We demonstrate a fiber-type optomechanical array consisting of elastically interconnected silica microbottle resonators with high-$Q$ optical and mechanical modes. In total, $50$ optomechanical resonators fabricated by fine glass processing are uniformly arrayed on a silica fiber. Evanescent coupling of a tapered optical fiber to an arbitrary resonator allows for highly sensitive readout and efficient actuation of mechanical motion at an arbitrary position in the array. Phonon propagation through the $50$ microbottles is achieved by both linearly and parametrically driving a mechanical mode at one end and by detecting it at the other end. This optomechanical array is scalable, tunable, and lithography-free and can be extended to fiber-based sensory applications with structural flexibility and operability in various environments.

Quantum digital signatures (QDSs) can provide information-theoretic security of messages against forgery and repudiation. Compared with previous QDS protocols that focus on signing one-bit messages, hash function–based QDS protocols can save quantum resources and are able to sign messages of arbitrary length. Using the idea of likely bit strings, we propose an efficient QDS protocol with hash functions over long distances. Our method of likely bit strings can be applied to any quantum key distribution–based QDS protocol to significantly improve the signature rate and dramatically increase the secure signature distance of QDS protocols. In order to save computing resources, we propose an improved method where Alice participates in the verification process of Bob and Charlie. This eliminates the computational complexity relating to the huge number of all likely strings. We demonstrate the advantages of our method and our improved method with the example of sending-or-not-sending QDS. Under typical parameters, both our method and our improved method can improve the signature rate by more than $100$ times and increase the signature distance by about $150\mathrm{km}$ compared with hash function–based QDS protocols without likely bit strings.

Controlling magnetic properties through the application of an electric field is a significant challenge in modern nanomagnetism. In this study, we investigate the magnetoionic control of magnetic anisotropy in the topmost Co layer in $\mathrm{Ta}$/$\mathrm{Pt}$/[$\mathrm{Co}$/$\mathrm{Al}$/$\mathrm{Pt}$]${}_n$/$\mathrm{Co}$/$\mathrm{Al}$/$\mathrm{Al}\mathrm{O}$${}_x$ multilayer stacks comprising $n+1$ Co layers and its impact on the magnetic properties of the multilayers. We demonstrate that the perpendicular magnetic anisotropy can be reversibly quenched through gate-driven oxidation of the intermediary $\mathrm{Al}$ layer between $\mathrm{Co}$ and ${\mathrm{Al}\mathrm{O}}_x$, enabling dynamic control of the magnetic layers contributing to the out-of-plane remanence—varying between $n$ and $n+1$. For multilayer configurations with $n=2$ and $n=4$, we observe reversible and nonvolatile additions of 1/3 and 1/5, respectively, to the anomalous Hall-effect amplitude based on the applied gate voltage. Magnetic imaging reveals that the gate-induced spin-reorientation transition occurs through the propagation of a single ${90}^{\circ }$ magnetic domain wall separating the perpendicular and in-plane anisotropy states. In the five-repetition multilayer, the modification leads to a doubling of the period of the magnetic domains at remanence. These results demonstrate that the magnetoionic control of the anisotropy of a single magnetic layer can be used to control the magnetic properties of coupled multilayer systems, extending beyond the gating effects on a single magnetic layer.

Metagratings are promising candidates for extreme-angle beam manipulation, due to two main features, namely, the ability to maintain very high efficiency in beam manipulation at extreme angles and the simplicity of the sparsely arranged structure for an easy physical implementation. Here, we propose to design transmissive metagratings, which can achieve highly efficient extreme-angle beam manipulation under normal incidence using a fully analytical design methodology, where information about the positions and impedance density values of the meta-atoms are derived from closed-form expressions. The design methodology is verified by full-wave simulations for extreme-angle anomalous refraction and reflection, where the simulations results are found to be consistent with the theoretical predictions. Proof-of-concept prototypes are fabricated and experimental demonstrations performed around 10 GHz are found to be in good agreement with the simulation results. The proposed metagratings open the door to high-efficiency functionalities in far-field beam manipulations and wireless power transfer.

Dispersive readout, the current standard for measuring superconducting qubits, relies on relatively easy to engineer, static transverse coupling between a qubit and a detuned cavity mode. However, this method requires circuit design trade-offs between qubit-cavity coupling, cavity-feedline coupling, detuning, and the resultant decoherence introduced by the cavity mode. Additionally, the criteria for performing quantum nondemolition measurements, including reduced cavity photon numbers, further limits speed and fidelity. In this work, we introduce a circuit with pulse-tunable qubit-cavity and cavity-feedline couplings so that the entire qubit-measurement process can be fully optimized. Here, the core measurement process resembles a quantum gate operation between the qubit and the cavity, with the cavity acting like an ancilla, storing the qubit-state information. To fully explore time-dependent effects of the full qubit-cavity dynamics, we go beyond the rotating-wave, Jaynes-Cummings, or dispersive approximations, to develop a pulsed-measurement protocol. In addition, we propose a realistic circuit architecture that can provide a very fast, high-fidelity, QND measurement with near-unit efficiency.

Ferroelectric domain wall (DW) conductivity (DWC) can be attributed to two separate mechanisms: (a) the injection/ejection of charge carriers across the Schottky barrier formed at the (metal-)electrode-DW junction and (b) the transport of those charge carriers along the DW. Current-voltage ($I$-$U$) characteristics, recorded at variable temperatures from ${\mathrm{Li}\mathrm{Nb}\mathrm{O}}_3$ (LNO) DWs, are clearly able to differentiate between these two contributions. Practically, they allow us to directly quantify the physical parameters relevant to the two mechanisms (a) and (b) mentioned above. These are, for example, the resistance of the DW, the saturation current, the ideality factor, and the Schottky barrier height of the electrode-DW junction. Furthermore, the activation energies needed to initiate the thermally activated electronic transport along the DWs can be extracted. In addition, we show that electronic transport along LNO DWs can be elegantly viewed and interpreted in an adapted semiconductor picture based on a double-diode, double-resistor equivalent-circuit model, the R2D2 model. Finally, our R2D2 model was checked for its universality by successfully fitting the $I$-$U$ curves of not only $z$-cut LNO bulk DWs, but equally of $z$-cut thin-film LNO DWs, and of $x$-cut thin-film DWs as reported in literature.

Doped semiconducting polymers are attractive for optoelectronic applications due to their solution processibility, low cost, flexibility, and potential for use in near-room-temperature thermoelectric generators. One way to improve charge transport in doped conjugated polymer films is to orient all the polymer chains in the same direction via rub-aligning, although aligned doped polymer films have an anisotropic electrical conductivity. Here, we compare the use of different electrode geometries to measure the anisotropic conductivity of rub-aligned doped poly(3-hexylthiophene-2,5,-diyl) ($\mathrm{P}3\mathrm{HT}$) films. We show that for $\mathrm{P}3\mathrm{HT}$ films doped both conventionally with 2,3,5,6-tetrafluoro-7,7,8,8-tetracyanoquinodimethane (${\mathrm{F}}_4\mathrm{TCNQ}$) and via anion exchange with ${\mathrm{F}}_4\mathrm{TCNQ}$ and lithium bis(trifluoromethanesulfonyl)imide, a rectangular electrode geometry in combination with the modified Montgomery method can determine the anisotropic conductivity as accurately as the use of the Hall bar geometry. This rectangular geometry provides two significant advantages. First, the rectangular electrode geometry is much simpler to fabricate than the standard Hall bar geometry. Second, the rectangular geometry requires only a single sample to obtain both components of the anisotropic conductivity, unlike the Hall bar geometry for which two separate samples are required. This is particularly important for rub-aligned doped conjugated polymer films, which have large sample-to-sample variabilities. We also explore anisotropic Hall effect measurements of rub-aligned doped polymer films, and examine the use of a four-line electrode geometry, which has been popular in the literature for measuring the anisotropic conductivities of rub-aligned doped conjugated polymer films. We find that the four-line geometry overestimates the conductivity by approximately 40%.

Inverted coplanar or bottom-gate bottom-contact (BGBC)-type thin-film transistors (TFTs) present several advantages for the manufacture and application of organic TFTs, although serious difficulties are encountered when trying to achieve sufficiently high performance. Recently, it was demonstrated that both high mobility and ideal on-off switching are attainable in BGBC-type printed organic TFTs with highly clean semiconductor-gate dielectric interfaces. However, an unknown channel material dependence in the device performance is found. Here, we show that the stability of semiconductor/metal/dielectric ternary interfaces is a crucial factor in the operation of BGBC-type organic TFTs. We fabricate single-crystal organic semiconductor (OSC) films with various numbers of layers using two different materials (phenyl/alkyl-substituted benzothieno[3,2-b]benzothiophene and phenyl/alkyl-substituted benzothieno[3,2-b]naphtho[2,3-b]thiophene) on highly lyophobic Cytop gate dielectric surfaces. The transfer characteristics exhibit notable time-dependent degradation, which clearly depends on the material, layer number, and encapsulation. Kelvin-probe force microscopy measurements reveal that the degradation is ascribed to contact resistance at the source electrodes, while it can be more suppressed in multilayer (two or more layers) OSCs. Atomic force microscopy and in-plane x-ray diffraction profiles present signs of the transformation in single molecular bilayer OSCs laid on the electrodes. The results suggest the importance of the quality of the OSC layer at ternary interfaces, providing a clue for improving the performance of BGBC-type organic TFTs.

Thermionic energy conversion has attracted significant attention in recent years thanks to improvements enabled by microfabrication techniques and advanced materials. As society moves towards a more-environment-friendly future, thermionic energy converters (TECs) could be a competitive option for clean energy generation by recycling wasted heat from various thermal systems. Reducing the interelectrode gap to a few micrometers has recently been proposed to improve the performance of TECs by mitigating the effect of negative space charge. Moreover, adding microscale features onto the surface of a TEC’s emitter is another way to improve the device’s performance significantly. The presence of microstructures on the emitter can increase the effective emitting surface area and enhance the effects of Schottky barrier lowering. Despite their advantages, no theoretical framework exists presenting an in-depth analysis of the effect of microscale features on a TEC. This study presents a theoretical framework considering microscale hyperboloidal emitter surface features for a TEC with a microscale interelectrode gap. Considering their impact on the maximum potential barrier, emitting surface area, and near-field radiative heat transfer, we show that the maximum output power density can be increased by a factor of approximately $2$ when the plane-to-plane gap distance and the periodicity are both $0.5\text{μ}\mathrm{m}$ compared with a TEC with a planar emitter. Despite the significant increase in output power density, the increase in efficiency is minimal due to the increase in the thermal radiative heat flux. The present work provides a theoretical framework for analyzing TECs equipped with surface-engineered emitters that can significantly increase and control the output power density, pushing thermionic energy conversion towards becoming a more-practical means of energy conversion.

Pulsed optically pumped (POP) Rb atomic clock is considered to be a powerful technique for the Rb atomic clock due to the capability of light-shift mitigation and atomic spectroscopy narrowing. Relevant investigations are conducted over two decades, however, no integrated prototype of such a kind of clock has been reported yet. Here, we present an integrated prototype of the POP Rb atomic clock and quantitatively characterize its physical properties. The atomic clock shows a short-term fractional frequency stability of $2.6\times {10}^{-13}/\sqrt{\tau }$ (where $\tau$ is the averaging time), and comes to $2.3\times {10}^{-15}$ with 40 000 s averaging (drift removed) under atmospheric conditions. Physical effects contributing to the long-term frequency stability are carefully analyzed and cavity-pulling shift is identified as the leading limit. The techniques presented here enable realizing a low ${10}^{-15}$ level vapor-cell atomic clock under atmospheric conditions, which could benefit a wide range of terrestrial applications, such as quantum communication, decimeter-level terrestrial positioning.

We report the experimental observation of an elastic bound mode in the continuum (BIC) in a compact region of an architected beam. We consider a long slender beam with rigid masses attached at periodic intervals, with a compact segment bounded by four protruding side beams. The key idea is to seek a mode where the side beams move out of phase with the compact region, thereby nullifying the forces and moments outside this region and resulting in a bound mode. The structure is modeled with use of Euler-Bernoulli beam theory and the side beams are designed by our imposing equilibrium constraints required for a BIC. Multiple bound modes in the continuum are found in the compact region, and for each BIC, we find a one-parameter family of BIC-supporting side-beam designs. The predictions are verified by three-dimensional finite-element simulations, followed by their experimental observation by laser Doppler vibrometry in a macroscale structure. Our approach allows one to achieve bound modes in the continuum in an arbitrary-sized compact region of the architected beam. Our findings may open avenues for confining elastic wave energy in compact regions for applications in sensors and resonators.

We propose a superconducting phase-controlled thermal switch based on a four-terminal graphene-superconductor system. By the coupling of two superconducting leads on a zigzag graphene nanoribbon, both the normal-transmission coefficient and the crossed-Andreev-reflection coefficient, which dominate the thermal conductivity of electrons in the graphene nanoribbon, can be well controlled simultaneously by the phase difference of the superconducting leads. As a result, the thermal conductivity of electrons in the graphene nanoribbon can be tuned and a thermal switching effect appears. Using the nonequilibrium Green’s function method, we verify this thermal switching effect numerically. At ambient temperatures less than about one tenth of the superconducting transition temperature, the thermal switching ratio can exceed 2000. The performance of the thermal switch can be regulated by the ambient temperature, and doping or gating can slightly increase the thermal switching ratio. The use of narrower graphene nanoribbons and wider superconducting leads facilitates the obtaining of larger thermal switching ratios. This switching effect of electronic thermal conductance in graphene is expected to be experimentally realized and applied.

We examined the temporal evolution of Josephson and resistive barriers created by a 30-keV focused helium ion beam in microbridges of epitaxially grown single-crystal ${\mathrm{YBa}}_2{\mathrm{Cu}}_3{\mathrm{O}}_{7-\delta }$ thin films. Repeated electric transport measurements at 4.2 K within 300 days after irradiation revealed an increase in the critical current density $j_{\mathrm{c}}$ for devices stored at room temperature under nitrogen atmosphere. This behavior can be described by a diffusion-based model of displaced chain oxygen moving back to original lattice sites, thus healing the barrier and partially restoring critical current. We find that $j_{\mathrm{c}}\propto \exp (-\sqrt{t/\tau })$ with time $t$. The relaxation time $\tau$ increases exponentially with helium irradiation dose and can exceed several hundred days for high-quality Josephson junctions. To achieve higher diffusion rates and thus shorter relaxation times, we annealed some devices in different oxygen partial pressures, right after irradiation. Within a week, those junctions relaxed to a quasistable state, making this a feasible option to achieve temporal stability of device parameters.

Operating the cavity-embedded Cooper-pair transistor (CCPT) in the Kerr bistable regime, we demonstrate single-shot resolution between two charge states that are $0.09e$ apart. The measurement is performed with $94\mathrm{\%}$ fidelity in a duration of around $3\text{μ}\mathrm{s}$. This is done with a drive power that corresponds to only 20 intracavity photons on average in the high oscillation amplitude state of the CCPT, which is orders of magnitude smaller than that in rf single electron transistors. We find that the limiting factor for this mode of operation of the CCPT is the spontaneous fluctuation-induced switching between the two metastable oscillation amplitude states. We present empirical data on the variation of the switching dynamics with drive parameters and CCPT dc bias.

The magnetic spin degrees of freedom in magnetic materials serve as an additional way to tune materials properties, thereby invoking a magneto-optical response. Herein, we report the magneto-optoelectronic properties of a family of lead-free magnetic double perovskites of the form ${\mathrm{Cs}}_2\mathrm{Ag}T{X}_6$ ($T=\mathrm{Sc},\mathrm{Ti},\mathrm{V},\mathrm{Cr},\mathrm{Mn},\mathrm{Fe},\mathrm{Co},\mathrm{Ni},\mathrm{Cu};X=\mathrm{Cl},\mathrm{Br},\mathrm{I}$). These provide an extremely fertile series, giving rise to potential candidate materials for photovoltaic applications. In conjunction with a high absorption coefficient and a high simulated power-conversion efficiency for photovoltaic applications, a few compounds in this series exhibit magnetic character useful for spintronic applications. The interaction between magnetism and light can have far-reaching effects on the photovoltaic properties as a consequence of the shift in the defect energy levels due to the Zeeman effect. This subsequently affects the recombination rate of minority carriers, and hence the photoconversion efficiency. Moreover, the distinct ferromagnetic and antiferromagnetic ordering driven by hybridization and the superexchange mechanism can play a significant role in breaking the time-reversal and/or inversion symmetry. Such a coalescence of magnetism and efficient optoelectronic response has the potential to trigger a magnetic/spin anomalous photovoltaic (nonlinear optical) effect in this ${\mathrm{Cs}}_2\mathrm{Ag}T{X}_6$ family. These insights can thus channelize the advancement of lead-free double perovskites in the magnetic/spin anomalous-photovoltaic-effect field as well.

Acoustic metamaterials introduce unprecedented ways by which to modulate acoustic waves in amplitude, phase, or both. Reconfigurable acoustic metamaterials yield advantages when compared to conventional metamaterials due to their flexible geometry and feasibility in realizing versatile functions. Herein, a reconfigurable acoustic metamaterial based on the angle-variant unit cells is proposed and demonstrated. Two orientation states of the V-shaped unit cell are selected to maximize the phase-shift-modulation effect. Furthermore, a metamaterial consisting of 18 components is fabricated, each of which is composed of 18 V-shaped unit cells in different orientation states. The components of the metamaterial may be readily reconfigured to deliver various phase-shift profiles in order to achieve on-demand acoustic functions. In addition, this rotation mechanism can be predictably improved by integrating motors for dynamic real-time reconfiguration. In this study, a series of acoustic functions, including acoustic focusing, splitting, and diffusion, are numerically and experimentally demonstrated. Additionally, an acoustic blocker design is also proposed. The results we present herein demonstrate the promise of our design in wave control. This work extends the realm of reconfigurable acoustic metamaterials and provides an alternative path for multifunctional acoustic wave modulation.

Two-dimensional (2D) hexagonal lattices of $\mathrm{Cu}$ disks are shown to induce orientation-dependent magnonic crystal (MC) modes for propagating forward volume spin waves in a single-crystal yttrium iron garnet (YIG) film. The width and depth of the magnonic band gaps are 0.022 GHz and –15.2 dB at the frequency of 1.815 GHz. Integrating differently oriented lattices on the same YIG film and positioning them between the microwave antenna surrounded by magnon absorbers consisting of $\mathrm{Au}$ films, we clearly resolve a characteristic frequency shift of the magnonic band gap by altering the incident angle of the spin waves to the 2D MC. The shift amounts to approximately 10 MHz when the incident angle is changed between 10° and 30°. The obtained results show a good agreement with calculations using the finite integration technique and are a step toward complete band-gap investigations in YIG of ultralow spin-wave damping.

The temperature-dependent magnetic moment of a magnetically ordered material is fundamental to all aspects of its technological applications. In ferrimagnetic materials with multiple sublattices containing different magnetic ions, the magnetization can vary nonmonotonically with temperature. Computational modeling of these materials provides insight into their sublattice occupancy and prediction of their behavior as a function of composition. Here we develop a python computer code called dionne that models the magnetism of rare earth (RE) iron garnets (${\mathrm{RE}}_3{\mathrm{Fe}}_5{\mathrm{O}}_{12}$, REIGs) using molecular field coefficient theory. The program calculates the exchange interactions and the magnetic moment of each sublattice to determine the net magnetization and angular momentum as a function of temperature. dionne accounts for site occupancy on each sublattice, including the effects of nonmagnetic and magnetic substitutions, vacancies, ${\mathrm{Fe}}^{2+}$, and deviations from the ideal $\mathrm{RE}:\mathrm{Fe}$ stoichiometry by considering their effects on the magnetization and exchange coupling. Unlike previous iterative methods, dionne recursively solves for the moment at each temperature, yielding an excellent match to magnetization versus temperature data for a range of bulk garnets. This work predicts magnetic properties of REIGs with a variety of compositions and point defect levels and enables design of ferrimagnets with useful properties.