Skyrmions are magnetic bubbles with nontrivial topology envisioned as data bits for ultrafast and power-efficient spintronic memory and logic devices. They may be stabilized in heavy-metal/ferromagnetic/oxide trilayer systems. The skyrmion chirality is then determined by the sign of the interfacial Dzyaloshinskii-Moriya interaction (DMI). Nevertheless, for apparently identical systems, there is some controversy about the DMI sign. Here, we show that the degree of oxidation of the top interface and the thickness of the ferromagnetic layer play a major role. Using Brillouin light-scattering measurements in $\mathrm{Ta}/\mathrm{Fe}$-$\mathrm{Co}$-$\mathrm{B}/{\mathrm{Ta}\mathrm{O}}_x$ trilayers, we demonstrate a sign change of the DMI with the degree of oxidation of the $\mathrm{Fe}$-$\mathrm{Co}$-$\mathrm{B}/{\mathrm{Ta}\mathrm{O}}_x$ interface. Using polar magneto-optical Kerr effect microscopy, we consistently observe a reversal of the direction of current-induced motion of skyrmions with the oxidation level of ${\mathrm{Ta}\mathrm{O}}_x$; this is attributed to their chirality reversal. In addition, a second chirality reversal is observed when changing the $\mathrm{Fe}$-$\mathrm{Co}$-$\mathrm{B}$ thickness, probably due to the proximity of the two $\mathrm{Fe}$-$\mathrm{Co}$-$\mathrm{B}$ interfaces in the ultrathin case. By properly tuning the chirality of the skyrmion, spin-transfer and spin-orbit torques combine constructively to enhance the skyrmion velocity. These observations thus allow us to envision an optimization of the material parameters to produce highly mobile skyrmions. Moreover, this chirality control enables a versatile manipulation of skyrmions and paves the way towards multidirectional devices.

Magnetic skyrmion technology is promising for the next-generation spintronics-based memory and neuromorphic computing due to their small size, nonvolatility and low depinning current density. However, the Magnus force originating from the skyrmion Hall effect causes the skyrmion to move along a curved trajectory, which may lead to the annihilation of the skyrmion in a nanotrack during current-induced skyrmion motion. Consequently, circuits utilizing skyrmionic motion need to be designed to limit the impact of the skyrmion Hall effect. In this work we propose a design of an artificial neuron, and a synapse using the bilayer device consisting of two antiferromagnetically exchange coupled ferromagnetic layers, which achieves robustness against the skyrmion Hall effect by nullifying the Magnus force. Using micromagnetic simulations, we show that the bilayer device can work as an artificial neuron and also as a synapse by modifying its uniaxial anisotropy. We also demonstrate that our proposed skyrmionic synapse has an intrinsic property of a perfectly linear and symmetric weight update, which is highly desirable for the synapse operation. We simulate a spiking neural network implemented using our proposed synapse and neuron and achieve a 96.23% accuracy in performing classification on the modified National Institute of Standards and Technology handwritten digit dataset.

Low-energy magnonic logic circuits are an actively developing field of modern magnetism. The potential benefits of magnonics for data processing are vitally dependent on units based on nonreciprocal propagation of spin waves, in analogy to semiconductor diodes and transistors in electronics. In this paper, we suggest the approach to realize nonreciprocal propagation of spin waves in a ferromagnetic metallic waveguide by exciting them with femtosecond laser pulse. Using micromagnetic modeling, we show that the combination of an external magnetic field and the position of the excitation laser spot across the waveguide leads to unidirectional propagation of the excited spin-wave packet. The results are meaningful for the design of hybrid magnonic-photonic circuits in future generations of data-processing devices.

We investigate native point defects and rare-earth (co)dopants in monoclinic $\mathrm{Sr}\mathrm{Al}$${}_2$$\mathrm{O}$${}_4$ using hybrid density-functional defect calculations. Europium ($\mathrm{Eu}$) and dysprosium ($\mathrm{Dy}$) are found to be mixed valence and energetically most favorable at the $\mathrm{Sr}$ lattice sites. However, unlike $\mathrm{Eu}$, where both $\mathrm{Eu}$${}^{2+}$ and $\mathrm{Eu}$${}^{3+}$ can be realized in synthesis, $\mathrm{Dy}$ is stable predominantly as $\mathrm{Dy}$${}^{3+}$ and the divalent $\mathrm{Dy}$${}^{2+}$ may only be photogenerated under irradiation. On the basis of an analysis of $\mathrm{Eu}$-related band–defect (including charge-transfer) and interconfigurational $5d$–$4f$ optical transitions, we assign the characteristic broad blue (445-nm) and green (520-nm) emission bands in $\mathrm{Eu}$${}^{2+}$-doped $\mathrm{Sr}\mathrm{Al}$${}_2$$\mathrm{O}$${}_4$ to the $4f^{6}5d^1\to 4f^7$ transition in $\mathrm{Eu}$${}^{2+}$ incorporated at the $\mathrm{Sr}$1 and $\mathrm{Sr}$2 sites, respectively. Strontium interstitials (${\mathrm{Sr}}_i$; not oxygen vacancies, in contrast to what is commonly believed) and substitutional Dy impurities (${\mathrm{Dy}}_{\mathrm{Sr}}$) can act as efficient electron traps for room-temperature persistent luminescence. This work calls for a reassessment of certain assumptions regarding specific carrier trapping centers made in all mechanisms previously proposed for the persistent luminescence in $\mathrm{Eu}$- and ($\mathrm{Eu}$,$\mathrm{Dy}$)-doped $\mathrm{Sr}\mathrm{Al}$${}_2$$\mathrm{O}$${}_4$. It also serves as a methodological template for the understanding and design of rare-earth doped phosphors.

Due to its unique features, like tunable gap and high absorption coefficient, type-II superlattices receive growing interest. Substantial progress has been made in the technology of these materials as well as in the processing of superlattice-based devices. On the theoretical side, whereas the methods for superlattice analysis are well developed, for the superlattice-based devices they are well behind. Usually, such devices are modeled with semiclassical methods, in which the superlattice is treated as a bulk material having effective parameters extracted from its analysis with full quantum methods. As there is little theoretical justification for such a substitution, attempts have been made to model a whole superlattice-based device on a fully quantum level. In this paper, such modeling is presented for a broken-gap type-II superlattice diode: the nonequilibrium Green’s function method is applied to the two-band model of an $\mathrm{In}\mathrm{As}$/$\mathrm{Ga}\mathrm{Sb}$ superlattice $p$-$i$-$n$ diode. The focus is paid on the band-to-band (BTB) tunneling with the aim of an assessment of equations used for its semiclassical description. The results of calculations presented in the paper demonstrate that, in the superlattice diode, the BTB tunneling occurs only for certain values of the in-plane momentum $k$, for which electronic and hole subbands cross. This is in contrast to the bulk materials, for which there is a range of such $k$ values. The simulations reveal much more differences. Accordingly, care must be taken when applying semiclassical models to describe the interband tunneling in superlattice devices.

In two-qubit gates activated by microwave pulses, by turning the pulse on or off, the state of qubits is swapped between entangled or idle modes. In either mode, the presence of stray couplings makes qubits accumulate coherent phase error. However, the error rates in the two modes differ because qubits carry different stray coupling strengths in each mode; therefore, eliminating stray coupling from one mode does not remove it from the other. We propose to combine such a gate with a tunable coupler and show that both idle and entangled qubits can become free from stray couplings. This significantly increases the operational switch fidelity in quantum algorithms. We further propose a weakly tunable qubit as an optimum coupler to bring the two modes parametrically near each other. This remarkably enhances the tuning process by reducing its leakage.

The $\mathrm{Ga}\mathrm{As}$/$(\text{Al,Ga})\mathrm{As}$ heterostructure with short-period superlattice (SPSL) doping possesses ultrahigh mobility of its two-dimensional electron gas by placing donors within the remote $\mathrm{Ga}\mathrm{As}$ layers. Here, we investigate its magnetotransport property under a heavily doped situation. After long enough illumination at cryogenic temperature, the change of the electron concentration inside the quantum well (QW) is only $5.9\mathrm{\%}$. Meanwhile, the quantum lifetime ${\tau }_{q,\text{QW}}$ of the electron shows an anomalous behavior. It increases slightly and then exhibits an exponential decay until saturation. This is different from the monotonic increase of ${\tau }_{q,\text{QW}}$ under illumination for the conventional doping situation. The increase of ${\tau }_{q,\text{QW}}$ originates from the larger donor filling-fraction-enhanced screening effect. Meanwhile, the decrease of ${\tau }_{q,\text{QW}}$ may be caused by stronger scattering of ionized $d^{+}$ states evolved from DX centers. The transfer of excess electrons between the $\mathrm{Al}\mathrm{As}$ layers can also cause the decrease of ${\tau }_{q,\text{QW}}$. This work provides an insight into the mechanism of DX centers on the quantum transport properties of SPSL-doped heterostructures.

Spin-polarized light-emitting diodes (spin LEDs) are essential for the optical transfer of carrier-spin information in spin-based electronic circuits. Efficient operation under high-bias conditions is required for high-speed transfer of spin information and strong light emission. However, achieving high-electroluminescence (EL) circular polarization is hampered by electric-field-induced spin relaxation during drift transport in undoped $\mathrm{Ga}\mathrm{As}$ barriers. Herein, we demonstrate the efficient room-temperature operation of a spin LED using ($\mathrm{In},\mathrm{Ga})\mathrm{As}$ quantum dots (QDs) tunnel coupled with a 5-nm-thick $\mathrm{Ga}(\mathrm{N},\mathrm{As})$ quantum well (QW). A high-EL circular polarization of approximately 7% is achieved, even under high-bias conditions, in comparison with the lower value of approximately 3% for the conventional QD spin LED without $\mathrm{Ga}(\mathrm{N},\mathrm{As})$. This is realized by an increase of spin polarization after injection into the QDs by utilizing remote spin filtering of QD electrons via an adjacent tunnel-coupled $\mathrm{Ga}(\mathrm{N},\mathrm{As})$ QW. These results demonstrate that the tunnel-coupled structure of ($\mathrm{In},\mathrm{Ga})\mathrm{As}$ QDs and $\mathrm{Ga}(\mathrm{N},\mathrm{As})$ QW is a promising active layer of spin LEDs for achieving both strong EL emission and high-EL circular polarization at room temperature.

In the work presented here, we systematically investigate the ultrafast optical properties of aluminum ($\mathrm{Al}$) thin films on silicon cantilevers using a microscopic femtosecond optical pump-probe technique to explore the effect of light irradiation upon cantilevers while considering radiation-pressure and photothermal effects. The ultrafast laser pulses used for the study are less than 30 fs in duration and 830 nm in wavelength, and the photon energy (1.49 eV) of the light pulses is close to the interband transition threshold (ITT) of $\mathrm{Al}$. Therefore, the change in the ITT due to the strain in the cantilever induced by the light irradiation can be detected through a change in the transient reflectivity, which is dominated by the thermalized electron signal. We reveal the position dependence of the transient-reflectivity change and the thermalized-electronic-signal amplitude for 100-nm-thick $\mathrm{Al}$ films on cantilevers of length $160$, $200$, and $240\text{μ}\mathrm{m}$, and these results are in excellent agreement with two-temperature-model-based curve fits. Furthermore, to understand the effect of light irradiation, we derive equations for the position-dependent radiation-pressure effect and the photothermal effect, and demonstrate that thermal-expansion-induced changes in the ITT dominate the position dependence of the signal intensity. Our findings offer avenues for exploring the effects of strain on ultrafast properties, and applications to ultrafast scanning probe microscopy.

The latest advances in topological physics have yielded a toolset for highly robust wave-propagation modalities for overcoming obstacles involving beam steering and lateral diffraction in surface acoustic waves (SAWs). However, extant proposals are limited to the exploitation of spin- or valley-polarized phases and rely on nonzero Berry curvature effects. Here, we propose and experimentally demonstrate a highly robust guiding principle, which instead employs an intrinsic chirality of phase vortices and maintains a zero Berry curvature for SAWs. Based on a line defect within a true triangular phononic lattice, the guided SAW mode spans a wide bandwidth [(Δω/ω${}_{\mathrm{center}}$) ∼ 10%] and is well confined in the lateral direction with 3-dB attenuation within half of a unit cell. SAW routing around sharp bends with negligible backscatter is demonstrated. The on-chip integrated design permits unidirectional SAW modes that can enable considerable miniaturization of SAW-based devices, with applications ranging from radio-frequency devices to quantum information transduction.

Quantum computation has demonstrated advantages over classical computation for special hard problems, where a set of universal quantum gates is essential. Geometric phases, which have built-in resilience to local noise, have been used to construct quantum gates with excellent performance. However, this advantage has been smeared in previous schemes. Here, we propose a state-independent nonadiabatic geometric quantum-gate scheme that is able to realize a more fully geometric gate than previous approaches, allowing for the cancelation of dynamical phases accumulated by an arbitrary state. Numerical simulations demonstrate that our scheme has significantly stronger gate robustness than the previous geometric and dynamical ones. Meanwhile, we give a detailed physical implementation of our scheme with the Rydberg atom system based on the Rydberg blockade effect, specifically for multiqubit control-phase gates, which exceeds the fault-tolerance threshold of multiqubit quantum gates within the considered error range. Therefore, our scheme provides a promising way for fault-tolerant quantum computation in atomic systems.

The presence of ferroelectric domain walls (DWs) can generate an above-band-gap photovoltage of a ferroelectric photovoltaic device, which is the anomalous photovoltaic (APV) effect, and its mechanism is still under debate. Here, the effective electric field and the local bulk photovoltaic (BPV) component at 71° DWs are reported by quantitatively analyzing the light polarization angle-dependent photovoltaic effect of nonperiodic DWs and periodic stripe DWs in ${\mathrm{Bi}\mathrm{Fe}\mathrm{O}}_3$ films. The photovoltaic measurement under white light illumination directly reveals a significantly enhanced electric field at stripe DWs in comparison with the domains. The BPV effect at stripe DWs is about 25 times as large as that of the domains. Furthermore, the defect states at the DWs may recombine the photogenerated carriers and drastically weaken the electric field of the DWs, whereas they negligibly mitigate the BPV effect. This work offers a deeper insight into the mechanism of the APV effect at ferroelectric DWs.

It has been recently reported that elastic waves induced by nanosecond light pulses can be used to drive nanomotion of micro-objects on frictional solid interfaces, a challenging task for traditional techniques using tiny optical force. In this technique, the main physical quantities and parameters involved are temporal width and energy of light pulses, thermal heating and cooling time, friction force, and elastic waves. Despite a few experimental observations based on microfiber systems, a microscopic theory, which reveals how these quantities collaboratively enable motion of the micro-objects and derives what the underlying manipulation principles emerge, is absent. In this paper, a comprehensive theoretical analysis—centralized around the above listed physical quantities, and illuminated by a single-friction-point model in conjunction with numerical simulations—is established to pedagogically clarify the physics. Our results reveal the two essential factors in this technique: (1) the use of short light pulses for rapid thermal expansion overwhelming friction resistance and (2) the timescale asymmetry in thermal heating and cooling for accumulating a net sliding distance. Moreover, we examine the effects of spatially distributed friction beyond the single-friction-point consideration, and show “tug-of-war”-like friction stretching in the driving process. Given these insights, we positively predict that this elastic-wave-based manipulation principle could be directly translated to micro- and nanoscale optical waveguides on optical chips, and propose a practical design. We wish that these results offer theoretical guidelines for ongoing efforts of optical manipulation on solid interfaces with light-induced elastic waves.

Graphitic carbon nitride ($g$-${\mathrm{C}}_3{\mathrm{N}}_4$) shows favorable performance as a photocatalyst and has attracted widespread attention in recent years. As its wide band gap of 2.70 eV limits light absorption in the visible range, many efforts have been made to optimize the band gap. In this report, pressure is used to engineer the band gap and photoelectric response of nitrogen-deficient $g$-${\mathrm{C}}_3{\mathrm{N}}_4$ nanoflakes. The band gap of the sample is first narrowed to 2.40 eV due to the introduction of nitrogen vacancies and then further narrowed to 1.70 eV by pressure, which is the lowest value reported in the literature for undoped $g$-${\mathrm{C}}_3{\mathrm{N}}_4$. Accordingly, the photoelectric response increases by nearly 50% because of the enhanced light absorption at high pressure. More interestingly, after depressurization to ambient pressure, the optimized band gap survives with a minimum value of 1.87 eV accompanied by enhanced photoelectric responsivity. In situ synchrotron x-ray diffraction and Raman spectra suggest that the tunable band gap originates from irreversible pressure-induced amorphization with the assistance of vacancies for $g$-${\mathrm{C}}_3{\mathrm{N}}_4$. The collaborative approach of introducing deficiency and pressure treatment adopted here shows the ability to engineer the band gap continuously over a prominently wider region than that for the single band-gap-narrowing technique, and thus, enhances the photoelectric performance for broadened semiconductors.

The variational quantum eigensolver (VQE) is a hybrid quantum classical algorithm designed for current and near-term quantum devices. Despite its initial success, there is a lack of understanding involving several of its key aspects. There are problems with VQE that forbid a favorable scaling towards quantum advantage. In order to alleviate the problems, we propose and extensively test a quantum annealing inspired heuristic that supplements VQE. The improved VQE enables an efficient initial state-preparation mechanism, in a recursive manner, for a quasidynamical unitary evolution. We conduct an in-depth scaling analysis of finding the ground-state energies with increasing lattice sizes of the Heisenberg model, employing simulations of up to 40 qubits that manipulate the complete state vector. In addition to systematically finding the ground-state energy, we observe that it avoids barren plateaus, escapes local minima, and works with low-depth circuits. For the current devices, we further propose a benchmarking toolkit using a mean-field model and test it on IBM Q devices. Realistic gate execution times estimate a longer computational time to complete the same computation on a fully functional error-free quantum computer than on a quantum computer emulator implemented on a classical computer. However, our proposal can be expected to help accurate estimations of the ground-state energies beyond 50 qubits when the complete state vector can no longer be stored on a classical computer, thus enabling quantum advantage.

Coherent coupling in magnon-based hybrid systems has many potential applications in quantum information processing. Magnons can propagate in magnetically ordered materials without any motion of electrons, offering a unique method to build low-power-consumption devices and information channels free of heat dissipation. In this paper, we demonstrate the coherent propagation of hybridized modes between spin waves and Love surface acoustic waves in a multiferroic ${\mathrm{Bi}\mathrm{Fe}\mathrm{O}}_3$ and ferromagnetic ${\mathrm{La}}_{0.67}{\mathrm{Sr}}_{0.33}{\mathrm{Mn}\mathrm{O}}_3$-based heterostructure. The magnetoelastic coupling enables a giant enhancement of the strength of the hybridized mode by a factor of 26 compared to that of the pure spin waves. A short wavelength down to 250 nm is demonstrated for the hybridized mode, which is desirable for nanoscale acoustomagnonic applications. Our combined experimental and theoretical analyses represent a step towards the coherent control in hybrid magnonics, which may inspire the study of magnon-phonon hybrid systems for coherent information processing and manipulation.