Herein, we propose a technique for megaelectronvolt ultrafast electron diffraction that can achieve a few-femtosecond time resolution. The technique modulates a relativistic electron beam using two radially polarized laser pulses of varying frequencies, which enable the condensation of the electrons into an ultrashort pulse. Moreover, our study shows that this technique can suppress the arrival-time jitter of the electron beam at the sample location. Numerical simulations show that using realistic laser parameters corresponding to the current gigawatt-power-level laser systems, a 3.3-fs (rms over 45% charge) electron beam can be obtained and the arrival-time jitter can be 1.35 fs (rms). This technique may have a considerable impact on ultrafast electron diffraction, microscopy, and other ultrafast research facilities that require few-femtosecond electron beams with a small time jitter.

Two-dimensional (2D) semiconductors offer a promising prospect for high-performance and energy-efficient devices especially in the sub-10-nm regime. Inspired by the successful fabrication of 2D β-${\mathrm{Te}\mathrm{O}}_2$ and the high on:off ratio and high air stability of fabricated FETs [Zavabeti et al., Nat. Electron. 4, 277 (2021)], we provide a comprehensive investigation of the electronic structure of monolayer β-${\mathrm{Te}\mathrm{O}}_2$ and the device performance of sub-10-nm MOSFETs based on this material. The anisotropic electronic structure of monolayer β-${\mathrm{Te}\mathrm{O}}_2$ plays a critical role in the anisotropy of transport properties for MOSFETs. We show that the 5.2-nm gate-length n-type MOSFET holds an ultrahigh on-state current exceeding 3700 µA/µm according to International Roadmap for Devices and Systems (IRDS) 2020 goals for high-performance devices, which is benefited by the highly anisotropic electron effective mass. Moreover, monolayer β-${\mathrm{Te}\mathrm{O}}_2$ MOSFETs can fulfill the IRDS 2020 goals for both high-performance and low-power devices in terms of on-state current, subthreshold swing, delay time, and power-delay product. This study unveils monolayer β-${\mathrm{Te}\mathrm{O}}_2$ as a promising candidate for ultrascaled devices in future nanoelectronics.

With continuing improvements on the quality of fabricated quantum devices, it becomes increasingly crucial to analyze noisy quantum process in greater details such as characterizing the non-Markovianity in a quantitative manner. In this work, we propose an experimental protocol, termed spectral transfer tensor maps (STTMs), to accurately predict the Rivas-Huelga-Plenio non-Markovian measure of any Pauli channels without state preparation and measurement errors. In fact, for Pauli channels, STTM even allows the reconstruction of a highly precise noise power spectrum for qubits. At last, we also discuss how STTM can be useful to approximately characterize non-Markovianity of non-Pauli channels via Pauli twirling in an optimal basis.

Current-induced spin-orbit torque (SOT) is regarded as a promising mechanism for driving neuromorphic behavior in spin-orbitronic devices. In principle, the strong SOT in a heavy-metal-based magnetic heterostructure is attributed to the spin-orbit coupling (SOC)-induced spin Hall effect and/or the spin Rashba-Edelstein effect. Recently, SOC-free mechanisms such as the orbital-angular-momentum-induced orbital Hall effect and/or the orbital Rashba-Edelstein effect have been proposed to generate sizable torques comparable to those from the conventional spin Hall mechanism. In this work, we show that the orbital current can be effectively generated by the nitrided light metal $\mathrm{Cu}$. The overall dampinglike SOT efficiency, which consists of both the spin and the orbital current contributions, can be tailored from ${\xi }_{\mathrm{DL}}\approx 0.06\text{ to }0.4$ in a $\mathrm{Pt}/\mathrm{Co}/{\mathrm{Cu}\mathrm{N}}_x$ magnetic heterostructure by tuning the nitrogen doping concentration. Current-induced magnetization switching further verifies the efficacy of such orbital current with a critical switching current density as low as $J_c$ ∼ 5 × ${10}^{10}{\mathrm{A}/\mathrm{m}}^2$. Most importantly, orbital-current-mediated memristive switching behavior can be observed in such heterostructures, which reveals that gigantic SOT and efficient magnetization switching are the trade-offs for the applicable window of memristive switching. Our work provides insights into the role that orbital current might play in SOT neuromorphic devices for making energy-efficient spin-orbitronic devices.

Although metal-organic frameworks attract much attention in electronic devices, the discontinuity at and weak interfacial contact with the substrate are still challenging. In this paper, an $\mathrm{Ag}$/(polyethylene glycol + zeolitic imidazole framework-8) @ (polyvinyl alcohol + 2-methylimidazole)/fluorine-doped tin oxide [$\mathrm{Ag}$/(PEG + ZIF8) @ (PVA + 2MIM)/FTO] structured resistive memory is fabricated. The interfacial contact and active site are enhanced by modifying FTO with (PVA + 2MIM). The continuity of ZIF-8 film is enhanced by compositing PEG. The resistive memory exhibits a lower set voltage of 1.3 V, a higher endurance of more than 140 cycles, and a longer retention time of over 10 000 s, in comparison with control devices of $\mathrm{Ag}$/ZIF-8/FTO and $\mathrm{Ag}/$ZIF-8 @ (PVA + 2MIM)/FTO. The mechanism of the resistive switching behavior is analyzed by conduction models, and explained by redox reactions of the top active electrode to form and rupture conductive filaments under the electric field. This study suggests a facial and general technology for synthesizing a continuous film with strong contact with the substrate, benefiting electronic devices.

Studies of micromechanical and acoustic modes in the quantum regime have shed light on quantum properties of massive objects. Integrating these systems into superconducting circuits shows great promise for applications as quantum memory elements, bosonic codes, or in frequency conversion. To this end, investigation of nonclassical properties of acoustic degrees of freedom is critical also for applications. Here, we investigate a strongly driven system consisting of a transmon qubit interacting with a high-overtone bulk acoustic resonator. We observe multiphonon transitions, which enable mapping the energy landscape in the coupled system. At a high driving amplitude comparable to the qubit-oscillator coupling, we observe a shift of the multiphonon spectral lines, reminiscent of Stark shift, which is explained using a simple model. Our work thus also provides insight in multiquanta transitions in other qubit-oscillator systems, not limited to acoustics or circuit quantum electrodynamics.

Synthesized three-dimensional space provides us with a platform to explore the intriguing acoustic valley Hall insulator. In this paper, a rotational degree of freedom is introduced to construct a synthetic three-dimensional space. Based on simulations, our results show that Weyl points and Fermi arcs and topologically nontrivial edge modes appear in a synthetic space. Not only rotational operation but also boundary truncation can modulate the frequencies of nontrivial edge states, which result in two completely different edge states at the upper and lower boundaries. Using frequencies of edge states protected by Weyl points, we devise two different acoustic topological rainbow devices. The two devices are able to trap states with different frequency components at different spatial positions: one device is capable of capturing low-frequency components at close range, while the other has the ability to confine low-frequency components at long distances. This work may boost further the development of topological rainbow devices in a synthetic space related to the acoustic valley Hall insulator.

Based on a quantum modeling of the electronic transport, this work shows that ultra-thin solar cells can exhibit an improved open-circuit voltage $V_{\mathrm{OC}}$, without current reduction. This improvement is obtained when an energy-selective contact is considered between the absorber and the reservoir, and is attributed to a hot-carrier effect. While extraction with a nonselective contact does not generate hot carriers, the use of energy-selective contact induces an increase of carrier temperature up to 130 K and a corresponding $V_{\mathrm{OC}}$ enhancement of 41 meV, considering an ($\mathrm{In},\mathrm{Ga})\mathrm{As}$ absorber. This enhancement agrees with a simple and general expression formulated in the quantum thermal machine field. Concerning the current, we show that current through an energy-selective contact is of the same order of magnitude as the one obtained without selectivity. This remarkable behavior, which is explained by the hybridization of states in the absorber with the state of the contact, requires a quantum confinement and thus an ultrathin absorber. Finally, we propose a simple rate model enabling an intuitive interpretation of the numerical results.

We report on a graphical domain engineering technique with the capability to fabricate macroscale domain structures with nanoscale spatial resolution in nonpolar-cut lithium-niobate thin film on insulators through the biased probe tip of scanning atomic force microscopy. It is found that the domain writing process is asymmetric with respect to the spontaneous polarization ${\mathrm{P}}_s$ even though the tip-induced poling field is mirror symmetric. Various domain structures, with a dimension larger than millimeters while consisting of nanoscale domain elements and with arbitrary domain-wall inclination angle with respect to ${\mathrm{P}}_s$ , are designed graphically and then written directly into nonpolar-cut lithium-niobate crystals. As a proof-of-principle demonstration, periodically poled $x$-cut lithium-niobate thin film on insulators with a period of 600 nm, a depth of 460 nm, and a length of approximately $1$ mm is fabricated. This technique could be useful for device applications in integrated optics and optoelectronics and domain-wall nanoelectronics based on lithium niobate on insulators.

We report on experimental and simulation-based results using $(\mathrm{In},\mathrm{Ga})\mathrm{N}$ alloy quantum barriers in c-plane green light-emitting diode (LED) structures as a means to improve vertical carrier transport and reduce forward voltage $(V_F)$. Three-dimensional device simulations that include random alloy fluctuations are used to understand carrier behavior in a disordered potential. The simulated current density–voltage (J-V) characteristics and modified electron-hole overlap $|F_{\mathrm{mod}}{|}^2$ indicate that increasing the indium fraction in the $(\mathrm{In},\mathrm{Ga})\mathrm{N}$ quantum barriers leads to a reduced polarization discontinuity at the interface between the quantum barrier and quantum well, thereby reducing $V_F$ and improving $|F_{\mathrm{mod}}{|}^2$. Maps of electron and hole current through the device show a relatively homogenous distribution in the $XY$ plane for structures using $\mathrm{Ga}\mathrm{N}$ quantum barriers; in contrast, preferential pathways for vertical transport are identified in structures with $(\mathrm{In},\mathrm{Ga})\mathrm{N}$ barriers as regions of high and low current. A positive correlation between hole (electron) current in the p-side (n-side) barrier and indium fraction reveals that preferential pathways exist in regions of high indium content. Furthermore, a negative correlation between the strain ${\epsilon }_{zz}$ and indium fraction shows that high indium content regions have reduced strain-induced piezoelectric polarization in the Z direction due to the mechanical constraint of the surrounding lower indium content regions. Experimentally, multiple quantum well green LEDs with $(\mathrm{In},\mathrm{Ga})\mathrm{N}$ quantum barriers exhibit lower $V_F$ and blue-shifted wavelengths relative to LEDs with $\mathrm{Ga}\mathrm{N}$ quantum barriers, consistent with simulation data. These results can be used to inform heterostructure design of low $V_F$, long-wavelength LEDs and provide important insight into the nature of carrier transport in III-nitride alloy materials.

The electric field tunable band gap and optical properties in low-dimensional materials (quantum-confined Stark effect) are very useful in applications of optoelectronics. In this paper, based on the many-body perturbation method, we investigate the evolution of the quasiparticle electronic structure, exciton, and optical properties of two-dimensional (2D) Hittorf’s phosphorene under an out-of-plane electric field. Compared to other 2D monolayers, the relatively large thickness of Hittorf’s phosphorene leads to a significant reduction in the quasiparticle band gap when an electric field is applied along the quantum confinement direction. The unique bilayer structure, on the other hand, guarantees a well spatial separation of photon-excited electron-hole pairs and, consequently, reduced exciton binding energy under an out-of-plane electric field. These combined effects lead to an almost fixed exciton energy and optical absorption edge at low applied electric fields. However, when the field is larger than 0.1 V/Å, substantial reductions in the exciton energy and optical absorption edge are identified. For the higher-order exciton states, the involvement of more complex band-to-band electron-hole pair formation results in a nonmonotonic electric field dependence. The effective optical modulation accompanied with these giant Stark effects shows potential applications of Hittorf’s phosphorene in 2D optoelectronic devices.

Field-of-view and resolution trade-offs in x-ray micro-computed-tomography (micro-CT) imaging limit the characterization, analysis, and model development of multiscale porous systems. To this end, we develop an applied methodology utilizing deep learning to enhance low-resolution (LR) images over large sample sizes and create multiscale models capable of accurately simulating experimental fluid dynamics from the pore (microns) to continuum (centimeters) scale. We develop a three-dimensional (3D) enhanced deep-superresolution (EDSR) convolutional neural network to create superresolution (SR) images from LR images, which alleviates common micro-CT hardware and/or reconstruction defects in high-resolution (HR) images. When paired with pore-network simulations and parallel computation, we can create large 3D continuum-scale models with spatially varying flow and material properties. We quantitatively validate the workflow at various scales using direct HR and SR image similarity, pore-scale material and/or flow simulations, and continuum-scale multiphase-flow experiments (drainage-immiscible flow pressures and 3D fluid-volume fractions). The SR images and models are comparable to the HR ground truth and generally accurate to within experimental uncertainty at the continuum scale across a range of flow rates. They are found to be significantly more accurate than their LR counterparts, especially in cases where a wide distribution of pore sizes are encountered. The applied methodology opens up the possibility to image, model, and analyze truly multiscale heterogeneous systems that are otherwise intractable.

In quasi-two-dimensional (quasi-2D) perovskites, inorganic quantum wells are sandwiched between organic spacers (barriers) from the energy-level perspective. In addition to quantum confinement, dielectric confinement occurs in the layered perovskites due to a large difference in the dielectric constants of the inorganic framework and the organic spacer layer. While quantum confinement increases the exciton binding energy, deconfinement in the dielectric constant results in a decrease in exciton binding energy and also an increase in carrier mobility. In this work, we investigate the effects of quantum confinement and dielectric deconfinement on the dynamic response of light-emitting diodes (LEDs) based on several series of quasi-2D perovskites and operated under an alternating voltage. With pulsed electroluminescence emission appearing in both bias sections of sinusoidal ac voltages, we show how the quantum confinement and dielectric deconfinement affect and increase the frequency range of ac LED operation, that is, the −3 dB cutoff frequency. This −3-dB frequency could be seen to increase due to a relaxation of quantum and dielectric confinements, leading to an enhancement of the effective carrier mobility in the active emitting material of the devices.

We examine coupling interactions used to synchronize macroscopic Josephson oscillations induced in intrinsic Josephson-junction (IJJ) mesa stacks made of a $\mathrm{Bi}2212$ single crystal. Synchronized radiations of terahertz electromagnetic (EM) waves are detected under common voltage and current bias operations of two connected mesas with close individual radiation frequencies, while uncoupled and bimodal radiations are frequently observed in two mesas with different individual radiation frequencies. Detailed observations of the polarizations of the EM waves emitted when two mesas are biased in parallel or series allow us to reveal the coupling matrix components, which include ratios of synchronized IJJs in the mesas and phase delay between the macroscopic Josephson oscillations. A frequency evolution of the phase delay implies that the coupling between the Josephson oscillations is mediated by the small amplitude Josephson plasmon inside the superconducting substrate. This finding stimulates systematic survey on polarization of EM wave emitted from synchronized multiple mesa devices in order to realize powerful terahertz emissions from superconductors.

Accurate control of light polarization represents a core building block in polarization metrology, imaging, and optical and quantum communications. Voltage-controlled liquid crystals offer an efficient way of polarization transformation. However, common twisted nematic liquid crystals are notorious for lacking an accurate theoretical model linking control voltages and output polarization. An inverse model, which would predict control voltages required to prepare a target polarization, is even more challenging. Here we report both the direct and inverse models based on deep neural networks, radial basis functions, and linear interpolation. We present an inverse–direct compound model solving the problem of control voltages ambiguity. We demonstrate an order of magnitude improvement in accuracy using deep learning compared to the radial basis function method and 2 orders of magnitude improvement compared to the linear interpolation. Errors of the deep neural network model also decrease faster than the other methods with an increasing number of training data. The best direct and inverse models reach average infidelities of $4\times {10}^{-4}$ and $2\times {10}^{-4}$, respectively, which are accuracy levels not reported yet. Furthermore, we demonstrate local and remote preparation of an arbitrary single-photon polarization state using deep learning models. The results will impact the application of twisted-nematic liquid crystals, increasing their control accuracy across the board. The presented bidirectional learning can be used for optimal classical control of complex photonic devices and quantum circuits beyond interpolation.

The perpendicular magnetic anisotropy at magnetic transition-metal–oxide interfaces is a key element in building out-of-plane magnetized magnetic tunnel junctions for spin-transfer-torque magnetic random access memory (STT MRAM). Size downscaling renders magnetic properties more sensitive to thermal effects. Thus, understanding the temperature dependence of the magnetic anisotropy becomes crucial. In this work, we theoretically address the correlation between the temperature dependence of magnetic anisotropy and magnetization in typical $\mathrm{Fe}$/$\mathrm{Mg}\mathrm{O}$-based structures. In particular, the possible mechanisms behind the experimentally reported deviations from the Callen and Callen scaling power law are analyzed. At ideal interfaces, first-principles calculations reveal (i) small high-order anisotropy constants compared to first order and (ii) enhanced exchange constants. Considering these two intrinsic effects in the atomistic simulations, the temperature-dependence of the total and layer-resolved anisotropy are found to follow the Callen and Callen scaling power law, thus ruling out an intrinsic microscopic mechanism underlying deviations from this law. Besides, two possible extrinsic macroscopic mechanisms are unveiled, namely the influence of the dead layer, often present in the storage layer of STT-MRAM cells and the spatial inhomogeneities of the interfacial magnetic anisotropy. About the first mechanism, we show that the presence of a dead layer tends to reduce the scaling exponents. In the second mechanism, increasing the percentage of inhomogeneity in the interfacial perpendicular magnetic anisotropy is revealed to decrease the scaling exponent. These results allow us to coherently explain the difference in scaling exponents relating anisotropy and magnetization thermal variations reported in earlier experiments. This is crucial for the understanding of the thermal stability of the storage-layer magnetization in STT-MRAM applications.