We present two configurations for analog 0 to 2π optical phase modulation using liquid crystals (LCs), each of which achieve switching times that are 1 ms or less. One configuration is based on the switching behavior of a so-called nematic pi cell, and the other is based on the flexoelectro-optic effect in chiral nematic LCs when operated in the uniform lying helix geometry. Both configurations exploit a multipass optical arrangement to enhance the available optical phase range, while maintaining a fast switching speed. Moreover, these devices can be operated at or close to room temperature. Experimental data are found to be in good agreement with results predicted from theory for these multipass phase-modulation configurations.

In this work, we report the dynamic response of alternating-current-driven light-emitting diodes based on a range of hybrid halide perovskites. Electroluminescence (EL), which appears only in the positive section of a dc voltage, arises in both cycles under a sinusoidal ac voltage. Appearance of EL emission in negative cycles of the ac voltage is explained in terms of available residual charges injected in the previous positive cycle of the sinusoidal voltage; the relative intensity of EL emission in the negative cycle can be correlated to the ambipolarity of charge carriers in the ${\mathrm{CH}}_3{\mathrm{NH}}_3{\mathrm{Pb}\mathrm{Br}}_x{\mathrm{I}}_{3\text{−}x}$ series. At low frequencies of the ac voltage, the dynamic response of EL emission in any cycle is in phase with the applied ac voltage; interestingly, the EL emission starts to lag the sinusoidal voltage at higher frequencies. The phase lag also has a correlation to the ambipolarity of the active perovskite material. Finally, the frequency dependence of EL emission under an ac voltage provides the limiting frequency of device operation (−3 dB frequency) and thereby the effective carrier mobility in the active heterostructure.

The presence of in-gap states, especially deep ones, that act as recombination centers in solar-cell absorbers influence the phototransport properties of the films, and thus, strongly affect the performance of the corresponding solar cells. The type and density of such defects is influenced by the deposition process. In particular, for $\mathrm{Cu}({\mathrm{In}}_{1\text{−}x}{\mathrm{Ga}}_x){\mathrm{Se}}_2$, it is well established that the presence of a small amount of alkali metal atoms (e.g., $\mathrm{Na}$) improves the performance of the solar cell. Furthermore, in a coevaporation process, a $\mathrm{Cu}$-rich [[$\mathrm{Cu}$]/([$\mathrm{In}$] + [$\mathrm{Ga}$]) > 1] intermediate step decreases the density of extended structural defects, such as stacking faults. Here, we apply temperature-dependent and intensity-dependent photoconductivity measurements, along with Shockley-Read-Hall model calculations, to study the phototransport properties of $\mathrm{Cu}({\mathrm{In}}_{1\text{−}x}{\mathrm{Ga}}_x){\mathrm{Se}}_2$ films with and without a $\mathrm{Cu}$-rich process step, as well as with and without $\mathrm{Na}$. Our experimental and theoretical results indicate a correlation between the presence of planar defects and the formation of a shallow recombination level. Our results further suggest that $\mathrm{Na}$ eliminates deep recombination centers, and thus, increases the lifetime of the photogenerated charge carriers.

Anisotropic phonon transport is observed along different lattice directions in two-dimensional (2D) and layered materials. However, this effect vanishes in homogeneous, covalently bonded films, as the thickness is increased beyond few atomic layers. Here we establish a fundamental mechanism to induce anisotropic phonon transport in quasi-2D materials with in-plane isotropic symmetry. The anisotropy is engendered by the resonant modes of surface nanostructures that hybridize with membrane modes. Using atomistic lattice dynamics and classical molecular dynamics we demonstrate that the thermal conductivity of silicon membranes with surface nanofins is larger by approximately $100\mathrm{\%}$ parallel to the fins than in the perpendicular direction. The primary advantage of these configurations is that they would be technologically viable for implementation to existing and innovative material architectures. We anticipate that our results will open up alternative research directions to control phonons of technology-enabling nanomaterials for a broad range of applications, including thermal management in three-dimensional interconnected nanoelectronics, thermoelectric conversion to IR sensing.

Gradual loss of the entanglement of a twin beam containing around 25 photon pairs with the increasing external noise is experimentally investigated. The entanglement is quantified by the nonclassicality depths and the nonclassicality counting parameters related to several nonclassicality criteria. The reduction of intensity moments of the analyzed multimode twin beams to single-mode ones allows determination of the negativity as another quantifier of the entanglement. Both the raw photocount histograms and the reconstructed photon-number distributions are analyzed in parallel.

We present a comprehensive model to analyze quantitatively and predict the process of degradation of organic light-emitting diodes (OLEDs) considering all possible degradation mechanisms, i.e., polarons, excitons, exciton-polaron interactions, exciton-exciton interactions, and impurity effects. The loss of efficiency during degradation is presented as a function of quencher density. The density and generation mechanisms of quenchers are extracted using a voltage-rise model. The comprehensive model is applied to stable blue phosphorescent OLEDs, and the results show that the model describes the voltage rise and external-quantum-efficiency loss very well, and that the quenchers in the emitting layer (EML) are generated mainly by polaron-induced degradation of dopants. Quencher formation is confirmed by mass spectrometry. The polaron density per dopant molecule in the EML is reduced by controlling the emitter doping ratio, resulting in the highest reported lifetime ${\mathrm{LT}}_{50}$ of 431 h for an initial brightness of $500\mathrm{cd}/{\mathrm{m}}^2$ with a commission internationale de l'éclairage y coordinate less than 0.25, and a high external quantum efficiency greater than 18%.

We show that the light shift in atomic clocks can be suppressed using time variation of the interrogation field intensity. By measuring the clock output at two intensity levels, error signals can be generated that simultaneously stabilize a local oscillator to an atomic transition and correct for the shift of this transition caused by the interrogating optical field. These methods are suitable for optical clocks using one- and two-photon transitions, as well as for microwave clocks based on coherent population trapping or direct interrogation. The proposed methods can be widely used both for high-precision scientific instruments and for a wide range of commercial clocks, including chip-scale atomic clocks.

Recent empirical results using quantum-annealing hardware have shown that midanneal pausing has a surprisingly beneficial impact on the probability of finding the ground state for a variety of problems. A theoretical explanation of this phenomenon has thus far been lacking. Here we provide an analysis of pausing using a master-equation framework, and derive conditions for the strategy to result in a success probability enhancement. The conditions, which we identify through numerical simulations and then prove to be sufficient, require that relative to the pause duration the relaxation rate is large and decreasing right after crossing the minimum gap, small and decreasing at the end of the anneal, and is also cumulatively small over this interval, in the sense that the system does not thermally equilibrate. This establishes that the observed success probability enhancement can be attributed to incomplete quantum relaxation, i.e., is a form of beneficial nonequilibrium coupling to the environment.

Measurement-based feedback control is central in quantum computing and precise quantum control. Here we realize a fast and flexible field-programmable-gate-array-based feedback control in an Xmon- superconducting-qubit system. The latency of room-temperature electronics is custom-optimized to be as short as 140 ns. Projective measurement of a signal qubit produces a feedback tag to implement a conditional pulse gate on the qubit. In a feedforward process, the measurement-based feedback tag is brought to a different target qubit for conditional control. In a two-qubit experiment, the feedback and feedforward controls are simultaneously implemented in consecutive steps. A quantum number is then generated by the signal qubit, and a random walk of the target qubit is correspondingly triggered and realized on the Bloch sphere. Our experiment provides a conceptually simple and intuitive benchmark for feedback control in a multiqubit system. The feedback system can be further scaled up for more-complex feedback-control experiments.

We demonstrate that, by using low-energy positive muon (${\mu }^{+}$) spin spectroscopy as a local probe technique, the profiles of free charge carriers can be directly determined in the accumulation-depletion surface regions of $p$- or $n$-type $\mathrm{Ge}$ wafers. The detection of free holes is accomplished by measuring the effect of the interaction of the free carriers with the ${\mu }^{+}$ probe spin on the observable muon spin polarization. By tuning the energy of the low energy ${\mu }^{+}$ between 1 and 20 keV, the near-surface region between 10 and 160 nm is probed. We find hole carrier depletion and electron accumulation in all samples with doping concentrations up to the ${10}^{17}{\mathrm{cm}}^{-3}$ range, which is opposite to the properties of cleaved $\mathrm{Ge}$ surfaces. By illumination with light, the hole carrier density in the depletion zone can be manipulated in a controlled way. Depending on the used light wavelength $\lambda$, this change can be persistent ($\lambda =405,457$ nm) or nonpersistent ($\lambda =635$ nm) at temperatures less than $270$ K. This difference is attributed to the different kinetic energies of the photoelectrons. Photoelectrons generated by red light do not have sufficient energy to overcome a potential barrier at the surface to be trapped in empty surface acceptor states. Compared to standard macroscopic transport measurements our contactless local probe technique offers the possibility of measuring carrier depth profiles and manipulation directly. Our approach may provide important microscopic information on a nanometer scale in semiconductor device studies.

Current density distributions in active integrated circuits result in patterns of magnetic fields that contain structural and functional information about the integrated circuit. Magnetic fields pass through standard materials used by the semiconductor industry and provide a powerful means to fingerprint integrated-circuit activity for security and failure analysis applications. Here, we demonstrate high spatial resolution, wide field-of-view, vector magnetic field imaging of static magnetic field emanations from an integrated circuit in different active states using a quantum diamond microscope (QDM). The QDM employs a dense layer of fluorescent nitrogen-vacancy (N-$V$) quantum defects near the surface of a transparent diamond substrate placed on the integrated circuit to image magnetic fields. We show that QDM imaging achieves a resolution of approximately $10\mu \mathrm{m}$ simultaneously for all three vector magnetic field components over the $3.7\times 3.7{\mathrm{mm}}^2$ field of view of the diamond. We study activity arising from spatially dependent current flow in both intact and decapsulated field-programmable gate arrays, and find that QDM images can determine preprogrammed integrated-circuit active states with high fidelity using machine learning classification methods.

Magnetic skyrmions have attracted considerable interest, especially after their recent experimental demonstration at room temperature in multilayers. The robustness, nanoscale size, and nonvolatility of skyrmions have triggered a substantial amount of research on skyrmion-based low-power, ultradense nanocomputing and neuromorphic systems such as artificial synapses. Room-temperature operation is required to integrate skyrmionic synapses in practical future devices. Here, we numerically propose a nanoscale skyrmionic synapse composed of magnetic multilayers that enables room-temperature device operation tailored for optimal synaptic resolution. We demonstrate that, when embedding such multilayer skyrmionic synapses in a simple spiking neural network (SNN) with unsupervised learning via the spike-timing-dependent plasticity rule, we can achieve only approximately a $78\mathrm{\%}$ classification accuracy in the Modified National Institute of Standards and Technology handwritten data set under realistic conditions. We propose that this performance can be significantly improved to approximately $98.61\mathrm{\%}$ by using a deep SNN with supervised learning. Our results illustrate that the proposed skyrmionic synapse can be a potential candidate for future energy-efficient neuromorphic edge computing.

Injecting spin current from ferromagnetic metals into two-dimensional (2D) transition-metal dichalcogenide semiconductors is expected to trigger a revolution in the next generation of energy-efficient spintronic and valleytronic devices. By investigating the spin transport properties in perpendicularly magnetized ${\mathrm{Mo}\mathrm{S}}_2/\mathrm{Pt}/[\mathrm{Co}/\mathrm{Ni}{]}_2$ and ${\mathrm{Mo}\mathrm{S}}_2/\mathrm{Pt}/\mathrm{Ni}/[\mathrm{Co}/\mathrm{Ni}{]}_2$ heterostructures, here we demonstrate a strong and controllable spin injection into the 2D ${\mathrm{Mo}\mathrm{S}}_2$ layer through a nonmagnetic $\mathrm{Pt}$ layer. Based on optical dynamic measurements, noticeable changes in the magnetic damping constant show that the spin injection intensity can be controlled by the $\mathrm{Pt}$ layer thickness. The improved spin mixing conductance of the interface is much higher than those reported in $\mathrm{Pt}/\mathrm{Ni}$/permalloy (Py) systems with in-plane magnetic anisotropy. Our results shed lights on the development of next-generation low-power spintronic devices employing both perpendicular magnetic anisotropy and 2D material.

Vortex $\gamma$-ray beams with large orbital angular momentum (OAM) have significant potential applications in material science, nuclear detection, and astrophysics. An all-optical scheme for production of intense vortex $\gamma$-ray beams using relativistic laser-plasma interactions is proposed, where the OAM of $\gamma$-photons is converted from the spin angular momentum of circularly polarized (CP) lasers. The spin-to-orbital angular momentum conversion is achieved by two processes: resonant acceleration of electrons in CP laser-plasma interactions and nonlinear Compton scattering (NCS) of another linearly polarized laser from the former. Three-dimensional particle-in-cell simulations show that vortex $\gamma$-ray beams with a large and controllable OAM of $2.5\times {10}^{18}\hslash$ and energy up to hundreds of $\mathrm{MeV}$ can be obtained by using two lasers at intensities of approximately ${10}^{22}{\mathrm{W}/\mathrm{cm}}^2$, which is much more practical and more efficient than previously proposed NCS of a Laguerre-Gaussian beam.

We report an experimental study on the early detection of cascade flutter in a model aircraft turbine using a methodology that combines complex networks and synchronization. The network topology constructed from the determinism in the cross-recurrence plot allows us to extract the primary hubs during a transition to cascade flutter. The node strength in the network topology and the synchronization parameter are valid for capturing a precursor of cascade flutter.

In this work, we realize a physical system—nanoengineered singly connected Josephson junction with a periodic superparamagnetic $\mathrm{Ni}/\mathrm{Al}$ multilayer—that manifests supercurrent-versus-magnetic field response typical of a dc superconducting quantum interference device (SQUID); however, unlike a SQUID, which involves a superconducting loop occupying significant space, our lumped device is more suitable for miniaturization. In addition, we show that it exhibits enhanced magnetic field sensitivity as compared with conventional superconductor-insulator-superconductor Josephson junctions. SQUID-like oscillatory response to external magnetic fields, analogous to the two-slit optical interference, is explained in terms of the dominance of Andreev bound states localized at the barrier edges in the comparatively thick and strongly anisotropic weak links. Our results may lead to significant advancement in development of nanoscale magnetic sensing techniques applicable to individual molecules or magnetic nanoparticles.

The optimal amount of dysprosium in the highly magnetostrictive rare-earth compounds ${\mathrm{Tb}}_{1-x}{\mathrm{Dy}}_x{\mathrm{Fe}}_2$ for room-temperature applications has long been known to be $x=0.73$ (Terfenol-D). Here, we derive this value from first principles by calculating the easy magnetization direction and magnetostriction as a function of composition and temperature. We use crystal-field coefficients obtained within density-functional theory to construct phenomenological anisotropy and magnetoelastic constants. The temperature dependence of these constants is obtained from disordered-local-moment calculations of the rare-earth magnetic order parameter. Our calculations find the critical $\mathrm{Dy}$ concentration required to switch the magnetization direction at room temperature to be $x_c=0.78$, with magnetostrictions ${\lambda }_{111}=2700$ and ${\lambda }_{100}=-430$ ppm, close to the Terfenol-D values.

We study electric dipole spin resonance caused by subterahertz radiation in a multilevel finite-size quantum dot formed in a nanowire, focusing on the range of driving electric field amplitudes where a strong interplay between the Rabi spin oscillations and tunneling from the dot to continuum states can occur. A strong effect of the tunneling on the spin evolution in this regime occurs due to the formation of mixed spin states. As a result, the tunneling strongly limits possible spin manipulation times. We demonstrate a backaction of the spin dynamics on the tunneling and position of the electron. The analysis of the efficiency of the spin manipulation in terms of the system energy shows that tunneling decreases this efficiency. Fourier spectra of the time-dependent expectation value of the electron position show a strong effect of the spin-orbit coupling on their low-frequency components. This result can be applied to operational properties of spin-based nanodevices and can extend the range of possible spin resonance frequencies to the subterahertz domain.

The rapid development of active acoustic metamaterials provides an alternative degree of freedom to manipulate scattering fields and acoustic radiation forces. With the multipole expansion method, acoustic radiation forces can be expanded in a series of coupling between adjacent pairs of scattering-field coefficients. Here, systems from two dimensional to one dimensional are explicitly considered. When shining a single acoustic plane wave, a necessary requirement to achieve negative force is that the particle has to be sufficiently active. A criterion of achieving the most negative or positive force by tuning the adjacent terms “in phase” or “out of phase” is derived. Thus, with the required scattering coefficients, the effective density and modulus corresponding to the largest pulling-pushing forces can be retrieved directly and function as a guide in the design of active metamaterials in one-dimensional systems. Our study can bridge the theoretical requirements of radiation force with the practical design of metamaterials.

We investigate surface-acoustic-wave- (SAW) induced ferromagnetic-resonance- (FMR) assisted spin-transfer-torque (STT) switching of perpendicular magnetic tunnel junctions (PMTJs) with inhomogeneities using micromagnetic simulations that include the effect of thermal noise. With suitable frequency excitation, the SAW can induce ferromagnetic resonance in magnetostrictive materials, and the magnetization can precess in a cone with high deflection from the perpendicular direction. With incorporation of inhomogeneity via lateral anisotropy variation as well as room-temperature thermal noise, the magnetization precession in different gains can be significantly incoherent. Interestingly, the precession in different grains are found to be in phase, even though the precession amplitude (angle of deflection from the perpendicular direction) varies across grains of different anisotropy. Nevertheless, the high mean deflection angle due to acoustically induced FMR can complement the STT switching by reducing the STT current significantly; even though the applied stress induced change in anisotropy is much lower than the total anisotropy barrier. This work indicates that SAW-induced FMR-assisted switching can improve energy efficiency while being scalable to very small dimensions, which is technologically important for STT random-access memory and elucidates the physical mechanism for the potential robustness of this paradigm in realistic scenarios with thermal noise and material inhomogeneity.