When performing precision measurements, the quantity being measured is often perturbed by the measurement process itself. Such measurements include precision frequency measurements for atomic clock applications carried out with Ramsey spectroscopy. With the aim of eliminating probe-induced perturbations, a method of generalized autobalanced Ramsey spectroscopy (GABRS) is presented and rigorously substantiated. The usual local-oscillator frequency control loop is augmented with a second control loop derived from secondary Ramsey sequences interspersed with the primary sequences and with a different Ramsey period. This second loop feeds back to a secondary clock variable and ultimately compensates for the perturbation of the clock frequency caused by the measurements in the first loop. We show that such a two-loop scheme can lead to perfect compensation for measurement-induced light shifts and does not suffer from the effects of relaxation, time-dependent pulse fluctuations and phase-jump modulation errors that are typical of other hyper-Ramsey schemes. Several variants of GABRS are explored based on different secondary variables including added relative phase shifts between Ramsey pulses, external frequency-step compensation, and variable second-pulse duration. We demonstrate that a universal antisymmetric error signal, and hence perfect compensation at a finite modulation amplitude, is generated only if an additional frequency step applied during both Ramsey pulses is used as the concomitant variable parameter. This universal technique can be applied to the fields of atomic clocks, high-resolution molecular spectroscopy, magnetically induced and two-photon probing schemes, Ramsey-type mass spectrometry, and the field of precision measurements. Some variants of GABRS can also be applied for rf atomic clocks using coherent-population-trapping-based Ramsey spectroscopy of the two-photon dark resonance.

Space-resolved Brillouin light-scattering spectroscopy and micromagnetic simulations are used to study the propagation of magnetostatic spin waves across nonidentical magnonic crystals within close proximity. We show that the characteristics of spin-wave modes in such structure depends on the geometry of adjacent magnonic crystals. In particular, our results demonstrate that the efficient spin-wave coupling at the frequency of the magnonic forbidden gap can be achieved, enabling the spin-wave power drop between asymmetric magnonic crystals. We demonstrate, that the combination of spatial-filtering features and the spin-wave coupling in the adjacent magnonic crystals leads to the realization of the frequency-selective magnonic drop filter which is expected to offer new functionalities for spin-wave demultiplexing applications within the complex magnonic circuitry.

Spin-orbit coupling of light, describing the interaction between the polarization (spin) and spatial degrees of freedom (orbit) of light, plays an important role in subwavelength scale systems and leads to many interesting phenomena, such as the spin Hall effect of light. Here, based on the spin-orbit coupling, we design and fabricate a helical tape waveguide (HTW), which can realize a circular-polarization-selective process. When the incident circularly polarized wave is of the same handedness as the helix of the HTW, a nearly complete transmission is observed; in contrast, a counterrotating circular polarization of incident wave results in a much lower transmission or is even totally blocked by the HTW. Indeed, both simulations and experiments reveal that the blocked component of power leaks through the helical aperture of the HTW and forms a conical beam analogous to helical Cherenkov radiation due to the conversion from the spin angular momentum to the orbital angular momentum. Our HTW structure demonstrates its potential as a polarization selector in a broadband frequency range.

Mutual synchronization of many Josephson junctions is required for superradiant enhancement of the emission power. However, the larger the junction array is, the more difficult is the synchronization, especially when the array size becomes much larger than the emitted wavelength. Here, we study experimentally Josephson emission from such larger-than-the-wavelength $\mathrm{Nb}/\mathrm{NbSi}/\mathrm{Nb}$ junction arrays. For one of the arrays we observe a clear superradiant enhancement of emission above a threshold number of active junctions. The arrays exhibit strong geometrical resonances, seen as steps in current-voltage characteristics. However, radiation patterns of the arrays have forward-backward asymmetry, which is inconsistent with the solely geometrical resonance (standing-wave) mechanism of synchronization. We argue that the asymmetry provides evidence for an alternative mechanism of synchronization mediated by unidirectional traveling-wave propagation along the array (such as a surface plasmon). In this case, emission occurs predominantly in the direction of propagation of the traveling wave. Our conclusions are supported by numerical modeling of Josephson traveling-wave antenna. We argue that such a nonresonant mechanism of synchronization opens a possibility for phase locking of very large arrays of oscillators.

Universal quantum gates and quantum error correction (QEC) lie at the heart of quantum-information science. Large-scale quantum computing depends on a universal set of quantum gates, in which some gates may be easily carried out, while others are restricted to certain physical systems. There is a unique three-qubit quantum gate called the Deutsch gate [$\mathbb{D}(\theta )$], from which a circuit can be constructed so that any feasible quantum computing is attainable. We design an easily realizable $\mathbb{D}(\theta )$ by using the Rydberg blockade of neutral atoms, where $\theta$ can be tuned to any value in $[0,\pi ]$ by adjusting the strengths of external control fields. Using similar protocols, we further show that both the Toffoli and controlled-not gates can be achieved with only three laser pulses. The Toffoli gate, being universal for classical reversible computing, is also useful for QEC, which plays an important role in quantum communication and fault-tolerant quantum computation. The possibility and speed of realizing these gates shed light on the study of quantum information with neutral atoms.

Considering the great success of scanning-tunneling-microscopy (STM) studies of graphene in the past ten years, it is quite surprising to notice that there is still a fundamental contradiction in the reported tunneling spectra of the quasifreestanding graphene monolayer. Many groups observed “$\mathsf{V}$-shaped” spectra with linearly vanishing density of states at the Dirac point, whereas others reported spectra with a gap of $\pm 60\mathrm{meV}$ pinned to the Fermi level in the quasifreestanding graphene monolayer. Here, we systematically study the two contradicting tunneling spectra of the quasifreestanding graphene monolayer on various substrates in the presence of different magnetic fields and demonstrate that both spectra are the “correct” spectra. However, the $\mathsf{V}$-shaped spectrum exhibits only the contribution of the low-energy Dirac fermions, whereas the gapped spectrum is contributed by both the low-energy Dirac fermions and the high-energy nearly free-electron states due to the existence of the inelastic tunneling process. Our results indicate that interaction with substrates plays a vital role in affecting the spectra of graphene. We also show that it is possible to switch the tunneling spectra between the two distinct features at the nanoscale through voltage pulses applied to the STM tip.

We investigate the use of ${\mathrm{Cu}}_{1-x}{\mathrm{Zn}}_x{\mathrm{Fe}}_2{\mathrm{O}}_4$ ferrites ($0.60<x<0.76$) as potential sensors for magnetic-resonance-imaging thermometry. Samples are prepared by a standard ceramic technique. Their structural and magnetic properties are determined using x-ray diffraction, scanning electron microscopy, superconducting quantum-interference device magnetometry, and Mössbauer and 3-T nuclear-magnetic-resonance spectroscopies. We use the mass magnetization of powdered ferrites and transverse relaxivity $r_2^{*}$ of water protons in Ringer’s-solution-based agar gels with embedded micron-sized particles to determine the best composition for magnetic-resonance-imaging (MRI) temperature sensors in the (280–323)-K range. A preclinical 3-T MRI scanner is employed to acquire $T_2^{*}$ weighted temperature-dependent images. The brightness of the MRI images is cross-correlated with the temperature of the phantoms, which allows for a temperature determination with approximately $1°\mathrm{C}$ accuracy. We determine that the composition of $0.65<x<0.70$ is the most suitable for MRI thermometry near human body temperature.

We present TEM images of the interior of metal whiskers (MWs) grown on electroplated Sn films. Along with earlier published information, our observations focus on a number of questions, such as, why MWs’ diameters are in the micron range (significantly exceeding the typical nanosizes of nuclei in solids), why the diameters remain practically unchanged in the course of MW growth, what the nature of MW diameter stochasticity is, and what the origin of the well-known striation structure of MW side surfaces is. In an attempt to address such questions, we perform an in-depth study of MW structure at the nanoscale by detaching a MW from its original film, reducing its size to a thin slice by cutting its sides by a focused ion beam, and performing TEM on that structure. Also, we examine the root of the MW and Cu-Sn interface for the intermetallic compounds. Our TEM observations reveal a rich nontrivial morphology suggesting that MWs may consist of many side-by-side grown filaments. This structure appears to extend to the outside whisker surface and be the reason for the striation. In addition, we put forward a theory where nucleation of multiple thin metal needles results in micron-scale and larger MW diameters. This theory is developed in the average field approximation similar to the roughening transitions of metal surfaces. The theory also predicts MW nucleation barriers and other observed features.

Antiferromagnets with zero net magnetic moment, strong anti-interference, and ultrafast switching speed are potentially competitive in high-density information storage. The body-centered tetragonal antiferromagnet ${\mathrm{Mn}}_2\mathrm{Au}$ with opposite-spin sublattices is a unique metallic material for Néel-order spin-orbit-torque (SOT) switching. We investigate the SOT switching in quasiepitaxial (103), (101) and (204) ${\mathrm{Mn}}_2\mathrm{Au}$ films prepared by a simple magnetron sputtering method. We demonstrate current-induced antiferromagnetic moment switching in all of the prepared ${\mathrm{Mn}}_2\mathrm{Au}$ films by using a short current pulse at room temperature, whereas differently oriented films exhibit distinguished switching characters. A direction-independent reversible switching is attained in ${\mathrm{Mn}}_2\mathrm{Au}$ (103) films due to negligible magnetocrystalline anisotropy energy, while for ${\mathrm{Mn}}_2\mathrm{Au}$ (101) and (204) films, the switching is invertible with the current applied along the in-plane easy axis and its vertical axis, but it becomes attenuated seriously during initial switching circles when the current is applied along the hard axis because of the existence of magnetocrystalline anisotropy energy. Besides the fundamental significance, the strong orientation-dependent SOT switching, which is not realized, irrespective of ferromagnet and antiferromagnet, provides versatility for spintronics.

We theoretically investigate the temperature-to-phase conversion (TPC) process occurring in dc superconducting quantum interferometers based on superconductor–normal-metal–superconductor ($S\text{−}N\text{−}S$) mesoscopic Josephson junctions. In particular, we predict the temperature-driven rearrangement of the phase gradients in the interferometer under the fixed constraints of fluxoid quantization and supercurrent conservation. This mechanism allows sizeable phase variations across the junctions for suitable structure parameters and temperatures. We show that the TPC can be a basis for sensitive single-photon sensors or bolometers. We propose a radiation detector realizable with conventional materials and state-of-the-art nanofabrication techniques. Integrated with a superconducting quantum-interference proximity transistor as a readout setup, an aluminum-based TPC calorimeter can provide a large signal-to-noise ratio $>100$ in the 10-GHz–10-THz frequency range and a resolving power larger than $10^2$ below 50 mK for terahertz photons. In the bolometric operation, electrical noise equivalent power of approximately ${10}^{-22}\mathrm{W}/\sqrt{\mathrm{Hz}}$ is predicted at 50 mK. This device can be attractive as a cryogenic single-photon sensor operating in the giga- and terahertz regime with applications in dark-matter searches.

We investigate the interaction of guided surface acoustic modes (GSAMs) in unconsolidated granular media with a metasurface, consisting of an array of vertical oscillators. We experimentally observe the hybridization of the lowest-order GSAM at the metasurface resonance, and note the absence of mode delocalization found in homogeneous media. Our numerical studies reveal how the stiffness gradient induced by gravity in granular media causes a down-conversion of all the higher-order GSAMs, which preserves the acoustic energy confinement. We anticipate these findings to have implications in the design of seismic-wave protection devices in stratified soils.

We investigate the microwave spectroscopies of a micrometer-sized single permalloy (Py) chiral structure on coplanar waveguides (CPWs). Under an external dc magnetic field applied in a direction perpendicular to the microwave propagation, the Py chiral structure loaded on the center of the CPW signal line shows Kittel-mode ferromagnetic resonance. Contrastingly, the structure on the signal-line edge highlights two additional resonances: spin-wave resonance at a higher frequency, and unique resonance at a lower frequency of approximately 7.8 GHz. The resonance signal at 7.8 GHz originates from magnetically induced, geometry-driven resonance, although the resonance frequency does not depend on the external magnetic field. Moreover, the displacement of the Py structures on the signal line results in nonreciprocal microwave transmission, which is traced back to the edge-guide mode.

We present a coherent switch for motion transduction based on dynamically localized mechanical modes in an optomechanical system consisting of two coupled cantilevers. By placing one of the cantilevers inside a harmonically oscillating optical trap, the effective coupling strength between the degenerate cantilevers can be tuned experimentally. In particular, when the coupling is turned off, we show that mechanical motion becomes tightly bounded to the isolated cantilevers rather than propagating away as a result of destructive Landau-Zener-Stückelberg–like interference. The effect of dynamical localization is adopted to implement a coherent switch, through which the tunneling oscillation is turned on and off with well-preserved phase coherence. We provide a simple yet efficient approach for full control of the coupling between mechanical resonators, which is highly desirable for coherent control of transport phenomena in a coupled-mechanical-resonator array.

The gap sizes between electrodes generated by typical methods are generally much larger than the dimension of a common molecule when fabricating a single-molecule junction, which dramatically suppresses the yield of single-molecule junctions. Based on the ab initio calculations, we develop a strategy named the field-aided method to accurately fabricate an atomic-sized gap between gold nanoelectrodes. To understand the mechanism of this strategy, configuration evolutions of gold nanojunction in stretching and compressing processes are calculated. The numerical results show that, in the stretching process, the gold atoms bridged between two electrodes are likely to form atomic chains. More significantly, lattice vacant positions can be easily generated in stretching and compressing processes, which make field-aided gap generation possible. In field-aided atom migration (FAAM), the external field can exert driving force, enhance the initial energy of the system, and decrease the barrier in the migration path, which makes the atom migration feasible. Conductance and stretching and compressing forces, as measurable variables in stretching and compressing processes, present very useful signals for determining the time to perform FAAM. Following this desirable strategy, we successfully fabricate gold nanogaps with a dimension of $0.38\pm 0.05\mathrm{nm}$ in the experiment, as our calculation simulates.

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

The utilization of defects in carbon nanotubes to improve their photoluminescence efficiency has become a widespread study of the realization of efficient light-emitting devices. Here, we report a detailed comparison of the defects in nanotubes (quantified by Raman spectroscopy) and photoluminescence (PL) intensity of individual suspended carbon nanotubes (CNTs). We also evaluate the impact of these defects on the electron or hole transport in the nanotubes, which is crucial for the ultimate realization of optoelectronic devices. We find that brightly luminescent nanotubes exhibit a pronounced D-band in their Raman spectra, and vice versa, dimly luminescent nanotubes exhibit almost no D-band. Here, defects are advantageous for light emission by trapping excitons, which extend their lifetimes. We quantify this behavior by plotting the PL intensity as a function of the $I_D/I_G$-band Raman intensity ratio, which exhibits a Lorentzian distribution peaked at $I_D/I_G=0.17$. For CNTs with a $I_D/I_G$ ratio $>0.25$, the PL intensity decreases, indicating that above some critical density, nonradiative recombination at defect sites dominates over the advantages of exciton trapping. In an attempt to fabricate optoelectronic devices based on these brightly luminescent CNTs, we transfer these suspended CNTs to platinum electrodes and find that the brightly photoluminescent nanotubes exhibit nearly infinite resistance due to these defects, while those without bright photoluminescence exhibit finite resistance. These findings indicate a potential limitation in the use of brightly luminescent CNTs for optoelectronic applications.

We consider sorting for the broad class of micromachines (also known as microswimmers, microrobots, micropropellers, etc.) propelled by rotating magnetic fields. We present a control policy that capitalizes on the variation in magnetic properties between otherwise-homogeneous micromachines to enable the sorting of a select fraction of a group from the remainder and prescribe its net relative movement, using a uniform magnetic field that is applied equally to all micromachines. The method enables us to accomplish this sorting task using open-loop control, without relying on a structured environment or localization information of individual micromachines. With our method, the control time to perform the sort is invariant to the number of micromachines. The method is verified through simulations and scaled experiments. Finally, we include an extended discussion about the limitations of the method and address open questions related to its practical application.

We model laser-induced transient-current waveforms in radiation coplanar-grid detectors. Poisson’s equation is solved by the finite-element method and currents induced by a photogenerated charge are obtained using the Shockley-Ramo theorem. The spectral response on a radiation flux is modeled by Monte Carlo simulations. We show a $10\times$ improved spectral resolution of the coplanar-grid detector using differential signal sensing. We model the current waveform dependence on the doping, depletion width, diffusion, and detector shielding, and their mutual dependence is discussed in terms of detector optimization. The numerical simulations are successfully compared to experimental data, and further model simplifications are proposed. The space charge below electrodes and a nonhomogeneous electric field on a coplanar-grid anode are found to be the dominant contributions to laser-induced transient-current waveforms.

We report a first-principles study of electronic structures, magnetic properties, and the tunneling-magnetoresistance (TMR) effect of a series of ferromagnetic nitride $M_4\mathrm{N}$ ($M=\mathrm{Fe}$, Co, Ni)-based magnetic tunnel junctions (MTJs). It is found that bulk ${\mathrm{Fe}}_4\mathrm{N}$ reveals a half-metal nature in terms of the ${\mathrm{\Delta }}_1$ state. A perpendicular magnetic anisotropy is observed in the periodic system ${\mathrm{Fe}}_4\mathrm{N}/\mathrm{MgO}$. In particular, the ultrahigh TMR ratio of over 24 000% is predicted in the ${\mathrm{Fe}}_4\mathrm{N}/\mathrm{MgO}/{\mathrm{Fe}}_4\mathrm{N}$ MTJ due to the interface resonance tunneling and relatively high transmission for states of other symmetry. Besides, the large TMR can be maintained with the change of atomic details at the interface, such as the order-disorder interface, the change of thickness of the MgO barrier, and different in-plane lattice constants of the MTJ. The physical origin of the TMR effect can be well understood by analyzing the band structure and transmission channel of bulk ${\mathrm{Fe}}_4\mathrm{N}$ as well as the transmission in momentum space of ${\mathrm{Fe}}_4\mathrm{N}/\mathrm{MgO}/{\mathrm{Fe}}_4\mathrm{N}$. Our results suggest that the ${\mathrm{Fe}}_4\mathrm{N}/\mathrm{MgO}/{\mathrm{Fe}}_4\mathrm{N}$ MTJ is a benefit for spintronic applications.

The coherent perfect absorber (CPA), a time-reversed counterpart to the laser emission, could cause all energy fed to the system to be absorbed. It can also be used as an absorptive interferometer, which could provide applications in controllable optical energy transfer. Here, in order to achieve a terahertz CPA, we propose a designed metasurface and experimentally demonstrate that it can serve as a polarization-insensitive CPA at a one-frequency channel under normal symmetric excitation, while a transverse-electric CPA at two-frequency channels around oblique 40° symmetric incidence. Such phenomena in this system can be attributed to Fano resonance consisting of interacting one bright and one dark mode under normal incidence and an additional operative dark mode under oblique symmetric excitation. The experimental results find good agreement with the fitted coupled-mode theory. Moreover, we show that the output amplitude can be effectively tuned from 0 to 1 only by varying the relative phase between the two input waves. The designed CPA could find potential application in effectively controlling absorption for terahertz imaging and terahertz switches.

We propose an on-chip optical-power delivery system for dielectric laser accelerators based on a fractal “tree-network” dielectric waveguide geometry. This system replaces experimentally demanding free-space manipulations of the driving laser beam with chip-integrated techniques based on precise nanofabrication, enabling access to orders-of-magnitude increases in the interaction length and total energy gain for these miniature accelerators. Based on computational modeling, in the relativistic regime, our laser delivery system is estimated to provide 21 keV of energy gain over an acceleration length of $192\mu \mathrm{m}$ with a single laser input, corresponding to a 108-MV/m acceleration gradient. The system may achieve 1 MeV of energy gain over a distance of less than 1 cm by sequentially illuminating 49 identical structures. These findings are verified by detailed numerical simulation and modeling of the subcomponents, and we provide a discussion of the main constraints, challenges, and relevant parameters with regard to on-chip laser coupling for dielectric laser accelerators.