Synthesis of hard/soft magnetically exchange-coupled heterostructures is one promising way to design energy-efficient rare-earth-free artificial magnetic materials for application as permanent magnets and in spintronics. As a model system, we experimentally investigate $\mathrm{Mn}\mathrm{Ga}$/$\mathrm{Fe}\mathrm{Co}$ bilayers and simulate their physical behavior in a combined density functional theory and micromagnetic approach. Using high-quality $L{1}_0\text{−}{\mathrm{Mn}}_{1.5}\mathrm{Ga}$ thin films with bulklike magnetic properties, we show that optimal coherent exchange coupling is obtained below a critical soft magnetic layer thickness that depends on the interface structure and composition. In particular, for atomically smooth and matched epitaxial interfaces of $L{1}_0\text{−}{\mathrm{Mn}}_{1.5}\mathrm{Ga}$ to a Co-terminated and Co-rich $\mathrm{Fe}\mathrm{Co}$ layer, coherent exchange coupling is observed for $\mathrm{Fe}\mathrm{Co}$ thicknesses below 2 nm. In optimized bilayers, the magnetic coercivity of $\mathrm{Mn}\mathrm{Ga}$ (approximately 6 kOe) can be fully conserved while the overall saturation magnetization is increased beyond $1000{\mathrm{emu}/\mathrm{cm}}^3$. Our model correlates interface structure and magnetic exchange coupling, providing guidelines to engineer high-performance exchange-coupled heterostructures for permanent magnets or spintronic devices.

Metasurfaces have introduced large flexibility in manipulating the impinging wavefront for light and sound by locally engineering the reflection and transmission coefficients based on generalized Snell’s laws. Local phenomena in each unit cell, however, are fundamentally limited in the level of efficiency with which anomalous wavefront transformations can be achieved. Here, we explore acoustic metasurfaces with suitably engineered nonlocality, obtained by coupling neighboring cells. We demonstrate that nonlocal passive metastructures can overcome the limitations of local designs, and mimic balanced gain and loss distributions, enabling unitary efficiency for extreme beam steering.

We report the experimental design of a one-dimensional stealth acoustic material, namely a material that suppresses the acoustic scattering for a given set of incident wave vectors. The material consists of multiple scatterers, rigid diaphragms, located in an air-filled acoustic waveguide. The position of the scatterers has been chosen such that in the Born approximation a suppression of the scattering for a broad range of frequencies is achieved and thus a broadband transparency. Experimental results are found in excellent agreement with the theory despite the presence of losses and the finite size of the material, features that are not captured in the theory. This robustness as well as the generality of the results motivates realistic potential applications for the design of transparent materials in acoustics and other fields of wave physics.

Nonlinear magneto-optical rotation (NMOR) is a sensitive technique for measuring magnetic fields. The polarization self-rotation (PSR) effect is a fundamental source of excess quantum noise that limits the sensitivity of this technique. In this paper, we study the influence of the PSR effect on the signal-to-noise ratio (SNR) of an NMOR magnetometer in the Earth-field range and find that the SNR highly depends on the quadrature phase of light with a magneto-optical rotation. We introduce a phase shift before the polarimeter to improve the SNR by controlling the phase. The implementation of the phase shifter results in an improvement in the SNR up to 10 dB, achieving a best sensitivity of $8\mathrm{pT}/\sqrt{\mathrm{Hz}}$ with an ordinary cell (without a paraffin coating or buffer gas). Our research should be useful for high-sensitivity magnetic field measurements in a real geomagnetic environment.

We suggest a single-photon thermal detector based on the abrupt jump of the critical current of a temperature-biased tunnel Josephson junction formed by different superconductors, working in the dissipationless regime. The electrode with the lower critical temperature is used as a radiation-sensing element, so it is thermally floating and is connected to an antenna or absorber. The warming up resulting from the absorption of a photon can induce a drastic measurable enhancement of the critical current of the junction. We propose a detection scheme based on a threshold mechanism for single- or multiphoton detection. This Josephson-threshold detector indeed has calorimetric capabilities, being able to determine the energy of the incident photon. So, for the realistic setup that we discuss, our detector can work efficiently as a calorimeter for photons from the midinfrared, through the optical, into the ultraviolet, specifically, for photons with frequencies in the range 30 THz to $9\times {10}^4\mathrm{THz}$. Over the whole range of detectable frequencies, we obtain a resolving power significantly larger than one. In order to reveal the signal, we suggest the fast measurement of the Josephson kinetic inductance. Indeed, the photon-induced change in the critical current affects the Josephson kinetic inductance of the junction, which can be read noninvasively through an $LC$ tank circuit that is inductively coupled to the junction. Finally, this readout scheme shows remarkable multiplexing capabilities.

Discharge and electron-impact excitation lead to the production of metastable helium atoms in two metastable states, $2^1{S}_0$ and $2^3{S}_1$. However, many applications require pure beams of one of these species or at least a detailed knowledge of the relative state populations. In this paper, we present the characterization of an original experimental scheme for the optical depletion of $\mathrm{He}(2^1{S}_0)$ in a supersonic beam which is based on the optical excitation of the $4^1{P}_1\leftarrow 2^1{S}_0$ transition at 397 nm using a diode laser. From our experimental results and from a comparison with numerical calculations, we infer a near-unit depletion efficiency at all beam velocities under study ($1070\mathrm{m/s}\le v\le 1750\mathrm{m/s}$). Since the technique provides a direct means to determine the singlet-to-triplet ratio in a pulsed supersonic helium beam, our results show that the intrabeam singlet-to-triplet ratio is different at the trailing edges of the gas pulse.

We describe the design and principle of operation of a fast and sensitive electrometer operated at millikelvin temperatures, which aims at replacing conventional semiconducting charge amplifiers in experiments needing low back action or high sensitivity. This electrometer consists of a Cooper-pair box (CPB) coupled to a microwave resonator, which converts charge variations to resonance frequency shifts. By analyzing in detail its sensitivity to various parameters, we find that the resonator nonlinearity induced by the CPB can be exploited to improve sensitivity. Using conventional nanofabrication and measurement techniques, a charge sensitivity down to ${10}^{-7}e/\sqrt{\mathrm{Hz}}$ with a megahertz bandwidth can be reached, which outperforms by 1 order of magnitude existing single-charge electrometers operated in the linear regime and opens up alternative possibilities in several fields such as mesoscopic and particle physics.

Chameleons have the ability to change their colors to adapt to nearby objects. Here, we introduce this behavior from optics to thermotics. By theory, simulation, and experiment, we design and fabricate a class of counterintuitive chameleonlike metashells: they exhibit adaptive thermal responses to changing objects in the vicinity (such objects are characterized by changing thermal conductivities and/or temperature distributions), even though their thermal conductivities are passive (without any preknowledge of the changing objects). We reveal the underlying mechanism by deriving the specific relations governing the components of tensorial conductivities of the metashells. Our metashells are passive, but they display adaptivity, which can work as a type of all-purpose material to meet different requirements of thermal conductivities under different conditions. The potential applications are to realize novel camouflaging, self-adaption, etc. This work gives a passive method for exploring intelligent thermal metamaterials in contrast to the existing active methods, and it also offers guidance for designing similar behaviors in electrostatics, magnetostatics, and particle diffusion.

When probed with current pulses, Josephson junctions and superconducting wires exhibit stochastic switching from a superconducting to a stable nonzero-voltage state. The electrical current dependence of the switching probability (the so-called $\mathsf{S}$ curve) or the switching-current distribution is a fingerprint of the physics governing the escape process. This work addresses the criterion of the independent switching event, which is important for the credibility of the switching measurements of superconducting wires and various Josephson junctions involving superconductor-insulator-superconductor tunnel junctions, proximity junctions, and Dayem nanobridges. Treating the Josephson junction as an electrical coin with a current-tuned switching probability, we investigate the effect of correlation between switching events on the switching statistics. We show that such a correlation originates from the thermal dynamics of the superconducting wire.

There is growing interest in exploring nanomagnetic devices as potential replacements for electronic devices (e.g., transistors) in digital switching circuits and systems. A special class of nanomagnetic devices are switched with electrically generated mechanical strain leading to electrical control of magnetism. Straintronic magnetotunneling junctions (SMTJs) belong to this category. Their soft layers are composed of two-phase multiferroics comprising a magnetostrictive layer elastically coupled to a piezoelectric layer. Here, we show that a single straintronic magnetotunneling junction with a passive resistor can act as a microwave oscillator whose traditional implementation would have required microwave operational amplifiers, capacitors, and resistors. This reduces device footprint and cost, while improving device reliability. This is an analog application of magnetic devices where magnetic interactions (interaction between the shape anisotropy, strain anisotropy, dipolar coupling field, and spin-transfer torque in the soft layer of the SMTJ) are exploited to implement an oscillator with reduced footprint.

We build a self-contained optical absolute frequency reference at 1064 nm based on the rovibronic transition R(56)32-0 in molecular iodine and operate this instrument in space on a sounding rocket mission. The frequency reference uses a microintegrated extended cavity diode laser and a quasimonolithic spectroscopy module for modulation transfer spectroscopy, providing frequency instability of $2\times {10}^{-13}$ for averaging times of 100 s. To demonstrate the autonomous operation of our reference, we perform an absolute frequency measurement using a chip-scale atomic clock and an optical frequency comb during a 6-min-long space fight.

The time response to an external signal is the main characteristic of optoelectronic devices that determines their maximum modulation frequency. The use of plasmonic structures provides the opportunity to significantly reduce the response time. In this paper, we study the temporal dynamics of a plasmonic distributed feedback laser consisting of a two-dimensional metallic structure coated with a layer of the active medium. Combining numerical simulations with an analytical evaluation, we show that the response time in the large-signal modulation regime strongly depends on the size of the pump beam. We demonstrate that the interaction of a large number of modes of the plasmonic structure with the pumped active medium leads to the formation of a single collective mode. If the size of the pump beam is larger than the propagation length of electromagnetic waves in the plasmonic structure, the collective mode is localized in the pumped area. In this case, the response time slightly increases with the size of the pump beam. In the opposite case, the mode is not localized inside the pumped area and the response time increases with the narrowing of the beam. We demonstrate that there is a minimum size of the pump beam for which the modulation speed is a maximum, and can achieve 1 THz. The results obtained opens the way to increasing both the modulation speed and the energy efficiency of plasmonic laser devices.

All-dielectric bianisotropic metasurfaces with tailored polarization and spectral responses are studied, and a systematic approach to their design is developed. It is shown that a wide range of polarization transformations over broad bandwidths, as well as multiple bands, can be implemented by cascading subwavelength dielectric gratings. The subwavelength gratings, modeled as anisotropic layers, are cascaded to form metasurfaces that control the polarization of impinging waves. These metasurfaces are a low-loss alternative to their lossy, plasmonic counterparts at optical frequencies, and can replace conventional, bulky optical components. The multifunctional performance and compactness (wavelength-sized thickness) of the proposed devices will also find use at millimeter-wave frequencies. Example metasurfaces are presented and their performance is verified through full-wave simulation. This work demonstrates the flexibility with which cascaded subwavelength gratings can realize unprecedented polarization control with varied spectral responses.

Electronic emulation of the biological synapse, which is the memory and learning element of the brain, is an important step towards the realization of brain-inspired computing systems. However, a complementary metal-oxide-semiconductor−based implementation of a synapse is not scalable due to the power and area inefficiency. It is therefore essential to develop alternative material and device concepts that can mimic the maximum number of synaptic functionalities in an energy-efficient way. Here, we demonstrate a nanosized energy-efficient three-terminal artificial synapse, with a separate read and write path, based on a magnetic material ($\mathrm{Co}$) that could generate both positive and negative synaptic outputs. The $\mathrm{Co}$ magnetization, which is representative of the synaptic weight, is modulated in an energy-efficient way by using an electric field to transport oxygen ions in and out of the $\mathrm{Co}$ layer. A wide range of synaptic functions such as synaptic potentiation and depression, spike magnitude, rate, and timing-dependent plasticity, and the transition from short-term to long-term plasticity are demonstrated. Our results suggest the viability of a spintronic synapse in neuromorphic computing.

The dynamics of Abrikosov vortices in superconductors is usually limited to vortex velocities $v\approx 1$ km/s, above which samples abruptly transit into the normal state. In the Larkin-Ovchinnikov framework, near the critical temperature, this transition is because of a flux-flow instability triggered by the reduction of the viscous drag coefficient due to the quasiparticles leaving the vortex cores. While the existing instability theories rely upon a uniform spatial distribution of vortex velocities, the measured (mean) value of $v$ is always smaller than the maximal possible one, since the distribution of $v$ never reaches the $\delta$-functional shape. Here, by guiding magnetic flux quanta at a tilt angle of ${15}^{\circ }$ with respect to a $\mathrm{Co}$ nanostripe array, we speed up vortices to 3–6 km/s. These exceed $v$ in the reference as-grown $\mathrm{Nb}$ films by almost an order of magnitude and are only a factor of about 3 smaller than the maximal vortex velocities observed in superconductors so far. We argue that such high $v$ values appear in consequence of a collective dynamic ordering when all vortices move in the channels with the same pinning strength and exhibit a very narrow distribution of $v$. Our findings render the well-known vortex guiding effect to open prospects for investigations of ultrafast vortex dynamics.

The coupling of spin-orbit materials to high-energy-barrier (approximately 40–60 $k_{B}T$) nanomagnets has recently attracted growing interest due to exciting new physics and various spintronic applications. We predict that a coupling between the spin-momentum locking (SML) observed in spin-orbit materials and low-energy-barrier magnets (LBMs) should exhibit a unique multiterminal rectification for arbitrarily small-amplitude channel currents. The basic idea is to measure the charge-current-induced spin accumulation in the SML channel in the form of a magnetization-dependent voltage using an LBM, either with an in-plane or perpendicular anisotropy (IMA or PMA, respectively). The LBM feels an instantaneous spin-orbit torque due to the accumulated spins in the channel, which causes the average magnetization to follow the current, leading to the nonlinear rectification. We discuss the frequency band of this multiterminal rectification, which can be understood in terms of the angular-momentum conservation in the LBM. For a fixed spin-current from the SML channel, the frequency band is same for LBMs with IMA and PMA, as long as they have the same total magnetic moment in a given volume. The proposed all-metallic structure could find application as a highly sensitive passive radio-frequency (rf) detector and as an energy harvester from weak ambient sources, where standard technologies may not operate.

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.