Dynamic hysteresis effects have been long known to occur in the current density-voltage characteristics of perovskite solar cells, with the ionic migration being identified as the primary factor. The hysteretic effects impacted early studies by the uncertainty in the evaluation of the power conversion efficiency, while currently, potential links to degradation mechanisms are the focus. Therefore, understanding ion migration is a central goal, typically addressed by performing a combined large and small signal analysis. The reported large capacitive and inductive effects created controversies with respect to the underlying mechanisms, yielding essentially two classes of models, one based on the large accumulation capacitances and the other based on the ionic modulation of the collected current. We introduce here an equivalent circuit model and interpret these phenomena in terms of recombination current modulation, identifying the distinct contributions from ion current and ionic charge accumulations. These contributions to the recombination current are associated with capacitive and inductive effects, respectively, and we corroborate the numerical simulations with electrochemical impedance spectroscopy measurements. These show the role of the recombination currents of photogenerated carriers in producing both capacitive and inductive effects as the illumination is varied. Moreover, we provide a bridging point between the two classes of models and suggest a framework of investigation of defect states based on the observed inductive behavior, which would further aid the mitigation of the degradation effects.

Two-dimensional (2D) magnetic semiconductor materials with the electric-field-tunable spin polarization and carrier-transport anisotropy are of great significance for the fundamental physics and device application. Here we propose a general strategy to tune the spin polarization and anisotropic effective mass in a 2D A-type antiferromagnetic (AFM) bilayer system. We take the A-type AFM bilayer $\mathrm{CrSBr}$ (with the intralayer ferromagnetic and interlayer AFM coupling) as an example to confirm this design principle. Firstly, a vertical electric field can lift the spin degeneracy in the A-type AFM bilayer $\mathrm{CrSBr}$. By flipping the direction of electric field, the spin polarization direction can be reversed. Secondly, in the A-type AFM bilayer $\mathrm{CrSBr}$ with an interlayer twist angle of 90°, both the spin direction and the carrier-transport anisotropy can be tuned simultaneously by applying a vertical electric field due to the in-plane anisotropic carrier effective mass in each $\mathrm{CrSBr}$ monolayer. This opens an opportunity for controlling the spin polarization as well as carrier-transport anisotropy in the 2D magnetic van der Waals layered materials by interlayer twist and electric field, and provides an idea for utilizing A-type AFM semiconductors to design spintronic devices.

The Airy beam has the characteristics of nondiffraction, self-bending, and self-healing, so it is of great significance to wireless power transfer (WPT) and wireless communication in the radio-frequency (rf) field. This paper proposes the generation and modulation method of the two-dimensional (2D) Airy beams based on a holographic tensor metasurface, which can generate 2D Airy beams with different trajectories. A multidimensional controllable tensor metasurface unit operating at 10 GHz is designed to concentrate the electromagnetic energy to achieve the linearly polarized Airy beam. The unit with tiny size can also precisely regulate both the amplitude and the phase of the electromagnetic wave so that the high-performance 2D Airy beams can be generated by the holographic tensor metasurface in the microwave band. Meanwhile, the Airy beams with varying deflected angles are designed based on beam-steering theory to obtain different energy transmission trajectories for WPT. The simulation and measurement results show that the deflected 2D Airy beam still maintains the characteristics of nondiffraction and self-bending.

Ferroelectric hafnium oxide films are one of the most promising functional materials for a new generation of nonvolatile memory for data storage because of a number of excellent performance features, including nanosecond speed, high endurance, low power consumption, full scalability, and perfect compatibility with Si technology. However, the commercialization of this memory is hindered by its limited retention time, which still does not meet the 10-year electronics-industry standard. To date, much effort has been focused on the improvement of retention via material and interface engineering. Meanwhile, the evaluation and comparison of engineering results have been hampered, since there is still no unified retention test. Indeed, a specific feature of the retention performance is that it cannot be measured directly, because 10 years is required to measure the actual polarization loss. In this paper, we present a physical model that predicts the retention loss in ${\mathrm{Hf}\mathrm{O}}_2$-based memory during long-term information storage. The time-dependent polarization loss originates from the phenomenon of polarization imprint, which consists in the temporal development of a preferential polarization state in a poled ferroelectric. For high reliability of the retention calculation, the model takes into account the specific features of imprint evolution in the ferroelectric material ${\mathrm{Hf}\mathrm{O}}_2$. Among these is the origin of the imprint in this material, which consists in the interplay of two physical mechanisms, specifically, charge injection into interface traps and migration of mobile charged defects across the functional layer. The second property of ${\mathrm{Hf}\mathrm{O}}_2$ taken into account is the different rates of imprint evolution in two types of ${\mathrm{Hf}\mathrm{O}}_2$ film that exist within each memory cell. These two types of film originate from two populations of domains that appear during crystallization of the ${\mathrm{Hf}\mathrm{O}}_2$ and persist during the whole lifetime of the memory cell. The high accuracy of the retention model makes it possible to implement an express retention test, i.e., to predict the retention time in record short time (1 h) for any particular sample, whereas usually a retention test takes at least several days. For ${\mathrm{Hf}}_{0.5}{\mathrm{Zr}}_{0.5}{\mathrm{O}}_2$-based capacitors, the retention model shows excellent agreement with experimental results. The proposed model can serve not only for the implementation of this express retention test, but also as a tool for intelligent material engineering in the field of nonvolatile ferroelectric memory.

Semiconductor spin qubits may be coupled through a superconducting cavity to generate an entangling two-qubit gate. However, the fidelity of such an operation will be reduced by a variety of error mechanisms such as charge and magnetic noise, phonons, cavity loss, transitions to nonqubit states, and, for electrons in silicon, excitation into other valley eigenstates. Here, we model the effects of these error sources and the valley degree of freedom on the performance of a cavity-mediated two-qubit $\mathrm{i}$swap gate. For valley splittings inadequately large relative to the interdot tunnel coupling within each qubit, we find that valley excitation may be a limiter to the fidelity of this two-qubit gate. In addition, we show trade-offs between gating times and exposure to various error sources, identifying optimal operating regimes and device improvements that would have the greatest impact on the fidelity of the cavity-mediated spin $\mathrm{i}$swap gate. Importantly, we find that, while the impact of charge noise and phonon relaxation favor operation in the regime where the qubits are most spinlike to reduce sensitivity to these sources of noise, the combination of hyperfine noise and valley physics shifts the optimal regime to chargelike qubits with stronger effective spin-photon coupling so that gate times can be made as short as possible. In this regime, the primary limitation is the need to avoid Landau-Zener transitions as the gate is implemented.

Imaging is indispensable for nearly every field of science, engineering, technology, and medicine. However, measurement noise and stochastic distortions pose fundamental limits to accessible spatiotemporal information despite impressive tools such as structured-illumination microscopy (SIM), stochastic optical reconstruction microscopy or photoactivated localization microscopy (STORM or PALM), and stimulated-emission-depletion (STED) microscopy. How to combat this challenge ideally has been an open question for decades. Inspired by a virtual-gain technique to compensate losses in metamaterials, active convolved illumination (ACI) has recently been proposed to significantly improve the signal-to-noise ratio and hence the data acquisition. In this technique, the light pattern of the object is superimposed with a correlated auxiliary pattern, the function of which is to reverse the adverse effects of losses, noise, and random distortion based on their spectral characteristics. Despite enormous implications in statistics, any experimental evidence verifying the theory of this novel technique has been lacking to date. We find experimentally that ACI boosts not just the resolution limit and image contrast but also the resistance to pixel saturation. The results confirm the previous theories and may open up horizons in a wide range of disciplines from atmospheric sciences, seismology, biology, statistical learning, finance, and information processing to quantum noise beyond the fundamental boundaries.

We demonstrate non-Hermitian topological phase transitions induced solely by imaginary disorder. Starting from the trivial phase in the well-known Benalcazar, Bernevig, and Hughes model, we find that adding sufficient imaginary disorder to the trivial bulk can lead to a higher-order topological insulator supporting topological corner-localized states. In experiments, we elaborately design a two-dimensional reconfigurable acoustic lattice with a loss configuration that can be randomly set. By increasing the strength of lossy disorder, we observe the transitions from a trivial to a higher-order topological phase and subsequently to a gapless phase. To further show that imaginary disorder offers an alternative degree of freedom for controlling topological phases, we propose a non-Hermitian topological disclination state induced by imaginary disorder. Our work demonstrates an example of a non-Hermitian higher-order topological Anderson insulator and offers a reconfigurable platform to study the interplay between non-Hermitian disorder and topological phases.

A method is presented for the determination of complex-valued compression and shear elastic moduli of polymers for ultrasound applications. The resulting values, which are scarcely reported in the literature, are found with uncertainties typically around 1% (real part) and 6% (imaginary part). The method involves a setup consisting of a cm-radius, mm-thick polymer ring glued concentrically to a disk-shaped piezoelectric transducer. The ultrasound electrical-impedance spectrum of the transducer is computed numerically and fitted to measured values as an inverse problem in a wide frequency range, typically from 500 Hz to 5 MHz, both on and off resonance. The method is validated experimentally by ultrasonic through transmission around 1.9 MHz. The method is low cost, not limited to specific geometries and crystal symmetries, and, given the developed software, easy to execute. The method has no obvious frequency limitations before severe attenuation sets in above 100 MHz.

Acoustic holography is widely used in particle manipulation. However, previous methods focus on modulating the amplitude and phase of the target sound field. In general, the amplitude gradient and phase gradient of the sound field correspond to the gradient force and the scattering force, respectively. But the relationship between the force and the aforementioned gradient is nonlinear even for a Rayleigh particle (whose radius is much smaller than the wavelength), not to mention the Mie particle(whose radius is comparable with the wavelength). If we want to control the continuous motion of a Mie particle quantitatively, we need to design force based on the relationship among the force, acceleration, and velocity. Here we propose to design radiation force along the path of a particle directly in holography methods. Compared to the usual modulation of amplitude and phase, the method designing radiation force directly can control the acceleration and orbital limiting velocity of a two-dimensional orbit rotation of a Mie particle with radius $a=0.4\mathrm{mm}$ (approximately $2/3$ wavelength) in simulation. The experiment verified its feasibility in controlling the continuous motion of particle quantitatively. This work shows its superiority over the pure acoustic holography methods in quantitative particle manipulation.

Machine learning finds application in the quantum control and readout of qubits. In this work we apply artificial neural networks to assist the manipulation and the readout of a prototypical molecular spin qubit-an oxovanadium(IV) moiety-in two experiments designed to test the amplitude and the phase recognition, respectively. We first successfully use an artificial network to analyze the output of a storage-retrieval protocol with four input pulses to recognize the echo positions and, with further post selection on the results, to infer the initial input pulse sequence. We then apply an artificial neural network to ascertain the phase of the experimentally measured Hahn echo, showing that it is possible to correctly detect its phase and to recognize additional single-pulse phase shifts added during manipulation.

The concept of birefringence, commonly treated as a property resulting from more than one polarization, is associated with different refractive indices, depending on polarization. Here, we propose a two-dimensional elastic metamaterial engineered to demonstrate birefringence of single polarization for flexural waves. By adopting a network of beams with “overpass” structures to upset the topology of unit cells from a conventional elastic plate, birefringence of single polarization at low frequencies can be realized and confirmed through measured band structures and double refraction of the flexural waves; this is further supported by full-wave simulations and models. Our approach can engineer birefringence at low frequencies in which the wavelength is at least a few times larger than the lattice constant, further allowing efficient control of beam splitting, filtering, and multiplexing with applications for elastic wave devices.

We study the stabilization and electrical manipulation of skyrmions in magnetic ultrathin films in the absence of an applied magnetic field. We show that this requires an increased magnetic anisotropy, controlled by the sample thickness, as compared to usual skyrmionic samples, so that the uniform state corresponds to the zero-field ground state and the skyrmions to metastable excitations. Although skyrmion stabilization at zero field is demonstrated over a broad range of thicknesses, electrical control appears to be more demanding to avoid skyrmion deformation. In the thinnest samples, the large magnetic anisotropy prevents skyrmion deformations and we show that they can be nucleated progressively by current pulses, which underlines that the only possible transition occurs between uniform and skyrmion states. The solitonic skyrmions in zero applied magnetic field have the same properties as compared to field-stabilized ones, with a long-term stability and high mobility when excited by a spin-orbit torque.

Electron spin transport in a two-dimensional $\mathrm{Si}$ inversion channel is experimentally and theoretically studied in terms of electron distribution in the subbands, electron momentum scattering processes, electron momentum lifetime, and spin lifetime. The electrical properties, electron charge transport, and spin transport are investigated by measuring a $\mathrm{Si}$-based spin metal-oxide-semiconductor field-effect transistor with a 10­-μm channel length under various bias and temperature conditions (4, 77, 150, and 295 K). In particular, in our unique procedure, the same device is measured before and after annealing to quantitatively clarify the change in the spin transport with lower and higher electron mobilities, by excluding device-to-device variability. Even when the distribution of electrons in the subbands, electron mobility, and temperature are significantly changed, the spin-flip probability, which is defined as the ratio of electron momentum lifetime to spin lifetime, is nearly constant at approximately 1/25 000, probably due to the Elliot-Yafet mechanism. The estimated spin conservation lengths at various temperatures are increased 2–50 times with increases in both the electron mobility and spin drift. With a high electron mobility of $3065{\mathrm{cm}}^2{\mathrm{V}}^{-1}{\mathrm{s}}^{-1}$ at 4 K, spin transport with a spin conservation efficiency of 94% is achieved through the 10-μm-long channel.

Passively mode-locked semiconductor disk lasers have received tremendous attention from both science and industry. Their relatively inexpensive production combined with excellent pulse performance and great emission-wavelength flexibility make them suitable laser candidates for applications ranging from frequency-comb tomography to spectroscopy. However, due to the interaction of the active medium dynamics and the device geometry, emission instabilities occur at high pump powers and thereby limit their performance potential. Hence, understanding those instabilities becomes critical for an optimal laser design. Using a delay-differential equation model, we are able to detect, understand, and classify three distinct instabilities that limit the maximum achievable pump power for the fundamental mode-locking state and link them to characteristic positive-net-gain windows. We furthermore derive a simple analytic approximation in order to quantitatively describe the stability boundary. Our results enable us to predict the optimal laser-cavity configuration with respect to positive-net-gain instabilities and therefore may be of great relevance for the future development of passively mode-locking semiconductor disk lasers.

We combine soliton dynamics in gas-filled hollow-core photonic crystal fibers with a state-of-the-art fiber laser to realize a turnkey system producing few-femtosecond pulses at 8-MHz repetition rate at pump energies as low as 220 nJ. Furthermore, by exploiting the soliton self-frequency shift in a second hydrogen-filled hollow-core fiber, we efficiently generate pulses as short as 22 fs, continuously tunable from 1100 to 1474 nm.