We numerically investigate the high-efficiency photovoltaic effect in a lateral $p$-$n$ homojunction based on monolayer black phosphorus (MBP) by using the nonequilibrium Green’s function combined with the density-functional theory. Because of the built-in electric field of the $p$-$n$ junction and the wrinkle structure of MBP, the photocurrent excited by either linearly or elliptically polarized light is significantly enhanced in a wide photon energy range. Moreover, because of the electron-photon interaction, the photocurrent is related to atomic orbitals through the polarizing angle of the polarized light. Therefore, we can read the orbital information of the band structure from the polarizing angular distribution of the photocurrent. These findings suggest the promising application of the MBP-based $p$-$n$ homojunction in high-efficiency photoelectric devices and orbital-resolved photovoltaic detection.

Since the first proof-of-principle experiments 25 years ago, quantum metrology has matured from fundamental concepts to versatile and powerful tools in a large variety of research branches, such as gravitational-wave detection, atomic clocks, plasmonic sensing, and magnetometry. At the same time, two-photon interferometry, which underpins the possibility of entanglement to probe optical materials with unprecedented levels of precision and accuracy, holds the promise to stand at the heart of innovative functional quantum sensing systems. We report a quantum based method for measuring the frequency dependence of the velocity in a transparent medium, i.e., chromatic dispersion (CD). This technique, using energy-time-entangled photons, allows straightforward access to CD value from the visibility of two-photon fringes recorded in a free-evolution regime. In addition, our quantum approach features all the advantages of classical measurement techniques, i.e., flexibility and accuracy, all in a plug-and-play system.

Dispersions around the Dirac points in sonic crystals give rise to various extraordinary transport properties of sound waves. In particular, with tilted Dirac cones, the acoustic Dirac crystals will exhibit obvious anisotropic properties. When the Dirac cone is titled to a certain degree, a unique type-III Dirac point emerges with the extreme anisotropic-wave-transport property, which features the crossing of flat bands and linear dispersions. Recently, some quantum and artificial structures have been proposed to realize different types of Dirac points, where the type-III Dirac point in sonic crystals has rarely been studied so far. Here, we utilize the orbital modes in an acoustic honeycomb lattice to obtain strictly flat bands through eliminating π bonding. The type-III Dirac points emerge with modulated intercell and intracell couplings, which are simulated with specific structural parameters. At type-III Dirac points, the acoustic wave propagation is extremely anisotropic and manifests as plaquette states that exist in the two-dimensional flat-band systems. Inspired by the unique properties at the type-III Dirac points, hybrid acoustic Dirac lattices of regions characterized by different wave-propagation directions are used to conduct the montage operation of plaquette states. The orbital type-III Dirac cones and the montage operation of plaquette states provide opportunities for manipulating energy flow in exotic ways and expanding the potential applications of Dirac materials.

In circuit quantum acoustodynamics (cQAD), superconducting circuits are combined with acoustic resonators to create and control nonclassical states of mechanical motion. Simulating these systems is challenging due to the extreme difference in scale between the microwave and mechanical wavelengths. All existing techniques simulate the electromagnetic and mechanical subsystems separately. However, this approach may not be adequate for all cQAD devices. Here, we demonstrate a single simulation of a superconducting qubit coupled to an acoustic and a microwave resonator and introduce two methods for using this simulation to predict the frequencies, coupling rates, and energy-participation ratios of the electromechanical modes of the hybrid system. We also discuss how these methods can be used to investigate important dissipation channels and quantify the nontrivial effects of mode hybridization in our device. Our methodology is flexible and can be extended to other acoustic resonators and quantum degrees of freedom, providing a valuable tool for designing hybrid quantum systems.

Nanomechanical resonators can serve as high-performance detectors and have the potential to be widely used in industry for a variety of applications. Most nanomechanical-sensing applications rely on detecting changes of resonance frequency. In commonly used frequency-tracking schemes, the resonator is driven at or close to its resonance frequency. Closed-loop systems can continually check whether the resonator is at resonance and adjust the frequency of the driving signal accordingly. In this work, we study three resonance-frequency-tracking schemes, a feedback-free (FF) scheme, a self-sustaining oscillator (SSO) scheme, and a phase-locked loop oscillator (PLLO) scheme. We improve and extend the theoretical models for the FF and the SSO tracking schemes and test the models experimentally with a nanoelectromechanical system (NEMS) resonator. We employ an SSO architecture with a pulsed positive-feedback topology and compare it to the commonly used PLLO and FF schemes. We show that all tracking schemes are theoretically equivalent and that they all are subject to the same speed versus accuracy trade-off characteristics. In order to verify the theoretical models, we present experimental steady-state measurements for all of the tracking schemes. The frequency stability is characterized by computing the Allan deviation. We obtain almost perfect correspondence between the theoretical models and the experimental measurements. These results show that the choice of the tracking scheme is dictated by cost, robustness, and usability in practice as opposed to fundamental theoretical differences in performance.

We use the principles of cavity optomechanics to design a resonant mechanical force sensor for atomic force microscopy. The sensor is based on a type of electromechanical coupling, dual to traditional capacitive coupling, whereby the motion of a cantilever induces surface strain that causes a change in the kinetic inductance of a superconducting nanowire. The cavity is realized by a compact microwave-plasma mode with an equivalent $LC$ circuit involving the kinetic inductance of the nanowire. The device is fully coplanar and we show how to transform the cavity impedance for optimal coupling to the transmission line and the following amplifier. For the device presented here, we estimate the bare kinetic inductive mechanoelectric coupling (KIMEC) rate $g_0/2\pi$ in the range 3–10 Hz. We demonstrate phase-sensitive detection of cantilever motion using a multifrequency pumping and measurement scheme.

The local ordering of atoms in alloys directly has a strong impact on their electronic and optical properties. This is particularly relevant in nonrandom alloys, especially if they are deposited far from the equilibrium processes, as is the case for epitaxial ${\mathrm{Ge}}_{1-x}{\mathrm{Sn}}_x$ layers. In this work, we investigate the arrangement of $\mathrm{Ge}$ and $\mathrm{Sn}$ atoms in optoelectronic grade ${\mathrm{Ge}}_{1-x}{\mathrm{Sn}}_x$ epitaxial layers featuring a $\mathrm{Sn}$ content in the 5–14% range by using polarization-dependent Raman spectroscopy and density-functional-theory calculations. The thorough analysis of the polarization-dependent spectra in parallel and perpendicular configuration allowed us to properly tag all the observed vibrational modes, and to shed light on that associated to disorder-assisted Raman transitions. Indeed, with the help of large-scale atomistic simulations, we were able to highlight how the presence of $\mathrm{Sn}$ atoms, that modify the local environments of $\mathrm{Ge}$ atoms, gives rise to two spectral features at different Raman shifts, corresponding to distortions of the atomic bonds. This analysis provides a valuable framework for advancing the understanding of the vibrational properties in ${\mathrm{Ge}}_{1-x}{\mathrm{Sn}}_x$ alloys, particularly with regard to the impact of local ordering of the different atomic species.

Despite extensive research interests in perovskite oxides, low-energy consumption, nondestructive, and manoeuverable methods for phase transition in perovskite oxides are still under exploration, and the underlying mechanisms remain ambiguous. Here, optical susceptibility, including electron and anharmonic phonon contributions, is used to evaluate Gibbs free-energy variations of ${\mathrm{Pb}\mathrm{Ti}\mathrm{O}}_3$ and ${\mathrm{Ba}\mathrm{Ti}\mathrm{O}}_3$ under terahertz irradiation. This corresponds to an off-resonant light-controlled phase transition, rather than the resonant approaches that excites hot carriers over electronic band or infrared-active vibrations in the phonon band. We show that intermediate terahertz light can trigger polarization change between ferroelectric orientation variants of ${\mathrm{Pb}\mathrm{Ti}\mathrm{O}}_3$ at room temperature. Similarly, the phase transformation from low-symmetric ferroelectric phase to high-symmetric paraelectric structure in ${\mathrm{Pb}\mathrm{Ti}\mathrm{O}}_3$ can be driven by changing the polarization and intensity of the incident light. Analogous results are obtained in ${\mathrm{Ba}\mathrm{Ti}\mathrm{O}}_3$. In detail, the phonon spectrum and optical susceptibility exhibit strong temperature dependence, in which we show significant effects of anharmonic vibration. In order to explicitly show its nonlinear optical nature, we perform an alternating electric field dressed ab initio molecular dynamics simulation, which maps onto the Raman-active phonon excitation under off-resonant terahertz irradiation.

Understanding the computational overheads imposed by classical control systems on quantum computing platforms becomes critically important as these quantum machines grow in scale and complexity. In this work, we calculate the overheads imposed by the implementation of real-time graph traversal algorithms needed to find computational paths through incomplete cluster states for the implementation of one-qubit gates; a necessary requirement for a realistic implementation of photonic measurement-based quantum computing. By implementing two different algorithms, a global breadth-first search that searches the entire cluster state and an incremental version that traverses a narrow subsection of the cluster state, we analyze the trade-off between the accuracy of finding viable paths and the speed at which this operation can be performed, which constrains the overall photonic clock cycle of the system. We also outline the broader implications of our results for implementing classical control systems for measurement-based photonic quantum computing.

Optically levitated microspheres are an excellent candidate for force and acceleration sensing. Here, we propose an acceleration-sensing protocol based on an optically levitated microsphere array (MSA). The system consists of an $N$-microsphere array levitated in a driven optical cavity via holographic optical tweezers. By positioning the microspheres suitably relative to the cavity, the center-of-mass (COM) mode of the collective motion of the MSA is coupled to the cavity mode. The optomechanical interaction encodes the information of acceleration that acts on the MSA onto the intracavity photons, which can then be detected directly at the output of the cavity. The optically levitated MSA distributes its mass into multiple optical traps, which not only circumvents the problem of levitating a large-mass microsphere but also leads to a significant improvement in sensitivity. Compared with the traditional single-microsphere measurement scheme, our method presents an improvement in sensitivity by a factor of $\sqrt{N}$.

We theoretically predict and experimentally observe the onset of weak-link physics in the dynamical response of transition-edge sensors (TESs). We develop a theoretical framework based on a Fokker-Planck description that incorporates both the TESs electrical response, stemming from Josephson phenomena, and the electrothermal effects due to coupling to a thermal bath. Our measurements of a varying dynamic resistance are in excellent agreement with our theory, thereby establishing weak-link phenomena as the main mechanism underlying the operation of TESs. Furthermore, our description enables the calculation of power spectral densities, paving the way for a more thorough investigation of the unexplained “excess noise” in long diffusive junctions and TESs reported in recent experiments.

By performing density-functional calculations, we find a giant magnetocrystalline anisotropy (MCA) constant in abundant element cerium ($\mathrm{Ce}$) substituted M-type hexaferrite, in the energetically favorable strontium site, assisted by a quantum confined electron transfer from $\mathrm{Ce}$ to a specific iron ($2a$) site. Remarkably, the calculated electronic structure shows that the electron transfer leads to the formation of ${\mathrm{Ce}}^{3+}$ and ${\mathrm{Fe}}^{2+}$ at the $2a$ site producing an occupied $\mathrm{Ce}$($4f^1$) state below the Fermi level that adds a significant contribution to MCA and magnetic moment. A half $\mathrm{Ce}$ substitution forms a metallic state, while a full substitution retains the semiconducting state of the strontium hexaferrite (host). In the latter, the band gap is reduced due to the formation of charge-transferred states in the gap region of the host. The optical absorption coefficient shows an enhanced anisotropy between light polarization in parallel and perpendicular directions. Calculated formation energies, including the analysis of probable competing phases, and elastic constants confirm that both compositions are chemically and mechanically stable. With successful synthesis, the $\mathrm{Ce}$ hexaferrite can be an alternative high-performing critical-element-free permanent magnet material adapted for use in devices such as automotive traction drive motors.

Acoustic beam-shaping techniques have a variety of application scenarios. Traditional phase-only modulation techniques are unable to accurately shape ultrasonic beams that require precise modulation of both amplitude and phase, such as nondiffracting beams. To provide more flexible, simple, and efficient shaping methods, we have developed the Fourier-transform acoustic beam-shaping system. Our proposed system enables the phase-only acoustic beam to reconstruct wave front and optimize its shape for achieving amplitude and phase modulation. We conducted experiments on generating different types of beams using the system. The feasibility of the system was established by generating multifocal beams. Additionally, nondiffracting Weber beams were constructed, with the findings showing that the beam possesses self-healing capability and can travel along a parabolic trajectory. Furthermore, by extending the Fourier transform to the two-dimensional space, we have successfully generated a perfect acoustic vortex beam, which has the same annular intensity profile for any given integer value of the order. Experiments showed that the perfect vortex beam enables the flexible and accurate manipulation of single and multiple particles. We conclude that the Fourier-transform acoustic system is a convenient way for acoustic beam shaping and has broad application prospects, such as drug delivery and high-speed acoustic communication.

Recently, much attention has been paid to second-order photonic topological insulators (SPTIs), because of their support for highly localized corner states with excellent robustness. SPTIs have been implemented in either transverse-magnetic (TM) or transverse-electric (TE) polarizations in two-dimensional photonic crystals (PCs), and the resultant topological corner states are polarization dependent, which limits their application in polarization-independent optics. However, to achieve polarization-independent corner states is not easy, since they are usually in-gap and the exact location in the topological band gap is not known in advance. Here we report on an SPTI based on a two-dimensional square-lattice PC made of an elliptic metamaterial, and we report that whether the band gap is topological or trivial depends on the choice of the unit cell. It is found that locations of topological band gaps of TM and TE polarizations in the frequency spectrum can be independently controlled by the out-of-plane permittivity ${\epsilon }_{\perp }$ and the in-plane permittivity ${\epsilon }_{\parallel }$, respectively, and more importantly, the location of in-gap corner states can also be separately manipulated by them. From this, we achieve topological corner states for both TM and TE polarizations with the same frequency in the PC by adjusting ${\epsilon }_{\perp }$ and ${\epsilon }_{\parallel }$, and their robustness with regard to disorders and defects is numerically demonstrated. The proposed SPTI provides a potential application scenario for polarization-independent topological photonic devices.

We experimentally verified our prior theoretical work [J. Opt. Soc. Am. B 39, 3012 (2022)] by employing matter-wave interference to measure the ultranarrow momentum width of an optically trapped Bose-Einstein condensate (BEC) in situ. Splitting the BEC wave packet into various diffraction orders through double stand-wave pulses, we calibrated the momentum width by fitting the oscillation curve of the population of the zero-momentum state. The observed interference fringes exhibited slight deviations from our simplified theory. We numerically calculated the Gross-Pitaevskii equation and utilized Wigner function to gain insight into the impact of the external trap potential and nonlinear term of BEC. We found that the discrepancy was caused by the combined effects of the mean-field interaction and spatial density modulation of BEC. This quantum thermometry is straightforward to apply and is especially well suited for temperature calibration in deep cooling experiments. For atomic samples at the pK level after deep cooling, interatomic interactions can be safely disregarded, enabling momentum-width calibration in a single shot.

The steering and the focusing of acoustic beams are of increasing interest for personalized-sound-zone applications. Sound zones require the generation of a strong contrast in sound pressure level between different regions of the space. A way to do so is to control and steer the acoustic field radiated from a source with use of metamaterial-based acoustic lenses. Predictive models based on numerical approaches such as the finite-element method are commonly used, but they are generally cumbersome and time-consuming. In contrast, explicit models rely on Snell’s law of refraction and phase gratings but do not account for the couplings between the metamaterial atoms. In this work, a metamaterial-based acoustic lens is proposed as a solution to steer and focus acoustic sources, with use of slits loaded with periodic Helmholtz resonators. We control the radiation of a baffled duct by a metalens located at the output of the waveguide and excited at the other end by a loudspeaker. A fully explicit two-dimensional predictive model is developed on the basis of the mode-matching technique, accounting for thermoviscous losses, interslit evanescent coupling, and high-order modes in the baffled duct. The pressure radiated by the metalens outside the baffled duct is then explicitly derived. The semianalytical results show excellent agreement when compared with the solutions obtained by the finite-element method, both in the near field and in the far field over a wide frequency range. An optimization strategy is then proposed, and the beam steering and focusing are reported experimentally on a three-dimensionally-printed prototype.

The superconducting fluxonium qubit has a great potential for high-fidelity quantum gates with its long coherence times and strong anharmonicity at the half-flux-quantum sweet spot. However, current implementations of two-qubit gates compromise fluxonium’s coherence properties by requiring either a temporary population of the noncomputational states or tuning the magnetic flux off the sweet spot. Here we realize a fast all-microwave cross-resonance gate between two capacitively coupled fluxoniums with the qubit dynamics well confined to the computational space. We demonstrate a direct controlled not (cnot) gate in 70 ns with fidelity up to $\mathcal{F}=0.9949(6)$ despite the limitations of a suboptimal measurement setup and device coherence. Our results project a possible pathway towards reducing the two-qubit error rate near to or below ${10}^{-4}$ with present-day technologies.

The propagation of magnetostatic surface spin waves in permalloy stripes was observed by high-resolution wide-field magneto-optical Kerr microscopy using the stroboscope technique. A long-time stable semiconductor-laser light source was employed and uniform illumination using a fiber speckle reducer and active stabilization of the focus and the sample position were achieved. These enabled the measurement of spin waves at a frequency of 5.6 GHz in a minimum stripe width of $2\text{μ}\mathrm{m}$ with a sensitivity of 1% saturation magnetization. A pronounced self-focusing effect was observed, which is explained by the peculiar dispersion relation of the spin wave. The capability of measuring all three vector components of magnetization allows direct observation of spin waves as a propagation of precessional motion.

We present all-electrical operation of a ${\mathrm{Fe}}_x{\mathrm{Cr}}_{1-x}$-based Curie switch at room temperature. More specifically, we study the current-induced thermally driven transition from ferromagnetic to antiferromagnetic Ruderman-Kittel-Kasuya-Yosida (RKKY) indirect coupling in a ${\mathrm{Fe}/\mathrm{Cr}/\mathrm{Fe}}_{17.5}{\mathrm{Cr}}_{82.5}/\mathrm{Cr}/\mathrm{Fe}$ multilayer. Magnetometry measurements at different temperatures show that the transition from the ferromagnetic to the antiferromagnetic coupling at zero field is observed at approximately $325$ K. Analytical modeling confirms that the observed temperature-dependent transition from indirect ferromagnetic to indirect antiferromagnetic interlayer exchange coupling originates from the modification of the effective interlayer exchange constant through the ferromagnetic-to-paramagnetic transition in the ${\mathrm{Fe}}_{17.5}{\mathrm{Cr}}_{82.5}$ spacer with minor contributions from the thermally driven variations of the magnetization and magnetic anisotropy of the Fe layers. Room-temperature current-in-plane magnetotransport measurements on the patterned ${\mathrm{Fe}/\mathrm{Cr}/\mathrm{Fe}}_{17.5}{\mathrm{Cr}}_{82.5}/\mathrm{Cr}/\mathrm{Fe}$ strips show the transition from the “low-resistance” parallel to the “high-resistance” antiparallel remanent magnetization configuration, upon increased probing current density. Quantitative comparison of the switching fields, obtained by magnetometry and magnetotransport, confirms that the Joule heating is the main mechanism responsible for the observed current-induced resistive switching.

Quasibrittle materials endowed with (statistically) self-similar hierarchical microstructures show distinct failure patterns that deviate from the standard scenario of damage accumulation followed by crack nucleation and growth. Here we study the failure of paper sheets with hierarchical slice patterns as well as nonhierarchical and unpatterned reference samples, considering both uncracked samples and samples containing a macroscopic crack. Failure is studied under displacement-controlled tensile loading as well as under creep conditions. Acoustic emission records and surface strain patterns are recorded alongside stress-strain and creep curves. The measurements demonstrate that hierarchical patterning efficiently mitigates against strain localization and crack propagation. In tensile loading, this results in a significantly increased residual strength of cracked samples. Under creep conditions, for a given range of lifetimes, hierarchically patterned samples are found to sustain larger creep strains at higher stress levels; their creep curves show unusual behavior characterized by multiple creep rate minima due to the repeated arrest of emergent localization bands.