroelectric diode that shows polarization-dependent hysteresis in its leakage currents. Our device comprises a metal/insulator/ferroelectric/metal structure (Pt/native oxide/Al0.64Sc0.36N/Pt) that is compatible with BEOL temperatures (≤ 350 °C) grown on top of a 4-in. silicon wafer. The device shows self-selective behavior as a diode with > 105 rectification ratio (for 5 V). It can suppress sneak currents without the need for additional access transistors or selectors. Furthermore, given the polarization-dependent leakage, the diode current–voltage sweeps are analogous to that of a memristor with an on/off ratio of ∼ 50 000 between low and high resistance states. Our devices also exhibit stable programed resistance states during DC cycling and a retention time longer than 1000 s at 300 K. These results demonstrate that this system has significant potential as a future high-performance post-CMOS compatible nonvolatile memory technology.

Reduction of vibration and sound energy in the form of traveling waves is of vital importance in many applications. Recent development of acoustic metamaterials opens up unusual ways for sound wave manipulation and control. Among acoustic metamaterials, a much newer concept, Acoustic Black Hole (ABH), has been drawing growing attention in recent years, which shows great potential for acoustic energy trapping and dissipation. In a duct ABH with a properly tailored continuous cross-sectional reduction and impedance variation, it is shown that the sound speed can be progressively reduced, which means that sound waves are eventually trapped in the structure. In this paper, such a wave trapping mechanism is further explored in the context of sound transmission problems, in which an exceptional phenomenon—simultaneous reduction of sound reflection and transmission—is realized. The archived trapping mechanism also ensures that little sound waves will be bounced back to the source to jeopardize the overall performance. Transfer matrix method simulations and impedance tube experiments are…

5G technology, both infrastructure and handsets, is being deployed throughout the world with television advertisements trumpeting faster download speeds, an end to dropped calls and buffering, high bandwidth communication systems enabling remote surgery, autonomous vehicles, and enhanced virtual reality experiences. In truth, the adoption of the 5G technology is in its infancy with numerous technical challenges ahead. In this perspective article, we will present an overview of the technology platforms that make up 5G and examine the role of ceramic oxide and nitride materials in enabling this technology. We will look at ceramic devices present in both handsets and the base station infrastructure in terms of its historical use in wireless telephony and its projected future use in 5G systems.

Through its dependence on low symmetry crystal phases, ferroelectricity is inherently a property tied to the lower temperature ranges of the phase diagram for a given material. This paper presents conclusive evidence that in the case of ferroelectric Al1−xScxN, low temperature has to be seen as a purely relative term, since its ferroelectric-to-paraelectric transition temperature is confirmed to surpass 1100 °C and thus the transition temperature of virtually any other thin film ferroelectric. We arrived at this conclusion through investigating the structural stability of 0.4–2 μm thick Al0.73Sc0.27N films grown on Mo bottom electrodes via in situ high-temperature x-ray diffraction and permittivity measurements. Our studies reveal that the wurtzite-type structure of Al0.73Sc0.27N is conserved during the entire 1100 °C annealing cycle, apparent through a constant c/a lattice parameter ratio. In situ permittivity measurements performed up to 1000 °C strongly support this conclusion and include what could be the onset of a diverging permittivity only at the very upper end of the…

We review the current status of mode-division multiplexing (MDM) techniques in fibers and on chips. Three system applications are introduced, including quasi-single mode transmission, multicore few-mode amplifier, and fiber sensing. We also discuss the technology development trend in terms of multiple-input-multiple-output-free MDM, economics of MDM, and quantum information processing. Finally, we provide perspectives on emerging applications beyond communications by leveraging the optical properties of high order modes, e.g., nonlinear optics in the visible regime, broadband frequency comb generation, and super resolution endoscopy.

Uniquely furnishing giant and nonvolatile modulation of optical properties and chalcogenide phase change materials (PCMs) have emerged as a promising material to transform integrated photonics and free-space optics alike. The surge of interest in these materials warrants a thorough understanding of their characteristics specifically in the context of photonic applications. This article seeks to clarify some commonly held misconceptions about PCMs and offer a perspective on new research frontiers in the field.

Flow-induced vibration (FIV) energy harvesting has attracted extensive research interest in the past two decades. Remarkable research achievements and contributions from different aspects are briefly overviewed. Example applications of FIV energy harvesting techniques in the development of Internet of Things are mentioned. The challenges and difficulties in this field are summarized from two sides. First, the multi-physics coupling problem in FIV energy harvesting still cannot be well handled. There is a lack of system-level theoretical modeling that can accurately account for fluid–structure interaction, the electromechanical coupling, and complicated interface circuits. Second, the robustness of FIV energy harvesters needs to be further improved to adapt to the uncertainties in practical scenarios. To be more specific, the cut-in wind speed is expected to be further reduced and the power output to be increased. Finally, Perspectives on the future development in this direction are discussed. Machine-learning approaches, the versatility of metamaterials, and more advanced interface…

Topological insulators have attracted intensive attention due to their robust properties of path defect immunity, with diverse applications in electromagnetic, acoustic, and elastic systems. The recent development of elastic topological insulators (ETIs), based on artificially structured phononic crystals, has injected new momentum into the manipulation of elastic waves. Earlier ETIs with unreconfigurable geometry and narrow frequency bandgaps hinder the exploration and design of adaptable devices. In this work, a tunable phononic crystal plate with Y-shaped prisms is designed to support valley transport of elastic waves, based on the analogy of the quantum valley Hall effect. By rotating the prisms to reconstruct the configuration, the mirror symmetry is broken to open a new bandgap. Based on this characteristic, we design an interface between two ETIs with different symmetry-broken geometries, which supports topologically protected edge states. We further design a reconfigurable device for elastic wave channel switching and beam splitting and demonstrate it both numerically and…

Digital computers store information in the form of bits that can take on one of two values 0 and 1, while quantum computers are based on qubits that are described by a complex wavefunction, whose squared magnitude gives the probability of measuring either 0 or 1. Here, we make the case for a probabilistic computer based on p-bits, which take on values 0 and 1 with controlled probabilities and can be implemented with specialized compact energy-efficient hardware. We propose a generic architecture for such p-computers and emulate systems with thousands of p-bits to show that they can significantly accelerate randomized algorithms used in a wide variety of applications including but not limited to Bayesian networks, optimization, Ising models, and quantum Monte Carlo.

Acoustic metasurfaces have enabled unprecedented control over acoustic waves, offering opportunities in areas such as holographic rendering, sound absorption, and acoustic communication. Despite the steady progress made in this field, most acoustic metasurface designs are passive in that they only provide static functionalities. Here, a reconfigurable active acoustic metalens is implemented to showcase scanning of the focus along arbitrary trajectories in free space with the help of a previously developed active acoustic metasurface platform. Each unit cell of the metasurface contains a cavity, whose size can be tuned continuously by a dynamic control system to adjust the phase of the reflected wave. While this work focuses on beam focusing, it could hold great promise for a wide range of applications including acoustic levitation and tweezers.

We report on the ferroelectric properties of single-phase wurtzite ScGaN grown on GaN by plasma-assisted molecular beam epitaxy. Distinct ferroelectric switching behavior was confirmed by detailed electrical characterization. Coercive fields in the range of 2.0–3.0 MV/cm and large, retainable remnant polarization in the range of 60–120 μC/cm2 are unambiguously demonstrated for ScGaN epilayers with Sc contents of 0.31–0.41. Taking advantage of the widely tunable energy bandgap of III-nitride semiconductors, the demonstration of ferroelectricity in ScGaN, together with the recently reported ferroelectric ScAlN, will enable a broad range of emerging applications with combined functionality in ferroelectric, electronic, optoelectronic, photovoltaic, and/or photonic devices and systems.

Quantum random number generators (QRNGs) can produce true random numbers. Yet, the two most important QRNG parameters highly desired for practical applications, i.e., speed and size, have to be compromised during implementations. Here, we present the fastest and miniaturized QRNG with a record real-time output rate as high as 18.8 Gbps by combining a photonic integrated chip and the technology of optimized randomness extraction. We assemble the photonic integrated circuit designed for vacuum state QRNG implementation, an InGaAs homodyne detector, and a high-bandwidth transimpedance amplifier into a single chip using hybrid packaging, which exhibits the excellent characteristics of integration and high-frequency response. With a sample rate of 2.5 GSa/s in a 10-bit analog-to-digital converter and subsequent paralleled postprocessing in a field programmable gate array, the QRNG outputs ultrafast random bitstreams via a fiber optic transceiver, whose real-time speed is validated in a personal computer.

Crystallographic characterization and the ferroelectric properties of 50 nm-thick sputter-deposited Al0.78Sc0.22N films deposited at room temperature (RT) and 400 °C are investigated. c-axis oriented growths were confirmed by x-ray diffraction patterns with rocking curve measurements for both samples. Al0.78Sc0.22N films were found to grow in the c-axis direction and showed poling-free ferroelectric properties, which are advantageous for practical memory and piezoelectric applications. Although the metal-ferroelectric-metal (MFM) capacitors represent low switching cycle endurance, MFM capacitors revealed remnant polarization (Pr) of 70 μC/cm2 and 113 μC/cm2 for RT- and 400 °C-deposited samples, respectively. Ferroelectric films with low-temperature process capability can open a wide range of applications.

A continuous-wave (CW) broadband high-level optical quadrature squeezer is essential for high-speed large-scale fault-tolerant quantum computing on a time-domain-multiplexed continuous-variable optical cluster state. CW THz-bandwidth squeezed light can be obtained with a waveguide optical parametric amplifier (OPA); however, the squeezing level has been insufficient for applications of fault-tolerant quantum computation because of degradation of the squeezing level due to their optical losses caused by the structural perturbation and pump-induced phenomena. Here, by using mechanical polishing processes, we fabricated a low-loss quasi-single-mode periodically poled LiNbO3 (PPLN) waveguide, which shows 7% optical propagation loss with a waveguide length of 45 mm. Using the waveguide, we assembled a low-loss fiber-pigtailed OPA module with a total insertion loss of 21%. Thanks to its directly bonded core on a LiTaO3 substrate, the waveguide does not show pump-induced optical loss even under a condition of hundreds of milliwatts pumping. Furthermore, the quasi-single-mode structure…

The single Helmholtz resonator obtains only one absorption peak in the broad frequency range, which limits its application in reducing the noise with multiple spectra. This paper reports an acoustic multi-layer Helmholtz resonance metamaterial, which can achieve multiple absorption peaks at given low-frequency targets. Meanwhile, through adjusting structural parameters of the multi-layer Helmholtz resonator, its impedance can be altered correspondingly to realize the absorption of noise with the multi groups of specific frequencies. In this paper, in order to achieve fine absorption performance with the specific frequencies of 100 and 400 Hz for a substation noise source, the sound absorption principle of a classical Helmholtz resonator with the embedded aperture is introduced theoretically, and then two series of multi-layer Helmholtz resonance structures with different parameters are designed. Thickness of the multi-layer structure is only 1/30th of the working wavelength, and two groups of resonance peaks are generated at 100 and 400 Hz, respectively. A finite element model of…

With exceptional electrical and mechanical properties and at the same time air-stability, layered MoSi2N4 has recently drawn great attention. However, band structure engineering via strain and electric field, which is vital for practical applications, has not yet been explored. In this work, we show that the biaxial strain and external electric field are effective ways for the bandgap engineering of bilayer MoSi2N4 and WSi2N4. It is found that strain can lead to indirect bandgap to direct bandgap transition. On the other hand, electric field can result in semiconductor to metal transition. Our study provides insights into the band structure engineering of bilayer MoSi2N4 and WSi2N4 and would pave the way for its future nanoelectronics and optoelectronics applications.

Quantum-limited microwave parametric amplifiers are genuine key pillars for rising quantum technologies and, in general, for applications that rely on the successful readout of weak microwave signals by adding only the minimum amount of noise allowed by quantum mechanics. In this Perspective, after providing a brief overview on the different families of parametric microwave amplifiers, we focus on traveling wave parametric amplifiers, underlining the key achievements of the last few years and the present open challenges. We also discuss possible new research directions beyond amplification such as exploring these devices as a platform for multi-mode entanglement generation and for the development of single photon detectors.

The generation and long-haul transmission of highly entangled photon pairs is a cornerstone of emerging photonic quantum technologies with key applications such as quantum key distribution and distributed quantum computing. However, a natural limit for the maximum transmission distance is inevitably set by attenuation in the medium. A network of quantum repeaters containing multiple sources of entangled photons would allow overcoming this limit. For this purpose, the requirements on the source's brightness and the photon pairs' degree of entanglement and indistinguishability are stringent. Despite the impressive progress made so far, a definitive scalable photon source fulfilling such requirements is still being sought after. Semiconductor quantum dots excel in this context as sub-Poissonian sources of polarization entangled photon pairs. In this work, we present the state-of-the-art set by GaAs based quantum dots and use them as a benchmark to discuss the challenges toward the realization of practical quantum networks.

The recent demonstration of MoSi2N4 and its exceptional stability to air, water, acid, and heat has generated intense interest in this family of two-dimensional materials. Among these materials, monolayers of NbSi2N4, VSi2N4, and VSi2P4 are semiconducting, easy-plane ferromagnets with negligible in-plane magnetic anisotropy. They, thus, satisfy a necessary condition for exhibiting a dissipationless spin superfluid mode. The Curie temperatures of monolayer VSi2P4 and VSi2N4 are determined to be above room temperature based on Monte Carlo and density functional theory calculations. The magnetic moments of VSi2N4 can be switched from in-plane to out-of-plane by applying tensile biaxial strain or electron doping.

Artificial magnetic molecules can contribute to progressing toward large scale quantum computation by (a) integrating multiple quantum resources and (b) reducing the computational costs of some applications. Chemical design, guided by theoretical proposals, allows embedding nontrivial quantum functionalities in each molecular unit, which then acts as a microscopic quantum processor able to encode error protected logical qubits or to implement quantum simulations. Scaling up even further requires “wiring-up” multiple molecules. We discuss how to achieve this goal by the coupling to on-chip superconducting resonators. The potential advantages of this hybrid approach and the challenges that still lay ahead are critically reviewed.

We demonstrate improved sensitivity of Rydberg electrometry based on electromagnetically induced transparency (EIT) with a ground state repumping laser. Though there are many factors that limit the sensitivity of radio frequency field measurements, we show that repumping can enhance the interaction strength while avoiding additional Doppler or power broadening. Through this method, we nearly double the EIT amplitude without an increase in the width of the peak. A similar increase in amplitude without the repumping field is not possible through simple optimization. We also establish that one of the key limits to detection is the photon shot noise of the probe laser. We show an improvement on the sensitivity of the device by a factor of nearly 2 in the presence of the repump field.

Motivated by the recent experimental realization of a two-dimensional (2D) BeN4 monolayer, in this study we investigate the structural, dynamical, electronic, and optical properties of a monolayer and few-layer BeN4 using first-principles calculations. The calculated phonon band dispersion reveals the dynamical stability of a free-standing BeN4 layer, while the cohesive energy indicates the energetic feasibility of the material. Electronic band dispersions show that monolayer BeN4 is a semi-metal whose conduction and valence bands touch each other at the Σ point. Our results reveal that increasing the layer number from single to six-layers tunes the electronic nature of BeN4. While monolayer and bilayer structures display a semi-metallic behavior, structures thicker than that of three-layers exhibit a metallic nature. Moreover, the optical parameters calculated for monolayer and bilayer structures reveal that the bilayer can absorb visible light in the ultraviolet and visible regions better than the monolayer structure. Our study investigates the electronic properties of…

Collapsing cavitation bubbles produce intense microscopic flows. Here, in an aqueous environment, we seed single laser-induced bubbles (diameter about one millimeter) in proximity to a solid surface, in a regime that has not been well explored before in order to generate a “needle jet.” The needle jet propagates at supersonic speed through the gas phase toward the solid. It reaches average velocities of more than 850 ms−1 and thus is an order of magnitude faster than the regular jets that have frequently been observed in cavitation bubbles. The dynamics leading to the needle jet formation are studied with high speed imaging at five million frames per second with femtosecond illumination. This highly repeatable, localized flow phenomenon may be exploited for injection purposes or material processing, and it is expected to generate significantly larger water hammer pressures and may also play a role in cavitation erosion and peening.

By exploring geometry-governed magnetic interactions, curvilinear magnetism offers a number of intriguing effects in curved magnetic wires and curved magnetic films. Recent advances in experimental techniques change the status of curvilinear magnetism, allowing the exploitation of 3D curved nanomagnets in emerging devices with numerous applications. Here, we provide our Perspective on the recent progress, challenges, and prospects of curvilinear magnetism with a special focus on novel physical effects caused by tailoring curvature and topology of conventional magnetic materials.

A strong spin-selective asymmetric transmission metasurface usually relies on a multilayer metal frame or compound all-dielectric system. By contrast, an all-silicon metasurface has advantages of simple process, low cost, and easy optical modulation. However, there is little work to explore the spin-selective asymmetric transmission in terahertz band based on the all-silicon metasurface due to weak interaction between all-silicon materials and terahertz waves. Herein, we have achieved excellent spin-selective terahertz asymmetric transmission in the all-silicon metasurface by the ingenious chirality structure design, and the maximum circular dichroism can reach 0.4. Meanwhile, 1064 nm continuous wave optical pumping modulates the asymmetric transmission amplitude of the all-silicon metasurface to achieve circular dichroism switch between “On” and “Off.” We have also developed terahertz chiral imaging applications based on the all-silicon metasurface with spin-selective asymmetric transmission, and the chiral imaging with On and Off switching characteristics can be realized via…

Invisibility has been a long-standing topic, owing to its crucial importance in both academia and industry. At the close of the last century, the advent of metamaterials unprecedentedly ignited the enthusiasm toward pursuing invisibility cloaks, but despite two decades of research, it has yet to mature into a practical method. In this Perspective, we first present a review of conventional and state-of-the-art invisibility methodologies, as well as their intrinsic challenges. To take a further step, we focus on the conjunction of two burgeoning disciplines—metamaterials and deep learning—to facilitate an intelligent self-adaptive invisibility cloak for dynamic surroundings. Finally, future directions are described, which include possible development directions and other practice-oriented metadevices driven by deep learning.

High-efficiency InGaN-based yellow light-emitting diodes (LEDs) with high brightness are desirable for future high-resolution displays and lighting products. Here, we demonstrate efficient InGaN-based yellow (∼570 nm) LEDs with optimized three-layer staggered quantum wells (QWs) that are grown on patterned sapphire substrates. Numerical simulations show that the electron–hole wavefunction overlap of staggered InGaN QWs with high In content exhibits a 1.7-fold improvement over that of square InGaN QWs. At the same injection current, LEDs with staggered QWs exhibit lower forward voltages and narrower full widths at half maximum than LEDs with square QWs. The light output power and external quantum efficiency of a staggered QW LED are 10.2 mW and 30.8%, respectively, at 15 mA. We combine atomic probe tomography (APT), time-resolved photoluminescence (TRPL), and transmission electron microscopy (TEM) with energy-dispersive x-ray (EDX) mapping spectroscopy to shed light on the origin of enhanced device performance. APT results confirm the staggered In profile of our designed staggered…

The fabrication of the C6N7 monolayer [Zhao et al., Sci. Bull. 66, 1764 (2021)] motivated us to discover the optical, structural, mechanical, and electronic properties of the C6N7 monolayer by employing the density functional theory (DFT) method. We find that the shear modulus and Young's modulus of the C6N7 monolayer are smaller than the relevant values of graphene. However, Poisson's ratio is more significant than that of graphene. Applying the PBE (HSE06) functional bandgap of the C6N7 monolayer is 1.2 (1.97) eV, and the electronic dispersion is almost isotropic around the Γ point. C6N7 is more active in the ultraviolet region as compared to the visible light region. This study provides outstanding results, highlighting the bright viewpoints for the applications of the C6N7 monolayer in electronic and optical systems.

It was first suggested that terahertz imaging has the potential to detect skin cancer twenty years ago. Since then, THz instrumentation has improved significantly: real time broadband THz imaging is now possible and robust protocols for measuring living subjects have been developed. Here, we discuss the progress that has been made as well as highlight the remaining challenges for applying THz imaging to skin cancer detection.

Quantum computers, much like their classical counterparts, will likely benefit from flexible qubit encodings that can be matched to different tasks. For trapped ion quantum processors, a common way to access multiple encodings is to use multiple, co-trapped atomic species. Here, we outline an alternative approach that allows flexible encoding capabilities in single-species systems through the use of long-lived metastable states as an effective, programmable second species. We describe the set of additional trapped ion primitives needed to enable this protocol and show that they are compatible with large-scale systems that are already in operation.

We introduce a simplified fabrication technique for Josephson junctions and demonstrate superconducting Xmon qubits with T1 relaxation times averaging above 50 μs (Q>1.5×106). Current shadow-evaporation techniques for aluminum-based Josephson junctions require a separate lithography step to deposit a patch that makes a galvanic, superconducting connection between the junction electrodes and the circuit wiring layer. The patch connection eliminates parasitic junctions, which otherwise contribute significantly to dielectric loss. In our patch-integrated cross-type junction technique, we use one lithography step and one vacuum cycle to evaporate both the junction electrodes and the patch. This eliminates a key bottleneck in manufacturing superconducting qubits by reducing the fabrication time and cost. In a study of more than 3600 junctions, we show an average resistance variation of 3.7% on a wafer that contains forty 0.5×0.5-cm2 chips, with junction areas ranging between 0.01 and 0.16 μm2. The average on-chip spread in resistance is 2.7%, with 20 chips varying between 1.4% and 2%.…

Epitaxial ferroelectric wurtzite films exhibiting clear polarization-electric field hysteresis behavior are presented. The coercive field of this epitaxial Al0.7Sc0.3N film on the W/c-sapphire substrate is 0.4 ± 0.3 MV cm−1 (8%) smaller than that of a conventional fiber textured film on a Pt/TiOx/SiO2/Si substrate, attributed to the 0.01 ± 0.007 Å smaller c-axis lattice parameter in the epitaxial film. The strain and decrease in the coercive field most likely originate from epitaxial strain rather than the mismatch in the thermal coefficient of expansion. These results provide insight for further coercive field reduction of wurtzite ferroelectrics using epitaxial mismatch strain.

Combinatorial optimization problems are known for being particularly hard to solve on traditional von Neumann architectures. This has led to the development of Ising Machines (IMs) based on quantum annealers and optical and electronic oscillators, demonstrating speed-ups compared to central processing unit (CPU) and graphics processing unit (GPU) algorithms. Spin torque nano-oscillators (STNOs) have shown GHz operating frequency, nanoscale size, and nanosecond turn-on time, which would allow their use in ultrafast oscillator-based IMs. Here, we show using numerical simulations based on STNO auto-oscillator theory that STNOs exhibit fundamental characteristics needed to realize IMs, including in-phase/out-of-phase synchronization and second harmonic injection locking phase binarization. Furthermore, we demonstrate numerically that large STNO network IMs can solve Max-Cut problems on nanosecond timescales.

Ionic thermoelectric (i-TE) materials, using ions as the energy carrier, can generate a voltage under a temperature difference, bearing similarities to the Seebeck effect of electrons and holes in solid-state materials. Recent experiments have demonstrated large thermopower of quasi-solid-state i-TE materials, which are attractive for harvesting ambient heat as large enough voltage can be generated under a small temperature difference to match the voltage input needs of sensors for internet-of-things applications. In this perspective article, we discuss similarities and differences of i-TE materials from electronic-based thermoelectric materials and also different i-TE thermoelectric effects including the thermodiffusion (Soret) effect and the thermogalvanic effect, in which the latter includes redox reaction entropy and the Soret effect. Strategies to improve performances of materials and devices are elaborated, together with needs for future research in understanding microscopic origins of different effects.

The photodetectors based on the wide bandgap semiconductor (WBS)/Si heterojunction have attracted more and more attention in recent years due to their excellent photoelectric characteristics and easy integration capabilities. In this work, we have demonstrated a self-powered solar-blind ultraviolet (UV) photodetector based on the ZnGa2O4/Si heterojunction. A typical rectification characteristic with a rectification ratio exceeding 103 within ±5 V can be obtained. At 0 V bias, the −3 dB cutoff wavelength of ∼255 nm and the UV-visible rejection ratio of ∼3 × 102 show that the device has excellent self-powered solar-blind UV detection performance. In addition, the responsivity and the response speed of ZnGa2O4/Si heterojunction can be efficiently enhanced by a transient spike current at 0 V bias when turning on and off the 254 nm UV light. The interface pyroelectric effect of the ZnGa2O4 film should be responsible for this transient spike photocurrent phenomenon. Our findings in this work pave a feasible way to realize high-performance WBS/Si heterojunction self-powered solar-blind…

Magnetic skyrmions are attractive for representing data in next-generation spintronic devices owing to their stability, small size, and ease of manipulation with spin torques. In order to realize such devices, it is necessary to be able to write, manipulate, and read back data by means of nucleating, propagating, and detecting skyrmions using an all-electrical approach. Here, we review the basic concepts underpinning magnetic skyrmions, describe our recent results on their electrical nucleation, propagation, and detection, and offer some perspectives for future research in this vibrant field.

Kagome lattice, made of corner-sharing triangles, provides an excellent platform for hosting exotic topological quantum states. Here, we systematically studied the magnetic and transport properties of RMn6Sn6 (R = Tb, Dy, Ho) with clean Mn kagome lattice. All the compounds have a collinear ferrimagnetic structure with different easy axis at low temperature. The low-temperature magnetoresistance (MR) is positive and has no tendency to saturate below 7 T, while the MR gradually declines and becomes negative with the increasing temperature. A large intrinsic anomalous Hall conductivity about 250, 40, and 95 Ω−1 cm−1 is observed for TbMn6Sn6, DyMn6Sn6, and HoMn6Sn6, respectively. Our results imply that RMn6Sn6 system is an excellent platform to discover other intimately related topological or quantum phenomena and also tune the electronic and magnetic properties in future studies.

Neuromorphic computing that mimics the biological brain has been demonstrated as a next-generation computing method due to its low power consumption and parallel data processing characteristics. To realize neuromorphic computing, diverse neural networks such as deep neural networks (DNNs) and spiking neural networks (SNNs) have been introduced. DNNs require artificial synapses that have analog conductance modulation characteristics, whereas SNNs require artificial synapses that have conductance modulation characteristics controlled by temporal relationships between signals, so the development of a multifunctional artificial synapse is required. In this work, we report a ferroelectric thin-film transistor (FeTFT) that uses zirconium-doped hafnia (HfZrOx) and indium zinc tin oxide (IZTO) for neuromorphic applications. With reliable conductance modulation characteristics, we suggest that the FeTFT with HfZrOx and IZTO can be used as an artificial synapse for both DNNs and SNNs. The linear and symmetric conductance modulation characteristics in FeTFTs result in high recognition accuracy…

We demonstrate optical coupling between a single tin-vacancy (SnV) center in diamond and a free-standing photonic crystal nanobeam cavity. The cavities are fabricated using quasi-isotropic etching and feature experimentally measured quality factors as high as ∼11 000. We investigate the dependence of a single SnV center's emission by controlling the cavity wavelength using a laser-induced gas desorption technique. Under resonance conditions, we observe an intensity enhancement of the SnV emission by a factor of 12 and a 16-fold reduction of the SnV lifetime. Based on the large enhancement of the SnV emission rate inside the cavity, we estimate the Purcell factor for the SnV zero-phonon line to be 37 and the coupling efficiency of the SnV center to the cavity, the β factor, to be 95%. Our work paves the way for the realization of quantum photonic devices and systems based on efficient photonic interfaces using the SnV color center in diamond.

Due to the “efficiency droop” effect of light-emitting diodes (LEDs) under high drive currents, laser diode (LD) driven light sources are considered to replace LED lighting for the next-generation solid-state-lighting technology. In this Perspective, the history of lighting technology and the intrinsic advantages of LDs lighting are outlined. We provide a comparative overview of several typical LD phosphors, such as single crystal, transparent ceramic, phosphor-in-glass, and film thereof, as well as surface texture modification of film-type phosphor plates. The relationship between the illumination characterizations (luminance saturation, conversion efficiency, extraction efficiency, illuminating directionality, and color spatial uniformity) with thermal conductivity, pores, defects, second phases, and surface roughing or patterning is discussed. Finally, the current challenges and prospects are highlighted to provide guidelines for future research on LD phosphors.

We investigate the emerging chemical states of TiN/HfO2/TiN capacitors and focus especially on the identification of vacancies and impurities in the ferroelectric HfO2 layers, which are produced either by physical vapor deposition (PVD) or atomic layer deposition (ALD). Depending on the specific growth conditions, we identify different mechanisms of oxygen vacancy formation. Corresponding spectral features are consistently observed for all HfO2- and TiN-related core levels by hard x-ray photoelectron spectroscopy (HAXPES). In ALD-grown samples, we find spectral signatures for the electronic interaction between oxygen vacancies and nitrogen impurities. By linking the HAXPES results to electric field cycling experiments on the TiN/HfO2/TiN capacitors, we discuss possible formation mechanisms and stabilization of the ferroelectric HfO2 phase directly related to specific PVD or ALD conditions.

Antiferromagnets are promising materials for future spintronic applications due to their unique properties including zero stray fields, robustness vs external magnetic fields, and ultrafast dynamics, which have attracted extensive interest in recent years. In this work, we investigate the dynamics of isolated skyrmions in an antiferromagnetic nanotrack with a voltage-gated region. It is found that the skyrmion can be jointly controlled by the driving current and the voltage-controlled magnetic anisotropy gradient. We further propose a design of logic computing gates based on the manipulation of antiferromagnetic skyrmions, which is numerically realized combining several interactions and phenomena, including the spin Hall effect, voltage-controlled magnetic anisotropy effect, skyrmion–skyrmion interaction, and skyrmion–edge interaction. The proposed logic gates can perform the basic Boolean operations of the logic AND, OR, NOT, NAND, and NOR gates. Our results may have a great impact on fundamental physics and be useful for designing future nonvolatile logic computing devices with…

Vertical β-Ga2O3 Schottky diodes from metal-organic chemical vapor deposition (MOCVD)-grown epitaxial films are reported in this paper for high-power application devices. The Schottky diode, fabricated with a field termination structure, showed a low differential specific on-resistance of 0.67 mΩ cm2. Furthermore, the MOCVD-grown β-Ga2O3 vertical Schottky diodes exhibited a punch-through breakdown and a higher Baliga's figure-of-merit compared to those from other epitaxial growth methods of similar drift layer thickness. This suggests that the MOCVD growth, supporting high-quality epitaxy, can be promising for high-performance β-Ga2O3-based high-power devices.

Superconducting nanowires have emerged as a powerful tool for detecting single photons in the visible and near-infrared range with excellent device performance metrics. We outline challenges and future directions related to the up-scaling of nanowire devices and detector systems toward widespread applications in demanding real-world settings. Progress on achieving superconducting single-photon detectors with a large active area and an increasing number of pixels is reviewed, comparing the recent literature in terms of the reported key detector parameters. Furthermore, we summarize currently available readout and multiplexing schemes for multi-pixel detector arrays and discuss implications of the recently discovered microwire-based detector geometries.

The bound states in a continuum (BICs) are objective physical phenomena that defy conventional wisdom, and they exist in the radiating continuous spectrum but remain perfectly localized with non-radiation, which is different from the traditional bound states. In this paper, we report in theory and experiment the high thickness all-silicon terahertz (THz) metasurfaces supporting BIC and quasi-BIC, which is realized by simple pairs of elliptical pillars. Meanwhile, we used an extra optical pump to modulate the transmission amplitude difference between BIC and quasi-BIC metasurfaces, to complete the active control from “On” state to “Off” state. We utilize the abundant amplitude gradation of metasurfaces with different asymmetric degrees to develop the polarization-dependent THz near-field displaying application that can make the grayscale characteristics of a photograph reappear and also be dynamically controlled at On state to Off state.

Flat optical components based on metasurfaces will appear in our daily life in the near future. Our discussion focuses on metasurface-based components consisting of sub-wavelength spaced dielectric nanostructures in the optical region. After an introduction to the underlying technology, the advantages of metasurfaces are highlighted and the efforts in the development of metasurface components is discussed. The metasurface not only promises a reduction in the size and complexity of optical components but also brings new functionalities. Examples of achromatic optical components, a full-Stokes metasurface camera, and a metasurface depth sensor with superior performance are highlighted. Finally, future trends and opportunities are discussed.

Red micro-light-emitting diodes (μLEDs) have been generated significant interest for the next generation μLEDs displays. It has been shown that the external quantum efficiency (EQE) of AlInGaP red μLEDs markedly decreases as the size goes to very small dimension. Here, we demonstrate size-independent peak EQE of 611 nm InGaN red μLEDs. Packaged μLEDs show a peak EQE varied from 2.4% to 2.6% as the device area reduces from 100 × 100 to 20 × 20 μm2. These results demonstrate the promising potential for realizing high efficiency red μLED with very small size using InGaN materials.

Due to the applications in meteorological broadcasts, radio navigation, and underwater communications, low-frequency (LF) receiving antennas have been actively studied. However, because the frequency range of LF antenna is 30 to 300 kHz, its electromagnetic wavelength is 1 to 10 km, which makes LF electromagnetic antennas difficult to be implemented in miniaturized or portable devices. This article presents a miniaturized LF magnetoelectric (ME) receiving antenna with an integrated DC magnetic bias. The antenna is based on the magnetoelectric effect and operates by resonance at its mechanical resonant frequency. Thus, compared with traditional LF wire antennas, the dimension of ME antenna is reduced significantly. Compared with prior art of ME antennas which do not have DC magnetic bias, higher performance can be achieved by integrating the miniaturized DC magnetic bias. Magnetostrictive TbDyFe2 (Terfenol-D) and piezoelectric lead zirconate titanate thin films are bonded together to form the 38 × 12 × 5.8 mm3 ME receiving antenna. Four 10 × 10 × 10 mm3 Rb magnets are implemented to…

Barocaloric materials have the potential to offer greener and more efficient alternatives to conventional refrigerants that exploit vapor compression for cooling and heating applications. The barocaloric effect, a temperature change in response to hydrostatic pressure, is widespread in materials; however, only in the last few years have giant and colossal barocaloric effects approaching those of commercial hydrocarbon refrigerants been discovered. These barocaloric effects are associated with large changes in the degrees of freedom within the material and can take various forms. Here, we discuss this variety and categorize barocaloric materials based on the degree of freedom that underpins the barocaloric effects. We use this framework to identify material families to explore for potential barocalorics.

Ultra-wide bandgap p-type α-(Ir,Ga)2O3 films with bandgaps of up to 4.3 eV have been obtained by unintentional doping or Mg doping. For Mg-doped films, Hall-effect measurements revealed a hole concentration of 9.9 × 1018 to 8.1 × 1019 cm−3 and a mobility of 0.13 − 0.92 cm2/V s, respectively. A preliminary test of a pn junction diode composed of p-type α-(Ir,Ga)2O3 and n-type α-Ga2O3 did not show catastrophic breakdown in the reverse direction until 100 V and the current on/off ratio at +3 V/−3V was 5 × 105. Since α-(Ir,Ga)2O3 and α-Ga2O3 take the same crystal structure and are well lattice-matched (with a lattice mismatch of <0.3%), the formation of a high-quality pn heterojunction is encouraged; this is one of the advantages of the corundum material system.

Killer defects are responsible for leakage current and breakdown in β-gallium oxide (β-Ga2O3) Schottky barrier diodes, which are crucial for power device applications. We have found that stacking faults in the halide vapor phase epitaxial (HVPE) (001) layer are killer defects. One type of defect is found to consist of (111) and (11¯1) stacking faults. The leakage current is 50 nA/defect at −200 V. This defect appears as a heart-shaped etch pit. Another type of defect is found to be a sequence of stacking faults from microparticles, which are formed at low gas flow rate during HVPE growth. The leakage current is 0.11–0.49 μA/defect at −200 V. This defect appears as a group of bullet-shaped etch pits.

We develop a hierarchical acoustic metamaterial with ultra-broadband sound absorption and promising applications at high temperatures as a super sound absorber. The proposed metamaterial is constructed through tailoring structural configurations by vertex-based hierarchy. The hierarchical design enables the acoustic metamaterial to achieve an excellent successive absorption spectrum in the low- to mid-frequency range from 0.58 kHz to 2.04 kHz in the first order and 0.64 kHz to 3.42 kHz in the second order, respectively. The sound absorption performance at high temperatures is predicted by means of theoretical and numerical approaches, and the models are validated by experimental measurements at room temperature. This work provides insight and opportunities into the role of structural hierarchy in designing acoustic metamaterials equipped with extraordinary absorption properties, illustrating an unprecedented capability of the hierarchical acoustic metamaterial.

Ultrawide-band-gap (UWBG) semiconductors are promising for fast, compact, and energy-efficient power-electronics devices. Their wider band gaps result in higher breakdown electric fields that enable high-power switching with a lower energy loss. Yet, the leading UWBG semiconductors suffer from intrinsic materials' limitations with regard to their doping asymmetry that impedes their adoption in CMOS technology. Improvements in the ambipolar doping of UWBG materials will enable a wider range of applications in power electronics as well as deep-UV optoelectronics. These advances can be accomplished through theoretical insights on the limitations of current UWBG materials coupled with the computational prediction and experimental demonstration of alternative UWBG semiconductor materials with improved doping and transport properties. As an example, we discuss the case of rutile GeO2 (r-GeO2), a water-insoluble GeO2 polytype, which is theoretically predicted to combine an ultra-wide gap with ambipolar dopability, high carrier mobilities, and a higher thermal conductivity than β-Ga2O3. The…

Efficient second harmonic generation (SHG) in nanophotonic designs based on all-dielectric nanostructures demands materials with large values of the quadratic nonlinear susceptibility, low dissipative losses, and high refractive index. One of the best materials meeting all these parameters is gallium phosphide (GaP). However, second-order nonlinearity requires high crystallinity and morphology quality of the GaP layer grown for further lithographic processing. Here we develop a method to fabricate high-quality crystalline GaP metasurfaces, which demonstrate pronounced linear and nonlinear optical properties. Direct growth of a GaP layer on a sapphire substrate tackles the previous problem of wafer bonding, because of high optical contrast between fabricated resonant nanoparticles and the substrate. As a result, the fabricated GaP metasurface supports bound state in continuum mode with an experimental quality factor around 100 yielding a strong enhancement of SHG in narrow spectral range. We believe that the developed approach will become a versatile platform for nonlinear…

Homoepitaxial film and semi-insulating bulk β-Ga2O3 with (001) orientation were studied using terahertz time-domain spectroscopy (THz-TDS) in the frequency region from 0.2 to 3.0 THz parallel to the [100] and [010] directions. The static permittivity of the bulk was determined to be 10.0 and 10.4 along the a-axis and b-axis, respectively, and the refractive index values at 0.2 THz are 3.17 and 3.23 for each axis. The electrical resistivity of the epilayer was extracted with good accuracy by employing the Drude–Lorentz model and without the use of electrical contacts. This noninvasive and contact-free material evaluation through THz-TDS proves to be a powerful tool for probing and obtaining various types of information about β-Ga2O3 materials such as bulk and thin films for the development of β-Ga2O3-based device applications.

Metalenses for optical beam manipulation have a significant impact in many exciting applications due their compact, planar geometry and ease of fabrication. However, the enormous physical size of metalenses relative to the optical wavelength provides a barrier to performing accurate simulations in a reasonable time frame. In principle, full-wave simulation techniques, such as the finite-difference time-domain (FDTD) method, are ideal for metalens modeling as they give an accurate picture of the device performance. However, when applied using traditional computing platforms, this approach is infeasible for large-diameter metalenses and requires hours and days to simulate even devices of modest size. To alleviate these issues, the standard approach has been to apply approximations, which typically employ simplified models of the metalens unit cells or ignore coupling between cells, leading to inaccurate predictions. In this Perspective, first, we summarize the current state of the art approaches in simulating large scale, three-dimensional metalenses. Then, we highlight that advances…

Molecular beam epitaxy of single-phase wurtzite N-polar ScxAl1−xN (x ∼ 0.11–0.38) has been demonstrated on sapphire substrates by locking its lattice-polarity to the underlying N-polar GaN buffer. Coherent growth of lattice-matched Sc0.18Al0.82N on GaN has been confirmed by x-ray diffraction reciprocal space mapping of the asymmetric (105) plane, whereas lattice-mismatched, fully relaxed Sc0.11Al0.89N and Sc0.30Al0.70N epilayers exhibit a crack-free surface. The on-axis N-polar crystallographic orientation is unambiguously confirmed by wet chemical etching. High electron mobility transistors using N-polar Sc0.18Al0.82N as a barrier have been grown on sapphire substrates, which present the existence of well confined two-dimensional electron gas. A Hall mobility of ∼564 cm2/V s is measured for a 15-nm-thick Sc0.18Al0.82N barrier sample with a sheet electron concentration of 4.1 × 1013 cm−2, and the corresponding sheet resistance is as low as 271 Ω/sq. The polarity-controlled epitaxy of ScxAl1−xN provides promising opportunities for applications in high-frequency and high-power…

This report details a deep learning approach to the forward and inverse designs of plasmonic metasurface structural color. Here, optimized Deep Neural Network models are presented to enable the forward and inverse mapping between metamaterial structure and corresponding color. The forward model is capable of predicting color with >96% accuracy, with a 105 order of magnitude decrease in computational time when compared to finite-difference time-domain simulations used in conventional design workflows. An inverse model is trained using a tandem autoencoder, employing the pre-trained forward model. Here, the use of synthetic training data for self-learning is reported, which results in an ≈15% improvement in training accuracy. The tightly constrained inverse model allows for the instantaneous design of metasurfaces, given a desired color, with an accuracy of >86%, making it suitable for commercial use as well as the acceleration of photonics research.

Moiré superlattices—twisted van der Waals (vdW) structures with small angles—are attracting increasing attention in condensed matter physics, due to important phenomena revealed therein, including unconventional superconductivity, correlated insulating states, and ferromagnetism. Moiré superlattices are typically comprised of atomic layers of vdW materials where the exotic physics arises from quantum electronic coupling between adjacent atomic layers. The abundant phenomena revealed in moire electronics have motivated intensive studies on their photonic counterpart. In addition to vdW materials, twisted photonic systems can also be comprised of metamaterials, metasurfaces, and photonic crystals, mediated by interlayer electromagnetic coupling instead. The interplay between short-ranged interlayer quantum and long-ranged electromagnetic coupling in twisted structures is expected to yield rich phenomena in nano-optics. This study reviews recent progress in twisted structures for nanophotonics and outlook for emerging topics, opportunities, fundamental challenges, and potential…

Here, we have explored the thermal stability of α-(Al,Ga)2O3 grown by the molecular-beam epitaxy on m-plane sapphire under high-temperature annealing conditions for various Al compositions (i.e., 0%, 46%, and 100%). Though uncapped α-Ga2O3 undergoes a structural phase transition to the thermodynamically stable β-phase at high temperatures, we find that an aluminum oxide cap grown by atomic layer deposition preserves the α-phase. Unlike uncapped α-Ga2O3, uncapped α-(Al,Ga)2O3 at 46% and 100% Al content remain stable at high temperatures. We quantify the evolution of the structural properties of α-Ga2O3, α-(Al,Ga)2O3, and α-Al2O3 and the energy bandgap of α-Ga2O3 up to 900 °C. Throughout the anneals, the α-Ga2O3 capped with aluminum oxide retains its high crystal quality, with no substantial roughening.

Inspired by the parallelism and efficiency of the brain, several candidates for artificial synapse devices have been developed for neuromorphic computing, yet a nonlinear and asymmetric synaptic response curve precludes their use for backpropagation, the foundation of modern supervised learning. Spintronic devices—which benefit from high endurance, low power consumption, low latency, and CMOS compatibility—are a promising technology for memory, and domain-wall magnetic tunnel junction (DW-MTJ) devices have been shown to implement synaptic functions such as long-term potentiation and spike-timing dependent plasticity. In this work, we propose a notched DW-MTJ synapse as a candidate for supervised learning. Using micromagnetic simulations at room temperature, we show that notched synapses ensure the non-volatility of the synaptic weight and allow for highly linear, symmetric, and reproducible weight updates using either spin transfer torque (STT) or spin–orbit torque (SOT) mechanisms of DW propagation. We use lookup tables constructed from micromagnetics simulations to model the…

Aluminum scandium nitride (Al1−xScxN/AlScN) (x = 0.32) Lamb wave resonators (LWR) have been fabricated and tested to demonstrate electromechanical coupling coefficients (kt2) in excess of 10%. The resonators exhibited an average kt2 and unloaded quality factor (Qu) of 10.28% and 711, respectively, when calculated from the measured data. Applying the Butterworth Van-Dyke (BVD) model to the measured data enabled the extraction of the resonator's lumped element parameters to calculate the motional quality factor (Qm), which neglects the contributions of the electrical traces. For the best measured resonator response, results from the BVD model showed a Qm of 1184 and a resulting figure-of-merit (FOM = K2 · Qm) of 100. Comparing the response of similar AlScN and AlN resonators shows that the AlScN LWR has a significantly lower motional resistance (Rm), suggesting that AlScN has a strong potential for use in piezoelectric microelectromechanical oscillators.

Thermoelectric (TE) properties of monolayered α-In2Se3 are investigated using the first-principles calculations based on the density functional theory and Boltzmann transport theory. The results show that monolayered α-In2Se3 is a great candidate for high-performance thermoelectric materials with the power factor PF and the figure of merit ZT as high as 0.02 W/mK2 and 2.18 at room temperature, respectively. We attribute such great TE performance to the large electrical conductivity and low lattice thermal conductivity, which originate from unique band structures of group III chalcogenides and anharmonic scattering. Furthermore, we prove that the quantum confinement effect can realize up to an order of magnitude enhancement in the PF. Our findings may open up new possibilities for two-dimensional thermoelectric materials in practical applications.

Bulk growth methods have made it possible to synthesize several allotropes of phosphorus such as black, white, red, and violet phosphorus. However, unlike exfoliated black phosphorus (BP), which has been extensively studied, much of the optoelectronic properties and stability of the other allotropes have yet to be comprehensively investigated. Here, we study the chemical degradation of exfoliated violet phosphorus (VP) in comparison to BP under ambient conditions using nanoscale infrared spectroscopy and imaging. We identify oxidized phosphorus species that result from chemical reaction processes on the surfaces of these phosphorus allotropes. We have found that VP exhibits a noticeably different and slower degradation process when compared to BP, establishing it as the more stable of the two allotropes. A better understanding of the stability of VP could lead to the further fundamental study of its monolayer form for potential future applications.

In this work, we investigate the electronic properties and thermoelectric performance of triangulene π-dimer junctions with the twist angle from 0° to 60° by using first-principles calculations in combination with a non-equilibrium Green's function method. It is found that the triangulene π-dimer can be transformed between nonmagnetic state and antiferromagnetic or ferromagnetic state by varying the twist angle. The reason is that the relative rotation between the monomers weakens the overlap of two single occupied molecular orbital. More importantly, our theoretical analysis shows that the ferromagnetic states of the triangulene π-dimer junctions at the twist angle of 20°, 30°, and 60° have outstanding thermoelectric performance. The corresponding ZT value is as high as around 6, which is mainly contributed from the spin splitting nature. This work is instructive to improve the thermoelectric properties of π-stacking molecular junctions or organic polymers.

The recognition, memory, and processing of light information are an important link in the development of artificial vision system. However, the traditional image recognition and memory unit of artificial vision systems need a complex circuit structure, which brings great challenges to the development of artificial vision. In this work, a CeOx/ZnO structure optoelectronic memristor based on a simple two-terminal structure was prepared. Through optical and electrical tests, 405 nm visible light recognition, storage, and processing were achieved, and at the same time, the response current has been greatly improved. And the response of 405 nm visible light was verified by using simulating memristor array, indicating potential application in the artificial vision system. Finally, the physical conduction mechanism of the device is explained combining with the adjustment of the height of the CeOx/ZnO interface barrier by photo-generated carriers. It provides an important reference for the simplification of the artificial vision system circuit structure in the future.

Quantum entanglement has been generated and verified in cold-atom experiments and used to make atom-interferometric measurements below the shot-noise limit. However, current state-of-the-art cold-atom devices exploit separable (i.e., unentangled) atomic states. This perspective piece asks the question: can entanglement usefully improve cold-atom sensors, in the sense that it gives new sensing capabilities unachievable with current state-of-the-art devices? We briefly review the state-of-the-art in precision cold-atom sensing, focusing on clocks and inertial sensors, identifying the potential benefits entanglement could bring to these devices, and the challenges that need to be overcome to realize these benefits. We survey demonstrated methods of generating metrologically useful entanglement in cold-atom systems, note their relative strengths and weaknesses, and assess their prospects for near-to-medium term quantum-enhanced cold-atom sensing.

Vacancy ordered halide perovskites have been extensively investigated as promising lead-free alternatives to halide perovskites for various opto-electronic applications. Among these, Cs2TiBr6 has been reported as a stable absorber with interesting electronic and optical properties, such as a bandgap in the visible, and long carrier diffusion lengths. Yet, a thorough theoretical analysis of the exhibited properties is still missing in order to further assess its application potential from a material's design point of view. In this Letter, we perform a detailed analysis for the established Ti-based compounds and investigate the less-known materials based on Zr. We discuss in detail their electronic properties and band symmetries, highlight the similarity between the materials in terms of properties, and reveal limits for tuning electronic and optical properties within this family of vacancy ordered double perovskites that share the same electron configuration. We also show the challenges to compute accurate and meaningful quasi-particle corrections at the GW level. Furthermore, we…

Rich dynamics in a living neuronal system can be considered as a computational resource for physical reservoir computing (PRC). However, PRC that generates a coherent signal output from a spontaneously active neuronal system is still challenging. To overcome this difficulty, we here constructed a closed-loop experimental setup for PRC of a living neuronal culture, where neural activities were recorded with a microelectrode array and stimulated optically using caged compounds. The system was equipped with first-order reduced and controlled error learning to generate a coherent signal output from a living neuronal culture. Our embodiment experiments with a vehicle robot demonstrated that the coherent output served as a homeostasis-like property of the embodied system from which a maze-solving ability could be generated. Such a homeostatic property generated from the internal feedback loop in a system can play an important role in task solving in biological systems and enable the use of computational resources without any additional learning.

A thin film of AlON with a nitrogen concentration of 13% was developed as the interfacial layer (IL) of HfZrOx-based ferroelectric field-effect transistor (FeFET) memory devices on a Si substrate. Compared to the conventional SiO2/SiON IL, due to a higher dielectric constant value that allows a smaller voltage drop across it and a larger valence band offset (ΔEv) with respect to Si along with prominent passivation of Si dangling bonds that effectively suppress hole trapping, memory devices with the AlON IL demonstrate a large memory window (MW) of 3.12 V by ±4 V gate voltage sweeping, robust endurance after 105 cycles with a long pulse width of 10−4 s, and a stable MW of 2.95 V up to 10 years, standing out from other HfZrOx-based FeFET memory reported in the literature. Furthermore, the AlON IL can be integrated with HfZrOx in the same atomic layer deposition step, which greatly simplifies the process. From the device performance and process integration points of view, the AlON IL unleashes the potential of FeFET memory by enabling high reliability with a large MW.

This Letter studies the reverse leakage and breakdown mechanisms of vertical GaN-on-Si Schottky barrier diodes (SBDs) with and without argon-implanted termination (ArIT). The electrical leakage characteristics in the vertical GaN-on-Si SBD without edge termination sequentially go through the thermionic field emission, variable range hopping (VRH), and trap-assisted tunneling conduction mechanisms as the reverse bias increases gradually. Its leakage and breakdown mechanisms are limited by the edge electric field crowding effect. While for the vertical GaN-on-Si SBD with ArIT (ArIT-SBD), the electrons conduction at a low reverse bias, following the space-charge-limited conduction (SCLC) model, is limited by the damage-induced traps in the implanted GaN. As the reverse bias increases up to the occurrence of breakdown, the VRH and SCLC dominate the leakage mechanism of the ArIT-SBD, which stem from intrinsic traps in GaN grown on Si. A rapidly growing leakage under a low reverse bias and enhanced breakdown voltage performance in the ArIT-SBD is attributed to the charging of the…

We present full-Maxwell topology-optimization design of a single-piece multilayer metalens, about 10 wavelengths λ in thickness, which simultaneously focuses over a 60° angular range and a 23% spectral bandwidth without suffering chromatic or angular aberration, a “plan-achromat.” At all angles and frequencies, it achieves diffraction-limited focusing (Strehl ratio >0.8) and an absolute focusing efficiency of >50%. Both 2D and 3D axisymmetric designs are presented, optimized over ∼105 degrees of freedom. We also demonstrate shortening the lens-to-sensor distance while producing the same image as for a longer “virtual” focal length and maintaining plan-achromaticity. These proof-of-concept designs demonstrate the ultra-compact multifunctionality that can be achieved by exploiting the full wave physics of subwavelength designs and motivate future work on design and fabrication of multilayer metaoptics.

Recently, rutile germanium dioxide (r-GeO2) has emerged as a novel ultra-wide bandgap semiconductor due to its theoretical excellent properties, that is, high thermal conductivity, ambipolar dopability, and high carrier mobility, in addition to its wide bandgap (4.44–4.68 eV). In this study, r-GeO2 thin films were grown on (001) r-TiO2 substrates by mist chemical vapor deposition. To optimize the growth conditions, we analyzed the decomposition processes of the Ge source (C6H10Ge2O7) by thermogravimetry-differential thermal analysis. It is found that GeO2 was synthesized from C6H10Ge2O7 at 553–783 °C in aqueous vapor. We accomplished fabrication of (001)-oriented r-GeO2 on r-TiO2 with a growth rate of 1.2–1.7 μm/h. On the other hand, under lower growth rate conditions (50 nm/h), the full width at half maximum of the r-GeO2 002 peak remarked a relatively small value of 560 arc sec. In addition, clear diffraction spots of r-GeO2 and r-TiO2 were observed at the r-GeO2/r-TiO2 interface, and the film was found to be significantly strained along the in-plane direction (∼2.3%) by…

Fundamental research and development of ultra-wide bandgap (UWBG) semiconductor devices are under way to realize next-generation power conversion and wireless communication systems. Devices based on aluminum gallium nitride (AlxGa1−xN, x is the Al composition), β-phase gallium oxide (β-Ga2O3), and diamond give promise to the development of power switching devices and radio frequency power amplifiers with higher performance and efficiency than commercial wide bandgap semiconductor devices based on gallium nitride (GaN) and silicon carbide (SiC). However, one of the most critical challenges for the successful deployment of UWBG device technologies is to overcome adverse thermal effects that impact the device performance and reliability. Overheating of UWBG devices originates from the projected high power density operation and poor intrinsic thermal properties of AlxGa1−xN and β-Ga2O3. This Perspective delineates the need and process for the “electro-thermal co-design” of laterally configured UWBG electronic devices and provides a comprehensive review of current state-of-the-art…

A new generation of ultrafast and low-noise supercontinuum (SC) sources is currently emerging, driven by the constantly increasing demands of spectroscopy, advanced microscopy, and ultrafast photonics applications for highly stable broadband coherent light sources. In this Perspective, we review recent progress enabled by advances in nonlinear optical fiber design, detail our view on the largely untapped potential for noise control in nonlinear fiber optics, and present the noise fingerprinting technique for measuring and visualizing the noise of SC sources with unprecedented detail. In our outlook, we highlight how these SC sources push the boundaries for many spectroscopy and imaging modalities and focus on their role in the development of ultrafast fiber lasers and frequency combs with ultra-low amplitude and phase noise operating in the 2 μm spectral region and beyond in the mid-IR.

How to explore the physical effect and characterize physical parameters in atomic scale crystals is still a challenging issue in modern optics. We detect the photonic spin Hall effect in monolayer MoS2 to study the atomic light-matter interaction. Due to the sensitive dependence of the spin-Hall shifts on the optical constants of MoS2, we propose this fundamental effect as a metrological tool for the parameter characterization. Combined with the weak-value technique, the experimental results show that the complex refractive index can be extracted by treating the atomically thin MoS2 as a homogeneous slab with an effective thickness. However, modeling the thin crystal as a surface current can determine the optical conductivity with a high measurement resolution. We believe this fast and precise measurement of optical parameters can provide a reliable approach to investigate optical properties for transition metal dichalcogenides.

We report a kind of sound absorbing metastructure based on micro-perforated panel (MPP) and impedance matching coiled-up cavity (IMCC), which can broaden the low-frequency working bandwidth tremendously using single unit (e.g., 3.13 times compared to conventional designs) within a deep sub-wavelength thickness (e.g., ∼λ/20 at 322 Hz). Physically, impedance matching stemming from the dramatic cross-sectional area contrast of coiled-up channels reduces the acoustic reactance variation with respect to frequency, leading to broader low-frequency bandwidth. Theoretical predictions of sound absorption coefficients of the metastructure coincide well with the numerical and experimental results. The band broadening characteristics of the proposed IMCC are confirmed in relatively wide range of MPP parameters and partition panel positions. This work contributes to expanding the designing degrees of freedom for coiled-up metastructures to manipulate low-frequency waves in broadband.

Elastocaloric cooling has been identified as a promising alternative to high global warming potential vapor compression cooling. Two key bottlenecks to adoption are the need for bulky/expensive actuators to provide sufficient uniaxial stress and inadequate elastocaloric material fatigue life. This paper defines the physics that govern the performance of axisymmetric flexural bending for use as an emerging low-force and low-fatigue elastocaloric heating and cooling mechanisms and further demonstrates a continuous rotary-driven cooling prototype using polycrystalline Ni50.75Ti48.74. Elastocaloric material performance is determined using infrared thermography during uniaxial-tension and four-point bending thermomechanical testing. A systematic study reveals the effects of strain rate (from 0.001 to 0.025 s−1), maximum strain (from 2 to 8%), and strain mode on the temperature evolution, mechanical response, and coefficient of performance. Four-point bending experiments demonstrate a temperature reduction of up to 11.3 °C, material coefficients of performance between 2.31 and 21.71, and…

There are pressing problems with traditional computing, especially for accomplishing data-intensive and real-time tasks, that motivate the development of in-memory computing devices to both store information and perform computation. Magnetic tunnel junction memory elements can be used for computation by manipulating a domain wall, a transition region between magnetic domains, but the experimental study of such devices has been limited by high current densities and low tunnel magnetoresistance. Here, we study prototypes of three-terminal domain wall-magnetic tunnel junction in-memory computing devices that can address data processing bottlenecks and resolve these challenges by using perpendicular magnetic anisotropy, spin–orbit torque switching, and an optimized lithography process to produce average device tunnel magnetoresistance TMR = 171% and average resistance-area product RA = 29 Ω μm2, close to the RA of the unpatterned film. Device initialization variation in switching voltage is shown to be curtailed to 7%–10% by controlling the domain wall initial position, which we show…

An oxidizable metal diffusion barrier inserted between the active metal electrode and the switching layer decreases the electroforming voltage and enhances the switching stability and synaptic performances in TaOx-based conducting bridge memristor devices. The TiW barrier layer avoids an excessive metal ion diffusion into the switching layer, while the TiWOx interfacial layer is formed between the barrier and the switching layer. It modulates the oxygen vacancy distribution at the top interface and contributes to the formation and rupture of the metal ion-oxygen vacancy hybrid conducting bridge. We observe that the device that relies upon non-hybrid (metal ions only) conducting bridge suffers from poor analogous performance. Meanwhile, the device made with the barrier layer is capable of providing 2-bit memory and robust 50 stable epochs. TaOx also acts as resistance for suppressing and a thermal enhancement layer, which helps to minimize overshooting current. The enhanced analog device with high linear weight update shows multilevel cell characteristics and stable 50 epochs. To…