Polarization conversion has been widely explored in the study of metasurfaces in order to achieve broad operation bandwidths, but its application is still restricted to a few practical scenarios. We design a broadband cross-polarization conversion metasurface and apply it to form a directionally radiated Cassegrain antenna with a low profile and high polarization purity. A quasi-I-shaped meta-atom is first optimized with both high cross-polarization conversion efficiency and a full span of phase modulation in a broad bandwidth. With a hyperbolic phase profile imposed on the surface, a metasurface is engineered that can conduct both focusing and polarization conversion into its cross-polarization component. Then, by introducing a polarized grating as a subreflector and simultaneously fixing the designed metasurface as the main reflector, a planar Cassegrain antenna is facilitated with a low profile, high gain, and high polarization purity in a broad frequency band. This strategy of broad-band polarization conversion and its attempt to design a Cassegrain antenna lays a foundation for high-performance functional devices, promising great potential in real-world applications.

Periodic hole-array phononic crystals (PnCs) can strongly modify phonon dispersion relations and have been shown to influence thermal conductance coherently, especially at low temperatures where bulk scattering is suppressed. One very important parameter influencing this effect is the period of the structure. Here, we measure the subkelvin thermal conductance of nanofabricated PnCs with identical hole-filling factors but three different periodicities, of 4, 8, and $16\mu \mathrm{m}$, using superconducting tunnel-junction thermometry. We find that all the measured samples can suppress thermal conductance by an order of magnitude and have a lower thermal conductance than the previously measured smaller-period 1-$\mu \mathrm{m}$ and 2.4-$\mu \mathrm{m}$ structures. The 8-$\mu \mathrm{m}$-period PnC gives the lowest thermal conductance of all the above samples and has the lowest specific conductance per unit heater length observed to date in PnCs. In contrast, coherent transport theory predicts that the longest period should have the lowest thermal conductance. Comparison with incoherent simulations suggests that diffusive boundary scattering is likely the mechanism behind the partial breakdown of the coherent theory.

Growth monitoring during the early stages of vapor deposition is of prime importance to understand the growth process, the microstructure, and thus the overall layer properties. We demonstrate that phonons can be used as an extremely sensitive probe to monitor the real-time evolution of film microstructure during growth, from incipient clustering to continuous film formation. For that purpose, a silicon nitride membrane-based sensor is fabricated to measure the in-plane thermal conductivity of thin film (conductive or nonconductive) samples. Operating with the $3\omega$-Völklein method at low frequencies, the sensor shows an exceptional resolution down to $\mathrm{\Delta }(\kappa \cdot t)=0.065\mathrm{W/m}\mathrm{K}\mathrm{nm}$, enabling accurate measurements even in poor conductive samples. Validation of the sensor performance is done by growth characterization of organic and metallic thin films to tackle the low to high thermal-conductivity range. In both cases, at early stages of growth, the extreme sensitivity of the technique has revealed an initial reduction of the effective thermal conductance of the supporting amorphous membrane, K, related to the surface phonon scattering enhanced by the incipient nanoclusters formation. As cluster coalescence advances, K reaches a minimum to rise up upon the percolation threshold. Subsequent island percolation produces a sharp increase of the conductance and once the surface coverage is completed K increases linearly with thickness. The thermal conductivity of the deposited films is also obtained from the slope of K with thickness.

Spin-orbit-torque- (SOT) induced magnetization switching in a $\mathrm{Co}$/$\mathrm{Pt}$/$\mathrm{Co}$ trilayer, with two $\mathrm{Co}$ layers exhibiting magnetization easy axes orthogonal to each other is investigated. A $\mathrm{Pt}$ layer is used as a source of spin-polarized current as it is characterized by relatively high spin-orbit coupling. The spin Hall angle of $\mathrm{Pt}$, $\theta =0.08$, is quantitatively determined using the spin-orbit-torque ferromagnetic resonance technique. In addition, $\mathrm{Pt}$ serves as a spacer between two $\mathrm{Co}$ layers and depending on its thickness, different interlayer-exchange-coupling (IEC) energy between ferromagnets is induced. Intermediate IEC energies, resulting in a top $\mathrm{Co}$ magnetization tilted from the perpendicular direction, allows for SOT-induced field-free switching of the top $\mathrm{Co}$ layer. The switching process is discussed in more detail, showing the potential of the system for neuromorphic applications.

Terahertz radiation generation from liquid water has long been considered impossible due to strong absorption. A few very recent works reported terahertz generation from water, but the mechanism is not clear and the efficiency demands to be enhanced. We show experimentally that strong single-cycle terahertz radiation with field strength of $0.2\mathrm{MV}{\mathrm{cm}}^{-1}$ is generated from a water line (or column) of approximately $200\mu \mathrm{m}$ in diameter irradiated by a mJ femtosecond laser beam. This strength is 100-fold higher than that produced from air using single-color pumping. We attribute the mechanism to the laser-ponderomotive-force-induced current with the symmetry broken around the water-column interface. This mechanism can explain our following observations: the radiation can be generated only when the laser propagation axis deviates from the column center; the deviation determines its field strength and polarity; it is always $p$ polarized no matter whether the laser is $p$ or $s$ polarized. This study provides a simple and efficient scheme of table-top terahertz sources based on liquid water.

There have been multiple attempts to demonstrate that quantum annealing and, in particular, quantum annealing on quantum-annealing machines, has the potential to outperform current classical optimization algorithms implemented on CMOS technologies. The benchmarking of these devices has been controversial. Initially, random spin-glass problems were used, however, these were quickly shown to be not well suited to detect any quantum speedup. Subsequently, benchmarking shifted to carefully crafted synthetic problems designed to highlight the quantum nature of the hardware while (often) ensuring that classical optimization techniques do not perform well on them. Even worse, to date a true sign of improved scaling with the number of problem variables remains elusive when compared to classical optimization techniques. Here, we analyze the readiness of quantum-annealing machines for real-world application problems. These are typically not random and have an underlying structure that is hard to capture in synthetic benchmarks, thus posing unexpected challenges for optimization techniques, both classical and quantum alike. We present a comprehensive computational scaling analysis of fault diagnosis in digital circuits, considering architectures beyond D-Wave quantum annealers. We find that the instances generated from real data in multiplier circuits are harder than other representative random spin-glass benchmarks with a comparable number of variables. Although our results show that transverse-field quantum annealing is outperformed by state-of-the-art classical optimization algorithms, these benchmark instances are hard and small in the size of the input, therefore representing the first industrial application ideally suited for testing near-term quantum annealers and other quantum algorithmic strategies for optimization problems.

Recently, multiferroic tunnel junctions (MFTJs) have gained significant attention in the literature due to their high tunneling electroresistance together with their nonvolatility. To analyze such devices and to have an insightful understanding of their characteristics, there is a need to develop a multiphysics modeling-and-simulation framework. The simulation framework discussed in this paper is motivated by the scarcity of such multiphysics studies in the literature. In this study, a theoretical analysis of MFTJs is demonstrated with use of a self-consistent analysis by a spin-based nonequilibrium Green’s function method to estimate the tunneling current, the Landau-Khalatnikov equation to model the ferroelectric polarization dynamics, and the Landau-Lifshitz-Gilbert equation to capture the magnetization dynamics. The spin-based nonequilibrium Green’s function method is equipped with a magnetization-dependent Hamiltonian that eases the modeling of the tunneling electroresistance, the tunneling magnetoresistance, and the magnetoelectric effect in MFTJs. Moreover, we perform first-principles calculations to estimate the screening lengths of the MFTJ electrodes that are necessary for estimation of the tunneling current. The simulation results of the proposed framework are in good agreement with the experimental results. Finally, a comprehensive analysis of tunneling electroresistance and tuneling magnetoresistance of MFTJs and their dependence on various device parameters is provided.

We construct a theory for the quantum efficiency of radiation in bulk semiconductor crystals as observed in PL measurements. The general form of the external quantum efficiency (EQE), which is directly measured from the PL intensity, is formulated by considering the self-absorption in the sample. For a better understanding of the physical concept behind the EQE, we derive a simple analytical formula for EQE in a weak photoexcitation condition and discuss the relation between EQE and internal quantum efficiency (IQE). We find that, in contrast to IQE, EQE is affected by two kinds of light-extraction efficiencies, ${\eta }_{\mathrm{opt}}$ and ${\eta }_{\mathrm{self}}$. ${\eta }_{\mathrm{opt}}$ gives the fraction of luminescence passing through the sample surface without the optical Fresnel reflection. Meanwhile, ${\eta }_{\mathrm{self}}$ gives the fraction of luminescence reaching the sample surface that overcomes the self-absorption process inside the sample. ${\eta }_{\mathrm{self}}$ strongly depends on the spatial carrier distribution, because of which carrier-diffusion phenomena have a large impact on EQE. We also show an example calculation of the theoretical calibration curve of EQE to determine the concentration of nonradiative recombination centers (NRCs) in semiconductor samples. The NRC concentration estimated from the calibration curve is consistent with the recent omnidirectional photoluminescence measurements, and the sensitivity is high enough to quantify defect concentrations for power-device applications.

Using first-principles calculations, we evaluate the electrochemical performance of heterostructures made up of ${\mathrm{Ti}}_2{\mathrm{CO}}_2$ and chemically modified graphene for $\mathrm{Li}$ batteries. We find that heteroatom doping and molecule intercalation have a significant impact on the storage capacity and $\mathrm{Li}$ migration barrier energies. While N and S doping do not improve the storage capacity, B doping together with molecule interaction make it possible to intercalate two layers of $\mathrm{Li}$, which stick separately to the surface of ${\mathrm{Ti}}_2{\mathrm{CO}}_2$ and B-doped graphene. The calculated diffusion-barrier energies ($E_{\mathrm{diff}}$), which are between 0.3 and 0.4 eV depending on $\mathrm{Li}$ concentration, are quite promising for fast charge and discharge rates. Besides, the predicted $E_{\mathrm{diff}}$ as much as 2 eV for the diffusion of the $\mathrm{Li}$ atom from the ${\mathrm{Ti}}_2{\mathrm{CO}}_2$ surface to the B-doped graphene surface significantly suppresses the interlayer $\mathrm{Li}$ migration, which diminishes the charge and discharge rates. The calculated volume and lattice parameter changes indicate that ${\mathrm{Ti}}_2{\mathrm{CO}}_2/$graphene hybrid structures exhibit cyclic stability against $\mathrm{Li}$ loading and unloading. Consequently, first-principles calculations we perform evidently highlight the favorable effect of molecular intercalation on the capacity improvement of ion batteries.

Recent measurements of an unusual high-thermal conductivity of around $1000\mathrm{W}{\mathrm{m}}^{-1}{\mathrm{K}}^{-1}$ at room temperature in cubic boron arsenide (BAs) confirm predictions from theory and suggest potential applications of this semiconductor compound for thermal management applications. Knowledge of the thermal expansion coefficient and Grüneisen parameter of a material contributes both to the fundamental understanding of its lattice anharmonicity and to assessing its utility as a thermal-management material. However, previous theoretical calculations of the thermal expansion coefficient and Grüneisen parameter of BAs have yielded inconsistent results. Here, we report the linear thermal expansion coefficient of BAs obtained from the x-ray diffraction measurements from 300 K to 773 K. The measurement results are in good agreement with our ab initio calculations that account for atomic interactions up to the fifth nearest neighbors. With the measured thermal expansion coefficient and specific heat, a Grüneisen parameter of BAs of 0.84 ± 0.09 is obtained at 300 K, in excellent agreement with the value of 0.82 calculated from first principles and much lower than prior theoretical results. Our results confirm that BAs exhibits a better thermal expansion coefficient match with commonly used semiconductors than other high-thermal conductivity materials such as diamond and cubic boron nitride.

We introduce a metamaterial-based self-deployable system with a rotational periodicity. As a demonstration, we propose an autonomous solar panel array that is programmed to self-deploy in response to changes in the surrounding temperature. We achieve shape reconfiguration and structural stability by exploiting the physical properties in the constituting material and the architecture of the wedge-shaped unit cell. The unit cell consists of one arm of the elastic “flasher” origami and a pair of scissor mechanisms. First, kinematic analysis shows the difference between the theoretical behavior and behavior considering the physical dimensions. This is used to optimize the expansion ratio. Second, the deployment mechanics are enabled through the shape-memory effect inherent in the underlying polymer. A viscoelastic constitutive model is constructed to accurately predict the self-expanding behavior. Lastly, the collapsing and deployment dynamics are discussed. Bifurcation is observed during folding, leading to two different end states, a disk or a cone. By investigating the energy landscape of the system, an apparatus is introduced to enable the disk-shaped folding. A two-stage expansion is observed during deployment. The system first rotates and then expands radially. The resulting system is three-dimensionally (3D) printed, achieves an expansion ratio of 1000% in under 40 s, and shows excellent agreement with simulation prediction both in the collapsed and expanded configurations.

We investigate the propagation dynamics of magneto-elastic waves in an out-of-plane (OP) magnetized film using time-resolved magneto-optical (TRMO) microscopy based on the Cotton-Mouton effect. In OP magnetized films, spin precession induces a temporal change in the in-plane component of magnetization (${\mathbf{m}}_{\perp }$), which can be measured through birefringence induced by ${\mathbf{m}}_{\perp }$, or the Cotton-Mouton effect (CME). Here we develop an imaging method based on CME, realizing the observation of stress-driven magneto-elastic waves in an OP magnetized film. This result expands the focus of TRMO microscopy, allowing investigation of magneto-elastic waves in OP magnetized films.

Ensembles of nitrogen-vacancy (N-$V$) centers in diamonds are widely utilized for magnetometry, magnetic field imaging, and magnetic resonance detection. At zero ambient field, Zeeman sublevels in the N-V centers lose first-order sensitivity to magnetic fields, as they are mixed due to crystal strain or electric fields. In this work, we realize a zero-field (ZF) magnetometer using polarization-selective microwave excitation in a ${}^{13}\mathrm{C}$-depleted crystal sample. We employ circularly polarized microwaves to address specific transitions in the optically detected magnetic resonance and perform magnetometry with a noise floor of $250\mathrm{pT}/\sqrt{\mathrm{Hz}}$. This technique opens the door to practical applications of N-$V$ sensors for ZF magnetic sensing, such as ZF nuclear magnetic resonance and investigation of magnetic fields in biological systems.

Known approaches to modeling terahertz plasmons in two-dimensional electron systems may differ significantly in their assumptions. There has, however, been little effort to analyze the differences between and application of different models to the same sets of structures. This paper discusses, develops, and compares several different theoretical approaches—namely, an effective-medium approximation, a transmission-line model, modal analysis, and full-wave simulations. In particular, we present a transmission-line model that takes into account the dielectric-air surrounding a two-dimensional system. Using modal analysis, we also solve analytically the problem of plasmon reflection and transmission when plasmons are incident on a junction between gated and ungated two-dimensional waveguides. Comparing the predictions made by the models for several structures, we find good agreement between full-wave simulations and both analytical and numerical modal analysis. The results of the effective-medium approximation and the transmission-line model also agree with each other, but differ quantitatively from those of the full-wave simulations and modal analysis. We attribute the differences to the phases of the plasmon reflection and transmission coefficients obtained with the different approaches. Our analytical expressions for the plasmon transmission and reflection coefficients represent a simple, yet accurate way to model plasmons in two-dimensional systems comprising both gated and ungated sections.

Autler-Townes splitting (ATS) and electromagnetically induced transparency (EIT) are similar phenomena but distinct in nature. They have been widely discussed and distinguished by employing the Akaike information criterion (AIC). However, such work is lacking in acoustic systems. In this work, the interaction of Love waves with a two-line pillared metasurface is numerically investigated by the finite-element method. An acoustic analog of ATS, Fabry-Perot resonance, and cavity modes are first demonstrated in two lines of identical pillars by varying the distance between the pillar lines. By detuning the radius of one line of pillars, Fabry-Perot resonance along with two different pillar resonances give rise to the acoustic analog of EIT (AIT) when the distance between the pillar lines is a multiple of half wavelength. ATS and AIT formula models are used to fit the transmission spectra, showing good agreements with numerical results. The quality of the fit models is quantitatively evaluated by resorting to the AIC. We show that theoretical and analytical discrimination between ATS and AIT are methodologically complementary. These results should have consequences for potential acoustic applications such as wave control, and the design of metamaterials and biosensors.

Interfaces of sapphire are of technological relevance as sapphire is used as a substrate in electronics, lasers, and Josephson junctions for quantum devices. In addition, its surface is potentially useful in catalysis. Using first-principles calculations, we show that, unlike bulk sapphire, which has inversion symmetry, the (0001) sapphire surface is piezoelectric. The inherent broken symmetry at the surface leads to a surface dipole and a significant response to imposed strain: the magnitude of the surface piezoelectricity is comparable to that of bulk piezoelectrics.

Gate-pulse-induced recombination, known as charge pumping (CP), is a fundamental carrier recombination process, and has been utilized as a method for analyzing the electrical properties of defects (or dangling bonds) at the transistor interfaces, which is now recognized to be a well-matured and conventional method. Nevertheless, neither the origin (the bonding configuration) of the defects responsible for the CP nor their detailed recombination sequence has been clarified yet for Si metal-oxide-semiconductor (MOS) interfaces. In order to address these problems, we investigate the CP under spin resonance conditions at temperatures ranging from 27 to 300 K in $\mathrm{Si}(100)$ n type MOS transistors. We obtain evidence that ${\mathrm{P}}_{\mathrm{b}0}$ and E′ centers, the two major dangling bonds at (and near) the $\mathrm{Si}(100)$ interface, participate in the CP recombination process. We also show that the spin-dependent CP process is explained by the formation of electron-electron spin pairs, which, in turn, reveals that the CP via ${\mathrm{P}}_{\mathrm{b}0}$ and E′ centers is inherently a two-electron process.

The compatibility of $\mathrm{Si}$ semiconductor technology with current electronic devices has led to a continuing search for new $\mathrm{Si}$-based materials with novel properties. The recent synthesis of penta-$\mathrm{Si}$ nanoribbons [Phys. Rev. Lett. 117, 276102 (2016); Nano Lett. 18, 2937 (2018); Nat. Commun. 7, 13076 (2016)] is a new addition to this family. However, pentasilicene, a two-dimensional (2D) sheet composed of only $\mathrm{Si}$ pentagons, was found to be unstable in pentagraphenelike configuration and could only be stabilized dynamically by surface decoration. Using first-principles calculations and a thorough analysis of its imaginary frequencies, we show that pentasilicene can indeed be made dynamically stable by tilting the $\mathrm{Si}$ dimers to reduce the Coulomb repulsion between them. The consequence of this tilting leads to a much more interesting discovery: the stabilized pentasilicene breaks the structural inversion symmetries, resulting in spontaneous electrical polarization and ferroelectricity with a high Curie temperature of 1190 K. This is the first report of a 2D ferroelectric system composed of a light single element, which can have potential applications in nonvolatile random access memory.

No existing model has completely explained the physics of the optical nonlinearity for epsilon-near-zero materials, which have recently emerged as candidates for design and fabrication of active nanophotonic devices. We propose to extend the Drude model by adopting a weighted electron effective mass, which takes into account the electron distribution in the nonparabolic conduction band. The extended Drude model is able to interpret the cause of the nonlinearity and provide a functional relationship between the refractive index and wavelength. Both the band nonparabolicity and the temperature-dependent mobility are responsible for the intraband-transition-induced optical nonlinearity. The spectrally resolved nonlinear refractive index and nonlinear susceptibilities are obtained from this model. The results can be applied to other transparent conducting oxides and will help in the design and modeling of spectral responses of nonlinear plasmonic devices.

Producing a metal photocathode with a low work function (WF), low emissivity, and high quantum efficiency is a matter of importance in the design of the next generation of free-electron laser facilities. General rules for the design of appropriate materials are currently unclear and difficult to elucidate from observations of structure-composition relationships of known photocathodes. In this work, high-quality density-functional-theory electronic structure calculations and a simple physical model are employed to develop design rules for photocathodes based on metallic alloys. A theoretical study of metal alloys for photocathode applications is presented, in which high WF, stable copper is paired with low WF, unstable barium in two alloys, ${\mathrm{Cu}}_{13}\mathrm{Ba}$ and $\mathrm{Cu}\mathrm{Ba}$. Surfaces terminating in a plane of $\mathrm{Ba}$ atoms have a lower computed surface energy than those terminating in $\mathrm{Cu}$ atoms due to surface segregation of the larger $\mathrm{Ba}$ atoms. This results in a significant surface dipole due to the interatomic charge transfer from the differences in electronegativity of the species. The details of the surface structure determine the direction of the dipole and thus have a strong influence on the computed WF. The computed WF of the ${\mathrm{Cu}}_{13}\mathrm{Ba}$ $\mathrm{Ba}$-terminated (100) surface is even lower than that of pure $\mathrm{Ba}$, at 1.95 eV. The computed quantum efficiency (QE) of the best-performing pure $\mathrm{Cu}$ surface is $5.86\times {10}^{-6}$, whereas the best-performing ${\mathrm{Cu}}_{13}\mathrm{Ba}$ surface terminates in a plane of $\mathrm{Ba}$ atoms and has a significantly increased QE of $5.09\times {10}^{-3}$. A surface terminating in two planes of $\mathrm{Ba}$ atoms, the (001) surface of $\mathrm{Cu}\mathrm{Ba}$, has an even higher computed QE of $1.38\times {10}^{-2}$.