The ratchet motion of magnetic skyrmions driven by microwave magnetic fields, with the motion direction and speed tunable by field parameters, provides a promising route to drive magnetic skyrmions in materials with poor conductivity. However, as an indirect motion, skyrmion ratchet-motion speed is generally low in comparison with the direct motions driven by currents. Toward practical applications, it is necessary to ask if there are mechanisms to realize ultrafast ratchet motion of magnetic skyrmions and how such a motion can be integrated into racetrack-type skyrmion devices. In this work, we explore the effects of defects and edges on the ratchet motion of magnetic skyrmions under time-varying magnetic fields in GHz. We demonstrate that the ratchet motion of skyrmion is not only guided along the defect tracks or edges, but also with a remarkable speed up (with a factor over 10) compared with that in the bulk region. The skyrmion ratchet-motion speed reaches 100 m/s along a straight defect track and edge and ${10}^9$ rad/s along a circular edge under a field of approximately 50 mT, comparable to those direct motions driven by currents. Moreover, the skyrmion ratchet motion along the defect track or edge can be facilely controlled by the field and defect parameters. Analysis based on the time-averaged Thiele equation of skyrmion verifies that such a speed-up effect is due to the increased time-averaged driving force perpendicular to the skyrmion motion when it approaches the defect track or edge, analogous to that discovered in direct motions driven by currents. The generality of our conclusions has been examined for the ratchet motion of Bloch and Néel-type skyrmions driven by a variety of time-varying magnetic fields, and for systems with open edges or defect tracks with modified Dzyaloshinskii-Moriya or exchange interactions and anisotropy.

Radiative heat transfer at the nanoscale has the potential to produce thermal emission with both spectral feature and intensity different from the bulk materials. Specifically, the idea of coupling the energy states of photons and phonons using surface phonon polaritons (SPhPs) blurs the boundary between light and heat, opening a path to the manipulation of heat transfer through photonic engineering and breaking the classical limit established by Planck’s law. SPhPs can generate a strong energy confinement effect near the surface; however, this effect is only valid within a narrow spectral regime, the so-called reststrahlen band. In this study, we employ a hybrid structure to broaden the effective energy regime for thermal emission by coupling SPhPs and surface plasmon polaritons (SPPs). We report remarkably enhanced thermal emission of a hybrid structure made of a ${\mathrm{Si}\mathrm{O}}_2$ nanoribbon decorated with Au nanoparticles by a factor of 4 over that of a bare ${\mathrm{Si}\mathrm{O}}_2$ nanoribbon. We introduce an experimental technique to quantify the emissivity from the complex nanostructured surface using frequency-dependent heat-transfer measurement. Not only does it enable us to detect feeble emission from a single nanoemitter, but it also differentiates two distinct heat-conducting processes of conduction and radiation, simultaneously. Our work may offer insights to engineer far-field thermal emission using hybrid structures combining the surface plasma and phonon polaritons.

The ferroelectricity of ${\mathrm{Hf}\mathrm{O}}_2$ thin films is technologically useful with various advantages compared to conventional ferroelectrics. However, the application of orthorhombic ${\mathrm{Hf}\mathrm{O}}_2$ has been limited by its large coercive field compared to perovskite-based ferroelectrics. Using first-principles calculations, we extensively search for 34 dopant elements to reduce the problematic coercive fields and discover that the coercive fields exhibit a simple volcano shape against the dopant’s size. We also discover that the Si dopant is a critical element in stabilizing tetragonal phase ${\mathrm{Hf}\mathrm{O}}_2$ (transition state) because of its intrinsic ${\mathrm{sp}}^3$ bond favoring characteristics with oxygen, thereby notably lowering the coercive fields. We provide an atomic scale picture to understand the excellent role of Si in effective ferroelectric switching and a simple rule to tune coercive fields with various doping agents.

The spin Hall effect in heavy-metal thin films is routinely used to convert charge currents into transverse spin currents and can be used to exert torque on adjacent ferromagnets. Conversely, the inverse spin Hall effect is frequently used to detect spin currents by charge currents in spintronic devices up to the terahertz frequency range. Numerous techniques to measure the spin Hall effect or its inverse have been introduced, most of which require extensive sample preparation by multistep lithography. To enable rapid screening of materials in terms of charge-to-spin conversion, suitable high-throughput methods for measuring the spin Hall angle are required. Here we compare two lithography-free techniques, terahertz emission spectroscopy and broadband ferromagnetic resonance, with standard harmonic Hall measurements and theoretical predictions using the binary-alloy series ${\mathrm{Au}}_x{\mathrm{Pt}}_{1-x}$ as a benchmark system. Despite their being highly complementary, we find that all three techniques yield a spin Hall angle with approximately the same $x$ dependence, which is also consistent with first-principles calculations. Quantitative discrepancies are discussed in terms of magnetization orientation and interfacial spin-memory loss.

In this article, three-dimensional cubic phase ${\mathrm{Cs}\mathrm{Pb}\mathrm{Cl}}_3$ perovskite is implemented in a quasi-solid-state $\mathrm{lithium}$-ion battery and dual-ion battery as the anode material. A half-cell lithium-ion battery developed using a gel polymer electrolyte and ${\mathrm{Cs}\mathrm{Pb}\mathrm{Cl}}_3$ anode achieves specific discharge capacities of 275.2, 362.4, 508.7 and 612.3 mA h/g at current rates of 250, 200, 100 and 50 mA/g, respectively, with an average Coulombic efficiency of 88% via the conversion process. A lithium-metal-free dual-ion battery, assembled using conventional graphite powder as a cathode and a high capacity ${\mathrm{Cs}\mathrm{Pb}\mathrm{Cl}}_3$ anode utilizing the same electrolyte, represents a potential battery concept for energy storage technologies. The operating voltage of the dual-ion battery is 0 to 4.0 V and delivers specific discharge capacities of 44.7, 59.3, 78.4 and 96.8 mA h/g at current rates of 400, 200, 100, and 50 mA/g, respectively, providing 70% Coulombic efficiency and an average voltage of 2.53 V.

Geometric phase is a promising element to induce high-fidelity and robust quantum operations due to its built-in noise-resilience feature. Unfortunately, its practical applications are usually circumscribed by requiring complex interactions among multiple levels or qubits and a longer gate time than the corresponding dynamical ones. Here, we propose a general framework of geometric quantum computation with the integration of the time-optimal control technique, where the shortest smooth geometric path is found to realize accelerated geometric quantum gates and thus greatly decreases the gate errors induced by both the decoherence effect and operational imperfections. Meanwhile, we faithfully implement our idea on a scalable platform of a two-dimensional superconducting transmon-qubit lattice, with simple and experimentally accessible interactions. In addition, numerical simulations show that our implemented geometric gates possess higher fidelities and stronger robustness, which outperform the best performance of the corresponding dynamical ones. Therefore, our scheme provides a promising alternative route toward scalable fault-tolerant solid-state quantum computation.

Strategically reducing the speed of waves, which greatly improves both the energy density and information capacity of carrier signals in space, is a key enabling factor for signal-processing devices. Among these devices, especially in the prosperous wireless communication industry, surface acoustic wave (SAW) devices based on interdigital transducers (IDTs) currently hold an essential status. However, velocity reduction in traditional IDT-based SAW devices can be achieved only by using specific substrate materials that are generally of lower hardness, which inevitably leads to an increase in device size and less-optimal electromechanical coupling coefficients. Here, we demonstrate a technological means of realizing slow on-chip SAWs that is relevant for practical rf signal processing, gyrometers, sensing, and transduction. This method takes advantage of the gradual flattening of a Rayleigh-type dispersion band due to the spatial lattice evolution of a surface phononic crystal. In our experiment, the speed of an ultraslow SAW is measured to be approximately 200 m/s, which is even slower than the speed of sound in air and equivalent to 1/17.4 of the speed of the original Rayleigh waves in ${\mathrm{Li}\mathrm{Nb}\mathrm{O}}_3$. Such ultraslow SAWs may have promising applications in time-dependent SAW modulation, high-sensitivity SAW sensors, and SAW nonlinear even quantum-dynamic systems. Additionally, our technique can be similarly applied to a broad range of other two-dimensional or quasi-two-dimensional wave structures, e.g., in electronic, optical, acoustic, and thermal systems.

Enhancing the measurement signal from solid-state quantum sensors such as the nitrogen-vacancy (N-$V$) center in diamond is a challenge for sensing and imaging of condensed-matter systems. Here we design and fabricate diamond scanning probes with a truncated parabolic profile that optimizes the photonic signal from single, near-surface, embedded N-$V$ centers, forming a high-sensitivity probe for nanoscale magnetic field imaging. These structures operate over the full N-$V$ photoluminescence spectrum with the emission being directed into a measured numerical aperture of 0.44. Furthermore, they yield a median saturation count rate of 2.1 MHz, a fivefold improvement in measurement signal over the state of the art in scanning-probe-based N-$V$ sensors. We additionally introduce a charge-state-registered pulsed-excitation method for measuring the detection efficiency of photons from the N-$V$’s metrologically relevant negative charge state, finding an overall detection efficiency of $12\mathrm{\%}$. We demonstrate the excellent properties of these diamond scanning probes by imaging thin-film ferromagnetic stripes with a spatial resolution better than 50 nm.

Transmon qubits are ubiquitous in the pursuit of quantum computing using superconducting circuits. However, they have some drawbacks that still need to be addressed. Most importantly, the scalability of transmons is limited by the large device footprint needed to reduce the participation of the lossy capacitive parts of the circuit. In this work, we investigate and evaluate losses in an alternative device geometry, namely, the merged-element transmon (mergemon). To this end, we replace the large external shunt capacitor of a traditional transmon with the intrinsic capacitance of a Josephson junction and achieve an approximately 100 times reduction in qubit dimensions. We report the implementation of the mergemon using a sputtered $\mathrm{Nb}$–amorphous-$\mathrm{Si}$–$\mathrm{Nb}$ trilayer film. In an experiment below 10 mK, the frequency of the readout resonator, capacitively coupled to the mergemon, exhibits a qubit-state-dependent shift in the low-power regime. The device also demonstrates the single-photon and multiphoton transitions that represent a weakly anharmonic system in two-tone spectroscopy. The transition spectra are explained well with master-equation simulations. A participation-ratio analysis identifies the dielectric loss of the amorphous-$\mathrm{Si}$ tunnel barrier and its interfaces as the dominant source for qubit relaxation. We expect the mergemon to achieve high coherence for relatively small device dimensions when implemented with a low-loss, epitaxially grown, and lattice-matched trilayer.

Metamaterials that exhibit a constitutive coupling between their momentum and strain, show promise in wave manipulation for engineering purposes and are called Willis materials. They were discovered using an effective-medium theory, showing that their response is nonlocal in space and time. Recently, we generalized this theory to account for piezoelectricity, and demonstrated that the effective momentum can depend constitutively on the electric field, thereby enlarging the design space for metamaterials. Here, we develop the mathematical restrictions on the effective properties of such generalized Willis materials, owing to passivity, reciprocity, and causality. The establishment of these restrictions is of fundamental significance, as they test the validity of theoretical and experimental results—and applicational importance, since they provide elementary bounds for the maximal response that potential devices may achieve.

We introduce a method to estimate unknown pure $d$-dimensional quantum states using the probability distributions associated with only three measurement bases. The measurement results of $2d$ projectors are used to generate a set of $2^{d-1}$ states, the value of the likelihood function of which is evaluated using the measurement results of the remaining $d$ projectors. The state with the highest value of the likelihood function is the estimate of the unknown state. The method estimates all pure states except for a null-measure set, which can be overcome by adapting two bases. The viability of the protocol is experimentally demonstrated using two different and complementary high-dimensional quantum information platforms. First, by exploring the photonic path-encoding strategy, we validate the method on a single eight-dimensional quantum system. Then, we resort to the five-superconducting-qubit IBM quantum processor to demonstrate the high performance of the method in the multipartite scenario.

The photogalvanic effect (PGE) occurring in noncentrosymmetric materials enables the generation of an open-circuit voltage that is much larger than the bandgap, making it rather attractive in solar cells. However, the magnitude of the PGE photocurrent is usually small, which severely hampers its practical application. Here we propose a mechanism to largely enhance the PGE photocurrent by mechanical strain based on the quantum transport simulations for the two-dimensional nickel-phosphorene-nickel photodetector. A broadband PGE photocurrent governed by the $C_s$ noncentrosymmetry is generated at zero bias under the illumination of linearly polarized light. The photocurrent depends linearly on the device asymmetry, while nonlinearly on the optical absorption. By applying the appropriate mechanical tension stress on the phosphorene, the photocurrent can be substantially enhanced by up to 3 orders of magnitude, which is primarily ascribed to the largely increased device asymmetry. The change in the optical absorption in some cases can also play a critical role in tuning the photocurrent due to the nonlinear dependence. Moreover, the photocurrent can be even further enhanced by mechanical bending, mainly owing to the considerably enhanced device asymmetry. Our results reveal the dependence of the PGE photocurrent on the device asymmetry and absorption in the transport process through a device, and also explore the potential of the PGE in self-powered low-dimensional flexible optoelectronics and low-dimensional photodetections with high photoresponsivity.

Three-dimensional harmless contactless manipulation and assembly of micro-objects and micro-organisms would open other horizons in microrobotics and microbiology, e.g., for microsystems assembly or tissue engineering. In this paper, we theoretically show that small particles compared to the wavelength can be trapped and assembled in three dimensions with synchronized spherical vortices. The particles can be approached both laterally and axially and, for each configuration, the maximum assembly speed can be determined by balancing the Stokes’ drag force and the critical radiation force. These theoretical results provide guidelines to design acoustical tweezers able to trap and assemble particles in three dimensions.

We propose and demonstrate a scheme for spatial mapping and direct reading of the nonlinear response of optical materials from the propagation of self-accelerating beam profiles. Through a single evolution measurement, a two-dimensional compact Airy beam is able to directly map the nonlinear response of a medium into spatial profiles. The most efficient mapping takes place along some particular transverse directions rather than the direction of the beam acceleration, depending on both the Airy intensity pattern and the type of nonlinearity. Our work may herald a simple yet effective way for measuring the nonlinear optical response, in contrast to the conventional Z-scan technique.

We analyze electric transport and noise properties at 4.2 K of self-shunted superconductor-normal metal-superconductor (SNS) sandwich-type Josephson junctions, comprising $\mathrm{Nb}$ as the superconductor and $\mathrm{Hf}$-$\mathrm{Ti}$ as the normal conducting material, with lateral dimensions down to approximately $80$ nm. The junctions are fabricated with an optimized multilayer $\mathrm{Nb}$ technology based on nanopatterning by electron-beam lithography and chemical-mechanical polishing. The dependence of transport properties on the junction geometry (lateral size and barrier thickness $d_{\mathrm{Hf}\mathrm{-}\mathrm{Ti}}$) is studied, yielding a characteristic voltage $V_c$ up to approximately $100\mu \mathrm{V}$ for the smallest $d_{\mathrm{Hf}\mathrm{-}\mathrm{Ti}}=17$ nm. The observed small hysteresis in the current-voltage curves of devices with high $V_c$ and large size can be attributed to self-heating of the junctions and fitted with an extended version of the resistively shunted junction model. Measurements of voltage noise of single junctions are consistent with the model including self-heating effects. The potential of our technology for further miniaturization of nanoscale superconducting quantum interference devices and for the improvement of their performance is discussed.

Quantum memory is the core device for the construction of large-scale quantum networks. For scalable and convenient practical applications, integrated optical memories, especially on-chip optical memories, are crucial requirements because they can be easily integrated with other on-chip devices. Here, we report the coherent optical memory based on a type-IV waveguide fabricated on the surface of a rare-earth ion-doped crystal (i.e., $\mathrm{E}{\mathrm{u}}^{3+}$:${\mathrm{Y}}_2\mathrm{Si}{\mathrm{O}}_5$). The properties of the optical transition (${}^7{\mathrm{F}}_0\to {}^5{\mathrm{D}}_0$) of the ${\mathrm{Eu}}^{3+}$ ions inside the surface waveguide are well preserved compared to those of the bulk crystal. Spin-wave atomic frequency comb storage is demonstrated inside the type-IV waveguide. The reliability of this device is confirmed by the high interference visibility of $97\pm 1\mathrm{\%}$ between the retrieval pulse and the reference pulse. The developed on-chip optical memory paves the way towards integrated quantum nodes.

Acoustic vortex beams have great potential for contactless particle manipulation and torque-based biomedical applications. However, focusing acoustic waves through highly aberrating layers such as the human skull at ultrasonic frequencies results in strong phase aberrations, which prevent the generation of sharp acoustic images. In the case of a wavefront containing phase dislocations, skull aberrations can inhibit the focusing of acoustic vortex beams inside the cranial cavity. In this paper, we demonstrate that phase-conjugated acoustic holograms can encode time-reversed fields, allowing compensation of the aberrations of the skull and, simultaneously, the generation of a focused vortex inside an ex vivo human skull. The method is applied to single-element geometrically focused sources and results in a very simple and compact ultrasonic system. This work will pave the way to designing low-cost particle-trapping systems, clot manipulation, and the exertion of acoustic-radiation forces and torques in the brain for biomedical applications.