In spintronics, one of the long-standing questions is why the $\mathrm{Mg}\mathrm{O}$-based magnetic tunnel junction (MTJ) is almost the only option for achieving a large tunneling magnetoresistance (TMR) ratio at room temperature, although this is not as large as the theoretical prediction. This study focuses on the development of an almost strain-free MTJ using metastable bcc ${\mathrm{Co}}_x{\mathrm{Mn}}_{100\text{−}x}$ ($\mathrm{Co}\text{−}\mathrm{Mn}$) ferromagnetic films. We investigate the degree of crystallization in MTJs consisting of $\mathrm{Co}\text{−}\mathrm{Mn}/\mathrm{Mg}\mathrm{O}/\mathrm{Co}\text{−}\mathrm{Mn}$ in relation to their TMR ratios. Cross-section high resolution transmission electron microscopy reveals that almost consistent lattice constants of these layers for 66 ≤ x ≤ 83 with large TMR ratios of 229% at room temperature, confirming the soft nature of the $\mathrm{Co}\text{−}\mathrm{Mn}$ layer with some dislocations at the $\mathrm{Mg}\mathrm{O}/{\mathrm{Co}}_{75}{\mathrm{Mn}}_{25}$ interfaces. Ab initio calculations confirm the crystalline deformation stability across a broad compositional range in $\mathrm{Co}\text{−}\mathrm{Mn}$, proving the advantage of a strain-free interface for much larger TMR ratios.

We present the design and characterization of a three-Josephson-junction superconducting loop circuit with three large shunt capacitors. Used as a qubit, the circuit shows long energy-relaxation times, of the order of $40\mu \mathrm{s}$ and a spin-echo dephasing time of $9.4\mu \mathrm{s}$. The circuit has high anharmonicity, of $2\pi \times 3.69\mathrm{GHz}$. We extract the multilevel relaxation and dephasing rates of the circuit used as a qutrit and discuss the possible sources for the decoherence. The high anharmonicity allows for fast qubit control with nanosecond-range gate durations and a measured average gate fidelity of 99.92%, characterized by randomized benchmarking. These results demonstrate interesting potential use for fast nanosecond-time-scale two-qubit gates and multilevel quantum logic.

The establishment of chiral coupling in thin magnetic films with inhomogeneous anisotropy has led to the development of artificial systems of fundamental and technological interest. The chiral coupling itself is enabled by the Dzyaloshinskii-Moriya interaction (DMI) enforced by the patterned noncollinear magnetization. Here, we create a domain wall track with out-of-plane magnetization coupled on each side to a narrow parallel strip with in-plane magnetization. With this we show that the chiral torques emerging from the DMI at the boundary between the regions of noncollinear magnetization in a single magnetic layer can be used to bias the domain wall velocity. To tune the chiral torques, the design of the magnetic racetracks can be modified by varying the width of the tracks or the width of the transition region between noncollinear magnetizations, reaching effective chiral magnetic fields of up to 7.8 mT. Furthermore, we show how the magnitude of the chiral torques can be estimated by measuring asymmetric domain wall velocities, and demonstrate spontaneous domain wall motion propelled by intrinsic torques even in the absence of any external driving force.

The characterization and radiography of special nuclear materials (SNMs) such as plutonium and uranium is essential in various scenarios in the context of nonproliferation and national security. However, existing techniques are not sufficient in and of themselves for rapid isotope-sensitive interrogation of SNMs. In the present study, we propose an interrogation method, called scattering-nuclear-resonance-fluorescence-based computer tomography (SNRF CT), to reveal concealed SNMs with simultaneous measurements of the isotopic composition and tomographic distribution. Monte Carlo simulations show that three NRF signatures at 1802, 1846, and 1862 keV from a uranium target composed of ${\hspace{0.17em}}^{235}\mathrm{U}$ and ${\hspace{0.17em}}^{238}\mathrm{U}$, irradiated by a quasimonochromatic $\gamma$-ray beam of high intensity, can be identified. For different target configurations, sinograms of the SNRF signatures are obtained, and the tomographic distributions are reconstructed with appreciable image contrast and spatial precision. It is found that the tomographic patterns of ${\hspace{0.17em}}^{235}\mathrm{U}$ and ${\hspace{0.17em}}^{238}\mathrm{U}$ hidden in a 30-mm-diameter iron rod can be clearly visualized. Compared with transmission NRF CT (TNRF CT), SNRF CT has the advantage that it does not need a witness target or calibration of the notch-refill effect. Furthermore, the imaging time for SNRF CT is 20 times shorter than that for TNRF CT, assuming a low missed-detection rate of ${10}^{-4}$. We conclude that, using an intense $\gamma$-ray beam, the proposed SNRF CT technique is capable of simultaneous identification and tomographic imaging of the isotopic content of SNMs in a realistic measurement time. This imaging method could be extended to other applications in spent-fuel management and screening of cargo for explosives, drugs, and toxic chemicals.

Scalable quantum information processing requires the ability to tune multiqubit interactions. This makes the precise manipulation of quantum states particularly difficult for multiqubit interactions because tunability unavoidably introduces sensitivity to fluctuations in the tuning parameters, leading to erroneous multiqubit gate operations. The performance of quantum algorithms may be severely compromised by coherent multiqubit errors. It is therefore imperative to understand how these fluctuations affect multiqubit interactions and, more importantly, to mitigate their influence. In this study, we demonstrate how to implement dynamical-decoupling techniques to suppress the two-qubit analogue of the dephasing on a superconducting quantum device featuring a compact tunable coupler, a trending technology that enables the fast manipulation of qubit-qubit interactions. The pure-dephasing time shows up to an approximate 14 times enhancement on average when using robust sequences. The results are in good agreement with the noise generated from room-temperature circuits. Our study further reveals the decohering processes associated with tunable couplers and establishes a framework to develop gates and sequences robust against two-qubit errors.

The transtor and memtranstor are the fourth basic linear and memory elements, respectively, which allow direct coupling of charge (q) to magnetic flux (φ) via linear and nonlinear magnetoelectric (ME) effects, respectively. It is found here that a large variation of magnetization by an electric field is realized in both linear and nonlinear hysteretic styles in a magnetoelectric Y-type hexaferrite ${\mathrm{Ba}}_{0.5}{\mathrm{Sr}}_{1.5}{\mathrm{Zn}}_2({\mathrm{Fe}}_{0.92}{\mathrm{Al}}_{0.08}{)}_{12}{\mathrm{O}}_{22}$ single crystal. Moreover, based on the spin-current model, the underlying microscopic mechanisms for generating the two types of linear and nonlinear M versus E curves are understood to be E-induced changes to the cone angle and sign of P, respectively, establishing the charge-driven transtor and memtranstor in the Y-type hexaferrite system. This work points to a promising pathway to develop alternative circuit functionalities using magnetoelectric materials.

We present the results of experiments on the generation of periodical trains of subnanosecond $K_a$-band pulses in the electron generator, which is based on passive mode locking. This technique is broadly used in laser physics for the generation of ultrashort pulses (USPs) but has not been previously employed in high-power microwave electronics. The microwave USP generator comprises a helical-waveguide gyrotron traveling wave tube and a saturable absorber in the feedback loop. Saturable absorption is implemented in an auxiliary section using the cyclotron resonance interaction with an initially rectilinear electron beam, for which nonlinear saturation is caused by the effect of the relativistic dependence of the gyrofrequency on the particle’s energy. In agreement with theoretical predictions, periodical trains of 0.4-ns pulses with a peak power of 100 kW and repetition of 2.5 ns are measured. Phase coherence of radiated pulses is demonstrated based on analysis of the autocorrelation function. The experiments performed open a wide range of possibilities for the generation of coherent broadband radiation, which is in high demand for numerous applications.

We can control the concentration of chemicals in microdroplets by using vertical contact control of droplets. When a microdroplet contacts an opposite microdroplet, chemicals diffuse between the droplets. Then, the coalescent droplet is separated into two microdroplets. Contact control is attractive for use in biochemical applications because it realizes three-dimensional microfluidic systems. However, in vertical contact control, gravity causes a volume difference of the microdroplets, ΔV, after separation. To solve the problem of gravity, we analyze ΔV quantitatively. In experiments, we measure the relationship between ΔV and the size of microdroplets, R. As a result, we find that ΔV increases proportionally to $R^5$, which agrees with a simple theoretical model based on the balance between gravity and surface tension. Our model suggests that ΔV of the microdroplets after vertical contact control can be predicted quantitatively with the material parameters of the density and surface tension of the liquid.

Highly efficient separation of tightly bound excitons (photoinduced electron-hole pairs) at an interface is crucial for high-performance two-dimensional (2D) heterostructure solar cells. Here we propose and demonstrate a strategy—dipole engineering—to effectively separate excitons at a 2D interface, which could evidently enhance power-conversion efficiency (PCE) via Heyd-Scuseria-Ernzerhof density functional calculations. The proposed dipole engineering is based on the synergetic effect of the intrinsic dipole in 2D Janus materials and interface dipole caused by charge transfer at the interface of 2D van der Waals heterostructures, in stark contrast to those strategies previously used, such as applying a strain or electric field. Taking the Janus ${\mathrm{Ga}}_2\mathrm{Se}\mathrm{Te}/\mathrm{In}\mathrm{S}$ heterostructure as an example, we demonstrate that the PCE can be promoted to a peak value of 23.2% when the intrinsic and interface dipoles are in the same direction, when accompanied by an appropriate applied compression strain. In addition, the Janus heterostructure possesses suitable band-edge positions of redox reaction potentials for full water splitting. Our work may herald an intrinsic yet effective way for enhancing the performance of optoelectronic devices, in contrast to the conventional external stimuli strategy.

Fixed-frequency qubits can suffer from always-on interactions that inhibit independent control. Here, we address this issue by experimentally demonstrating a superconducting architecture using qubits that comprise of two capacitively shunted Josephson junctions connected in series. Historically known as tunable coupling qubits (TCQs), such two-junction qubits support two modes with distinct frequencies and spatial symmetries. By selectively coupling only one type of mode and using the other as our computational basis, we greatly suppress crosstalk between the data modes while permitting all-microwave two-qubit gates.

We report on the impact of structural defects on mid-infrared intersubband (ISB) properties of $\mathrm{Ga}\mathrm{N}/(\mathrm{Al},\mathrm{Ga})\mathrm{N}$ heterostructures grown by ammonia molecular beam epitaxy (${\mathrm{NH}}_3$ MBE). Twenty-period $\mathrm{Ga}\mathrm{N}/(\mathrm{Al},\mathrm{Ga})\mathrm{N}$ multi-quantum-well (MQW) heterostructures are grown on co-loaded a-plane freestanding $\mathrm{Ga}\mathrm{N}$ substrates and heteroepitaxial a-plane $\mathrm{Ga}\mathrm{N}$ on r-plane sapphire templates (a-$\mathrm{Ga}\mathrm{N}$/r-sap) for three different quantum-well (QW) widths (3.0, 3.3, and 3.7 nm). Co-loaded structures grown on freestanding a-plane with no basal-plane stacking faults (BSFs), prismatic stacking faults (PSFs), and partial dislocations (PDs), with low threading dislocation (TD) densities of about ${10}^5{\mathrm{cm}}^{-2}$ are compared with those grown on a-$\mathrm{Ga}\mathrm{N}$ templates on $(10\overline{1}2)$ r-sapphire with BSF, PSF, PD, and TD densities of about 4 × ${10}^5$ to ${10}^6{\mathrm{cm}}^{-2}$, 5 × ${10}^3$ to 2 × ${10}^4{\mathrm{cm}}^{-2}$, about 9 × ${10}^{10}$ to 2 × ${10}^{11}{\mathrm{cm}}^{-2}$, and about ${10}^{10}{\mathrm{cm}}^{-2}$, respectively. Fourier-transform infrared absorption spectroscopy indicates ISB transition energies in the range of about 250–300 meV (wavelength range 4.1–4.8 µm) for MQWs with different QW widths. The ISB absorption spectra indicate about 5% smaller transition energies and only about 10%–20% larger spectral linewidths for structures grown on a-$\mathrm{Ga}\mathrm{N}/$r-sapphire templates compared with those on freestanding $\mathrm{Ga}\mathrm{N}$ substrates. The strong defect tolerance in the nonpolar a-plane ISB structures could be due to the nature of defects and their energy levels with respect to the conduction-band minima, which do not affect the ISB properties. Our results pave the way toward the production of low-cost scalable nonpolar III-nitride MQW heterostructures for a variety of passive and active optical materials and devices based on intersubband transitions.

Classical simulations of time-dependent quantum systems are widely used in quantum control research. In particular, these simulations are commonly used to host iterative optimal control algorithms. This is convenient for algorithms that are too onerous to run in the loop with current-day quantum hardware, as well as for researchers without consistent access to hardware. However, if the model used to represent the system is not selected carefully, an optimized control protocol may be rendered futile when applied to hardware. We present a series of models, ordered in a hierarchy of progressive approximation, which appear in quantum control literature. The validity of each model is characterized experimentally by designing and benchmarking control protocols for an IBMQ cloud quantum device. This result demonstrates error amplification induced by the application of a first-order perturbative approximation. Furthermore, the emergence of errors that cannot be corrected by simple amplitude scaling of control pulses is demonstrated in simulation, due to an underlying mistreatment of noncomputational dynamics. Finally, an evaluation of simulated control dynamics reveals that despite the substantial variance in numerical predictions across the proposed models, the complexity of discovering local optimal control protocols appears invariant in the simple control scheme setting.

Recently, space-time plasma-steering source has been proposed as an innovative microwave plasma source to meet the challenge of controlling plasmas in overmoded cavities. This concept has been successfully demonstrated experimentally, allowing the space-time control of nanosecond microwave plasmas on initiators. This paper gives insights into the path that shall be taken to reach full space-time control of plasmas in overmoded cavities. To that end, a key criterion, namely the “plasma-steering criterion,” is introduced and verified with a numerical model. This criterion must be respected for an accurate space-time control of plasmas in overmoded cavities. The usefulness of the plasma density (depending on its value with respect to the critical plasma density) on the plasma-control capabilities is also highlighted.

Scanning tunneling microscope lithography can be used to create nanoelectronic devices in which dopant atoms are precisely positioned in a $\mathrm{Si}$ lattice within approximately $1$ nm of a target position. This exquisite precision is promising for realizing various quantum technologies. However, a potentially impactful form of disorder is due to incorporation kinetics, in which the number of P atoms that incorporate into a single lithographic window is manifestly uncertain. We present experimental results indicating that the likelihood of incorporating into an ideally written three-dimer single-donor window is $63\pm 10\mathrm{\%}$ for room-temperature dosing, and corroborate these results with a model for the incorporation kinetics. Nevertheless, further analysis of this model suggests conditions that might raise the incorporation rate to near-deterministic levels. We simulate bias spectroscopy on a chain of comparable dimensions to the array in our yield study, indicating that such an experiment may help confirm the inferred incorporation rate.