Zinc tin nitride ($\mathrm{Zn}\mathrm{Sn}{\mathrm{N}}_2$) is attracting growing interest as a nontoxic and earth-abundant photoabsorber for thin-film photovoltaics. Carrier transport in $\mathrm{Zn}\mathrm{Sn}{\mathrm{N}}_2$ and consequently cell performance are strongly affected by point defects with deep levels acting as carrier recombination centers. In this study, the point defects in $\mathrm{Zn}\mathrm{Sn}{\mathrm{N}}_2$ are revisited by careful first-principles modeling based on recent experimental and theoretical results. It is shown that $\mathrm{Zn}\mathrm{Sn}{\mathrm{N}}_2$ does not have low-energy defects with deep levels, in contrast to previously reported results. Therefore, $\mathrm{Zn}\mathrm{Sn}{\mathrm{N}}_2$ is more promising as a photoabsorber material than formerly considered.

Decoherence remains a major challenge in quantum computing hardware, and a variety of physical-layer controls provide opportunities to mitigate the impact of this phenomenon through feedback and feed-forward control. In this work, we compare a variety of machine-learning algorithms derived from diverse fields for the task of state estimation (retrodiction) and forward prediction of future qubit-state evolution for a single qubit subject to classical, non-Markovian dephasing. Our approaches involve the construction of a dynamical model capturing qubit dynamics via autoregressive or Fourier-type protocols using only a historical record of projective measurements. A detailed comparison of achievable prediction horizons, model robustness, and measurement-noise-filtering capabilities for Kalman filters (KFs) and Gaussian process regression (GPR) algorithms is provided. We demonstrate superior performance from the autoregressive KF relative to Fourier-based KF approaches and focus on the role of filter optimization in achieving suitable performance. Finally, we examine several realizations of GPR using different kernels and discover that these approaches are generally not suitable for forward prediction. We highlight the linkages between predictive performance and kernel structure, and we identify ways in which forward predictions are susceptible to numerical artifacts.

We provide detailed insights into a link between structural and optical disorder of resonant optical structures, in particular, distributed Bragg reflectors (DBRs) and resonant microcavities ($\mu \mathrm{Cs}$). The standard (targeted) DBR structures have periodic square-wave–like refractive-index profiles, and their optical performance is determined by the refractive-index ratio of the two applied materials ($n_{12}=n_1/n_2$, $n_1>n_2$) and the number of DBR periods ($N$). It is well known that its structural disorder strongly affects its optical properties, but, despite that, this influence has not been quantitatively addressed in the literature. We propose a precise quantitative definition for a structural disorder of a single DBR unit cell (disorder factor $D_F$), completing the set of DBR fundamental parameters ($n_{12}$, $N$, $D_F$). Then we expose the basis for the effective refractive-index approximation (ERIA), showing that, as long as DBR optical properties are concerned, the influence of increasing structural disorder ($D_F\uparrow$) is virtually identical to the influence of decreasing refractive-index ratio ($n_{12}\downarrow$), with the latter influence being easily quantified. Making use of the ERIA method, simple analytical formulas, which enable rapid insights into the reflectivity and stop-band width of DBRs with different types of transient layers at the heterointerfaces, are derived and the results validated, via both transfer-matrix simulations and direct experimental measurements of imperfect DBRs. The insights of the ERIA method are then further applied on resonant $\mu \mathrm{Cs}$, providing a comprehensive link between their structural disorder and subsequent deterioration of their quality ($Q$) factor.

Electrical manipulation of antiferromagnets with specific symmetries offers the prospect of creating antiferromagnetic spintronic devices. Such devices aim to make use of the insensitivity to external magnetic fields and the ultrafast dynamics at the picosecond timescale intrinsic to antiferromagnets. The possibility to electrically switch antiferromagnets was first predicted for ${\mathrm{Mn}}_2\mathrm{Au}$ and then experimentally observed in tetragonal CuMnAs. Here, we report on the electrical switching and detection of the Néel order in epitaxial films of ${\mathrm{Mn}}_2\mathrm{Au}$. The exponential dependences of the switching amplitude on the current density and the temperature are explained by a macroscopic thermal-activation model taking into account the effect of the Joule heating in Hall cross devices, and we observe that the thermal activation plays a key role in the reorientation process of the Néel order. Our model analysis shows that the electrically set Néel state is long-term stable at room temperature, paving the way for practical applications in memory devices.

Low-damping spin sources are critical to efficient spin-current generation, but low-damping magnetic insulators have not been systematically explored so that the controlling parameters for efficient spin-current generation are not well understood. The choice of magnetic insulators with sufficiently low damping has been largely limited to ${\mathrm{Y}}_3{\mathrm{Fe}}_5{\mathrm{O}}_{12}$ (YIG), whose compatibility with existing microelectronics is problematic at best. Therefore, an alternative material or family of magnetic insulators with low damping would provide not only fundamental insight into the underlying mechanisms for low-damping magnetic insulators but also the foundation for a spin-current-based electronics future. The family of spinel ferrites includes a wide variety of magnetic insulators, but high damping in conventional spinel ferrites has made them poor spin-current sources. In this study, we demonstrate that microwave excitation of low-damping $(\mathrm{Ni},\mathrm{Zn},\mathrm{Al})$ ferrite (NZAFO) efficiently generates spin current. Spin pumping from the ferrite to an adjacent metal layer is manifest in both an increase in Gilbert damping and the emergence of a voltage peak that occurs at ferromagnetic resonance (FMR). Magnetotransport measurements suggest negligible contributions from a proximity-induced magnetic layer in the metal. From FMR and magnetotransport measurements, we estimate the spin-mixing conductance at the $\mathrm{NZAFO}/\mathrm{Pt}$ interface to be approximately ${10}^{14}{\mathrm{\Omega }}^{-1}{\mathrm{m}}^{-2}$, on the same order of magnitude as the often-studied $\mathrm{YIG}/\mathrm{Pt}$ interface. These results indicate that doped spinel ferrites can be efficient spin-pumping sources with a potential for highly tunable magnetic properties and coherent integration with a diverse range of complex oxides for all-oxide spintronics.

We study the evolution of the surface-plasmon resonances of individual ion-beam-shaped prolate gold nanoparticles embedded in a dielectric ${\mathrm{SiO}}_2$ environment by electron-energy-loss spectroscopy mapping in a scanning transmission electron microscope. The controlled symmetric dielectric environment obtained through the ion-beam-shaping method allows a direct quantitative comparison with numerical results obtained through simulations (auxiliary differential-equation finite-difference time-domain and boundary-element method) and with theoretical results obtained through analytical models (quasistatic model for prolate nanoellipsoids and waveguide model for infinite one-dimensional plasmonic waveguides), with which our experimental results are in very good agreement. We confirm the accuracy of state-of-the-art numerical tools and analytical theories that establish ion-beam shaping as a very promising method to design metal-dielectric nanocomposites with well-predicted optical properties, and with many possible applications in surface-enhanced Raman spectroscopy and second-harmonic generation, as well as in conventional applications of metamaterials like negative refraction, superimaging, and invisibility cloaking.

We demonstrate experimentally that single-photon detection can be achieved in micrometer-wide NbN bridges, with widths ranging from 0.53 to $5.15\mu \mathrm{m}$ and for photon wavelengths of 408 to 1550 nm. The microbridges are biased with a dc current close to the experimental critical current, which is estimated to be about 50% of the theoretically expected depairing current. These results offer an alternative to the standard superconducting single-photon detectors, based on nanometer-scale nanowires implemented in a long meandering structure. The results are consistent with improved theoretical modeling based on the theory of nonequilibrium superconductivity, including the vortex-assisted mechanism of initial dissipation.

We present a porous electrode theory for capacitive deionization with electrodes containing nanoparticles that consist of a redox-active intercalation material. A geometry of a desalination cell is considered which consists of two porous electrodes, two flow channels, and an anion-exchange membrane, and we use the Nernst-Planck theory to describe ion transport in the aqueous phase in all these layers. A single-salt solution is considered, with unequal diffusion coefficients for anions and cations. Similar to previous models for capacitive deionization and electrodialysis, we solve the dynamic two-dimensional equations by assuming that the flow of water, and thus the advection of ions, is zero in the electrode, and in the flow channel only occurs in the direction along the electrode and membrane. In all layers, diffusion and migration are only considered in the direction perpendicular to the flow of water. Electronic as well as ionic transport limitations within the nanoparticles are neglected, and instead the Frumkin isotherm (or regular solution model) is used to describe local chemical equilibrium of cations between the nanoparticles and the adjacent electrolyte, as a function of the electrode potential. Our model describes the dynamics of key parameters of the CDI process with intercalation electrodes, such as effluent salt concentration, the distribution of intercalated ions, cell voltage, and energy consumption.

We experimentally demonstrate that a linear dipole is not restricted to emit linearly polarized light, provided that it is embedded in the appropriate nanophotonic environment. We observe emission of various elliptical polarizations by a linear dipole, including circularly polarized light, without the need for birefringent components. We further show that the emitted state of polarization can theoretically span the entire Poincaré sphere. The experimental demonstration is based on elongated gold nanoparticles (nanorods) deposited on an optical nanofiber and excited by a free-space laser beam. The light directly collected in the guided mode of the nanofiber is analyzed in regard to the azimuthal position and orientation of the nanorods, observed by means of scanning electron microscopy. We demonstrate a mapping between purely geometrical degrees of freedom of a light source and all polarization states that could open the way to alternative methods for polarization control of light sources at the nanoscale.

We demonstrate a liquid-crystal (LC)-tuned Tamm-plasmon (TP) resonance device. The term TP polariton refers to plasmonic resonance at the interface between a photonic crystal (PC) and a metallic film. We formulate a precise optical analysis of the proposed device using the transfer-matrix method. A single resonance dip in the photonic band-gap region guarantees that the thickness of the LC layer between the PC and the metallic film can be as small as about 180 nm. Tuning phase retardation in the LC layer is shown to shift the resonance wavelength tens of nanometers, depending on the birefringence and width of the LC layer. Simulation results are in good agreement with the experimental data.

We show experimentally that the spin direction of the spin current generated by spin-orbit interactions within a ferromagnetic layer can be reoriented by turning the magnetization direction of this layer. We do this by measuring the fieldlike component of spin-orbit torque generated by an exchange-biased ${\mathrm{Fe}}_{95}{\mathrm{Gd}}_5$ thin film and acting on a nearby ${\mathrm{Co}}_{40}{\mathrm{Fe}}_{40}{\text{B}}_{20}$ layer. The relative angle of the ${\mathrm{Co}}_{40}{\mathrm{Fe}}_{40}{\text{B}}_{20}$ and ${\mathrm{Fe}}_{95}{\mathrm{Gd}}_5$ magnetic moments is varied by applying an external magnetic field. We find that the resulting torque is in good agreement with predictions that the spin current generated by the anomalous Hall effect from the ${\mathrm{Fe}}_{95}{\mathrm{Gd}}_5$ layer depends on the ${\mathrm{Fe}}_{95}{\mathrm{Gd}}_5$ magnetization direction ${\widehat{m}}_{\mathrm{FeGd}}$ according to $\overrightarrow{\sigma }\propto (\widehat{y}·{\widehat{m}}_{\mathrm{FeGd}}){\widehat{m}}_{\mathrm{FeGd}}$, where $\widehat{y}$ is the in-plane direction perpendicular to the applied charge current. Because of this angular dependence, the spin-orbit torque arising from the anomalous Hall effect can be nonzero in a sample geometry for which the spin Hall torque generated by nonmagnetic materials is identically zero.

Two important goals for emerging spintronic applications exploiting current-induced magnetization switching are reducing the critical current to switch the magnetization and reducing or eliminating the need for an external in-plane magnetic field for deterministic magnetization reversal. Here, we present experimental studies of the heavy-metal-ferrimagnetic bilayer system, $\mathrm{W}/{\mathrm{Co}}_x{\mathrm{Tb}}_{1-x}$ characterized using magnetometry and anomalous Hall resistance measurements for temperatures ranging from 10 to 350 K. The current-induced-switching experiments are performed in the spin-orbit torque geometry where the current pulses are injected in plane and the magnetization reversal is detected by the measurement of the Hall resistance. The full magnetization reversal has been observed in all samples. Despite its large perpendicular magnetic anisotropy we find magnetic reversal for a strongly reduced in-plane magnetic field which is due to thermal contribution to switching. We find a characteristic switching temperature $T_{\text{switch}}$ induced by Joule heating which is above the magnetic, $T_{M\mathrm{comp}}$, and angular, $T_{A\mathrm{comp}}$, compensation temperatures but below its Curie temperature.

Elastic waves propagating in piezoelectric materials are accompanied by a time-varying electric potential, which is of critical importance for acoustoelectronic applications. The spatial mapping of such a potential at microwave frequencies is challenging since the characteristic length scale is determined by the acoustic wavelength of several micrometers. We report on the visualization of surface acoustic waves (SAWs) on ferroelectric samples by transmission-mode microwave impedance microscopy. The SAW potential launched by the interdigital transducer is detected by the tip and demodulated by the microwave electronics as time-independent spatial patterns. Wave phenomena such as interference and diffraction are imaged and the results are in excellent agreement with the theoretical analysis. Our work opens up an avenue to study various electromechanical systems in a spatially resolved manner.

We report on the conversion to telecommunication wavelengths of single photons emitted by a nitrogen-vacancy (NV) defect in diamond. By means of difference frequency generation, we convert spin-selective photons at 637 nm, associated with the coherent NV zero-phonon line, to the target wavelength of 1588 nm in the $L$-telecommunication band. The successful conversion is evidenced by time-resolved detection revealing a telecommunication-photon lifetime identical to that of the original 637-nm photon. Furthermore, we show by second-order correlation measurements that the single-photon statistics are preserved. The overall efficiency of this one-step conversion reaches 17% in our current setup, along with a signal-to-noise ratio of approximately 7 despite the low probability $(<{10}^{-3})$ of an incident 637-nm photon. This result shows the potential for efficient telecommunication-photon–NV-center interfaces and marks an important step towards future long-range entanglement-based quantum networks.

A photon-number-resolving transition edge sensor (TES) is used to measure the photon-number distribution of two microcavity lasers. The investigated devices are bimodal microlasers with similar emission intensity and photon statistics with respect to the photon autocorrelation. Both high-$\beta$ microlasers show partly thermal and partly coherent emission around the lasing threshold. For higher pump powers, the strong mode of microlaser $\mathit{A}$ emits Poissonian distributed photons, while the emission of the weak mode is thermal. By contrast, laser $\mathit{B}$ shows a bistability resulting in overlayed thermal and Poissonian distributions. While a standard Hanbury Brown and Twiss experiment cannot distinguish between the simple thermal emission of laser $\mathit{A}$ and the temporal mode switching of the bistable laser $\mathit{B}$, TESs allow us to measure the photon-number distribution, which provides important insight into the underlying emission processes. Indeed, our experimental data and their theoretical description by a master equation approach show that TESs are capable of revealing subtle effects like the mode switching of bimodal microlasers. As such, we clearly demonstrate the benefit and importance of investigating nanophotonic devices via photon-number-resolving transition edge sensors.

We propose a quantum algorithm for simulating spin models based on the periodic modulation of transmon qubits. Using the Floquet theory, we derive an effective time-averaged Hamiltonian, which is of the general $XYZ$ class, different from the isotropic $XY$ Hamiltonian typically realized by the physical setup. As an example, we provide a simple recipe to construct a transverse Ising Hamiltonian in the Floquet basis. For a 1D system, we demonstrate numerically the dynamical simulation of the transverse Ising Hamiltonian and quantum annealing to its ground state. We benchmark the Floquet approach with a digital simulation procedure and demonstrate that it is advantageous for limited resources and finite anharmonicity of the transmons. The described protocol represents a hardware-efficient quantum software and can serve as a simple yet reliable path towards configurable quantum simulators with currently existing superconducting chips.

The mechanical quality factor $Q_m$ in ferroelectric materials can be improved via acceptor doping to extend their application in high-power devices. It is generally believed that the improvement of $Q_m$ originates from the pinning effect of internal bias. However, the contributions from restricted spontaneous polarization ($P_S$) rotation and clamped domain-wall motion are difficult to separate, leading to some ambiguities about the effect of doping. We report on a strong anisotropic pinning effect in Mn-doped $0.27\mathrm{Pb}({\mathrm{In}}_{1/2}{\mathrm{Nb}}_{1/2}){\mathrm{O}}_3\text{−}0.46\mathrm{Pb}({\mathrm{Mg}}_{1/3}{\mathrm{Nb}}_{2/3}){\mathrm{O}}_3\text{−}0.27{\mathrm{PbTiO}}_3$ single crystals and investigate the related mechanism. We observe that the domain-wall motion is not clamped by the internal bias but are strongly pinned by charged defects at the charged domain walls, whereas the $P_S$ rotation is suppressed by the internal bias. We also confirm that the enhancement of longitudinal factor $Q_{33}$ is attributed mainly to the restricted polarization rotation, whereas the enhancement of shear factor $Q_{15}$ is ascribed to both the restricted polarization rotation and the clamped domain-wall motion. Our results reveal the fundamental mechanism of the acceptor doping effects in relaxor-based ferroelectric materials, and they also provide important design principles to achieve high-$Q_m$ piezoelectric materials to satisfy the demands of high-power electromechanical devices.

We propose a method for nondestructive characterization of progressive fatigue damage in solid plates, using a zero-group-velocity (ZGV) Lamb mode generated and detected by lasers. Our experimental results depict a nonmonotonous change in the ZGV mode frequency with an increasing number of loading cycles, and more importantly its drastic decrease prior to the specimen failure. We report three experiments on three specimens made of aluminum, which fail after different numbers of loading cycles. Despite this difference, the three nonmonotonous variations of the ZGV mode frequency with an increasing number of loading cycles are superimposed when plotted as a function of the normalized fatigue lifetime. This feature stresses out the potential of the technique to locate fatigue damage, to predict the fatigue lifetime, and to qualitatively, and even quantitatively, assess the different stages of fatigue damage in micrometer- to potentially centimeters-thick solid plates.

We study the magnetization reversal of $\mathrm{Co}\text{−}\mathrm{Fe}\text{−}\mathrm{B}/\mathrm{MgO}$ nanodots with perpendicular anisotropy of size ranging from $w=400\mathrm{nm}$ to $1\mu \mathrm{m}$. Contrary to previous experiments, the switching field distribution is shifted toward lower magnetic fields as the size of the elements is reduced with a mean switching field varying as $1/w$. We show that this mechanism can be explained by the nucleation of a pinned magnetic domain wall (DW) at the edges of the nanodots where damages are introduced by the patterning process. As the surface tension (Laplace pressure) applied on the DW increases when reducing the size of the nanodots, we demonstrate that to reverse the entire elements, the depinning field varies as $1/w$. These results suggest that the presence of DWs must be considered in the switching process of nanoscale elements and open a path toward scalable spintronic devices.

The spin-transfer-torque effect is predicted and subsequently demonstrated in a giant magnetoresistive (GMR) structure with current-perpendicular-to-plane geometry. In this work, we report that an unconventional out-of-plane magnetic field is created and exerts torque on a free magnet while in-plane current flows through the GMR sample. This strong effective magnetic field completely dominates the dampinglike torque which is expected to arise due to the anomalous Hall effect of a ferromagnet (analogous to the spin Hall effect in heavy metal). This anomalous magnetic field shows a very unusual angular dependence, which indicates broken-mirror symmetry in the lateral dimension. This anomalous torque may open an additional functionality for memory application.