Superconducting metamaterials are a promising resource for quantum-information science. In the context of circuit QED, they provide a means to engineer on-chip dispersion relations and a band structure that could ultimately be utilized for generating complex entangled states of quantum circuitry, for quantum-reservoir engineering, and as an element for quantum-simulation architectures. Here we report on the development and measurement at millikelvin temperatures of a particular type of circuit metamaterial resonator composed of planar superconducting lumped-element reactances in the form of a discrete left-handed transmission line that is compatible with circuit QED architectures. We discuss the details of the design, fabrication, and circuit properties of this system. As well, we provide an extensive characterization of the dense mode spectrum in these metamaterial resonators, which we conduct using both microwave-transmission measurements and laser-scanning microscopy. Results are observed to be in good quantitative agreement with numerical simulations and also an analytical model based upon current-voltage relationships for a discrete transmission line. In particular, we demonstrate that the metamaterial mode frequencies, spatial profiles of current and charge densities, and damping due to external loading can be readily modeled and understood, making this system a promising tool for future use in quantum-circuit applications and for studies of complex quantum systems.

Magnonic quasicrystals exceed the possibilities of spin-wave (SW) manipulation offered by regular magnonic crystals, because of their more complex SW spectra with fractal characteristics. Here, we report the direct x-ray microscopic observation of propagating SWs in a magnonic quasicrystal, consisting of dipolar coupled permalloy nanowires arranged in a one-dimensional Fibonacci sequence. SWs from the first and second band as well as evanescent waves from the band gap between them are imaged. Moreover, additional mini band gaps in the spectrum are demonstrated, directly indicating an influence of the quasiperiodicity of the system. Finally, the localization of SW modes within the Fibonacci crystal is shown. The experimental results are interpreted using numerical calculations and we deduce a simple model to estimate the frequency position of the magnonic gaps in quasiperiodic structures. The demonstrated features of SW spectra in one-dimensional magnonic quasicrystals allow utilizing this class of metamaterials for magnonics and make them an ideal basis for future applications.

Quantum-limited Josephson parametric amplifiers are crucial components in circuit QED readout chains. The dynamic range of state-of-the-art parametric amplifiers is limited by signal-induced Stark shifts that detune the amplifier from its operating point. Using a superconducting nonlinear asymmetric inductive element (SNAIL) as an active component, we show the ability to in situ tune the device flux and pump to a dressed Kerr-free operating point, which provides a 10-fold increase in the number of photons that can be processed by our amplifier, compared to the nominal operating point. Our proposed and experimentally verified methodology of Kerr-free three-wave mixing can be extended to improve the dynamic range of other pumped operations in quantum superconducting circuits.

The validity of Fourier’s law in telescopic nanowires is tested by means of molecular dynamics simulations. We observe that the radial dependence of the heat-flux profile, the temperature jump, and the appearance of vorticity obtained in molecular dynamics telescopic wires near the contact point acquire a hydrodynamic character, and we show that they are incompatible with Fourier’s law. We propose the Guyer-Krumhansl equation as a generalization capable of capturing these hydrodynamic effects. Lattice-dynamics results show that the thermal average of the confined modes shows no radial dependence of the vibrating energy inside the wire. This means that hydrodynamic effects could not be related to the confinement effects in these small systems.

Dilute magnetic semiconductors (DMSs) have attracted much attention because of their huge potential applications in spintronics. The simple band-coupling model suggests that isovalent doping of semiconductors (e.g., $\mathrm{Mn}$-doped II-V semiconductors) should be antiferromagnetic due to the dominant superexchange interaction, which is consistent with experimental observations on most $\mathrm{Mn}$-doped II-VI and $\mathrm{Fe}$-doped III-V semiconductors (FDMSs). However, recently, it has been reported experimentally that some FDMSs are ferromagnetic with a very high Curie temperature (T_C) of over 300 K, but the underlying mechanism is not clear. Here, we reveal that the unusual ferromagnetism in FDMSs originates from a unique p-d coupling–induced band crossing and the resulting charge transfer from anion p to unoccupied Fe 3d orbitals. This result suggests that the ferromagnetism can be more easily realized by $\mathrm{Fe}$ doping in III-V semiconductors with high anion p orbital energies, such as $\mathrm{Ga}\mathrm{Sb}$, instead of those with small band gaps, such as $\mathrm{In}\mathrm{As}$. Moreover, we illustrate that the isovalent character guarantees low self-compensation in FDMSs, which sets a major advantage to the realization of high T_C in FDMSs. Our finding can well explain the recent experimental observations and suggests a new avenue for the future design of high-T_C DMSs.

We propose a two-scale architecture by simultaneously exploiting both the Rayleigh anomaly and localized surface plasmon resonance in colloidal directly assembled oligomers to achieve giant field enhancement, overcoming limitations by losses and the intrinsic nonlocality of the dielectric response of metal nanoparticles. We show that the total field enhancement in the nanogap hotspots of this two-scale structure is the product of the enhancements of each individual geometrical scale. Under this scheme, chemically assembled metallic oligomers enable strong enhancement of the near-field in resultant few-nanometer gaps at their plasmonic resonance. An extra enhancement factor results from combining the plasmonic resonance enhancement of these oligomers (nanometer scale) with a Rayleigh anomaly caused by a one-dimensional (1D) periodic set of nanorods (micrometer scale) fabricated using a lithographic method. This extra field enhancement cannot be obtained by further reducing the nanoantennas’ gaps, since they are already in the few-nm ranges thanks to direct chemical assembly of oligomers. Experimental results verify that surface-enhanced Raman spectroscopy (SERS) signal is enhanced by more than one order of magnitude due to the Rayleigh anomaly as compared to the signal taken by using only metallic oligomers without periodic nanorods. The proposed two-scale structure could serve as a substrate for different spectroscopy techniques including SERS and can improve the sensitivity of spectroscopy for ultrasensitive biomolecule detection and single molecule spectroscopy studies.

Large-scale fiber-based quantum networks will likely employ telecommunication-wavelength photons of around 1550 nm wavelength to exchange quantum information between remote nodes, and quantum memories, ideally operating at the same wavelength, that allow the transmission distances to be increased, as key elements of a quantum repeater. However, the development of a suitable memory remains an ongoing challenge. Here, we demonstrate the storage and reemission of single heralded 1532-nm-wavelength photons using a crystal waveguide. The photons are emitted from a photon-pair source based on spontaneous parametric down-conversion and the memory is based on an atomic frequency comb of 6 GHz bandwidth, prepared through persistent spectral-hole burning of the inhomogeneously broadened absorption line of a cryogenically cooled erbium-doped lithium niobate waveguide. Despite currently limited storage time and efficiency, this demonstration represents an important step toward quantum networks that operate in the telecommunication band and the development of integrated (on-chip) quantum technology using industry-standard crystals.

Curved reflective mirrors play an indispensable role in widely employed Cassegrain-type optical devices, which unfortunately are inevitably bulky and thus neither integrable nor well extendable. Through substituting geometric phase-based flat reflective metasurfaces for curved mirrors, here planar Cassegrain-type Schwarzschild objectives are obtained, and their achromatic focusing and imaging functionalities of high efficiency experimentally verified. The planar reflective objectives achieved are intrinsically free of residual light and thus impose less stringent restrictions on illuminating light and nanofabrication precision compared to their refractive counterparts. Our planar Cassegrain-type designs can potentially reshape and simplify the core components of many microscope and telescope systems working over broadband spectral regimes, and thus open up alternative avenues for designing ultracompact imaging systems.

We propose a paradigm to realize nonreciprocal wavefront engineering using time-modulated gradient metasurfaces. The essential building block of these surfaces is a subwavelength unit cell whose reflection coefficient oscillates at low frequency. We demonstrate theoretically and experimentally that such modulation permits tailoring the phase and amplitude of any desired nonlinear harmonic and determines the behavior of all other emerging fields. By appropriately adjusting the phase delay applied to the modulation of each unit cell, we realize time-modulated gradient metasurfaces that provide efficient conversion between two desired frequencies and enable nonreciprocity by (i) imposing drastically different phase gradients during the up/down conversion processes and (ii) exploiting the interplay between the generation of certain nonlinear surface and propagative waves. To demonstrate the performance and broad reach of the proposed platform, we design and analyze metasurfaces able to implement various functionalities, including beam steering and focusing, while exhibiting strong and angle-insensitive nonreciprocal responses. Our findings open an alternative direction in the field of gradient metasurfaces, in which wavefront control and magnetic-free nonreciprocity are locally merged to manipulate the scattered fields.

Mechanically driven assembly of three-dimensional (3D) mesostructures by compressive buckling represents an area of increasing attention, owing to the excellent compatibility with planar fabrication technologies and the broad applicability to a variety of material types (from semiconductors to metals and dielectrics) and length scales (from nanometers to centimeters). As a representative class of 3D geometries, the assembly of filamentary 3D ribbon configurations has been studied extensively, either to analyze the deformation features or to explore the applications in various functional devices. Previous studies mainly focused on the modeling of the “forward problem” that enables the prediction of the assembled 3D ribbon configuration for a prescribed 2D precursor. Development of an efficient method to solve the “inverse problem” that maps the target 3D geometry onto an unknown 2D precursor design remains a challenge. In the framework of adaptive-genetic-algorithm optimization, this paper proposes a systematic computational method to solve the inverse design problem of ribbon-shaped 3D structures assembled through compressive buckling. Based on a theoretical analysis on the inherent geometric relationship between the deformed and the initial ribbon configurations, we establish an optimization-based formulation of the inverse design problem to determine the initial geometries of 2D precursors, with the aid of finite-element analyses. Both the convergence and accuracy of the developed method are verified through a set of illustrative examples. Computational and experimental studies over approximately 20 ribbon structures, each with comparison between the target and assembled configurations, show the broad applicability of the inverse design method. Demonstrated capabilities to form 3D structures that resemble the sketches of real objects (e.g., peaked hat, butterfly, and frog) suggest promising utilities in the accurate design of functional devices.

$\mathrm{Cu}(\mathrm{In},\mathrm{Ga}){\mathrm{S}}_2$-based solar cells are interesting tandem partners for $\mathrm{Si}$ or chalcopyrite solar cells, but suffer from a low open-circuit voltage. Recently, record efficiencies have been achieved by using higher growth temperatures for the absorber. To understand the effect of higher growth temperatures, we investigate the structural and electronic properties of ${\mathrm{Cu}\mathrm{In}\mathrm{S}}_2$ absorbers. By investigating the absorber alone as opposed to complete solar cells, we can separate changes in the absorber from effects of the interface properties. We show that the quasi-Fermi-level splitting, which indicates the maximum open-circuit voltage an absorber is capable of, increases with higher growth temperature. The quasi-Fermi-level splitting is limited by a deep defect, the concentration of which decreases with higher growth temperature and is less prominent in $\mathrm{Cu}$-rich films. Thus, we demonstrate that the open-circuit voltage of ${\mathrm{Cu}\mathrm{In}\mathrm{S}}_2$-based solar cells is limited to below 850 mV by the absorber itself, independent of the interface. In contrast to the changes in the electronic properties, the structural properties are rather independent of temperature within the range investigated but are significantly influenced by the composition.

The metasurface has aroused broad enthusiasm due to its splendid properties for electromagnetic (EM) wave manipulations. However, most previous metasurfaces, including passive and active metasurfaces, are almost energy losing when controlling EM waves. Some amplifying metasurfaces have been proposed, but the amplification function is uncontrollable. Here, we propose a spatial-energy digital-coding metasurface with active amplifiers to realize arbitrary editing of the energy of spatial propagating waves. Based on the proposed metasurface, the spatial energy of a linearly polarized propagating wave can not only be amplified, but can also be edited into arbitrary magnitudes (including amplification and reduction) by controlling the voltage. A metasurface element integrated with an amplifier chip is designed for energy modulations in the microwave frequency range. The numerical simulations and experimental results show good consistency, validating the feasibility of our design.

Precise knowledge of the dependence of elastic modulus and $Q$ factor on the amplitude of excitation is a prerequisite for the development and validation of models to explain the hysteresis observed in quasistatic experiments for various media, i.e., the different deformations at the same applied stress observed when stress change rate is positive or negative. Separation of different contributions to dynamic nonlinearity (e.g., those due to nonequilibrium effects, often termed conditioning) and independent estimation of nonlinearities originated by the strain dependence of velocity and the damping factor are required, which is often not possible with standard approaches. Here we propose and validate a method that, measuring the response of a sample to a monochromatic excitation at different amplitudes, allows fast, continuous, and quasi-real-time monitoring of the dependence of the material elastic properties on amplitude: dynamic elastic modulus (related with velocity through density) and $Q$ factor of the mechanical resonances (related with wave-amplitude attenuation parameters).

Conversions between charge and spin currents by spin-orbit interactions have become usual in today's spintronics, either by using the spin Hall effect (SHE) and inverse spin Hall effect (ISHE) of heavy metals or the Edelstein effect and inverse Edelstein effect of a two-dimensional (2D) electron gas. These conversions can be characterized by the spin Hall angle for SHE or ISHE and by the conversion parameters $q_{\mathrm{IC}}$ for EE and ${\lambda }_{\mathrm{IEE}}$ for IEE. Using typical experimental values of these different parameters for heavy metals and 2D electrons of topological insulators (TIs), we show that, compared to heavy metals, the TIs lead to a gain of about an order of magnitude in the conversion between spin to charge or charge to spin currents and even more in the production of voltage or electrical power by conversion of spin currents. Thus, we anticipate a giant spin Seebeck power generated in devices designed with an insulating magnetic layer and 2DEGs of a TI as α-$\mathrm{Sn}$ or Rashba 2DEGs. We also discuss the detrimental role of additional shunts in these conversion experiments.

The spin Hall effect describes interconversion between charge currents and spin currents through the spin-orbit interaction. Recently, various spin-current generation mechanisms in addition to the spin Hall effect have been studied. They include the anomalous Hall effect, spin swapping, planar Hall current, and interface-generated spin current. Spin currents generated by these mechanisms carry a spin polarization whose direction differs from that originating from the bulk spin Hall effect. In this work, we theoretically and numerically investigate antiferromagnetic spin oscillations induced by a spin current carrying an arbitrary spin polarization direction. We find that the threshold current to excite antiferromagnetic oscillations shows a nonmonotonic dependence on the spin polarization direction. Moreover, we show how material parameters and imperfections affect properties of antiferromagnetic oscillators. We expect that our result will be useful for applications of antiferromagnetic oscillators and understanding antiferromagnetic spin dynamics induced by spin currents originating from various spin-current generation mechanisms.

The reliable estimation of the spin-orbital-torque (SOT) efficiency in ferromagnetic multilayers with small coercive force is a nontrivial task, but extremely demanded for implementation of current-induced magnetization reversal memory cells. Here, we report on results of an experimental study of current-induced magnetization reversal of the $\mathrm{Ru}/\mathrm{Co}/\mathrm{Ru}$ and $\mathrm{Ru}/\mathrm{Co}/\mathrm{Ru}/\mathrm{W}$ structures. In the considered structures, because of the small value of the coercive force comparable in magnitude to the Oersted field and the SOT effect field, the magnetization reversal is carried out by moving a domain wall parallel to the direction of current injection. For such a case, an alternative method for estimating the effective field of SOT based on the analysis of the domain-wall position considering the distribution of the Oersted field is proposed. This method allows us to determine the effective longitudinal field ${\mu }_0{H}_L/j=1.6\times {10}^{-11}\mathrm{mT/A}{\mathrm{m}}^{-2}$ and the efficiency of SOT ${\xi }_L=0.03$ in the quasisymmetric $\mathrm{Ru}/\mathrm{Co}/\mathrm{Ru}$ structure. An addition of a capping layer of $\mathrm{W}$ enhances ${\xi }_L$ by 5 times.

We propose the concept of and experimentally demonstrate a metasurface-based smart table for wireless power transfer. The proposed structure supports propagating surface modes that can be excited by the near field of wireless devices. As a result, if two devices are placed anywhere on a metasurface table, a substantial enhancement of the effective near-field coupling can be achieved. This enhancement leads to a demonstrated power-transfer efficiency of 80% for the receiver placed in any location with respect to the transmitter on the table. The proposed concept enables a variety of applications from consumer electronics to charging electric cars.

The nature of the mechanical-strain-mediated converse magnetoelectric effect (CME) is studied in multiferroic composites of single-crystal-like thin films of nickel ferrite (NFO) and polycrystalline lead zirconate titanate (PZT). Ferrite films of thickness 0.45–1 micron are prepared by pulsed-laser deposition on lattice-matched (100)- and (110)-oriented substrates of magnesium gallate (MGO) and cobalt gallate (CGO) that result in elimination of antiphase boundaries and magnetic parameters comparable to bulk single crystals. Ferromagnetic resonance under a static electric field E is utilized for studies of CME effects in composites of PZT and NFO films on the substrates. The in-plane static magnetic field H is applied along the principal crystallographic axes of the ferrite film to study its influence on the CME. The E-induced magnetoelectric (ME) anisotropy field $H_{\mathrm{ME}}$ is estimated from ferromagnetic resonance data based on the shift in resonance frequency with E in order to determine the ME constant $A=H_{\mathrm{ME}}/E$. In composites with NFO films on (110)-oriented substrates (i) the ME coupling is stronger for films on CGO possibly due to a better lattice match and weaker substrate clamping than for films on MGO, (ii) A values are the highest for H || [1,−1,0], and (iii) a negative A value is inferred for H || [001]. For composites with NFO on (100)-oriented substrates the strongest ME coupling is measured for H along the [001] direction. A model for CME that takes into consideration both compressive and bending deformation in the composites is developed and the results of the theory are in agreement with both the sign and magnitude of the measured ME coefficient A. The results of the studies presented here indicate the potential for use of the composites in self-biased E-tunable microwave devices.

Different types of particle and/or cell patterning in acoustic cavities produced by the radiation force of ultrasonic standing waves have been observed. However, most explanations of this phenomenon are constrained to particles much smaller than the wavelength (i.e., the so-called Rayleigh regime). Here, we present a theoretical model for acoustic trapping and patterning of particles/cells in a rectangular cavity beyond the Rayleigh regime. A simple closed-form expression of the radiation-force potential for particles of virtually any size immersed in a fluid is obtained. Particles with a size comparable to one wavelength (Mie particles) can be trapped in acoustic potential wells, whose stability is quantified by the trap stiffness. Our findings reveal that an acoustic trap can occur at a pressure node, antinode, and midpoint (i.e., a point midway between two nodes). These locations depend on the acoustic parameters of the particle and surrounding fluid (density, longitudinal, and shear speed of sound) and the ratio of particle size to wavelength. We also investigate the effects of the secondary radiation forces on trapping stability. We determine the possible acoustic patterns formed with polystyrene particles and osmotically swollen red blood cells (SRBCs). The conditions that may lead to one particle/cell per acoustic well patterning are discussed. A set of patterning experiments is performed with an acoustofluidic rectangular device, operating at 6.5-MHz frequency, using polystyrene particles with diameters of $10\mu \mathrm{m}$ (Rayleigh particles) and $75\mu \mathrm{m}$ (Mie particles) immersed in distilled water. The obtained experimental results are consistent with our theoretical predictions. The present study can help in designing acoustofluidic devices with the ability to spatially arrange larger, or more closely spaced particles, cells, and other micro-organisms.

Optical studies of single self-assembled semiconductor quantum dots (QDs) have been a topic of intensive investigation over the past two decades. Due to their solid-state nature, their electronic and optical emission properties are affected by the particular crystal structure as well as many-body-carrier interactions and dynamics. In this work, we use a master equation for microstates (MEM) model to study the carrier capture and escape from single QDs under optical nonresonant excitation and under the influence of a two-dimensional (2D) carrier reservoir (the wetting layer). This model reproduces carrier dynamics from power-dependent and time-resolved microphotoluminescence experiments. Due to the random nature of the carrier capture and escape processes, when a single QD is pumped with enough excitation power, the carrier redistribution across the available QD microstates produces an effective double-peaked excitonic decay. This double peak is characterized by a first ultrafast (subnanosecond) and a second conventional (approximately nanosecond) decay. The effective transient photoluminescence shape of the population dynamics is governed by the wetting-layer radiative decay and the exciton capture time.