A 3D-printed gradient-index phononic crystal structure based on air inclusions in a polymer is introduced to manipulate underwater acoustic waves. The proposed approach enables a simple configuration without heavy inclusions or geometrically complex unit cells to create the required refractive-index profile and focus incident plane waves propagating in a known direction. Based on the band-structure analysis, a gradient-index lens is designed, fabricated, and tested, yielding an excellent agreement between the experimental results and finite-element simulations of underwater acoustic wave focusing. The proposed concept and the resulting lens design can find applications spanning from power enhancement in acoustic energy transfer and harvesting to signal enhancement in sensing and imaging.

Binding energy calculation in two-dimensional (2D) materials is crucial in determining their electronic and optical properties pertaining to enhanced Coulomb interactions between charge carriers due to quantum confinement and reduced dielectric screening. Based on full solutions of the Schrödinger equation in a screened hydrogen model with a modified Coulomb potential ($1/r^{\beta -2}$), we present a generalized and analytical scaling law for the exciton binding energy, $E_{\beta }=E_0(a{\beta }^b+c)(\mu /{ϵ}^2)$, where $\beta$ is a fractional-dimension parameter that accounts for the reduced dielectric screening. The model is able to provide accurate binding energies, benchmarked using the reported Bethe-Salpeter equation and experimental data, for 58 monolayer 2D and eight bulk materials, respectively, through $\beta$. For a given material, $\beta$ is varied from $\beta =3$ for bulk three-dimensional materials to a value lying in the range 2.55–2.7 for 2D monolayer materials. With ${\beta }_{\mathrm{mean}}=2.625$, our model improves the average relative mean square error by a factor of 3 in comparison to existing models. The results can be used for Coulomb engineering of exciton binding energies in the optimal design of 2D materials.

The aim of this work is to clarify the scintillation-yield enhancement in $\mathrm{Lu}\mathrm{YAG}:\mathrm{Pr}$ scintillators obtained by $\mathrm{Li}$ codoping via integrated study of the valence state of activators, the preferential site occupancy of $\mathrm{Li}$ codopants, and defect structures from experimental and theoretical insights. With $\mathrm{Li}$ codoping, the light yield and energy resolution of $10\times 10\times 10{\mathrm{mm}}^3$ $\mathrm{Lu}\mathrm{YAG}:\mathrm{Pr}$ samples are improved from 15 600 to 24 800 photons/MeV, and 5.3 to 4.3% at 662 keV, respectively. The optical absorption spectra indicate that $\mathrm{Li}$ codoping does not induce conversion of stable ${\mathrm{Pr}}^{3+}$ to ${\mathrm{Pr}}^{4+}$ in $\mathrm{Lu}\mathrm{YAG}:\mathrm{Pr}$ single crystals. Based on the formation energies of substitutional and interstitial $\mathrm{Li}$ sites using density-functional-theory (DFT) calculations and the ${}^7\mathrm{Li}$ nuclear magnetic resonance results, it is shown that the $\mathrm{Li}$ ions prefer to dominantly occupy the fourfold coordinated interstitial sites and fourfold coordinated $\mathrm{Al}$ sites. The systematic analysis of thermoluminescence glow curves, positron annihilation lifetime spectroscopies, and defect formation energies derived from DFT calculations reveals that the concentration of isolated $\mathrm{Lu}$ and $\mathrm{Al}$ vacancies as dominant acceptor defects is reduced by $\mathrm{Li}$ codoping, whilst the shallow ${\mathrm{Li}}_i$ interstitial defects and the deep $V_O$ oxygen vacancies are introduced simultaneously. We propose that the lowering of hole trapping at defects resulting from $\mathrm{Li}$ codoping contributes to the scintillation-yield enhancement.

The quartz-crystal microbalance (QCM) has become an important technique for the detection of biomolecules and their interactions. Acoustic sensing of DNA requires hybridizing the oligonucleotide chain with a nanoparticle that acts as an acoustic amplifier. In this device, the amplifier remains suspended in the liquid, nanometers away from the surface. The hydrodynamics of nonadsorbed nanoparticles in QCM sensing devices has, however, been overlooked in the literature. Here, we show that the acoustic response (shifts in frequency, $\mathrm{\Delta }f$, and dissipation, $\mathrm{\Delta }D$) of the amplifier-target complex is determined by the hydrodynamic wall stress. Our study focuses on an alternative type of DNA sensor based on liposome-DNA complexes and provides a theoretical analysis backed up by experiments with 1-palmitoyl-2-oleoyl-sn-glycero-3-phosphocholine liposomes. The target DNA strands are attached on one side to surface-adsorbed neutravidin through a biotin linker, while the other side is anchored to a liposome acting as an amplifier. A range of liposome radii and double-stranded-DNA contour lengths are studied, covering sizes up to the wave penetration depth ($\delta \sim 100\mathrm{nm}$). Simulations, based on the immersed boundary method and an elastic-network model, are in excellent agreement with the experiments. We derive an analytical expression for the acoustic impedance, which reflects strong signal amplification as the liposome gets closer to the wall. This enhancement is due to hydrodynamic reflection: the ambient flow induces a stress on the liposome surface, which propagates back to the resonator. For this reason, acoustic signals are extremely sensitive to the distribution of the liposome-wall distance (which we determine from the mechanical properties of double-stranded-DNA chains). Our approach helps in deciphering the role of hydrodynamics in acoustic sensing and reveals the role of parameters hitherto largely unexplored. A practical consequence may be improvements in the design of biosensors and detection schemes.

We predict the emergence of second-order topological insulators in the dynamics of spin-texture-based metamaterials. We propose that the quantized Chern number and the ${\mathbb{Z}}_6$ Berry phase can completely characterize the nontrivial topology. By studying the collective gyration of magnetic vortices in a breathing honeycomb lattice, we derive the full phase diagram and show that the topological “zero-energy” corner mode is protected by a generalized chiral symmetry in the sexpartite lattice, leading to particular robustness against disorder and defects. Interestingly, we observe corner states at either obtuse-angled or acute-angled corners, depending on whether the lattice boundary has an armchair or a zigzag shape. Full micromagnetic simulations confirm the theoretical predictions with good agreement. Our findings open up a promising route to realizing higher-order symmetry-protected corner states in magnetic systems and to finally achieving topological spintronic memories, imaging, and computing.

Forces induced by quantum fluctuations of the electromagnetic field control adhesion phenomena between rough solids when the bodies are separated by distances of approximately $10\mathrm{nm}$. However, this distance range remains largely unexplored experimentally in contrast with the shorter (van der Waals forces) or the longer (Casimir forces) separations. The reason for this is the pull-in instability of the systems with the elastic suspension that poses a formidable limitation. In this paper we propose a genuine experimental configuration that does not suffer from the short distance instability. The method is based on the adhered cantilever, whose shape is sensitive to the forces acting near the adhered end. The general principle of the method, its possible realization, and feasibility are extensively discussed. The dimensions of the cantilever are determined by the maximum sensitivity to the forces. If the adhesion is defined by strong capillary or chemical interactions, the method loses its sensitivity. Special discussion is presented for the determination of the minimum distance between the rough solids upon contact, and for the compensation of the residual electrostatic contribution. The proposed method can be applied to any kind of solids (metals, semiconductors, or dielectrics) and to any intervening medium (gas or liquid).

As an essential building block for developing a large-scale brain-inspired computing system, we propose a highly scalable and energy-efficient artificial neuron device composed of an ovonic threshold switch (OTS) and a few passive electrical components. It is found that the proposed neuron device shows not only the basic integrate-and-fire function and the rate-coding property, but also the spike-frequency-adaptation (SFA) property and the chaotic activity of biological neurons, the most common features found in mammalian cortex, but they have been hard to achieve up to now. In addition, it is shown that the energy consumption of the OTS-based neuron device scales with the size of the OTS device, extrapolating both the size and the energy efficiency to the level of a biological neuron in a human brain with state-of-the-art technology. Finally, using the OTS-based neuron device combined with the reservoir computing technique, the spoken-digit recognition task has been performed with a considerable degree of recognition accuracy (94%). These results demonstrate that our OTS-based artificial neuron device is promising for the application in the development of a large-scale brain-inspired computing system.

With the optimization of the back contact, improvement of carrier lifetime, and the use of $\mathrm{Se}$-alloyed $\mathrm{Cd}\mathrm{Te}$ as the absorber, the power conversion efficiency (PCE) of $\mathrm{Cd}\mathrm{Te}$ solar cells has now risen beyond 22%. However, further improvement of the PCE of $\mathrm{Cd}\mathrm{Te}$-based solar cells by optimization of the window layer has not been fully discussed. In this work, we propose that ${\mathrm{Zn}}_x{\mathrm{Cd}}_{1-x}{\mathrm{S}}_y{\mathrm{Se}}_{1-y}$ alloys can be tuned as promising window layers of both $\mathrm{Cd}\mathrm{Te}$ and ${\mathrm{Cd}\mathrm{Se}}_x{\mathrm{Te}}_{1\text{−}x}$ solar cells when band alignment and lattice match are taken into account. Using first-principles calculations and the special quasirandom structure approach, we investigate the compositional dependence of formation energies, lattice parameters, band-gap energies, and band alignments of ${\mathrm{Zn}}_x{\mathrm{Cd}}_{1-x}{\mathrm{S}}_y{\mathrm{Se}}_{1-y}$ alloys. We find that the interaction parameters and thus the miscibility temperatures of ${\mathrm{Zn}}_x{\mathrm{Cd}}_{1-x}{\mathrm{S}}_y{\mathrm{Se}}_{1-y}$ alloys are directly proportional to the lattice mismatch between the mixing elements, and are larger for mixed-cation alloys than for mixed-anion alloys. The lattice parameter of ${\mathrm{Zn}}_x{\mathrm{Cd}}_{1-x}{\mathrm{S}}_y{\mathrm{Se}}_{1-y}$ exhibits a linear dependence to its composition. The bowing of band-gap energies of ${\mathrm{Zn}}_x{\mathrm{Cd}}_{1-x}{\mathrm{S}}_y{\mathrm{Se}}_{1-y}$ alloys is quite large, due to the symmetry-reduction-induced intraband coupling in quaternary alloys. Finally, we illustrate the optimal composition range of ${\mathrm{Zn}}_x{\mathrm{Cd}}_{1-x}{\mathrm{S}}_y{\mathrm{Se}}_{1-y}$ alloys as the window layer for both $\mathrm{Cd}\mathrm{Te}$ solar cells and ${\mathrm{Cd}\mathrm{Se}}_x{\mathrm{Te}}_{1\text{−}x}$ solar cells. Our work provides a guideline for further improving the performance of $\mathrm{Cd}\mathrm{Te}$ and ${\mathrm{Cd}\mathrm{Se}}_x{\mathrm{Te}}_{1\text{−}x}$ solar cells by adopting ${\mathrm{Zn}}_x{\mathrm{Cd}}_{1-x}{\mathrm{S}}_y{\mathrm{Se}}_{1-y}$ alloys as promising window layers.

We combine steady-state photoconductivity and laser-induced transient current measurements under above-band-gap illumination to study space-charge formation in ($\mathrm{Cd},\mathrm{Zn})\mathrm{Te}$. Analytical and numerical models describing space-charge-limited photocurrents are developed and excellent agreement with measured data is obtained, especially with the Drift-diffusion model. A linear rise of photocurrent at low voltage is observed and ascribed to the trapping of injected holes in the region close to the cathode side. The influence of space-charge formation, the influence of photoconductive gain, and the contribution of shallow and deep levels to photocurrent-voltage characteristics are numerically simulated. According to measurements and calculations, recent principles used to evaluate detector properties, mainly the mobility-lifetime product, via the photoconductivity are critically assessed.

Recently, all-inorganic two-dimensional (2D) Ruddlesden-Popper (RP) halide perovskites have drawn much attention due to their excellent stability in ambient air. However, their electronic and optical performance resulting from a wide bandgap and low carrier mobility have hindered their use in photodetectors. To overcome these limitations, in this work, taking advantage of hexagonal indium selenide ($\mathrm{In}\mathrm{Se}$) with a high electron mobility, we construct an atomically thin heterostructure. Density-functional-theory (DFT) calculations of the electronic and optical properties are performed for these heterostructures. The results demonstrate that the photodetection response spectrum of the heterostructures is significantly broadened as the bandgap decreases from 2.17 to 0.40 eV for the $\mathrm{In}\mathrm{Se}$/${\mathrm{Cs}}_2{\mathrm{Sn}\mathrm{I}}_2{\mathrm{Cl}}_2$ heterostructure. Moreover, the electron effective mass, $m_e^{\ast }$, is reduced from 1.13 $m_0$ to 0.41 $m_0$ for the $\mathrm{In}\mathrm{Se}$/${\mathrm{Cs}}_2{\mathrm{Ge}\mathrm{I}}_2{\mathrm{Cl}}_2$ heterostructure. The significant reductions in both the band gap and effective mass are determined to be related to the type-II band alignment, which favors the carrier separation at the interface. The physical mechanisms related to the usage of this material in photodetectors are also discussed. The proposed III–VI semiconductor InSe and all-inorganic 2D RP perovskite ${\mathrm{Cs}}_{2}X{\mathrm{I}}_2{\mathrm{Cl}}_2$ (X = $\mathrm{Pb}$, $\mathrm{Sn}$, and $\mathrm{Ge}$) heterostructures provide challenges and opportunities for designing high-performance photodetectors.

The ability to control the directional and spectral properties of thermal emission is of fundamental importance for many applications. Here we propose a method to design directional and broadband thermal emitters (DBTEs) and directional and narrowband thermal emitters (DNTEs) based on classic thermal emitters and angular selectors. DBTEs exhibit strong thermal emission at a specific angle and within a broadband wavelength range. Besides, DBTEs achieve better radiative cooling performance than classic thermal emitters without special angular control when there are surrounding buildings. On the other hand, DNTEs can confine thermal emission within a narrow bandwidth and a narrow angular width. DNTEs show higher energy conversion efficiency than conventional narrowband thermal emitters, and have potential application in high-efficiency infrared light sources.

Ferrimagnetic insulators (FiMI) have been intensively used in microwave and magneto-optical devices as well as spin caloritronics, where their magnetization direction plays a fundamental role on the device performance. The magnetization is generally switched by applying external magnetic fields. Here we investigate current-induced spin-orbit-torque switching of the magnetization in ${\mathrm{Y}}_3{\mathrm{Fe}}_5{\mathrm{O}}_{12}$ ($\mathrm{YIG}$)/$\mathrm{Pt}$ bilayers with in-plane magnetic anisotropy, where the switching is detected by spin Hall magnetoresistance. Reversible switching is found at room temperature for a threshold current density of ${10}^7\mathrm{A}{\mathrm{cm}}^{-2}$. The $\mathrm{YIG}$ sublattices with antiparallel and unequal magnetic moments are aligned parallel or antiparallel to the direction of current pulses, which is consistent to the Néel-order switching in an antiferromagnetic system. It is proposed that such a switching behavior may be triggered by the antidamping torque acting on the two antiparallel sublattices of FiMI. Our finding not only broadens the magnetization switching by electrical means and promotes the understanding of magnetization switching, but also paves the way for all-electrically modulated microwave devices and spin caloritronics with low power consumption.

The geometric magnetoresistance effect in semiconductors has remained a heated discussion for many years. However, there are few reports on laser-triggered geometric magnetoresistance in traditional structures. In this work, we use a laser to change the carrier concentration to obtain a large magnetoresistance (212.6%) under a low magnetic field (1 T) at 150 K in a Corbino disk with $\mathrm{Co}$-$\mathrm{Ag}$ films. One unanticipated finding is that the large positive magnetoresistance does not change monotonously with temperature, which is different from previous research. Theoretical calculation reveals that the interaction among a photogenerated carrier, bending of the current path, and magnetic nanoparticles in low temperature improves the magnetoresistance in a Corbino disk. These findings reveal an important strategy for creating laser-trigged nanoscale magnetoresistance devices, while presenting a wide range of possibilities for exploring the dependence of photogenerated carriers on temperature under magnetic field.

Manipulation of evanescent waves in a waveguide with gradient-index metamaterials is analyzed. It is shown that evanescent waves excited by a cutoff waveguide can be efficiently transmitted over a long distance by a mode-conversion mechanism. The experimental results are in good agreement with simulated results, and the manipulation is of broadband frequencies. The results suggest potential applications in subwavelength microwave transmission.

The nonlinear interaction of a time-harmonic acoustic wave with an anisotropic particle gives rise to the radiation force and torque effects. These phenomena are at the heart of the acoustofluidics technology, where microparticles such as cells and micro-organisms are acoustically manipulated. We present a theoretical model considering a generic acoustic beam interacting with a subwavelength spheroidal particle in a nonviscous fluid. Concise analytical expressions of the radiation force and torque are obtained in the scattering dipole approximation. The radiation force is given in terms of a gradient and scattering force; while the radiation torque has two fundamental contributions, namely, the momentum arm and acoustic spin (spin-torque effect). As a practical example, we use the theory to describe the interaction of two crossed plane waves and a prolate spheroidal particle. The results reveal the particle is transversely trapped in a pressure node and is axially pushed by the radiation force. Also, the momentum arm aligns the particle in the axial direction. At certain specific positions, only the spin-torque occurs. Our findings are remarkably consistent with finite-element simulations. The success of our model enables its use as an investigation tool for the manipulation of anisotropic microparticles in acoustofluidics.

Subwavelength acoustic scatterers with extraordinary multipole scattering properties have attracted much research interest. We introduce an ultrasparse anisotropic acoustic scatterer array, which has degenerate monopole and dipole resonances, for total absorption of sound based on scattering cancellation of monopole and dipole moments. The dipolar scattering property of the scatterer is extremely anisotropic so that the dipole resonance can be switched off by rotating the scatterer about its own center by ${90}^{\circ }$. This feature leads to the scatterer-orientation dependence of mode degeneracy as well as the sound-absorption capability. Simulation and experimental results demonstrate the orientation-dependent sound-isolation property of the scatterer array. Moreover, the scatterer array is ultrasparse in that the spatial period exceeds the limit bounded by 4 times the absorption cross section of a subwavelength resonator ($d>4{\sigma }_{\mathrm{abs}}=2\lambda /\pi$). This ultrasparse and controllable sound-isolating material may have applications in many areas such as infrastructures, consumer electronics, and the automotive industry.