At low temperatures, microwave cavities are often preferred for the readout and control of a variety of systems. In this paper, we present design and measurements on an optomechanical device based on a three-dimensional rectangular waveguide cavity. We show that by suitably modifying the electromagnetic field corresponding to the fundamental mode of the cavity, the equivalent circuit capacitance can be reduced to 29 fF. By coupling a mechanical resonator to the modified electromagnetic mode of the cavity, we achieve a capacitance participation ratio of $43\mathrm{\%}$. We demonstrate an optomechanical cooperativity, $C\sim$ 40, characterized by performing measurements in the optomechanically induced absorption limit. In addition, due to a low-impedance environment between the two halves of the cavity, our design has the flexibility of incorporating a dc bias across the mechanical resonator, often a desired feature in tunable optomechanical devices.

Midinfrared devices based on metalenses are of great utility for numerous applications, including free-space communication, imaging, and molecule sensing. However, because of the intrinsic dispersion of their building bricks, metalenses significantly suffer from large chromatic aberration. In this paper, we propose an achromatic metalens that can operate over a broad band of wavelengths in the infrared region from 3.7 to 4.5 $\mu \mathrm{m}$ for circularly polarized incidences. The Pancharatnam-Berry phase and the propagation phase are used to control the wavefront of light and eliminate the chromatic aberration, respectively. As a proof-of-principle demonstration, we numerically study two devices: an achromatic focusing lens and a beam deflector. The simulation results demonstrate the devices have a focusing efficiency of about $20.7\mathrm{\%}$ in the infrared region from 3.7 to 4.5 $\mu \mathrm{m}$. Our proposed approach may pave the way toward practical application of midinfrared devices.

We present an optical approach to compensating for spatially varying ac-Stark shifts that appear on atomic ensembles subject to strong optical control or trapping fields. The introduction of an additional weak light field produces an intentional perturbation between atomic states that is tuned to suppress the influence of the strong field. The compensation field suppresses sensitivity in one of the transition frequencies of the trapped atoms to both the atomic distribution and motion. We demonstrate this technique in a cold rubidium ensemble and show a reduction in inhomogeneous broadening in the trap. This two-color approach emulates the magic-trapping approach that is used in modern atomic-lattice clocks but provides greater flexibility in choice of atomic species, probe transition, and trap wavelength.

Magnetic skyrmions are chiral spin structures recently observed at room temperature in multilayer films. Their topological stability will enable high scalability in confined geometries—a sought-after attribute for device applications. Despite numerous theoretical studies examining sub-100-nm Néel skyrmions in nanostructures, in practice their ambient stability and evolution with confinement and their magnetic parameters remain to be established. Here we present the zero-field stabilization of sub-100-nm room-temperature Néel-textured skyrmions confined in $\mathrm{Ir}$/$\mathrm{Fe}$$(x)$/$\mathrm{Co}$$(y)$/$\mathrm{Pt}$ nanodots over a wide range of magnetic and geometric parameters. The zero-field skyrmion size, here as small as approximately 50 nm, can be tailored by a factor of 4 with variation of dot size and magnetic interactions. Crucially, skyrmions with differing thermodynamic stability exhibit an unexpected dichotomy in confinement phenomenologies. These results establish skyrmion phenomenology in multilayer nanostructures, and prompt the synergistic use of magnetic and geometric parameters to achieve desired properties in devices.

Recent advances in nanotechnology have enabled researchers to manipulate small collections of quantum-mechanical objects with unprecedented accuracy. In semiconductor quantum-dot qubits, this manipulation requires controlling the dot orbital energies, the tunnel couplings, and the electron occupations. These properties all depend on the voltages placed on the metallic electrodes that define the device, the positions of which are fixed once the device is fabricated. While there has been much success with small numbers of dots, as the number of dots grows, it will be increasingly useful to control these systems with as few electrode voltage changes as possible. Here, we introduce a protocol, which we call the “compressed optimization of device architectures” (CODA), in order both to efficiently identify sparse sets of voltage changes that control quantum systems and to introduce a metric that can be used to compare device designs. As an example of the former, we apply this method to simulated devices with up to 100 quantum dots and show that CODA automatically tunes devices more efficiently than other common nonlinear optimizers. To demonstrate the latter, we determine the optimal lateral scale for a triple quantum dot, yielding a simulated device that can be tuned with small voltage changes on a limited number of electrodes.

We propose an approach for real-time manipulation of low-frequency phononic band gaps in a metamaterial without affecting the material geometry, microarchitecture, or the crystal structure of the base material. Metamaterials with tunable band gaps are realized by introducing periodically arranged negative-stiffness inclusions, the modulus of which can be varied in time in order to modify the metamaterial macroscopic stiffness in certain directions without bringing the material to the point of elastic instability or inducing extreme geometric change. The evolution of band gaps is investigated numerically, and the proposed concept is verified experimentally in a lattice prototype with magnetic elements functioning as negative-stiffness units. Design guidelines for achieving a real-time tunable phononic band gap are also presented.

In the last 60 years conventional solid and electrolyte gating has allowed sizable modulations of the surface carrier concentration in metallic superconductors, resulting in tuning of their conductivity and change of their critical temperature. Recent conventional gating experiments on superconducting metal nanostructures showed full suppression of the critical current without variation of the normal-state resistance and the critical temperature. These results still lack a microscopic explanation. In this article, we describe a complete set of gating experiments on $\mathrm{Ti}$-based superconducting Dayem-bridges and a proposed classical thermodynamic model that seems to account for several of our experimental findings. In particular, zero-bias resistance and critical-current $I_C$ measurements highlight the following: the suppression of $I_C$ with both polarities of gate voltage, the surface nature of the effect, the independence of critical temperature from the electric field, and the gate-induced growth of a subgap dissipative component. In addition, the temperature dependence of the Josephson critical current seems to show a transition from ballistic Kulik-Omelyanchuk behavior to Ambegaokar-Baratoff tunnellike behavior on increase of the electric field. Furthermore, the $I_C$ suppression persists in the presence of sizable perpendicular-to-plane magnetic fields. We propose a classical thermodynamic model able to describe some of the experimental observations of the present and previous work. Above all, the model grabs the bipolar-electric-field-induced suppression of $I_C$ and the emergence of a subgap dissipative component near full suppression of the supercurrent. Finally, applications using the effect discussed are proposed.

The energy loss experienced by organic photovoltaics (OPVs) is the difference between the lowest photogenerated exciton energy of a donor or acceptor and the open circuit energy. It sets a fundamental limit to the open-circuit voltage and hence the efficiency of OPVs. This loss can be as large as 0.7 eV for fullerene acceptors, although nonfullerene acceptors (NFAs) reduce this to ≤0.6 eV. Here, we systematically quantify the relationship between charge transfer energy loss $(\mathrm{\Delta }{E}_{\mathrm{CT}})$, nonradiative recombination loss, exciton binding energy, and intra- and intermolecular electron-phonon couplings. Density functional theory and comprehensive quantum mechanical modeling are used to associate molecular volume, effective conjugation length, and the nonbonding character of molecules to these types of energy losses. Nonradiative recombination in donor-NFA heterojunctions is quantified by the charge transfer state emission quantum yield and its Frank-Condon shift. Our analytical results are consistent with measurements where $\mathrm{\Delta }{E}_{\mathrm{CT}}$ is varied between 0 and 0.6 eV using a variety of fullerene derivatives and thiophene-based NFAs paired with donor molecules. Molecular design rules to decrease the energy loss in OPVs derived from our analysis are provided.

Potential applications of ${\mathrm{Fe}}_3{\mathrm{O}}_4$ nanostuctures can be enhanced by preparing nanohollow spheres (NHSs) instead of nanoparticles and doping $\mathrm{Zn}$ to replace $\mathrm{Fe}$. Detailed temperature-dependent magnetic studies of ${\mathrm{Zn}}_x{\mathrm{Fe}}_{3-x}{\mathrm{O}}_4$ (x = 0.0, 0.1, 0.2, 0.3, 0.4, 1.0) NHSs indicate their increase in saturation magnetization ($M_S$) with $\mathrm{Zn}$ doping, attaining a maximum at x = 0.2 ($M_s=92.52\mathrm{emu}/\mathrm{g}$ at room temperature), due to the replacement of antiferromagnetically coupled ${\mathrm{Fe}}_{\mathrm{A}}^{3+}$ ions on the A site by nonmagnetic ${\mathrm{Zn}}^{2+}$ ions and the transformation of ${\mathrm{Fe}}_B^{2+}$ to ${\mathrm{Fe}}_B^{3+}$ at the B site for maintaining the charge neutrality. The dynamic magnetic properties, studied by analyzing the real and imaginary parts of ac susceptibility, reveal the spin cluster interaction in our systems. Moreover, the investigation of dielectric properties as a function of temperature over a frequency range of 10 KHz to 100 MHz indicates the reduction in dielectric permittivity and electrical conductivity with increasing $\mathrm{Zn}$ content. Enhanced magnetism with a decrease in conductivity, permittivity, and dipolar interaction enables ${\mathrm{Zn}}_x{\mathrm{Fe}}_{3-x}{\mathrm{O}}_4$ NHSs to be an excellent material for high-frequency and biomedical applications.

For recently emerging ferroelectric resistive memories, for example, ferroelectric diodes, retention is one of the most critical concerns and understanding its underlying physics is of importance. It is proposed that in a ferroelectric diode, the depolarization field ($E_{\mathrm{dp}}$)-driven polarization relaxation can modify the conduction against time, because the metal/ferroelectric Schottky barrier is greatly influenced by the polarization. Based on this effect, a theoretical model combining both the $E_{\mathrm{dp}}$-driven polarization relaxation kinetics and polarization-controlled Schottky emission is established. Simultaneously, we experimentally investigate the retention characteristics of the metal/$\mathrm{Bi}\mathrm{Fe}{\mathrm{O}}_3$ (BFO)/${\mathrm{La}}_{0.7}{\mathrm{Sr}}_{0.3}\mathrm{Mn}{\mathrm{O}}_3$ (LSMO) ferroelectric diodes with various BFO thicknesses and top electrodes in order to verify this model. It is revealed that all the samples exhibit switchable diodelike resistive switching behaviors. The measured current-time [I(t)-t] characteristics show that the current in the low-resistance state decreases with time, indicating resistance relaxation. Moreover, the resistance relaxation becomes faster as the BFO thickness decreases from 40 to 10 nm and as the top electrode changes from $\mathrm{Ag}$ to $\mathrm{Au}$ and to $\mathrm{Co}$. Using appropriate parameters, all the I(t)-t characteristics and their dependences on the BFO thickness and the top electrode's screening ability can be well described by the proposed retention model, in which the key role of the $E_{\mathrm{dp}}$ is demonstrated. It is, therefore, deduced that the retention loss in a ferroelectric diode is mainly caused by the $E_{\mathrm{dp}}$-driven polarization relaxation and its effects on the Schottky emission. In addition to these fundamental insights, our study also provides suggestions for the technical aspects, such as improving the retention properties and nondestructively monitoring the polarization over time.

We report the enhanced spin-orbit torque (SOT) effect due to the rare-earth metal $\mathrm{Tb}$ in a $\mathrm{Pt}/[\mathrm{Co}/\mathrm{Ni}{]}_2/\mathrm{Co}/\mathrm{Tb}$ multilayer with perpendicular magnetic anisotropy (PMA). Due to the large spin-orbit coupling in $\mathrm{Tb}$, the effective dampinglike efficiency, ${\xi }_{\mathrm{DL}}^{\mathrm{eff}}$, is determined to be 0.55 for 9-nm $\mathrm{Tb}$ as compared to ${\xi }_{\mathrm{DL}}^{\mathrm{eff}}=0.18$ for the reference $\mathrm{Pt}/[\mathrm{Co}/\mathrm{Ni}{]}_2/\mathrm{Co}/\mathrm{Ta}$ stack. A relatively large effective spin Hall efficiency of $\mathrm{Tb}$, ${\theta }_{\mathrm{SH,Tb}}^{\mathrm{eff}}$, is determined to be −0.480 for $\mathrm{Tb}$ with a 9-nm thickness through a H_z-biased harmonic measurement. Enhanced magnetization switching efficiency is observed with increasing $\mathrm{Tb}$ thickness, affirming the sizable antidamping torque responsible for deterministic switching. Furthermore, due to a more substantial interfacial coupling in $\mathrm{Co}/\mathrm{Tb}$ interface than in $\mathrm{Co}/\mathrm{Ta}$ interface, strong angular dependence of the dampinglike and fieldlike terms is, but diminishes with increasing $\mathrm{Tb}$ thickness. These results affirm that $\mathrm{Tb}$ plays a significant role in tuning the SOT efficiency in these structures as the increased $\mathrm{Tb}$ concentration leads to an enhancement of the effective spin Hall angle and switching efficiency.

Phosphorene is a promising single-elemental two-dimensional layered semiconductor with huge potential for future nanoelectronics and spintronics applications. In this work, we investigate the effect of an organic molecule (benzene) in the close proximity of a phosphorene nanoribbon. Our extensive calculations reveal that the semiconducting nature of phosphorene remains unaffected as a result of the molecular adsorption while the transport properties go through drastic changes. Under the influence of dopant atoms and external strain, colossal changes in the conductivity are observed, with a maximum enhancement $>1500\mathrm{\%}$. This effect is pretty robust against (i) variation of the system size, (ii) the type, location, and concentration of dopants and (iii) the nature and magnitude of the external strain. Furthermore, we demonstrated how a gate voltage can be used to fine tune the enhanced conductivity response in a field-effect transistor (FET) structure. Our results provide direction for phosphorene-based nanoelectronics in applications such as sensing, switching where a higher level of conduction can offer better resolution, a higher on:off ratio, and superior energy efficiency.

We present a terahertz (THz) radiation source combining the principles of spintronics and vacuum electronics: the electronic-spintronic THz emitter (E-STE). Different from the traditional laser-driven spintronic THz emitters, E-STE is driven by low-energy electron beams. Working through four steps: the electron-beam excitation of hot electrons, the electron superdiffusion, the inverse spin Hall effect, and the diffraction radiation of the periodic gratings, E-STE is able to emit continuous, monochromatic THz radiation. Detailed theoretical modeling and numerical calculation show that E-STE can cover the whole THz-frequency regime (0.1–10 THz) by modifying the period of the gratings. The electron beam voltage and current are relatively low (voltage U = 5 V, 25 V, peak current I∼100 mA), while the radiation power density and working efficiency are relatively high. (The power density can reach ${10}^8{\mathrm{W}/\mathrm{cm}}^2$, the working efficiency can reach 25%.) This compact and high-power E-STE may contribute to the development of a THz-radiation source integrated on chip.

The investigation of surface-induced lattice-strain effects in free-standing silicon nanocrystals ($\mathrm{Si}$ NCs) is fundamental in view of the materialization of nanosilicon-based technologies that exploit the unique properties of silicon at the nanoscale. In this work, a comprehensive investigation in free-standing $\mathrm{Si}$ NCs is performed, aimed at unveiling physical phenomena related to surface oxidation upon exposure to air, from which strongly surface-dependent lattice strain is found. Raman and infrared spectroscopy techniques are used to monitor the time evolution of the oxidation, through which a clear correlation between the formation of a superficial native oxide and the appearance of compressive strain in the nanocrystals’ core is found, which increases continuously as the oxidation progresses. By comparing experimental data with simulations using an improved phonon confinement model, it is concluded that strain is negligible in $\mathrm{H}$-terminated nanocrystals with sizes of approximately $3$ nm. After long-term natural oxidation, the compressive stress imposed by the native oxide shell is estimated to be 1.2 GPa. The results presented here link the time-dependent oxidation phenomenon with the experimental observation of compressive strain in free-standing $\mathrm{Si}$ NCs, clarifying contradicting results found in the literature, and demonstrate a simple route for the deconvolution of confinement and strain effects in low-dimensional structures using Raman spectroscopy.

Thermal maintenance costs 15% of global energy. The concept of an “energy-free thermostat” was accordingly proposed to change environmental temperature gradients, which can maintain a certain space at constant temperature without the need to consume additional energy according to a temperature-trapping theory [Shen et al., Phys. Rev. Lett. 117, 055501 (2016)]. Here we develop the theory by introducing thermoelectric effects, and we then propose a “negative-energy thermostat” that generates electricity associated with energy-free maintenance of a constant ambient temperature. The thermostat further helps to design a different thermoelectric cloak: for changing ambient temperature gradients, its central region has an approximately constant temperature and potential while simultaneously generating thermoelectromotive forces. Our finite-element simulations confirm the effects predicted by the theory developed, and we also discuss the possibility of experimental demonstration with natural materials. This work not only has relevance for expanding the temperature-trapping theory by achieving thermoelectric conversion synergistically but also offers a different method to manipulate coupled fields in electrothermotics; it also gives hints on how to control other coupled fields in photothermotics, photoelectrics, electromagnetics, thermomagnetics, and thermomechanics.

We present elastic phased diffraction gratings for passive manipulation of ultrasonic guided waves in solids by engineering the guided wave dispersion and diffraction wavefields. Such gratings are easy to fabricate, interchangeable, and feasible for a wide range of ultrasonic guided modes. We selectively prove the capability for modulating the commonly used antisymmetric A, symmetric S, and surface Rayleigh modes. Multiple functions including steering plane waves, as well as generating and steering focused, tweezerlike, and bottle-shaped guided wave beams are experimentally demonstrated simply by redesigning and changing the grating's superstrates. Our grating provides an alternative paradigm for manipulating multimodal ultrasonic guided waves in solids and will significantly advance a broad range of applications, such as nondestructive evaluation, structural health monitoring, energy harvesting, and acoustic tweezers.

Formation and dissolution of skyrmions via local modulation of magnetic anisotropy is theoretically explored in ferromagnetic (FM) and antiferromagnetic (AFM) thin-film structures. By a micromagnetic simulation, the spatial evolution and temporal evolution of spin textures are analyzed, while the effect of externally controlled effective magnetic anisotropy is introduced for a finite duration. The simulation results clearly indicate that the Néel-type skyrmions can be excited and destroyed in a FM layer or an AFM layer in selected parameter windows, enabling deterministic encoding of logical states “1” and “0” in this physical system. The characteristic times for the dynamical responses are much faster in the AFM structures, particularly in the creation process. The size of the excitation region can be reduced to the sub-10-nm range for the radius. The investigation also examines the effect of inhomogeneous in-plane magnetic properties, further elucidating the feasibility in realistic conditions. Considering the low-power and locally controlled nature of magnetic anisotropy modulation, this approach is expected to provide a highly efficient means for achieving skyrmion-based devices. The AFM structures offer a particularly promising opportunity.

High input intensities are usually required to efficiently produce nonlinear optical effects in ultrathin structures due to their extremely weak nature. This problem is particularly critical at low terahertz frequencies because high-input-power terahertz sources are not available. The demonstration of enhanced nonlinear effects at terahertz frequencies is particularly important since these nonlinear mechanisms promise to play a significant role in the development and design of new reconfigurable planar terahertz nonlinear devices. In this work, we present an alternative class of ultrathin nonlinear hybrid planar terahertz devices based on graphene-covered plasmonic gratings exhibiting very large nonlinear response. The robust localization and enhancement of the electric field along the graphene monolayer, combined with the large nonlinear conductivity of graphene, can lead to boosted third-harmonic-generation and four-wave-mixing nonlinear processes at terahertz frequencies. These interesting nonlinear effects exhibit very high nonlinear conversion efficiencies and are triggered by realistic input intensities with relatively low values. In addition, the third-harmonic-generation and four-wave-mixing processes can be significantly tuned by the dimensions of the proposed hybrid structures, the doping level of graphene, or the input intensity, whereas the nonlinear radiated power remains relatively insensitive to the incident angle of the excitation source. The nonlinear hybrid graphene-covered plasmonic gratings presented have a relative simple geometry and, as a result, can be used to realize efficient third-order nonlinear terahertz effects with a limited fabrication complexity. Several new nonlinear terahertz devices are envisioned on the basis of the proposed hybrid nonlinear structures, such as frequency generators, all-optical signal processors, and wave mixers. These devices are expected to be useful for nonlinear terahertz spectroscopy, noninvasive terahertz subwavelength imaging, and terahertz communication applications.

We study the possibility of counteracting the line broadening of atomic magnetic resonances due to inhomogeneities of the static magnetic field by means of spatially dependent magnetic dressing, driven by an alternating field that oscillates much faster than the Larmor precession frequency. We demonstrate that an intrinsic resonance linewidth of 25 Hz that has been broadened up to hundreds of hertz by a magnetic field gradient can be recovered by the application of an appropriate inhomogeneous dressing field. The findings of our experiments may have immediate and important implications, because they enable the use of atomic magnetometers as robust, high-sensitivity sensors to detect in situ the signal from ultralow-field NMR-imaging setups.

Ferroelectric materals are an important platform for the realization of nonvolatile memories. So far, existing ferroelectric memory devices have utilized out-of-plane polarization in ferroelectric thin films. In this paper, we propose a type of random-access memory (RAM) based on ferroelectric thin films with in-plane polarization, called an “in-plane ferroelectric tunnel junction.” Apart from nonvolatility, lower power usage, and a faster writing operation compared with traditional dynamic RAMs, our proposal has the advantage of a faster reading operation and a nondestructive reading process, thus overcoming the write-after-read problem that exists widely in current ferroelectric RAMs. The recent discovered room-temperature ferroelectric IV-VI semiconductor thin films are a promising material platform for the realization of our proposal.