In this work, we propose and explore a double-stage phononic crystal (PnC) consisting of a top suspended $\mathrm{ZnO}$ layer, an array of sandwiched $\mathrm{Si}$ pillars, and a bottom $\mathrm{ZnO}$ layer on a $\mathrm{Si}$ substrate. The proposed double-stage PnC exhibits interesting behavior of routing the incident surface acoustic waves (SAWs) based on polarization and frequency, so that it can be considered as an efficient building block for alternative multistage SAW components. Here, we design the double-stage PnC to behave as an efficient vertical SAW splitter in a Mach-Zehnder interferometer, using the excitation of appropriate surface-coupled modes through the periodic locally resonating pillars. Benefiting from the acoustoelectric effect in $\mathrm{ZnO}$, the exposed top $\mathrm{ZnO}$ layer serves as the sensing branch, and the bottom $\mathrm{ZnO}$ layer serves as the reference branch in the presented vertical Mach-Zehnder interferometer. The vertical configuration of the proposed design allows exposure of the top $\mathrm{ZnO}$ layer to external stimulations, such as exposure to ultraviolet illumination or a hydrogen environment, while protecting the underlying parts from being exposed. This structural aspect simplifies the experimental implementation of our design in comparison with the conventional planar Mach-Zehnder interferometers by avoiding the necessity of in-plane focusing of external stimulation on the sensing branch. Our optimized Mach-Zehnder interferometer shows an output transmission signal of −8 dB and a high extinction ratio of 23 dB at 8.6 GHz when the conductivity of the top $\mathrm{ZnO}$ layer is increased from about 1 S/m to 100 S/m via external stimulation. Here, we propose a highly sensitive miniature vertical surface acoustic Mach-Zehnder interferometer, which may open up alternative horizons for future multistage SAW components.

The ability to transmit quantum states over long distances is a fundamental requirement of the quantum internet and is reliant upon quantum repeaters. Quantum repeaters involve entangled photon sources that emit and deliver photonic entangled states at high rates and quantum memories that can temporarily store quantum states. Improvement of the entanglement distribution rate is essential for quantum repeaters, and multiplexing is expected to be a breakthrough. However, limited studies exist on multiplexed photon sources and their coupling with a multiplexed quantum memory. Here, we demonstrate the storing of a frequency-multiplexed two-photon source at telecommunication wavelengths in a quantum memory accepting visible wavelengths via wavelength conversion after 10-km distribution. To achieve this, quantum systems are connected via wavelength conversion with a frequency-stabilization system and a noise-reduction system. The developed system is stably operated for more than 42 h. Therefore, it can be applied to quantum repeater systems comprising various physical systems requiring long-term system stability.

We demonstrate the manipulation of perpendicular magnetic anisotropy (PMA) in magnetic tunnel junctions (MTJs) by varying the layer stacks and temperature. PMA is tuned to compensate the shape anisotropy, giving a nonhysteretic magnetic response, a noteworthy sensitivity enhancement, and a field detectability of $1.8\mathrm{nT}/\sqrt{\mathrm{Hz}}$ at 100 kHz. Such a method is further exemplified in multiple MTJs, providing a solution to obtain desired sensitivities and operating temperatures. Additionally, the electronic noise of this MTJ is revealed as a random telegraph noise (RTN) due to a generation-recombination process. The observed voltage-dependent RTN could potentially be applied to true random number generators.

We report the realization of a strong coupling between a $\mathrm{Gd}$${}^{3+}$ spin ensemble hosted in a scheelite ($\mathrm{CaWO}$${}_4$) single crystal and the resonant mode of a coplanar-strip-line superconducting cavity leading to a large separation of spin-photon states of 146 MHz. The interaction is well described by the Dicke model and the crystal-field Hamiltonian of the multilevel spin system. We observe a change of the crystal-field parameters due to the presence of photons in the cavity, which generates a significant perturbation of the crystal ground state. Using finite-element calculations, we numerically estimate the cavity sensing volume as well as the average spin-photon coupling strength of $g_0/2\pi \approx 620\mathrm{Hz}$. Lastly, the dynamics of the spin-cavity states are explored via pulsed measurements by recording the cavity ring-down signal as a function of the pulse length and the amplitude. The results indicate a potential method to initialize this multilevel system in its ground state via an active cooling process.

In this work, we study a series of tunable flux qubits inductively coupled to a coplanar waveguide resonator fabricated on a sapphire substrate. Each qubit includes an asymmetric superconducting quantum interference device, which is controlled by the application of an external magnetic field and acts as a tunable Josephson junction. The tunability of the qubits is typically $\pm 3.5\mathrm{GHz}$ around their central gap frequency. The measured relaxation times are limited by dielectric losses in the substrate and can attain $T_1\sim 8\text{μ}\mathrm{s}$. The echo dephasing times are limited by flux noise even at optimal points and reach $T_{2E}\sim 4\text{μ}\mathrm{s}$, almost an order of magnitude longer than state of the art.

Memristive devices are promising elements for energy-efficient neuromorphic computing and future artificial-intelligence systems. For diffusive memristors, resistive switching occurs because of the sequential formation and rupture of conduction filaments between device electrodes due to drift and diffusion of silver nanoparticles in the dielectric matrix. This process is governed by the applied electric voltage. Here, both in experiment and in simulations we demonstrate that varying temperature offers an efficient control of memristor states and charges transport in the device. By raising and lowering the device temperature it is shown that the memristive state can be reset, even if it cannot be done by varying the applied voltage. In addition, a change in the spiking regime is observed when the spiking is generated in the memristive circuit at a constant applied voltage, but different device temperatures. Our simulations demonstrate a good qualitative agreement with the experiments, and help to explain the effects reported.

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.