Frequency nonreciprocity of wave phenomena describes the situation where the wave dispersion depends on the sign of the wave vector, i.e., counterpropagating waves exhibit different wavelengths for the same frequency. Such behavior has recently been observed in heavy-metal–ferromagnetic interfaces with Dzyaloshinskii-Moriya coupling, and is also known for coupled magnetic bilayers, where the nonreciprocity is enhanced when the two layers are aligned antiparallel. Besides the conventional uses of spin waves, nonreciprocity adds further functionalities, such as its potential applications in communication technologies and logic operations. In the current paper, we thus examine the spin-wave nonreciprocity induced by dipolar interactions in a coupled bilayer consisting of two ferromagnetic layers separated by a nonmagnetic spacer. We derive an easy-to-use formula to estimate the frequency difference provided by the nonreciprocity, which allows one to choose an optimal system in order to maximize the effect. For small wave numbers, the nonreciprocity scales linearly, while for larger wave vectors the nonreciprocity behaves nonmonotonically, with a well-defined maximum. The study is carried out by means of analytical calculations that are complemented by micromagnetic simulations. Furthermore, we confirm our model by experimental investigation of the spin-wave dispersion in a prototype antiparallel-coupled bilayer system. Since the relative magnetic orientation can be controlled through a bias field, the magnon nonreciprocity can then be turned on and off, which lends an important functionality to the coupled ferromagnetic bilayers.

In the past decades, ferromagnet-metalloid alloy films of $\mathrm{Co}\text{−}\mathrm{Fe}\text{−}\mathrm{B}$ have been widely used in alternative magnetic devices due to their excellent performance, such as easy industrial-scale fabrication, considerable ability for tunneling magnetoresistance and perpendicular magnetic anisotropy. However, the insufficient thermal tolerance and interfacial state densities in the typical $\mathrm{Co}\text{−}\mathrm{Fe}\text{−}\mathrm{B}/\mathrm{Mg}\mathrm{O}$ system limits the devices’ optimization. Because of the improvement in thermal stability and interfacial properties by carbon element replacement, alternative theoretical and experimental work on $\mathrm{Co}\text{−}\mathrm{Fe}\text{−}\mathrm{C}$ alloy film properties have been reported. Here, we report on the magnetostrictive behavior, soft magnetism, and microwave properties of a series of $({\mathrm{Co}}_{0.5}{\mathrm{Fe}}_{0.5}{)}_x{\mathrm{C}}_{1-x}$ films grown on silicon (001) substrates. The addition of carbon changes the $\mathrm{Co}\text{−}\mathrm{Fe}\text{−}\mathrm{C}$ films from nanocrystalline body-centered-cubic to an amorphous phase and leads to a high saturated magnetostriction constant of 75 ppm, high piezomagnetic coefficient of 10.3 ppm/Oe, excellent magnetic softness with a low coercivity less than 2 Oe, narrow ferromagnetic resonance line width of 25 Oe at the X band, extremely low Gilbert damping of 0.002, and up to 500 °C thermal stability. The large saturated magnetostriction constant and piezomagnetic coefficient result from the coexistence of nanocrystalline body-centered-cubic and amorphous phases. The extremely low Gilbert damping is related to the minimized density of states around the Fermi energy of the alloys induced by carbon doping. The combination of these properties makes $\mathrm{Co}\text{−}\mathrm{Fe}\text{−}\mathrm{C}$ films promising candidates to be widely used in voltage-tunable magnetoelectric devices and microwave magnetic devices.

Magnetoelastic (straintronic) switching of bistable magnetostrictive nanomagnets is an extremely energy-efficient switching methodology for (magnetic) binary switches that has recently attracted widespread attention because of its potential application in ultra-low-power digital computing hardware. Unfortunately, this modality of switching is also very error prone at room temperature. Theoretical studies of switching error probability of magnetoelastic switches have predicted probabilities ranging from ${10}^{-8}$ to ${10}^{-3}$ at room temperature for ideal, defect-free nanomagnets, but experiments with real nanomagnets show a much higher probability that exceeds 0.1 in some cases. The obvious spoilers that can cause this large difference are defects and nonidealities. We theoretically study the effect of common defects (that occur during fabrication) on magnetoelastic switching probability in the presence of room-temperature thermal noise. Surprisingly, we find that even small defects increase the switching error probabilities by orders of magnitude. There is usually a critical stress that leads to the lowest error probability and its value increases enormously in the presence of defects. All this could limit or preclude the application of magnetoelastic (straintronic) binary switches in either Boolean logic or memory, despite their excellent energy efficiency, and restrict them to non-Boolean (e.g., neuromorphic, stochastic) computing applications. We also study the difference between magnetoelastic switching with a stress pulse of constant amplitude and sinusoidal time-varying amplitude (e.g., due to a surface acoustic wave) and find that the latter method is more reliable and generates lower switching error probabilities in most cases provided the time variation is reasonably slow.

We perform a comparative study on thermoelectric performance of antiperovskite oxides $A{e}_{3}Tt\mathrm{O}$ and nitrides $A{e}_{3}Pn\mathrm{N}$ ($Ae=\mathrm{Ca},\mathrm{Sr},\mathrm{Ba}$; $Tt=\mathrm{Ge},\mathrm{Sn},\mathrm{Pb}$; $Pn=\mathrm{As},\mathrm{Sb},\mathrm{Bi}$) by means of first-principles calculation. As for the oxides with the cubic structure, ${\mathrm{Ca}}_3\mathrm{Ge}\mathrm{O}$ with a sizable band gap exhibits high thermoelectric performance at high temperatures, while ${\mathrm{Ba}}_3\mathrm{Pb}\mathrm{O}$ with Dirac cones without the gap is favorable at low temperatures. The latter high performance owes to high valley degeneracy, including the multiple Dirac cones and the valleys near the $\mathrm{\Gamma }$ and R points. For the nitrides with the cubic structure, an insulator with strong quasi-one-dimensionality exhibits high thermoelectric performance. We also find that the orthorhombic structural distortion sometimes sizably enhances thermoelectric performance, especially for ${\mathrm{Ba}}_3\mathrm{Ge}\mathrm{O}$ and ${\mathrm{Sr}}_3\mathrm{As}\mathrm{N}$, where the high valley degeneracy is realized in the $Pnma$ phase. Our calculation reveals that antiperovskites offer a fertile playground of various kinds of characteristic electronic structures, which enhance thermoelectric performance, and provide promising candidates for high-performance thermoelectric materials.

Ferroelectric tunnel barriers in between two ferromagnetic electrodes (multiferroic tunnel junctions, or MFTJs) hold great promise for future microelectronic devices. Here, we utilize ${\mathrm{Hf}}_{0.5}{\mathrm{Zr}}_{0.5}{\mathrm{O}}_2$ (HZO) tunnel barriers with an ultralow thickness of only 2 nm, epitaxially grown on ${\mathrm{La}}_{0.7}{\mathrm{Sr}}_{0.3}{\mathrm{Mn}\mathrm{O}}_3$ ferromagnetic bottom electrodes and with cobalt top electrodes. Both tunneling electroresistance and tunneling magnetoresistance effects are observed, demonstrating four nonvolatile resistance states in HZO-based junctions. The large band gap and excellent homogeneity of the HZO tunnel barriers enable a high yield of working devices, as well as devices with sizes of tens of micrometers. This allows working with fixed electrodes, as opposed to the use of scanning probes, bringing MFTJs closer to applications.

Unintentional doping of the active layer is a source for lowered device performance in organic solar cells. The effect of doping is to induce a space-charge region within the active layer, generally resulting in increased recombination losses. The impact of a doping-induced space-charge region on the current-voltage characteristics of low-mobility solar cell devices is clarified by means of analytical derivations and numerical device simulations. It is found that in the case of a doped active layer, the collection efficiency of photogenerated charge carriers is independent of the light intensity and exhibits a distinct voltage dependence, resulting in an apparent electric field dependence of the photocurrent. Furthermore, an analytical expression describing the behavior of the photocurrent is derived. The validity of the analytical model is verified by numerical drift-diffusion simulations and demonstrated experimentally on solution-processed organic solar cells. Based on the theoretical results, conditions of how to overcome charge collection losses caused by doping are discussed. Furthermore, the presented analytical framework provides tools to distinguish between different mechanisms leading to voltage-dependent photocurrents.

Control of nonlinear processes in multimode fibers and their applications have seen a recent resurgence because of the large dimensionality afforded by the platform. We show that by engineering the relative group velocity within spatial dimensions, we can tailor the phase-matching condition of orbital angular momentum (OAM) modes. We demonstrate efficient and low-cross-talk intramodal four-wave mixing processes between OAM modes in a selfdesigned and fabricated vortex fiber. The fiber is designed using a genetic algorithm such that it can both support stable OAM mode transmission and meet the phase-matched condition of intramodal four-wave mixing (FWM) during multimodes. We experimentally characterize the gain spectral bandwidth of different modes and find that they depend on multimode dispersion properties of the designed fiber, which agrees well with our theoretical calculation.

Complex electrical behavior, including the field-screening distribution and possible electrochemical effects, makes electromechanical imaging by piezoresponse force microscopy in conductive liquids a critical challenging issue in studying the functionalities and performance of piezoelectric materials. Here, we propose a theoretical model to directly quantify the electric-field-screening effect inside the tip-sample junction in a variety of electrolytes of varying ionic strength. The field-screening effect resulting from an electric double layer in the tip-surface system offers the benefit of better understanding of the central physical mechanism in electrical probing by scanning probe microscopy (SPM) in a polar liquid medium. Experimentally, an intrinsic piezoresponse in ferroelectric antiparallel-polarization regions is observed in electrolytes with various assemblies of valences and with various molarities through updating the design of a nanoelectrode probe. Accompanied by the unveiling of the underlying physical mechanism, these experimental results pave the way to the broad application of in vivo or in operando electrical analysis on the nanoscale by the SPM technique in conductive-liquid environments.

The possibility of tuning the Dzyaloshinskii-Moriya interaction (DMI) by electric (E)-field gating in ultrathin magnetic materials has opened up new perspectives in terms of controlling the stabilization of chiral spin structures. The most recent efforts have used voltage-induced charge redistribution at the interface between a metal and an oxide to modulate the DMI. This approach is attractive for active devices but tends to be volatile, making it energy-demanding, and it is limited by Coulomb screening in the metal. Here we demonstrate nonvolatile E-field manipulation of the DMI by ionic-liquid gating of $\mathrm{Pt}/\mathrm{Co}/{\mathrm{Hf}\mathrm{O}}_2$ ultrathin films. The E-field effect on the DMI scales with the E-field exposure time, and we propose that it is linked to the migration of oxygen species from the ${\mathrm{Hf}\mathrm{O}}_2$ layer into the $\mathrm{Co}$ and $\mathrm{Pt}$ layers and subsequent anchoring. This effect permanently changes the properties of the material, showing that E fields can be used not only for local gating in devices but also as a highly scalable materials design tool for postgrowth tuning of the DMI.

Spin current induced by a charge current has been extensively explored to electrically manipulate magnetism and high charge-to-spin conversion efficiency is expected to enable a low power spin-orbit torque (SOT) switching mechanism. We report on experimental results that demonstrate a charge current flowing in-plane in an oxide Rashba-Edelstein two-dimensional electron gases (2DEGs) interface that can efficiently generate a spin current, and a strong SOT in an adjacent permalloy layer is observed by spin-torque ferromagnetic resonance. The charge-to-spin conversion efficiency $|{\theta }_{\mathrm{CS}}|$ is up to a value of 0.9 for a simple ${\mathrm{Ar}}^{+}$-etched ${\mathrm{Sr}\mathrm{Ti}\mathrm{O}}_3/\mathrm{Ni}\mathrm{Fe}$ structure, which is much larger than that of heavy meals, and the structure can be easily fabricated by magnetron sputtering. In addition, the spin-orbital coupling in the ${\mathrm{Sr}\mathrm{Ti}\mathrm{O}}_3/{\mathrm{La}\mathrm{Al}\mathrm{O}}_3$ interface is more efficient and $|{\theta }_{\mathrm{CS}}|$ can be increased up to 1.8 for the ${\mathrm{Sr}\mathrm{Ti}\mathrm{O}}_3/{\mathrm{La}\mathrm{Al}\mathrm{O}}_3/\mathrm{Ni}\mathrm{Fe}$ device, whereas it is only 0.6 for a ${\mathrm{Sr}\mathrm{Ti}\mathrm{O}}_3/{\mathrm{La}\mathrm{Ti}\mathrm{O}}_3/\mathrm{Ni}\mathrm{Fe}$ device. These results will enable the realization of SOT devices based on ${\mathrm{Sr}\mathrm{Ti}\mathrm{O}}_3$-based 2DEGs for low power nonvolatile memory and logic.

Terahertz (THz) radiation sources ranging from 0.3 to 10 THz are the last frontier of the light spectrum because of the relative lack of physical systems that can operate in this regime, the well-known “THz gap.” We propose a mechanism for frequency down-conversion to produce THz radiation covering most of the THz gap by utilizing the approximately equidistant helical edge states in topological insulator quantum dots. If the edge-state level separation inside the phonon gap is chosen to suppress phonon-induced nonradiative decay, a topological insulator quantum dot array in a resonant cavity can form a continuous-wave THz laser operating at room temperature, with an output power reaching 28 mW at $\sim 3$ THz and an energy conversion efficiency of $\sim 14$%. Our work may pave the way to applications of topological insulator quantum dots for THz emission.

We study the temperature and voltage dependence of current density in hybrid poly(3,4-ethylenedioxythiophene) polystyrene sulfonate (PEDOT:PSS)/$\mathrm{Si}$ heterojunctions. It is shown that charge transport follows distinct mechanisms at reverse and forward bias. The current is found to be injection limited at reverse bias owing to the quasi-Schottky-barrier at the organic-inorganic interface. At forward bias, the carrier transport is governed by space-charge-limited current in PEDOT:PSS. Trap-limited current at low forward bias is associated with trap states in the organic layer. The total density and the characteristic energy of localized states of PEDOT:PSS are estimated as $N_t\simeq 5.6\times {10}^{15}{\mathrm{cm}}^{-3}$ and $E_{\mathrm{ch}}\simeq 0.107\mathrm{eV}$, respectively. The commonly observed double-logarithmic character at forward bias is interpreted as the signature of transition from the Mark-Helfrich regime to the Mott-Gurney regime.

Nonreciprocal devices that allow unidirectional wave propagation are indispensable in many scientific fields, including, for example, optics, acoustics, electronics, and thermodynamics. Here, we propose to realize the nonreciprocal transmission of a microwave field in a two-cavity magnonics system consisting of two microwave cavities and a yttrium iron garnet (YIG) sphere, which is placed in one of them. We discover that magnon Kerr nonlinearity leads to the occurrence of magnon bistability and different magnonic responses for the forward and backward propagation of a microwave field. The different magnonic responses play a crucial role in the nonreciprocal transmission, which essentially breaks the space-reversal symmetry of transmission of the microwave field. More importantly, by adjusting the external bias magnetic field, which determines the frequency of the magnon, we can obtain a variable isolation of the microwave field or even its transmission in only one direction, which may be utilized to achieve light diodes. In addition, by increasing the strength of a microwave source applied to drive the magnon mode, stronger nonreciprocal transmission can be observed. These results may open a route to exploiting various nonreciprocal effects in cavity magnonics systems.

Within the context of spin-related optical phenomena, near-field directionality is generally understood from the viewpoint of the quantum spin Hall effect of light, according to which the transverse spin of surface or guided modes is locked to the propagation direction. Most previous works have been focused on the spin properties of circularly polarized dipolar sources. However, in near-field optics, higher-order multipole sources (e.g., quadrupole, octupole, and so on) might become relevant, so a more in-depth formulation would be highly valuable. Building on the angular-spectrum representation, we provide a general, analytical, and ready-to-use treatment in order to address the near-field directionality of any multipole field, particularizing to the electric quadrupole case. Besides underpinning and upgrading the current framework on spin-dependent directionality, our results may open up interesting perspectives for engineering light-matter coupling at the nanoscale.

Electromagnetic waves exhibiting vortex properties from orbital angular momentum (OAM) modes that are inherently orthogonal show promising potential for a wide range of applications. However, classic OAM-wave generators in the radio-frequency band are generally limited by a narrow operation bandwidth, complicated feeding structures, or high losses. In this work, we propose a method for OAM-wave generation using the concept of spatial transformation at microwave frequencies. A reflection-type vortex-beam generator is designed and simulated. Then an all-dielectric proof-of-concept prototype for a +1 topological charge is physically implemented and the complete characterization of the device is reported. Both full-wave simulations and experimental measurements successfully validate the spiral-shaped phase fronts of the vortex wave. Furthermore, the all-dielectric implementation of the device enables operation over a broad frequency bandwidth. The proposed method provides an efficient approach to generate vortex waves carrying OAM modes and illustrates the practicality of using spatial transformations to achieve an alternative class of optical devices at microwave frequencies.

To obtain room-temperature ferromagnetic semiconductors and to realize the room-temperature quantum anomalous Hall effect (QAHE) have been big challenges for a long time. Here we report that, on the basis of first-principles calculations, ${\mathrm{Pd}\mathrm{Br}}_3$, ${\mathrm{Pt}\mathrm{Br}}_3$, ${\mathrm{Pd}\mathrm{I}}_3$, and ${\mathrm{Pt}\mathrm{I}}_3$ monolayers are ferromagnetic semiconductors that could exhibit a high-temperature QAHE. The Curie temperatures estimated by Monte Carlo simulations are 350 and 375 K for ${\mathrm{Pd}\mathrm{Br}}_3$ and ${\mathrm{Pt}\mathrm{Br}}_3$ monolayers, respectively. The band gaps of ${\mathrm{Pd}\mathrm{Br}}_3$ and ${\mathrm{Pt}\mathrm{Br}}_3$ are found to be 58.7 and 28.1 meV, respectively, with the generalized-gradient approximation and 100.8 and 45 meV, respectively, with the HSE06 method, being quite well in favor of observing the room-temperature QAHE. It is shown that the large band gaps are induced from multiorbital electron correlations. By carefully studying the stabilities of the four aforementioned monolayers, we unveil that they could be feasible for use in experiments. The present work sheds light on the development of spintronic devices by use of room-temperature ferromagnetic semiconductors and the implementation of dissipationless devices by application of the room-temperature QAHE.

Quantum memories for light are important components for future long-distance quantum networks. We present on-chip quantum storage of telecommunication-band light at the single-photon level in an ensemble of erbium-167 ions in an yttrium orthosilicate photonic crystal nanobeam resonator. Storage times of up to $10\mu \mathrm{s}$ are demonstrated with an all-optical atomic-frequency-comb protocol in a dilution refrigerator under a magnetic field of 380 mT. We show this quantum-storage platform to have high bandwidth, high fidelity, and multimode capacity, and we outline a path toward an efficient erbium-167 quantum memory for light.

The sending-or-not-sending protocol of the twin-field quantum key distribution (TFQKD) has its advantage of unconditional security under any coherent attack and fault tolerance to a large misalignment error. Here, we consider the complete finite-key effects for the protocol and we calculate the secure key rate with a fixed security coefficient. The numerical simulation shows that the protocol with a typical finite number of pulses in practice can produce an unconditional secure final key under general attack, including all coherent attacks. It can exceed the secure distance of 500 km in a typical finite number of pulses in practice even with a large misalignment error.

The relative stability of $\mathrm{Si}\mathrm{C}$ polytypes, changing with temperature, has been considered a paradox for about 30 years, due to discrepancies between theory and experiments. Based on ab initio calculations including van der Waals corrections, a temperature-dependent polytypic diagram consistent with the experimental observations is obtained. Results are easily interpreted based on the influence of the hexagonality on both cohesive energy and entropy. Temperature-dependent stability of stacking faults is also analyzed and found to be in agreement with experimental evidence. Our results suggest that lower temperatures during $\mathrm{Si}\mathrm{C}$ crystal deposition are advantageous in order to reduce ubiquitous stacking faults in $\mathrm{Si}\mathrm{C}$-based power devices.

The effect of thermal fluctuation on the spin-chirality-induced anomalous Hall effect in itinerant magnets is studied theoretically. Considering a triangular-lattice model as an example, we find that a multiple-spin scattering induced by the fluctuating spins increases the Hall conductivity at a finite temperature. The temperature dependence of anomalous Hall conductivity is evaluated by a combination of an unbiased Monte Carlo simulation and perturbation theory. Our results show that the Hall conductivity can increase up to ${10}^3$ times the ground-state value; we discuss that this is a consequence of a skew-scattering contribution. This enhancement shows that the thermal fluctuation significantly affects the spin-chirality-related Hall effect. Our results are potentially relevant to the thermal enhancement of the anomalous Hall effect often seen in experiments.