The figures-of-merit for reservoir computing (RC), using spintronics devices called magnetic tunnel junctions (MTJs), are evaluated. RC is a type of recurrent neural network (RNN). The input information is stored in certain parts of the reservoir and computation can be performed by optimizing a linear transform matrix for the output. While all the network characteristics should be controlled in a general RNN, such optimization is not necessary for RC. The reservoir only has to possess a nonlinear response with memory effect. In this paper, macromagnetic simulation is conducted for the spin dynamics in MTJs for RC. It is determined that the MTJ system possesses the memory effect and nonlinearity required for RC. With RC using 5–7 MTJs, high performance can be obtained, similar to an echo-state network with 20–30 nodes, even if there are no magnetic and/or electrical interactions between the magnetizations.

Photon recycling has been proven to be an important process in metal halide perovskite thin films. We propose a model of photon recycling based on the photon diffusion concept that can interpret experimental observations on the photoluminescence and transient spectroscopy in perovskite layers. Our model shows how the photon recycling can enhance the apparent photocarrier lifetime and thereby slow down the photoluminescence decay dynamics. We also discuss the interplay between photon recycling and the photocarriers diffusive transport and demonstrate that at high carrier concentrations, photon recycling dominates the diffusion, conveying the carriers inside the absorbing layer over long distances. We also provide a quantitative determination of a diffusion length that is considerably expanded by the photon recycling process. The effect of the photon recycling process on the Auger recombination rate is also addressed here. The results provide insights for both interpreting the experimental observations and also designing experimental schemes.

For a conventional higher-order Poincaré (HOP) sphere beam that generalizes optical vortices and vector beams, its donutlike intensity profile varies significantly with topological charges, which is undesirable for many applications in optical manipulations and communications. Recently, the perfect optical vortex and perfect vector beam have been reported to provide a solution. Until now, this strategy has not been extended to the whole HOP sphere. In this work, the concept of a perfect HOP sphere is proposed for the first time and realized by digitalizing the spiral geometric phase with specific structures of circular Dammann gratings (CDGs), which are demonstrated in liquid crystals through dynamic photo-patterning. Via selecting the incident spin, any point on the perfect HOP sphere can be obtained. Properly programming CDGs makes both single- and multi-ringed perfect HOP sphere beams achievable with merits of high efficiency and uniform energy distribution. All experimental results are qualitatively consistent with the theoretical calculations. This integrated liquid crystal device paves the way for perfect optical angular momentum manipulation and facilitates numerous cutting-edge applications.

Nonlocal (spatial-dispersion) effects in multilayered metamaterials composed of periodic stacks of alternating, deeply subwavelength dielectric layers are known to be negligibly weak. Counterintuitively, under certain critical conditions, weak nonlocality may build up strong boundary effects that are not captured by conventional (local) effective-medium models based on simple mixing formulas. Here we show that this phenomenon can be fruitfully studied and understood in terms of error propagation in the iterated maps of the trace and antitrace of the optical transfer matrix of the multilayer. Our approach effectively parameterizes these peculiar effects via remarkably simple and insightful closed-form expressions, which enable direct identification of the critical parameters and regimes. We also show how these boundary effects can be captured by suitable nonlocal corrections.

We introduce a concept to generate spin-polarized currents in graphene nanoribbons by using light radiation and a significant electron-electron interaction. For this purpose, we design a phototransistor based on the sawtoothlike graphene nanoribbons (SGNRs). The structures, with their intrinsic magnetic moments, have a narrow spin-dependent band around their Fermi energy that can enhance interband transitions and produce a spin photocurrent at room temperature without applying any magnetic field and without using any element. Interestingly, the changes in the size parameters and the gate voltage modify the magnitude and position of optical absorption peaks and optical spin polarization and the gate voltage individually switches the sign of the spin photocurrent. Finally, the fully spin-polarized photocurrent, the high quantum efficiency with a maximum of approximately 40%, the wide-wavelength-range operation from ultraviolet to infrared and optical spin-filtering effects, that are tunable with size and gate voltage, pave the way toward the improved design and performance of spin-optoelectronic devices based only on carbon atoms.

The processes of hole localization in ${\mathrm{Y}}_3{\mathrm{Al}}_5{\mathrm{O}}_{12}$ and ${\mathrm{Lu}}_3{\mathrm{Al}}_5{\mathrm{O}}_{12}$ single crystals are investigated by electron paramagnetic resonance (EPR) and thermally stimulated luminescence (TSL). It is found that holes created by x-ray irradiation at 77 K are predominantly self-trapped at regular oxygen ions forming an ${\mathrm{O}}^{-}$ hole center. This self-trapped hole (STH) center is thermally stable to about 100 K in both yttrium aluminum garnet ${\mathrm{Y}}_3{\mathrm{Al}}_5{\mathrm{O}}_{12}$ (YAG) and lutetium ${\mathrm{Lu}}_3{\mathrm{Al}}_5{\mathrm{O}}_{12}$ garnet ($\mathrm{Lu}\mathrm{AG}$) crystals. At higher temperatures, thermally liberated holes are retrapped at oxygen ions in the vicinity of an acceptor ion such as ${\mathrm{Mg}}^{2+}$ and ${\mathrm{Al}}_{\mathrm{Y}}$ or ${\mathrm{Al}}_{\mathrm{Lu}}$ antisite ions which leads to an increase in the thermal stability of the trapped hole approximately equal to 150 K. TSL measurements show two composite glow peaks in the temperature range of 77–280 K, the temperature positions of which correlate well with the thermal stability of the ${\mathrm{O}}^{-}$ centers. The hole thermal ionization energy is determined from a numerical fit of the TSL peaks within the model of second-order kinetics. It is in the range of 0.25–0.26 eV for an ${\mathrm{O}}^{-}$ STH center, and increases to 0.41–0.45 eV for an ${\mathrm{O}}^{-}$ center stabilized by the acceptor. Revealed ${\mathrm{O}}^{-}$ centers can be attributed to ${\mathrm{O}}^{-}$ small polarons formed mainly due to hole stabilization by short-range interactions with the surrounding lattice. Knowledge of the nature of color centers and related trapping sites is critically important for further optimization of all garnet scintillation materials.

We explore pumping and digitization of two-phase flow patterns inside a microchannel with the help of a unidirectional and oscillating Lorentz force. For this purpose, an electric field has been coupled with an oscillating (or unidirectional) magnetic field to generate a sinusoidal (or unidirectional) Lorentz force in the channel filled with a pair of Newtonian, immiscible, and electrically conducting fluids. Application of the steady or oscillating Lorentz force is found to enhance the throughput of a pressure-driven flow in conjunction with the mixing of the phases by creating discrete and miniaturized flow structures. Numerical simulations show that the application of Lorentz force in an oil-water stratified flow leads to the digitization of the flow patterns together with enhanced transport due to the magnetohydrodynamic pumping of the fluids. The size and frequency of the flow patterns and the throughput of the flow can be noninvasively altered by tuning the intensity of the electric or magnetic field, frequency of the magnetic field, and fluid properties. An oscillatory Lorentz force with periodic change in direction can lead to time-periodic forward and backward motions of the fluids to prompt a unique reciprocating motion of the flow features while they translate along the channel. The oscillation frequency of some of the flow features is found to follow a linear correlation with the frequency of the magnetic field suitable for pumping applications. The proposed pumping and digitization strategies can be of significance in the design and development of next-generation microscale reactors, mixers, pumps, and microelectromechanical systems (MEMS) devices.

The picosecond dynamics in a $\mathrm{Ga}\mathrm{As}$ solar cell is studied using time-resolved photoemission spectroscopy. At a pump fluence of $0.081\mu {\mathrm{J}/\mathrm{cm}}^2$, photovoltage builds up in approximately 23 ps. The subsequent decay of the voltage is found to occur over 7 ns. These quantities reflect the time scales of carrier transport and recombination, both of which are critical factors for the conversion efficiencies of solar cells. We demonstrate that the on-site properties of solar cells relevant to conversion efficiency are directly accessible with picosecond temporal resolution using the photoemission technique. Application of the present technique will open a new route in the pursuit of optimal designs for solar-cell structures.

Cleaning pollution from a heterogeneous flow environment is far from simple. We consider the flow capture problem, which has flows and sinks in a heterogeneous environment, and investigate the problem of positioning pollutant capture units. We show that arrays of capture units carry a high risk of failure without accounting for environmental heterogeneity and chaos in their placement, design, and operation. Our idealized two-dimensional models reveal salient features of the problem. Maximum capture efficiency depends on the required capture rate: long-term efficiency decreases as the number of capture units increases, whereas short-term efficiency increases. If efficiency is important, the capture process should begin as early as feasible. Knowledge of transport controlling flow structures offers predictability for unit placement. We demonstrate two heuristic approaches to near-optimally position capture units.

Magnetic resonance coupling theory for connected metamaterial resonators is studied and experimentally demonstrated. The subwavelength resonators have dominantly conductive coupling through the current exchange. Two distinct interactions are observed in two different connection configurations. Based on the strong coupling, one-dimensional localized surface magnetic modes propagating along a chain of subwavelength resonators are proposed. By changing the connection configuration, the surface wave can be switched between the forward and backward modes. The numerical simulations and experiments are in good agreement. The proposed mechanism of energy transport in the subwavelength scale has potential applications in integrated devices and circuits.

We suggest an ultrasensitive detector of electromagnetic fields exploiting the giant thermoelectric effect recently found in superconductor-ferromagnet hybrid structures. Compared with other types of superconducting detectors where the detected signal is based on variations of the detector impedance, the thermoelectric detector has the advantage of requiring no external driving fields. This is especially relevant in multipixel detectors, where the number of bias lines and the heating induced by them are an issue. We propose different material combinations to implement the detector and provide a detailed analysis of its sensitivity and speed. In particular, we perform a proper noise analysis that includes the cross correlation between heat and charge current noise and thereby describes also thermoelectric detectors with a large thermoelectric figure of merit.

Spin-orbit torque (SOT) switching is an energy-efficient method to control magnetization in magnetic memory devices. However, deterministically switching perpendicular memory bits usually requires the application of an additional bias field to break lateral symmetry. Here we present a field-free deterministic perpendicular switching approach using a strain-mediated SOT switching method. The strain-induced magnetoelastic anisotropy breaks the lateral symmetry, and the resulting symmetry breaking is controllable. A finite-element model and a macrospin model are used to numerically simulate the strain-mediated SOT switching mechanism. The results show that a relatively small voltage (±0.5 V) along with a current ($5\times {10}^7\mathrm{A}/{\mathrm{cm}}^2$) can produce a 180° perpendicular magnetization reversal. The switching direction (“up” or “down”) is dictated by the voltage polarity (positive or negative) applied to the piezoelectric layer in the magnetoelastic/heavy metal/piezoelectric heterostructure. The switching speed can be as fast as 10 GHz. More importantly, this control mechanism can be potentially implemented in a magnetic random-access memory system with a small footprint, high endurance, and high tunnel magnetoresistance readout ratio.

A simple device based on the principle of frustrated total internal reflection is used to image the regions of contact between rubber objects and a large-area perspex waveguide. Measurements of the intensity of light scattered at the interface are found to depend upon the magnitude of the applied force, the mechanical properties of the contacting material, and the roughness of the contacting objects. The intensity-force response is found to have the same functional dependence irrespective of the position on the waveguide surface, but to scale by an amount that is proportional to the local intensity of light incident on the perspex-object interface. Once this spatial variation in intensity is calibrated, the waveguide can be used to perform optical measurements of the forces/pressures exerted on the surface of the waveguide and to generate spatial maps of the pressure at frame rates up to 200 Hz. The resulting optical force platform is used to measure the time-dependent evolution of the pressure distribution beneath a foot and a sports shoe during a foot-strike event. A simple theory is developed to describe the light-scattering phenomenon and to explain the relationship between the scattered light intensity and the applied force.

We describe and implement a family of entangling gates activated by radio-frequency flux modulation applied to a tunable transmon that is statically coupled to a neighboring transmon. The effect of this modulation is the resonant exchange of photons directly between levels of the two-transmon system, obviating the need for mediating qubits or resonator modes and allowing for the full utilization of all qubits in a scalable architecture. The resonance condition is selective in both the frequency and amplitude of modulation and thus alleviates frequency crowding. We demonstrate the use of three such resonances to produce entangling gates that enable universal quantum computation: one $i\mathrm{SWAP}$ gate and two distinct controlled-Z gates. We report interleaved randomized benchmarking results indicating gate error rates of 6% for the $i\mathrm{SWAP}$ (duration 135 ns) and 9% for the controlled-Z gates (durations 175 and 270 ns), limited largely by qubit coherence.

Confocal micro-Raman microscopy performed in the transparent spectral region of a semiconductor can, in principle, be used for operando three-dimensional (3D) thermometry with optical diffraction-limit spatial resolution. However, when applied to high-power $\mathrm{Ga}\mathrm{N}$-based light-emitting diodes (LEDs), the applicability is hindered by the often strong secondary electroluminescence in the visible spectral region that overwhelms the Raman signal. We develop a “split-time-window” scheme that can mimic the continuous wave operation but without the interference of the secondary emission, which allows us to carry out noninvasive 3D temperature profiling and comprehensive thermal analyses of the whole device at any operation current. The technique is applied to an $({\mathrm{In}}_x{\mathrm{Ga}}_{1-x})\mathrm{N}/\mathrm{GaN}$ LED to extract its 3D temperature distribution when operated at 350 mA with µm-scale resolution when using a 532-nm laser. We show that although a conventional technique can yield a reliable average temperature difference between the heat sink and the LED junction (a few degrees), the spatial fluctuations are much larger than the average difference. Furthermore, we show that using the anti-Stokes to Stokes Raman intensity ratio as a metric can yield more reliable and accurate results than using the Raman frequency shift.

There has been tremendous interest in manipulating electron and hole spin states in low-dimensional structures for electronic and spintronic applications. By first-principles calculations we study the edge magnetic anisotropy and exchange coupling in zigzag nanoribbons of stanene, a graphenelike honeycomb material with strong spin-orbit coupling. We find that the zigzag stanene nanoribbon is always insulating and its band gap depends on the magnetization direction and the interedge magnetic coupling. Moreover, we propose a strategy whereby both the magnetic anisotropy and the exchange coupling are controlled by electron and hole doping, demonstrating an experimentally feasible gate-induced modulation of the magnetization. Another interesting finding is that for distinct magnetizations, spin or valley degeneracies are broken under the action of the spin-orbit coupling and further tuned by a transverse electric field, giving full play to spin and valley degrees of freedom. These intriguing features offer a practical avenue for designing energy-efficient devices based on magnetoelectric couplings and multiple electronic degrees of freedom.

We theoretically and numerically investigate the switching of perpendicular magnetization by ac spin-orbit torque. We find that the threshold switching current for ac spin-orbit torque is much smaller than that for dc spin-orbit torque. Moreover, the ac spin-orbit torque acts like an effective out-of-plane field in the rotating frame and thus allows field-free deterministic switching of perpendicular magnetization. The ac spin-orbit torque scheme will be useful for energy-efficient operation of spin-orbit torque devices.

Materials with tunable thermal conductivity play a critical role in advanced thermal management, including building and vehicle heating and cooling, solar energy harvesting, and energy storage. 2D materials provide a platform for controlling heat transport via surface modification. Using density functional theory and the Boltzmann transport equation for phonons, we demonstrate the ability to tune thermal conductivity in a 2D material ${\mathrm{C}}_3\mathrm{N}$ by adding and removing $\mathrm{C}$ atoms: (1) although ${\mathrm{C}}_3\mathrm{N}$ is even stiffer than graphene, its lattice thermal conductivity at 300 K is nearly ten times lower (482 W/mK); (2) by adding $\mathrm{C}$ atoms, the formation of a more complex dumbbell structure results in more tortuous paths for phonons, but counterintuitively, the room-temperature thermal conductivity increases; (3) by removing $\mathrm{C}$ atoms, the thermal conductivity at 300 K further reduces to 34 W/mK. Moreover, ${\mathrm{C}}_3\mathrm{N}$ shows much lower thermal conductivity than the dumbbell structures at low temperature but much higher thermal conductivity at high temperature, rendering ${\mathrm{C}}_3\mathrm{N}$ and its derivatives highly promising for various applications at ambient temperature. All these distinct behaviors are related to the localized out-of-plane phonon modes that rely on chemical bonding. Our work provides an intuitive understanding of how structure influences heat transport and demonstrates the possibility to control heat transport precisely via structural modification.

Exceptional points (EPs) are non-Hermitian degeneracies that can enable various intriguing applications such as enhanced sensing and asymmetric mode switching. Here, we theoretically propose the design of a 1 × 2 terahertz switch by employing the topological structure of the energy surfaces (i.e., eigenvalue distributions in a two-variable parameter space) near an EP. The switch consists of a pair of terahertz waveguides, one of which has a monolayer graphene laid on the bottom. The wave transmission through the system can be understood by considering an adiabatic trajectory on self-intersecting Riemann sheets due to the existence of an EP. The trajectory can be tuned by changing the chemical potential of graphene, and as a result, we can choose in which waveguide the power exits to achieve the switching functionality. The physics behind the switch lies in the fact that the trajectories on different sides of the EP lead to different final states due to the unique topological structure of energy surfaces near the EP. We perform numerical simulations to optimize the performance of the terahertz switch.

We demonstrate a vector magnetometer that simultaneously measures all Cartesian components of a dynamic magnetic field using an ensemble of nitrogen-vacancy (NV) centers in a single-crystal diamond. Optical NV-diamond measurements provide high-sensitivity, broadband magnetometry under ambient or extreme physical conditions and the fixed crystallographic axes inherent to this solid-state system enable vector sensing free from heading errors. In the present device, multichannel lock-in detection extracts the magnetic-field-dependent spin-resonance shifts of NVs oriented along all four tetrahedral diamond axes from the optical signal measured on a single detector. The sensor operates from near dc (5 Hz) up to a 12.5-kHz measurement bandwidth and simultaneously achieves approximately $50\mathrm{pT}/\sqrt{\mathrm{Hz}}$ magnetic-field sensitivity for each Cartesian component, which is, to date, the highest demonstrated sensitivity of a full vector magnetometer employing solid-state spins. Compared to optimized devices interrogating the four NV orientations sequentially, the simultaneous vector magnetometer enables a $4\times$ measurement speedup. This technique can be extended to pulsed-type sensing protocols and parallel wide-field magnetic imaging.