We employ the phase-field method to assess electromigration (EM) damage in wide polycrystalline interconnects due to grain-boundary grooving. An interplay of surface and grain-boundary diffusion is shown to drastically influence the mode of progressive EM damage. Rapid atomic transport along the surface leads to shape-preserving surface drift reminiscent of Blech drift-velocity experiments. On the other hand, a comparatively faster grain-boundary transport localizes the damage, resulting in the proliferation of intergranular slits with a shape-preserving tip. At steady state, the two regimes exhibit exponents of 1 and $3/2$, respectively, in Black’s law. While surface drift obeys an inverse scaling with grain size, slits exhibit a direct relationship at small sizes, with the dependence becoming weaker at larger ones. Furthermore, we explain the influence of curvature- or EM-mediated healing fluxes running along the surface on groove replenishment. Insights derived from phase-field simulations of EM in bicrystals are extended to investigate the multiphysics of mixed-mode damage of a polycrystalline interconnect line that is characterized by a drift of small grain surfaces, slit propagation, and coarsening. The triple and quadruple junctions are identified as prominent sites of failure.

We demonstrate the ability to reduce the carrier concentration of thin films of the topological insulator (TI) ${\mathrm{Bi}}_2{\mathrm{Se}}_3$ by utilizing a nonvolatile electrostatic gating via corona charging of electret polymers. Sufficient electric field can be imparted to a polymer-TI bilayer to result in significant electron density depletion, even without the continuous connection of a gate electrode or the chemical modification of the TI. We show that the Fermi level of ${\mathrm{Bi}}_2{\mathrm{Se}}_3$ is shifted toward the Dirac point with this method. Using terahertz spectroscopy, we find that the surface chemical potential is lowered into the bulk band gap (approximately 50 meV above the Dirac point and 170 meV below the conduction-band minimum), and it is stabilized in the intrinsic regime while enhancing electron mobility. The mobility of surface state electrons is enhanced to a value as high as approximately $1600{\mathrm{cm}}^2/\mathrm{V}\mathrm{s}$ at 5 K.

The generation of true optical time standards will require the conversion of the highly stable optical-frequency output of an optical atomic clock to a high-fidelity time output. We demonstrate a comb-based clockwork that phase-coherently integrates $\sim 7\times {10}^{20}$ optical cycles of an input optical frequency to create a coherent time output. We verify the underlying stability of the optical timing system by comparing two comb-based clockworks with a common input optical frequency and show $<20\mathrm{fs}$ total time drift over the 37-day measurement period. Both clockworks also generate traditional timing signals including an optical pulse per second and a 10-MHz rf reference. The optical pulse-per-second time outputs remain synchronized to 240 attoseconds (240 as) at 1000 s. The phase-coherent 10-MHz rf outputs are stable to near a part in ${10}^{19}$. Fault-free timekeeping from an optical clock to femtosecond level over months is an important step in replacing the current microwave time standard by an optical standard.

Strong photon-photon interactions are a required ingredient for deterministic two-photon optical quantum logic gates. Multiphoton transitions in dense atomic vapors have been shown to be a promising avenue for producing such interactions. The strength of a multiphoton interaction can be enhanced by conducting the interaction in highly confined geometries such as small-cross-section optical waveguides. We demonstrate, both experimentally and theoretically, that the strength of such interactions scale only with the optical mode diameter, $d$, not $d^2$ as might be initially expected. This weakening of the interaction arises from atomic motion inside the waveguides. We create an interaction between two optical signals, at 780 and 776 nm, using the $5S_{1/2}\to 5D_{5/2}$ two-photon transition in rubidium vapor within a range of hollow-core fibers with different core sizes. The interaction strength is characterized by observing the absorption and phase shift induced on the 780-nm beam, which is in close agreement with theoretical modeling that accounts for the atomic motion inside the fibers. These observations demonstrate that transit-time effects upon multiphoton transitions are of key importance when engineering photon-photon interactions within small-cross-section waveguides that might otherwise be thought to lead to enhanced optical nonlinearity through increased intensities.

High-efficiency emission of multipoles is unachievable by a source much smaller than the wavelength, preventing compact acoustic devices for generating directional sound beams. Here, we present a primary scheme towards solving this problem by numerically and experimentally enclosing a monopole sound source in a structure with a dimension of around $1/10$ sound wavelength to emit a dipolar field. The radiated sound power is found to be more than twice that of a bare dipole. Our study of efficient emission of directional low-frequency sound from a monopole source in a subwavelength space may have applications such as focused ultrasound for imaging, directional underwater sound beams, miniaturized sonar, etc.

We fabricate a surface acoustic wave (SAW) device on a multiferroic ${\mathrm{BiFeO}}_3$ crystal while SAW devices are usually fabricated on nonmagnetic piezoelectrics and commercially available as bandpass filters. By using the time-domain technique, we demonstrate the SAW excitation on ${\mathrm{BiFeO}}_3$. The amplitude and phase of the SAW signal are modulated by the external magnetic field reflecting the multiferroicity of ${\mathrm{BiFeO}}_3$. The magnetic controllability of the multiferroic SAW device seems useful for the further functionalization of the SAW device.

Violating time-reversal symmetry enables one to engineer nonreciprocal structures for isolating and rectifying sound and mechanical vibrations. Rectifying sound is commonly achieved in nonlinear media, but the operation is inherently linked to weak and distorted signals. Here, we show how a pronounced electron-phonon coupling in linear piezophononic media under electrical bias can generate full mechanical rectification of broad spectral width, which permits the isolation of pulsed vibrations while keeping the wave-front shape fully intact. In this context, we deliberately show how the acoustoelectric effect can provide active loss compensation against lattice anharmonicity and thermoelastic damping. Further, our predictions confirm tunable nonreciprocity at an ultralarge contrast ratio, which should open the doors for future mechanical diodes and compact ultrasonic transducers for sensing and imaging.

Topological protected wave engineering in artificially structured media is at the frontier of ongoing metamaterials research that is inspired by quantum mechanics. Acoustic analogues of electronic topological insulators have recently led to a wealth of new opportunities in manipulating sound propagation with strikingly unconventional acoustic edge modes immune to backscattering. Earlier fabrications of topological insulators are characterized by an unreconfigurable geometry and a very narrow frequency response, which severely hinders the exploration and design of useful devices. Here we establish topologically protected sound in reconfigurable phononic crystals that can be switched on and off simply by rotating its three-legged “atoms” without altering the lattice structure. In particular, we engineer robust phase delay defects that take advantage of the ultrabroadband reflection-free sound propagation. Such topological delay lines serve as a paradigm in compact acoustic devices, interconnects, and electroacoustic integrated circuits.

An optoelectromechanical system formed by a nanomembrane capacitively coupled to an $LC$ resonator and to an optical interferometer has recently been employed for the highly sensitive optical readout of rf signals [T. Bagci et al., Nature (London) 507, 81 (2013)]. We propose and experimentally demonstrate how the bandwidth of such a transducer can be increased by controlling the interference between two electromechanical interaction pathways of a two-mode mechanical system. With a proof-of-principle device operating at room temperature, we achieve a sensitivity of $300\mathrm{nV}/\sqrt{\mathrm{Hz}}$ over a bandwidth of 15 kHz in the presence of radio-frequency noise, and an optimal shot-noise-limited sensitivity of $10\mathrm{nV}/\sqrt{\mathrm{Hz}}$ over a bandwidth of 5 kHz. We discuss strategies for improving the performance of the device, showing that, for the same given sensitivity, a mechanical multimode transducer can achieve a bandwidth significantly larger than that for a single-mode one.

We present a fully fiber-coupled source of high-fidelity single photons. An (In,Ga)As semiconductor quantum dot is embedded in an optical Fabry-Perot microcavity with a robust design and rigidly attached single-mode fibers, which enables through-fiber cross-polarized resonant laser excitation and photon extraction. Even without spectral filtering, we observe that the incident coherent light pulses are transformed into a stream of single photons with high purity (97%) and indistinguishability (90%), which is measured at an in-fiber brightness of 5% with an excellent cavity-mode-to-fiber coupling efficiency of 85%. Our results pave the way for fully fiber-integrated photonic quantum networks. Furthermore, our method is equally applicable to fiber-coupled solid-state cavity-QED-based photonic quantum gates.

We show that Co spins in $\mathrm{Co}/\mathrm{FeRh}$ epitaxial bilayers grown on W(110) switch reversibly between the two orthogonal in-plane directions as the FeRh layer undergoes temperature-driven antiferromagnetic-ferromagnetic (AFM-FM) phase transition. The switching of Co magnetization is characterized by a hysteretic behavior owing to a temperature hysteresis of the AFM-FM transition in FeRh. The spin reorientation of Co is driven by the evolution interfacial exchange coupling to the FeRh system across the AFM-FM process. Our results provide a method of writing information purely by a local temperature change.

Self-organizing logic is a recently suggested framework that allows the solution of Boolean truth tables “in reverse”; i.e., it is able to satisfy the logical proposition of gates regardless to which terminal(s) the truth value is assigned (“terminal-agnostic logic”). It can be realized if time nonlocality (memory) is present. A practical realization of self-organizing logic gates (SOLGs) can be done by combining circuit elements with and without memory. By employing one such realization, we show, numerically, that SOLGs exploit elementary instantons to reach equilibrium points. Instantons are classical trajectories of the nonlinear equations of motion describing SOLGs and connect topologically distinct critical points in the phase space. By linear analysis at those points, we show that these instantons connect the initial critical point of the dynamics, with at least one unstable direction, directly to the final fixed point. We also show that the memory content of these gates affects only the relaxation time to reach the logically consistent solution. Finally, we demonstrate, by solving the corresponding stochastic differential equations, that, since instantons connect critical points, noise and perturbations may change the instanton trajectory in the phase space but not the initial and final critical points. Therefore, even for extremely large noise levels, the gates self-organize to the correct solution. Our work provides a physical understanding of, and can serve as an inspiration for, models of bidirectional logic gates that are emerging as important tools in physics-inspired, unconventional computing.

The $3\omega$ method is a standard way to measure the thermal conductivity of thin films. In this study, we apply the method to read the magnetic state of a perpendicularly magnetized CoTb ferrimagnetic Hall cross using a thermal excitation. In order to generate the thermal excitation, an oscillating current at an $\omega$ frequency is applied to the Hall cross using different geometries. The magnetic signals oscillating at $\omega$, $2\omega$, and $3\omega$ are probed using a lock-in technique. From the analysis of the power dependence, we can attribute the $3\omega$ response to the temperature oscillation and the $2\omega$ to the temperature-gradient oscillation. Finally, the frequency dependence of the magnetic signals can be understood by considering the heat diffusion in a two-dimensional model.

In optical diffraction tomography, the multiply scattered field is a nonlinear function of the refractive index (RI) of the object. The Rytov method relies on a single-scattering propagation model and is commonly used to reconstruct images. Recently, a reconstruction model was introduced based on the beam propagation method that takes multiple scattering into account. We refer to this method as learning tomography (LT). We carry out simulations and experiments in order to assess the performance of LT over the iterative single-scattering propagation method. Each algorithm is rigorously assessed for spherical and cylinderical objects, with synthetic data generated using Mie theory. By varying the RI contrast and the size of the objects, we show that the LT reconstruction is more accurate and robust than the reconstruction based on the single-scattering propagation model. In addition, we show that LT is able to correct distortions that are evident in the Rytov-approximation-based reconstructions due to limitations in phase unwrapping. More importantly, the ability of LT to handle multiple scattering is demonstrated by simulations of multiple cylinders using Mie theory and is confirmed by experiment.

Since AlN has emerged as an important piezoelectric material for a wide variety of applications, efforts have been made to increase its piezoelectric response via alloying with transition metals that can substitute for Al in the wurtzite lattice. We report on density functional theory calculations of structure and properties of the ${\mathrm{Cr}}_x{\mathrm{Al}}_{1-x}\mathrm{N}$ system for Cr concentrations ranging from zero to beyond the wurtzite-rocksalt transition point. By studying the different contributions to the longitudinal piezoelectric coefficient, we propose that the physical origin of the enhanced piezoelectricity in ${\mathrm{Cr}}_x{\mathrm{Al}}_{1-x}\mathrm{N}$ alloys is the increase of the internal parameter $u$ of the wurtzite structure upon substitution of Al with the larger Cr ions. Among a set of wurtzite-structured materials, we find that ${\mathrm{Cr}}_x{\mathrm{Al}}_{1-x}\mathrm{N}$ has the most sensitive piezoelectric coefficient with respect to alloying concentration. Based on these results, we propose that ${\mathrm{Cr}}_x{\mathrm{Al}}_{1-x}\mathrm{N}$ is a viable piezoelectric material whose properties can be tuned via Cr composition. We support this proposal by combinatorial synthesis experiments, which show that Cr can be incorporated in the AlN lattice up to 30% before a detectable transition to rocksalt occurs. At this Cr content, the piezoelectric modulus $d_{33}$ is approximately 4 times larger than that of pure AlN. This finding, combined with the relative ease of synthesis under nonequilibrium conditions, may position ${\mathrm{Cr}}_x{\mathrm{Al}}_{1-x}\mathrm{N}$ as a prime piezoelectric material for applications such as resonators and acoustic wave generators.

A bipedal DNA nanowalker was recently reported to convert chemical energy into directional motion autonomously and efficiently. To elucidate its chemomechanical coupling mechanisms, three-dimensional molecular modeling is used to obtain coarse-grained foot-track binding potentials of the DNA nanowalker via unbiased and biased sampling techniques (for the potentials’ basin and high-energy edges, respectively). The binding state that is protected against fuel-induced dissociation responds asymmetrically to forward versus backward forces, unlike the unprotected state, demonstrating a mechanosensing capability to gate fuel binding. Despite complex DNA mechanics, the foot-track potential exhibits a surprisingly neat three-part profile, offering some general guidelines to rationally design efficient nanowalkers. Subsequent modeling of the bipedal walker attached to the track gives estimates of the free energy for each bipedal state, showing how the mechanosensing foot-track binding breaks the symmetry between the rear and front feet, enabling the rear foot to be selectively dissociated by fuel and generating efficient chemomechanical coupling.

Nanoelectromechanical systems provide ultrahigh performance in sensing applications. The sensing performance and functionality can be enhanced by utilizing more than one resonance mode of a nanoelectromechanical-systems device. However, it is often challenging to measure mechanical modes at high frequencies or modes that couple weakly to output transducers. In this paper, we propose the use of intermodal coupling as a mechanism to enable the detection of such modes. To implement this method, a probe mode is continuously driven and monitored using a phase-locked loop, while an auxiliary drive signal scans for other modes. Each time the auxiliary drive signal excites the corresponding mode by matching the mechanical frequency, the effective tension within the structure increases, which in turn causes a frequency shift in the probe mode. The location and width of these frequency shifts can be used to determine the frequency and quality factor of mechanical modes indirectly. Intermodal coupling can be used as a tool to obtain the spectrum of a mechanical structure even if some of these modes cannot be detected conventionally.

We find that the misfit strain may lead to the oscillatory size distributions of heteroepitaxial nanostructures. In heteroepitaxial FePt thin films grown on single-crystal MgO substrate, $⟨110⟩$-oriented mazelike and granular patterns with “quantized” feature sizes are realized in scanning-electron-microscope images. The physical mechanism responsible for the size oscillations is related to the oscillatory nature of the misfit strain energy in the domain-matching epitaxial $\mathrm{FePt}/\mathrm{MgO}$ system, which is observed by transmission electron microscopy. Based on the experimental observations, a model is built and the results suggest that when the FePt island sizes are an integer times the misfit dislocation period, the misfit strain can be completely canceled by the misfit dislocations. With applying the mechanism, small and uniform grain is obtained on the TiN (200) polycrystalline underlayer, which is suitable for practical application. This finding may offer a way to synthesize nanostructured materials with well-controlled size and size distribution by tuning the lattice mismatch between the epitaxial-grown heterostructure.

Although various defect centers have displayed promise as either quantum sensors, single photon emitters, or light-matter interfaces, the search for an ideal defect with multifunctional ability remains open. In this spirit, we study the dichroic silicon vacancies in silicon carbide that feature two well-distinguishable zero-phonon lines and analyze the quantum properties in their optical emission and spin control. We demonstrate that this center combines 40% optical emission into the zero-phonon lines showing the contrasting difference in optical properties with varying temperature and polarization, and a 100% increase in the fluorescence intensity upon the spin resonance, and long spin coherence time of their spin-$3/2$ ground states up to 0.6 ms. These results single out this defect center as a promising system for spin-based quantum technologies.

This paper is a contribution to the Physical Review Applied collection in memory of Mildred S. Dresselhaus.

We present a graphene-based metasurface that can be actively tuned between different regimes of operation, such as anomalous beam steering and focusing, cloaking, and illusion optics, by applying electrostatic gating without modifying the geometry of the metasurface. The metasurface is designed by placing graphene ribbons on a dielectric cavity resonator, where interplay between geometric plasmon resonances in the ribbons and Fabry-Perot resonances in the cavity is used to achieve a $2\pi$ phase shift. As a proof of concept, we demonstrate that the wave front of the field reflected from a triangular bump covered by the metasurface can be tuned by applying electric bias so as to resemble that of a bare plane and of a spherical object. Moreover, reflective focusing and the change of the reflection direction for the above mentioned cases are also shown.