$1T\text{−}\mathrm{Ta}{\mathrm{S}}_2$ is a layered material that exhibits charge density wave (CDW) -induced distinct electrical resistivity phases and has attracted a lot of attention for interesting device applications. However, such resistivity switching effects are often weak, and cannot be modulated by an external gate voltage — limiting their widespread usage. Using a back-gated $1T\text{−}\mathrm{Ta}{\mathrm{S}}_2/2H\text{−}\mathrm{Mo}{\mathrm{S}}_2$ heterojunction, we show that the usual resistivity switching in $\mathrm{Ta}{\mathrm{S}}_2$ due to different phase transitions is accompanied with a surprisingly strong modulation in the Schottky barrier height (SBH) at the $\mathrm{Ta}{\mathrm{S}}_2/\mathrm{Mo}{\mathrm{S}}_2$ interface — providing an additional knob to control the degree of the phase-transition-driven resistivity switching by an external gate voltage. In particular, the commensurate (C) to triclinic (T) phase transition results in an increase in the SBH owing to a collapse of the Mott gap in $\mathrm{Ta}{\mathrm{S}}_2$. The change in SBH allows us to estimate an electrical Mott-gap opening of approximately 71 ± 7 meV in the C phase of $\mathrm{Ta}{\mathrm{S}}_2$. On the other hand, the nearly commensurate (NC) to incommensurate (IC) phase transition results in a suppression in the SBH, and the heterojunction shows a gate-controlled resistivity switching ratio up to 17.3, which is approximately $14.5$ times higher than that of stand-alone $\mathrm{Ta}{\mathrm{S}}_2$. The findings mark an important step forward showing a promising pathway to externally control as well as amplify the CDW-induced resistivity switching. This will boost device applications that exploit these phase transitions, such as ultra-broadband photodetection, negative differential conductance, fast oscillator and threshold switching in neuromorphic circuits.

We present a phononic crystal based on $\mathrm{Zn}\mathrm{O}$ pillars on a $\mathrm{Si}$ substrate that creates local-resonance band gaps, prohibiting surface-acoustic-wave propagation. The created local-resonance band gaps originate from Fano-resonance between the pillars and the substrate, and are shown to be highly influenced by the structural parameters of the pillars. Regarding surface acoustic waveguiding, we introduce a line defect to the crystal where elastic energy can be confined, leading to defect modes with slow group velocity. It is shown that the emerged defect guiding modes can be conducted into the local-resonance gap by adjusting the structural parameters of the defects. Moreover, it is proven that hollow cylinders in the linear defect introduce higher degrees of freedom in the structural parameters of the defect for adjusting the guiding mode, and can lead to narrow-band guiding modes in comparison with the gap width at the vicinity of the mid gap. The achieved narrow-band guiding modes are attributed to the reduced number of allowed resonance frequencies in hollow cylinders due to their reduced effective radius and inherent spatial confinement. Additionally, benefiting from acoustoelectric interaction in piezoelectric materials, we can modulate the elasticity of the $\mathrm{Zn}\mathrm{O}$ structures by varying their conductivity, which can lead to efficient tuning of the designed guiding modes. The required conductivity modulation in the $\mathrm{Zn}\mathrm{O}$ structures is proposed to achieved by UV illumination so that the photogenerated excess carriers screen the electric field associated with the elastic wave, perturbing the wave propagation and affecting the defect guiding modes. The proposed system opens up perspectives for designing reconfigurable, high-quality-factor surface-acoustic-wave devices such as filters and duplexers. The achieved maximum shift of the guiding frequency is about 100 MHz for a defect frequency of 4.18 GHz and a bandwidth of about 100 MHz, suitable for wireless communication applications.

Two reciprocity theorems are important for fundamental understanding of the solar cell operation and applications to device evaluation: (1) the carrier-transport reciprocity connecting the dark-carrier injection with the short-circuit photocarrier collection and (2) the optoelectronic reciprocity connecting the electroluminescence with the photovoltaic quantum efficiency at short circuit. These theorems, however, fail in devices with thick depletion regions such as p-i-n junction solar cells. By properly linearizing the carrier-transport equation in such devices, we report that the dark-carrier injection is related to the photocarrier collection efficiency at the operating voltage, not at short circuit as suggested in the original theorem. This leads to the general form of the optoelectronic reciprocity relation connecting the electroluminescence with the voltage-dependent quantum efficiency, providing a correct interpretation of the optoelectronic properties of p-i-n junction devices. We also discuss the validity of the well-known relation between the open-circuit voltage and the external luminescence efficiency. The impact of illumination intensity and device parameters on the validity of the reciprocity theorems is quantitatively investigated.

$\mathrm{Fe}\mathrm{Pt}$ is the primary material being considered for the development of information storage technologies based on heat-assisted magnetic recording (HAMR). A practical realization of HAMR requires understanding the high-temperature phase transition behavior of $\mathrm{Fe}\mathrm{Pt}$, including critical exponents and Curie temperature distributions as the fundamental HAMR media design characteristics. The studies so far found a significant degree of variability in the values of critical exponents of $\mathrm{Fe}\mathrm{Pt}$ and remain controversial. Here, we show that at the heart of this variability is the phase transition crossover phenomenon induced by two-ion anisotropy of $\mathrm{Fe}\mathrm{Pt}$. Through Monte Carlo simulations based on a realistic $\mathrm{Fe}\mathrm{Pt}$ effective Hamiltonian, we demonstrate that in order to identify the critical exponents accurately, it is necessary to base the analysis on field-dependent magnetization data. We have developed a two-variable finite-size scaling method that accounts for the field effect. Through the use of this method, we show unambiguously that true critical exponents of $\mathrm{Fe}\mathrm{Pt}$ are fully consistent with the three-dimensional Heisenberg universality class.

We study spin-wave propagation using a phased array antenna. A phase-controlled spin-wave source array generates a spin wave propagating in any direction over a wide angle. The magnitude and directivity of the spin wave are found to be enhanced by increasing the number of individual elements, suggesting that the unidirectional high-power spin-wave beam can be possible using a phased array antenna. Furthermore, the phased array enables to focus the spin wave at a specific position, leading to a local confinement of the spin wave with an enhanced amplitude. The proposed spin-wave array antenna can be applied to the development of spin-wave demultiplexers.

Width modulation has long been proven to be an effective method for band-gap engineering of graphene nanoribbons (GNRs). The success of bottom-up GNR synthesis technology has led to the realization of width modulation within a nanoribbon, which is regarded as a GNR heterojunction (HJ). However, the HJ “shape” does not guarantee an ideal HJ band structure. We report a first-principles investigation of GNR HJs composed of narrow and wide GNR segments. Two possible coupling modes exist when two GNR segments with different widths are connected. We find that the electronic states near the Fermi level are localized in the small band-gap segment for one coupling mode, whereas they are almost equally distributed in the two segments for the other one. Consequently, only the former mode can produce a HJ band structure. The underlying mechanisms are analyzed using two hypothetical experiments. The delocalized electronic states are generated by the interface edge $\mathrm{C}—\mathrm{C}$ bonds. The stabilities of the two coupling modes are verified by molecular dynamics simulations. To exclude other factors and demonstrate the universality of this finding, GNR HJs with varied lengths and widths are also studied. These investigations provide insights into GNR HJ constructions.

We study spatial intensity distributions in plasmonic distributed feedback lasers (DFB) composed of metal nanoparticle arrays. Real-space distributions give direct access to “coupling-strength” parameters that quantify DFB performance in the framework of coupled-wave theory (CWT). We observe that CWT indeed parametrizes real-space intensity distributions and extract coupling-strength parameters that quantify the plasmonic feedback mechanism. These coupling-strength parameters differ from those required to parametrize the plasmonic band structures of the system, counter to the common result for dielectric DFB lasers, where CWT describes both real-space and $k$-space physics. Also, the measured coupling constants are significantly smaller than would be expected from estimates on the basis of the unit-cell geometry. We conclude that while CWT is successful as a generic description of any system with forward and backward waves with gain, matching this model to photonic band structures, or to common parameter-estimate approaches, fails because the underlying assumption that a perturbative plane-wave expansion applies is not valid for plasmonic antenna arrays.

The possibility of realizing an electrostatically induced phononic crystal is numerically investigated in an acoustic waveguide based on a graphene sheet that is suspended over periodically arrayed electrodes. The application of dc voltage to these electrodes exerts an electrostatic force on the graphene and this results in the periodic formation of stress in the waveguide structure in a noninvasive way, unlike the cases with mass loading and air holes. This noninvasive scheme enables a band gap, namely a phononic crystal, to be created in the waveguide that can be used to dynamically tune the acoustic transparency in the medium. Our approach will allow the dispersion relation to be locally modified, thus modulating the response of traveling acoustic phonon waves. This alternate phonon architecture is promising in terms of realizing advanced control of phonon dynamics such as waveform engineering.

Domain-wall memory devices, in which the information is stored in nanowires, are expected to replace hard disk drives. A problem that remains to be solved in domain-wall memory is to pin the domain walls in a controllable manner at the nanometer scale using simple fabrication. We demonstrate the possibility to stabilize domain walls by making staggered nanowires. Controllable domain-wall movement is exhibited in permalloy nanowires using magnetic fields where the pinning field is about 10 mT. The pinning field and stability of the domain walls can be increased by adjusting the offset dimensions of the staggered nanowires. Domain-wall velocities of about 200 m/s are computed for the experimentally used permalloy nanowires. Domain-wall velocities are found to be independent of pinning strength and stability, providing a way to tune the pinning without compromising domain-wall velocities.

Manipulation of acoustic waves, in particular for perfect absorption through a subwavelength medium, is of great importance in scientific and practical aspects. Conventional absorbers (e.g., thick foams or dense media) can sufficiently absorb acoustic waves, but at the same time, block fluid flow and visible light. Although sparsely arranging acoustic resonators will permit fluid flow and light propagation, this sparse arrangement leads to high acoustic transmission because of acoustic radiation symmetry. We analytically and numerically demonstrate that a sparsely distributed resonator array consisting of pairs of lossy and lossless resonators has perfect absorption at resonance with ultrahigh sparsity (a volume-filling ratio of <5%) owing to the symmetry breaking induced by the asymmetric loss. We find that the maximum period for ultrahigh sparsity is bounded by four times the absorption cross-section limit of a single-resonance subwavelength resonator, independent of its physical dimensions. Therefore, one can, in principle, obtain arbitrarily high sparsity. We experimentally demonstrate that our sparse absorber array exceeds 90% absorption at resonance with a volume-filling ratio of approximately 26%. In addition, we show that the ultrasparse absorber enables air-flow-direction control and optical cloaking. These results open up an area of multidisciplinary, multifunctional acoustic metamaterials.

Current-driven motion of ferromagnetic domain walls has attracted intensive attention due to its rich physics and the potential applications in racetrack nonvolatile memory. However, the interplay between antiferromagnetic-ferromagnetic (AFM-FM) phase boundaries and current has remained elusive. Here, electrical control of anisotropic FM domains is investigated in AFM domains during the AFM-FM phase transition of $\mathrm{Pd}$-doped $\mathrm{Fe}\mathrm{Rh}$ films. For the as-grown state, the AFM-FM phase boundaries are initially aligned along the [100] axis with weak anisotropy. A stronger anisotropy is introduced by applying a current with a density of approximately ${10}^6\mathrm{A}/{\mathrm{cm}}^2$. Moreover, the phase boundaries can be reversibly oriented by the applied (writing) current, giving rise to an orthogonal alignment between the phase boundaries and the current. Alignments of anisotropic FM domains generate multilevel resistance. Overall, the results confirm electrical control of domains in magnets, and provide an alternative way to create multilevel memories.

With the increasing crossover between quantum information and machine learning, quantum simulation of neural networks has drawn unprecedentedly strong attention, especially for the simulation of associative memory in Hopfield neural networks due to their wide applications and relatively simple structures that allow easier mapping to the quantum regime. Quantum stochastic walk, a strikingly powerful tool to analyze quantum dynamics, has recently been proposed to simulate the firing pattern and associative memory with a dependence on the Hamming distance. We successfully map the theoretical scheme into a three-dimensional photonic quantum chip and realize quantum-stochastic-walk evolution through well-controlled detunings of the propagation constant. We demonstrate a good match rate of the associative memory between the experimental quantum scheme and the expected result for Hopfield neural networks. The ability of quantum simulation for an important feature of a neural network combined with the scalability of our approach through low-loss-integrated-chip and straightforward Hamiltonian engineering provides a primary but steady step toward photonic artificial-intelligence devices for optimization and computation tasks with greatly increased efficiencies.

We propose a method for switching the Néel vector of an antiferromagnetic thin film by the application of an ultrashort electric field pulse. The electric field induces a reorientation of the antiferromagnetic order parameter due to the voltage-induced modification of the magnetic anisotropy. When the electric field pulse is timed to half the oscillation period of the terahertz antiferromagnetic dynamics, it induces a picosecond-timescale reversal of the Néel vector. Importantly, the electric field required to induce this reversal is as small as approximately 100 mV/nm, comparable to fields used for switching of ferromagnetic tunnel junctions in earlier works. This electric field is determined by the anisotropy of the antiferromagnet, while the much larger exchange field determines the frequency of the resulting dynamics (and hence the switching time). Our results indicate the possibility to switch a 50-nm circular antiferromagnetic element with an energy dissipation of 250 aJ in less than 30 ps and in the absence of any current-induced torque. The electric-field-induced switching of the Néel vector opens an alternative route toward energy-efficient and ultrafast magnetic memories and computing devices based on antiferromagnets.

Suppression of boundary-driven Rayleigh streaming has recently been demonstrated for fluids of spatial inhomogeneity in density and compressibility owing to the competition between the boundary-layer-induced streaming stress and the inhomogeneity-induced acoustic body force. To understand the implications of this for acoustofluidic particle handling in the submicrometer regime, we here characterize acoustic streaming by general defocusing particle tracking inside a half-wavelength acoustic resonator filled with two miscible aqueous solutions of different density and speed of sound by adjusting the mass fraction of solute molecules. We follow the temporal evolution of the system as the solute molecules become homogenized by diffusion and advection. The acoustic streaming is suppressed in the bulk of the microchannel for 70–200 s, depending on the choice of inhomogeneous solutions. From confocal measurements of the concentration field of fluorescently labeled Ficoll solute molecules, we conclude that the temporal evolution of the acoustic streaming depends on the diffusivity and the initial distribution of these molecules. Suppression and deformation of the streaming rolls are observed for inhomogeneities in the solute mass fraction down to $0.1\mathrm{\%}$.

Intermetallic compounds are usually poor thermoelectrics due to the high electronic densities of states at the Fermi level and concomitantly low Seebeck coefficients. However, intermetallic $B20$-type $\mathrm{Co}\mathrm{Si}$ has been experimentally shown to attain remarkably large negative values of the Seebeck coefficient. We provide a theoretical explanation for this surprising fact using state-of-the-art first-principles calculations with explicit treatment of electron-phonon interactions. We find that the unique band structure of $\mathrm{Co}\mathrm{Si}$, which has both massless and heavy fermion bands near the Fermi level, facilitates strong scattering of the low-energy electrons by phonons into the heavy band, resulting in effective energy filtering and high thermal voltage. Our study predicts that a very large thermoelectric power factor of 80 $\mu \mathrm{W}{\mathrm{cm}}^{-1}{\mathrm{K}}^{-2}$ or higher is experimentally accessible in the $300$–$600$ K range and highlights a general principle for identifying intermetallic compounds with large thermopower.

Recent advances in metasurfaces have shown the importance of controlling the bianisotropic response of the constituent meta-atoms for maximum efficiency wave-front transformation. By carefully designing the bianisotropic response of the metasurface, full control of the local transmission and reflection properties is enabled, opening new design avenues for creating reciprocal metasurfaces. Despite recent advances in the highly efficient transformation of both electromagnetic and acoustic plane waves, the importance of bianisotropic metasurfaces for transforming cylindrical waves is still unexplored. Motivated by the possibility of arbitrarily controlling the angular momentum of cylindrical waves, we develop a design methodology of a bianisotropic cylindrical metasurface that enables transformation of cylindrical waves for both acoustic and electromagnetic waves with theoretically 100% power efficiency. This formalism is further validated in the acoustic scenario where an experimental demonstration of highly efficient angular momentum transformation is shown.

Understanding the carrier excitation and transport processes at the single-charge level plays a key role in quantum-dot-based solar cells and photodetectors. Here we report on Coulomb-induced giant photocurrent enhancement of positively charged trions ($X^{+}$) in a single self-assembled $\mathrm{In}\mathrm{As}$/$\mathrm{Ga}\mathrm{As}$ quantum dot embedded in an n-i Schottky device by high-resolution photocurrent spectroscopy. The Coulomb repulsion between the two holes in $X^{+}$ increases the tunneling rate of one hole, and the remaining hole can be reused as the initial state to regenerate $X^{+}$. This process results in the photocurrent amplitude of $X^{+}$ being up to 30 times larger than that of the neutral exciton. The analysis of the hole tunneling time gives the equivalent change of hole tunnel barriers caused by Coulomb interaction between two holes as 8.05 meV during the tunneling process. Our work provides a fundamental understanding of energy conversion for solar cells on the nanoscale to increase internal quantum efficiency for energy harvesting.

Finding out when cracks become unstable is at the heart of fracture mechanics. Cracks often grow by avalanches and when a sample fails depends on its past avalanche history. We study the prediction of sample failure in creep fracture under a constant applied stress and induced by initial flaws. Individual samples exhibit fluctuations around a typical rheological response or creep curve. Predictions using the acoustic emission from the intermittent crack growth are not feasible until well beyond the sample-dependent minimum strain rate. Using an optical speckle analysis technique, we show that predictability is possible later because of the growth of the fracture process zone.

The silicon monovacancy in $4H$-$\mathrm{Si}\mathrm{C}$ is a promising candidate for solid-state quantum information processing. We perform high-resolution optical spectroscopy on single $V2$ defects at cryogenic temperatures. We find favorable low-temperature optical properties that are essential for optical readout and coherent control of its spin and for the development of a spin-photon interface. The common features among individual defects include two narrow, nearly lifetime-limited optical transitions that correspond to $m_s=\pm 3/2$ and $m_s=\pm 1/2$ spin states with no discernable zero-field-splitting fluctuations. Initialization and readout of the spin states are characterized by time-resolved optical spectroscopy under resonant excitation of these transitions, showing significant differences between the $\pm 3/2$ and $\pm 1/2$ spin states. These results are well described by a theoretical model that strengthens our understanding of the quantum properties of this defect.

Monolayer gallium selenide ($\mathrm{Ga}\mathrm{Se}$) is a promising material for nanoscale electronic, spintronic, and optoelectronic applications. Its electrical conductivity and optical properties were reported to be largely tuned by strain engineering, where the assessment of the applied strain is crucial. In this work, we apply first-principles calculations to unveil the strain-induced phonon frequency shifts in monolayer $\mathrm{Ga}\mathrm{Se}$, showing its promising application as a strain probe. Furthermore, we find that the uniaxial-, biaxial-, and shear-strain onset for strain-induced lattice instability are 22%, 16%, and 5%, respectively, indicating it could be used as a handle in flexible electronics. We find monolayer $\mathrm{Ga}\mathrm{Se}$ to be about 2 times softer than monolayer ${\mathrm{Mo}\mathrm{S}}_2$ and 4 times softer than monolayer graphene, and that its Raman fingerprint modes can resolve strain values in the low-strain regime up to 1.0%. These results suggest its use as a strain sensor in fragile applications. Our results provide insight into the use of Raman and infrared spectroscopies to probe the strain in monolayer $\mathrm{Ga}\mathrm{Se}$, which is of paramount importance for further developments in strain engineering and flexible electronics in $\mathrm{Ga}\mathrm{Se}$-based devices.