Two-dimensional (2D) materials that exhibit ferroelectric, ferromagnetic, or topological order have been a major focal topic of nanomaterials research in recent years. The latest efforts in this field explore 2D quantum materials that host multiferroic or concurrent ferroic and topological order. We present a computational discovery of multiferroic state with coexisting ferroelectric and ferromagnetic order in recently synthesized ${\mathrm{CH}}_2\mathrm{O}{\mathrm{CH}}_3$-functionalized germanene. We show that an electric-field-induced rotation of the ligand ${\mathrm{CH}}_2\mathrm{O}{\mathrm{CH}}_3$ molecule can serve as the driving mechanism to switch the electric polarization of the ligand molecule, while unpassivated $\mathrm{Ge}$ $p_z$ orbits generate ferromagnetism. Our study also reveals coexisting ferroelectric and topological order in ligand-functionalized arsenene, which possesses a switchable electric polarization and a Dirac transport channel. These findings offer insights into the fundamental physics underlying these coexisting quantum orders and open avenues for achieving states of matter with multiferroic or ferroic-topological order in 2D-layered materials for innovative memory or logic device implementations.

We theoretically predict the upper limit of magnetic damping of the order of $1.0$ in ultrathin ${\mathrm{Co}}_2\mathrm{Mn}\mathrm{Si}$-based spin valves on the basis of a first-principles scattering theory; this is orders of magnitude larger than that in a bulky ferromagnet and a nobel-metal/ferromagnet interface. Huge angular dependence in spin-dependent scattering related to the interfacial resonant state should be responsible for the giant magnetic damping, which is sensitive to point defects, interfacial disorder, spin disorder, and thermal lattice disorder.

We report on a Stirling-cooled compact ${\mathrm{Bi}}_2{\mathrm{Sr}}_2\mathrm{Ca}{\mathrm{Cu}}_2{\mathrm{O}}_{8+\delta }$ intrinsic Josephson-junction stack with very high critical current density and improved cooling, operating at bath temperatures $T_b$ up to 86 K. The square stand-alone stack is embedded between two sapphire substrates. For bath temperatures between 27.8 and 86 K emission is observed at frequencies from 0.356 to 2.09 THz. The emission power exceeds $1\mu \mathrm{W}$ at bath temperatures between 60 and 80 K for emission frequencies between 0.5 and 0.88 THz. A record high value of 0.577 THz is obtained for the emission frequency at $T_b=80\mathrm{K}$, which is important for potential applications using liquid nitrogen as a coolant. We also compare our experimental results with numerical simulations based on three-dimensional coupled sine-Gordon equations combined with heat diffusion equations.

A neural-network computation scheme is proposed based on a perceptron model having processing units that consist of spin-wave-coupled spin-torque oscillators. This is an oscillatory neural network, where the relative phase of the oscillators is controlled by tuning the Dzylaoshinshkii-Moriya interaction and applying an oscillating magnetic field. Each processing unit receives an input signal that is an oscillating magnetic field and transmits alternating current. The alternating current is a function of the relative phase and generates an oscillating magnetic (Oersted) field around the wire through which the current flows. The generated Oersted field then becomes an input signal to the next processing unit. By solving the Landau-Lifshitz-Gilbert equation, we obtain an activation function of the processing unit. Finally, an artificial neural network is constructed using the obtained activation function to recognize the handwritten digits in the Modified National Institute of Standards and Technology database.

We investigate the lasing dynamics of optically-pumped submicrowatt-threshold Raman silicon lasers that employ a high-quality (high-Q) nanocavity design. The measurements reveal that free carriers generated by two-photon absorption induce dynamic effects during the initial lasing process, even at the very low threshold power of 0.12 µW. At higher excitation powers, the Raman laser signal exhibits a significant reduction within a few hundred nanoseconds after the initial rise, followed by clear oscillations. We find that the temporal behavior of the laser signal strongly depends on the excitation wavelength. However, the Raman laser signal converges to a stable continuous operation within a few microseconds after the initial rise for any excitation power employed in this work. Numerical simulations indicate that the oscillations reflect the dynamic shift of the resonant wavelength due to the thermo-optic effect and the carrier-plasma effect, which are induced by free carriers generated via two-photon absorption. These results are useful for understanding the correct design of future devices that employ Raman silicon nanocavity lasers.

Transient photovoltage and photocurrent responses of organic solar cells provide valuable information regarding device performance and charge-carrier dynamics. However, the techniques used for analyzing these data have only been shown to be valid in a limited set of circumstances. Here we make use of a quasi-three-dimensional model to simulate these measurements in order to quantitatively evaluate the validity of the parameters extracted. With the experimentalist in mind, we produce a set of criteria that can be easily tested and demonstrate a good degree of generality for different sample parameters. This approach allows us to explore the use of “large perturbation” conditions in order to increase the signal-to-noise ratio and speed of the measurement. We propose the introduction of an “effective fixed capacitance,” which is shown to significantly reduce the uncertainty associated with the deviation from the small-perturbation regime.

High-mobility field-effect transistors (FETs) can serve as resonant detectors of terahertz radiation due to excitation of plasmons in the channel. The modeling of these devices previously relied either on approximate techniques or on complex time-consuming full-wave simulations. In this paper, we obtain an exact solution for driven electrical oscillations in a plasmonic FET with realistic contact geometry. The solution obtained highlights the importance of evanescent plasma waves excited near the contacts, which qualitatively modify the detector responsivity spectra. We also derive the boundary condition on the ac floating electrodes that generalizes the open-circuit (Dyakonov-Shur) and short-circuit (fixed-voltage) conditions and reduces to them in limting cases. In both limits, the FET photovoltage has resonant fringes; however, the absolute value of the voltage is greater in the open-circuit case.

We present a device to continuously focus microparticles in a liquid-filled open channel subjected to lateral vibration at frequencies of the order of 10 Hz. The vibration generates a capillary wave at the liquid-air interface. This capillary wave leads to a net motion of microparticles over multiple cycles, causing collection under the displacement nodes of the capillary wave. These accumulated particles are observed as a concentrated stream in the presence of a continuous flow along the open channel, which means that the channel can be designed such that the focused particle stream exits through one outlet, while the filtrate is removed via interspaced outlets on each side of the particle stream. A numerical model is proposed, which superimposes the periodic flow field due to the capillary wave and the inlet-induced transverse flow field between the inlet and the outlet. The model is utilized to predict the smallest distance from the inlet at which the focused stream of particles is obtained, termed here the collection length. In addition, experiments are performed for different channel and inlet sizes, vibration actuation amplitudes, flow rates, and particle sizes. By considering the design factors extracted from the modeling data, we demonstrate that the resulting device is capable of continuous particle collection down to $1\mu \mathrm{m}$ diameter, at flow rates of up to 1.2 ml/min.

Synthetic antiferromagnets with high Curie temperature (approximately 290 K) are realized among all-perovskite ${\mathrm{La}}_{0.7}{\mathrm{Sr}}_{0.3}\mathrm{Mn}{\mathrm{O}}_3$ $(\mathrm{LSMO})/\mathrm{Ca}{\mathrm{Ru}}_{0.5}{\mathrm{Ti}}_{0.5}{\mathrm{O}}_3$ (CRTO) superlattices, which show antiferromagnetic (AF) interlayer exchange coupling (IEC) rather than traditional interfacial exchange coupling that is usually observed under the Curie temperature of the spacer. The system shows layer-resolved magnetic switching, resulting in sharp steplike hysteresis loops with magnetization plateaus depending on the repetition number of the stacking bilayers. This bilinear IEC can be easily changed to biquadratic IEC by change of orthorhombic $\mathrm{Nd}\mathrm{Ga}{\mathrm{O}}_3(001)$ substrate to cubic $(\mathrm{La}\mathrm{Al}{\mathrm{O}}_3{)}_{0.3}(\mathrm{Sr}{\mathrm{Al}}_{0.5}{\mathrm{Ta}}_{0.5}{\mathrm{O}}_3{)}_{0.7}(001)$ substrate. The strength of AF IEC increases with decrease of temperature, and the modeling results suggest that both magnetic LSMO and the CRTO space layer play a role in this behavior. Remarkably, the LSMO/CRTO system shows controllability of AF-IEC behavior under a moderate magnetic field of hundreds of oersteds. These observations may have potential applications in future oxide spintronic devices.

Nonlinear thermal transport can arise naturally in materials with strongly temperature-dependent thermal conductivities; however, this is exceedingly rare and weak. If a general strategy could be devised to yield nonlinear thermal transport, it would provide an avenue to controlling heat flow and realizing nonlinear thermal devices. Phase-change materials, which can exist in two states with distinct thermal conductivities, provide a unique opportunity to realize nonlinear thermal transport. In this work, we develop an analytical framework upon which we propose a material architecture for actualizing one type of nonlinear thermal transport, thermal rectification, where heat flux is biased in one direction. Our findings show that a heterojunction of two tunable phase-change materials can demonstrate strong thermal rectification. The magnitude of thermal rectification is analyzed as a function of the phase-change properties of each material, and we determine the fundamental heat-flux relations for each direction in these heterojunctions and criteria to separate the various phase situations that can occur. Finally, the analytical framework is applied to a junction comprising two phase-change materials, polyethylene and vanadium dioxide, and demonstrate a maximal theoretical rectification factor of approximately 140%. This analysis provides an important analytical tool in helping researchers design thermal circuits or advanced thermal energy storage media.

Nondegenerate two-photon absorption (ND TPA) is studied in a nanophotonic silicon waveguide in a configuration such that the dispersion of the nonlinear absorption and refraction cannot be ignored. It is shown that a signal wave around 1330 nm can be strongly absorbed by ND TPA with a pulse of 6-W peak power at 1850 nm. Experiments are performed with pulses of about 200 fs in a dispersion engineered waveguide in which the pump and signal pulses propagate at a similar group velocity. The experimental results are very well reproduced by numerical simulations of two coupled generalized nonlinear Schrödinger equations (GNLSEs). We show that the nonlinear dynamics can be well described by a single GNLSE despite the wavelength separation between the pump and the signal waves. We also demonstrate that in silicon nanophotonic waveguides, the dispersion of the nonlinear absorption is much larger than the dispersion of the Kerr effect. This could have an impact in the design of all-optical functions based on ND TPA, as well as on the study of supercontinuum and frequency-comb generation in integrated semiconductor-on-insulator platforms.

Both experiments and simulations show that the polarization state and propagation path of the Bloch surface waves sustained on a dielectric multilayer can be manipulated with the grooves inscribed on this multilayer. These grooves can be easily producible, accessible, and controllable. Various nanodevices for the Bloch surface waves, such as the launcher, beam splitter, reflector, polarization rotator, and even the photonic single-pole double-throw switch, are all experimentally realized with the properly designed grooves, which are consistent with the numerical simulations. The proposed devices will be basic elements for the two-dimensional photonic system and will find numerous applications, including integrated photonics, molecular sensing, imaging, and micromanipulation.

The alloy $\mathrm{Co}\text{−}\mathrm{Fe}\text{−}\mathrm{B}$ finds extensive application in spintronics, and in particular the perpendicular magnetic anisotropy characteristic of $\mathrm{Co}\text{−}\mathrm{Fe}\text{−}\mathrm{B}\text{−}\mathrm{Mg}\mathrm{O}$ systems is of great interest. While some efforts have been made to examine the effect of composition on magnetic properties of $\mathrm{Co}\text{−}\mathrm{Fe}\text{−}\mathrm{B}$ materials, the magnetic-property–composition relationship for the $\mathrm{Co}\text{−}\mathrm{Fe}\text{−}\mathrm{B}\text{−}\mathrm{Mg}\mathrm{O}$ system is still not fully understood. Therefore, it is fundamentally and practically important to understand the $\mathrm{Co}\text{−}\mathrm{Fe}\text{−}\mathrm{B}$ composition dependence of the magnetic properties of $\mathrm{Co}\text{−}\mathrm{Fe}\text{−}\mathrm{B}\text{−}\mathrm{Mg}\mathrm{O}$ systems. This work focuses on the data-storing free layer of double-barrier magnetic tunnel junctions with perpendicular magnetic anisotropy (PMTJs), which include $({\mathrm{Co}}_x{\mathrm{Fe}}_{1-x}{)}_{80}{\mathrm{B}}_{20}$ ultrathin films sandwiched between two $\mathrm{Mg}\mathrm{O}$ layers and a W insert layer. We study magnetic properties of various $({\mathrm{Co}}_x{\mathrm{Fe}}_{1-x}{)}_{80}{\mathrm{B}}_{20}$ compositions at different annealing conditions and find x ∼ 35% to have the highest anisotropy energy to ensure high thermal stability, while maintaining the lowest Gilbert damping parameter which is essential to achieve a low critical switching current. This composition also shows the highest thermal stability at elevated temperatures. In addition, we perform first-principle calculations to explain the anomalous composition trends of the magnetization and Gilbert damping parameter. Moreover, we find that the conventional Slater-Pauling curve is not applicable, and it is necessary to consider the magnetization’s dependence on the magnetic anisotropy which in turn depends on $({\mathrm{Co}}_x{\mathrm{Fe}}_{1-x}{)}_{80}{\mathrm{B}}_{20}$ composition and the oxide interface. Our results provide a perspective for a better understanding of metal-oxide systems with desirable properties for DBL PMTJ applications.

Achieving exact balance between spatially separated gain and loss is generally regarded as a necessary condition for parity-time- (PT-)symmetric optical systems. We introduce generalized PT- (GPT-)symmetric optical structures, which have an asymmetric and unbalanced gain-loss profile, while exhibiting similar scattering properties and phase transitions as their traditional PT-symmetric counterparts. In particular, we show that the concept of GPT symmetry may help to reduce the threshold gain in achieving newly discovered PT-enabled applications, such as the coherent perfect absorber laser and exceptional points. The concept proposed herein will facilitate the practice of PT-symmetric optical and photonic devices by offering greater design flexibility to tailor gain-loss profiles and their thresholds.

We study spin-orbit-torque-driven ferromagnetic resonance (FMR) in ferromagnetic (FM) bilayers, consisting of $\mathrm{Co}$ and permalloy ($\mathrm{Py}$) sandwiched between $\mathrm{Pt}$ and $\mathrm{Mg}\mathrm{O}$ layers. We find that the FM layer in contact with the $\mathrm{Pt}$ layer dominantly determines the spin Hall angle, which is consistent with the spin-transparency model. By contrast, the FMR linewidths are considerably influenced not only by the spin-pumping effect across the $\mathrm{Pt}/\mathrm{FM}$ interface but also by the spin relaxation such as two-magnon scattering at the $\mathrm{FM}/\mathrm{Mg}\mathrm{O}$ interface. The $\mathrm{Co}/\mathrm{Mg}\mathrm{O}$ interface leads to notably increased FMR linewidths, while the $\mathrm{Py}/\mathrm{Mg}\mathrm{O}$ interface has less effect. The different contributions of each interface to the spin Hall angle and dissipation parameter suggest that the stack configuration of $\mathrm{Pt}/\mathrm{Co}/\mathrm{Py}/\mathrm{Mg}\mathrm{O}$ requires less writing energy than $\mathrm{Pt}/\mathrm{Py}/\mathrm{Co}/\mathrm{Mg}\mathrm{O}$ in spin-orbit-torque-driven magnetic switching. Our approach offers a practical method to optimize material parameters by engineering either interfaces in contact with the heavy metal or the oxide layer.

Inserting graphene into van der Waals (vdW) heterostructures (HTSs) may result in unprecedented opportunities to design high-performance nanodevices, although the underlying physical mechanisms are unclear. Here, we present theoretical studies on electronic structures, carrier concentrations, and electronic transport of the $\mathrm{Mo}{\mathrm{S}}_2/\mathrm{graphene}/\mathrm{W}{X}_2$ ($X=\mathrm{S},\mathrm{Se}$) vdW HTSs. The studies show that the $\mathrm{Mo}{\mathrm{S}}_2/\mathrm{graphene}/\mathrm{W}{X}_2$ sandwiched systems have the desired characteristics of highly thermal stability to at least 500 K, staggered band alignments, and intact Dirac cone. In particular, the transport currents of the $\mathrm{Mo}{\mathrm{S}}_2/\mathrm{graphene}/\mathrm{W}{X}_2$ HTS are two orders of magnitude higher than that of the $\mathrm{Mo}{\mathrm{S}}_2/\mathrm{W}{X}_2$ HTS, which indicates that the insertion of graphene obviously enhances the transport properties. Moreover, the external electric field can induce p-type doping of graphene and remarkably enhance the carrier concentration from approximately ${10}^{11}$ to approximately ${10}^{13}{\mathrm{cm}}^{-2}$. The results reveal the physical nature of $\mathrm{Mo}{\mathrm{S}}_2/\mathrm{graphene}/\mathrm{W}{X}_2$ vdW HTSs, which are helpful in providing a route to design two-dimensional semiconductor-based high-performance optoelectronic devices.

We report a mode-locked laser design using $\mathrm{Yb}$-doped fiber lasers for low-noise frequency-comb generation. The frequency comb covers the spectral range from 700 to 1400 nm. Although this range is more practical for many measurements than that produced by the more commonly used Er-fiber lasers, it has been addressed in only a handful of reports, mainly due to the difficulty of generating a fully coherent supercontinuum at $1\mu \mathrm{m}$. We overcome this difficulty by a tailored design of the mode-locking dynamics that succeeds in generating energetic 33-fs-long pulses without even using higher-order-dispersion compensation, while ensuring that the laser operates with net zero cavity dispersion for low-noise supercontinuum generation. After locking to a $\mathrm{Cs}$ atomic clock, this frequency comb is used for absolute-frequency measurements of a $\mathrm{Nd}:\mathrm{YAG}\text{−}{\mathrm{I}}_2$ laser to verify its accuracy by comparison with results from the International Committee for Weights and Measures. After this verification, it is further used to measure the absolute frequency of a 543-nm two-mode stabilized He-Ne laser, which is routinely used for length measurements in our institute, thus verifying its practical utility in metrology applications. The entire setup is built with readily available components for easy duplication by other researchers.

$\mathrm{Ce}$-activated $A^{2+}{B}^{4+}{\mathrm{O}}_3$ perovskites represent a class of compounds currently under active exploration for their potential as scintillators. Depending on the chemistry and synthesis conditions, perovskites can crystallize in multiple crystal structures, and a $\mathrm{Ce}$ substitutional dopant in an $AB{\mathrm{O}}_3$ perovskite can adopt different charge states (i.e., ${\mathrm{Ce}}^{3+}$ or ${\mathrm{Ce}}^{4+}$) as well as different substitutional sites (namely, the 12-fold-coordinated $A$ site or the octahedrally coordinated $B$ site). Here, we use first-principles density-functional-theory- and hybrid-functional-based computations to study relative trends in the structure, energetics, and electronic structure of bulk $AB{\mathrm{O}}_3$ perovskites, where $A=\mathrm{Ca},\mathrm{Sr}$, or $\mathrm{Ba}$ and $B=\mathrm{Hf}$ or $\mathrm{Zr}$. Subsequently, we consider the relative energetics of preferential solution sites for $\mathrm{Ce}$ as a function of charge states, chemical potential, and defect configurations. Our results reveal that while ${\mathrm{Ce}}^{3+}$ or ${\mathrm{Ce}}^{4+}$ defects can be thermodynamically stable, depending on the choice of the substitutional site and synthesis conditions (i.e., prevailing chemical potential), only ${\mathrm{Ce}}^{3+}$ dopant at the $A$ site leads to an electronic structure that can exhibit scintillation. Our comparative analysis shows that while the positions of the $5d^1$ and $4f$ levels of ${\mathrm{Ce}}^{3+}$ as a dopant at the $A$ site are favorably placed in the band structure, these levels are consistently higher for the ${\mathrm{Ce}}^{4+}$ charge state and are unlikely to manifest any luminescence. The findings of this study are also discussed in relation to previously reported results and display excellent agreement with past experimental observations. In general, it is demonstrated that control of the $\mathrm{Ce}$ charge state and local chemical environment can be used—in addition to band-gap and band-edge engineering—to manipulate the relative position of scintillating states with respect to the valence-band maximum and conduction-band minimum. While this study specifically focuses on perovskites, the results (in particular, the relative alignment of the positions of the $5d^1$ and $4f$ levels of $\mathrm{Ce}$ dopant as a function of the activator’s charge state) are expected to be general and thus transferable to other chemistries.

Driven in part by the extreme requirements of gravitational-wave detection, recent research about interferometric phase estimation has focused on the use of nonclassical states of light in order to beat the shot-noise limit, allowing further improvements in sensitivity once the detected optical power has been maximized. Still, for a large class of interferometric applications, the efficient use of these states remains prohibitively complicated. Seeking simple improvements to instruments illuminated by classical laser light, we revisit the passive or unmodulated Mach-Zehnder interferometer by structuring the estimation problem in the form of a linear model for which an unbiased estimator attaining the Cramér-Rao bound is easily computed. This paper compares the performance of such linear and efficient estimators for several detection schemes used with two-output and four-output (in-phase and quadrature) interferometers. We find that the independent monitoring of all available output ports leads to an overall sensitivity that is, in most cases, better than that of balanced detection and single-output detection. In addition, it allows the cancellation of the technical amplitude noise contribution at all operating phases without resorting to modulation techniques. This seldom used but simple detection scheme should therefore be considered when designing instruments for operation close to and perhaps below the shot-noise limit.

This paper proposes a 2D single-point-fed metamaterial-like transmission-line- (TL-)grid antenna design that achieves extremely high aperture efficiency levels. Suitable unloaded transmission-line grids exhibit Dirac dispersion cones at the $\mathrm{\Gamma }$ point when accidental degeneracy of the three eigenmodes of interest is achieved, similar to the case of photonic crystals. The proposed antenna design employs a particular $\mathrm{\Gamma }$-point eigenmode of such unloaded transmission-line grids that is shown to be symmetric with respect to the unit cell and is invariant to the direction of wave propagation. The design is scalable to millimeter-wave and terahertz frequencies as it is simply a grid of interconnected meandered transmission lines. The TL-grid structure is center fed from the back using a coaxial cable and is operated as a standing-wave antenna. The entire structure is also of extremely low profile and printed using microstrip technology. The proposed antenna is shown to be versatile in the sense that it achieves this high aperture efficiency for various array sizes, providing customized levels of gain. The proposed antenna generates a broadside pencil beam with extremely high aperture efficiency, low sidelobe levels along the principle planes, low cross-polarization levels and an almost circularly symmetric beamwidth. The generated beam is maintained over a reasonable bandwidth and does not squint or split with frequency. The design is fabricated and experimentally tested confirming the high level of aperture efficiency achieved and proving the successful excitation of the symmetric eigenmode at the $\mathrm{\Gamma }$ point.