The strength and angular dependence of the spin-orbit torques (SOTs) in $\mathrm{Pt}/\mathrm{Co}/\mathrm{Mg}\mathrm{O}$ trilayers are obtained by analyzing harmonic Hall voltage measurement results. The detailed analytical equations that consider both the isotropic and angle-dependent torques are derived and then used to characterize the angular dependence of the SOTs. The results show that both the dampinglike and fieldlike torques are strongly dependent on the polar angle of the magnetization and furthermore, the angle-dependent torques, combined with a strong contribution of the planar Hall effect, bring about the sign change of the out-of-phase Hall voltage signals. The SOT efficiencies are also investigated as a function of the $\mathrm{Mg}\mathrm{O}$ sputtering power density ($P_{\mathrm{Mg}\mathrm{O}}$) and post annealing temperature in order to find the correlation between these parameters and the strength of the SOTs. A large change in SOT is observed depending on $P_{\mathrm{Mg}\mathrm{O}}$ and the fieldlike torque efficiency, for example, it is approximately 1.8 times higher at $P_{\mathrm{Mg}\mathrm{O}}=4.39\mathrm{W}/{\mathrm{cm}}^2$ than at $P_{\mathrm{Mg}\mathrm{O}}=1.10\mathrm{W}/{\mathrm{cm}}^2$ in the annealing temperature range of ${250}^{\circ }\mathrm{C}-{350}^{\circ }\mathrm{C}$.

A one-dimensional magnonic crystal for forward volume spin waves (FV SWs) is demonstrated using eight pairs of $\mathrm{Cu}$ stripes fabricated on a 1-mm wide × 14-mm long × 10.2-µm thick yttrium iron garnet (YIG) waveguide and a SW absorber of 30-nm-thick $\mathrm{Au}$ film. The development of this crystal is challenging due to strong spectral oscillations caused by edge reflections and process difficulties associated with YIG magnonic crystals. A magnonic band gap with a depth of −4.6 dB is observed at a frequency of 1.80 GHz for a FV SW excited by a 50-µm-wide microstrip line, which is in good agreement with simulation results using three-dimensional modeling in the radio frequency region. The obtained performance is illustrated by plotting the depth of the respective band gaps vs the pair number of the stripes. This value is compared with that obtained in previous studies.

The operation and readout of a nonvolatile current-modulated four-state magnetoplasmonic metal-insulator-metal (M-I-M) waveguide device is modeled as a function of device and material parameters. Current pulses applied to electrically isolated stacks containing a ferromagnet and an active layer (either a metal with large spin-orbit coupling or a topological insulator) embedded within a plasmonic metal ($\mathrm{Au}$) situated on either side of the insulating core can switch the orientation of the magnetic layers via the spin-transfer torque mechanism. The four possible magnetic-orientation combinations interact with the surface-plasmon-polariton mode to produce four distinguishable values (on the order of 0.1–1%) of the effective index of refraction. By overcoming the optical losses traditionally associated with magnetic materials within a plasmonic environment, this four-state optical memory element is able to exploit both the robust magnetic stability of metallic magnets and the spin-torque switching mechanism that can reorient these magnets via on-chip currents without requiring external magnetic fields. Using this element, we propose a 2-stage plasmonic memory register with a range of states that can scale as $4^n$ for $n$ stages.

Dephasing induced by residual thermal photons in the readout resonator is a leading factor limiting the coherence times of qubits in the circuit quantum electrodynamics architecture. This residual thermal photon, on the order of ${10}^{-1}$–${10}^{-3}$, is suspected to arise from noise impinging on the resonator from its input and output ports. To address this problem, we designed and tested a new type of band-pass microwave attenuator that consists of a dissipative cavity well thermalized to the mixing chamber stage of a dilution refrigerator. By adding such a cavity attenuator in-line with a three-dimensional superconducting cavity housing a transmon qubit, we have reproducibly measured increased qubit coherence times. At base temperature, through a Hahn echo experiment, we measured $T_{2e}/2T_1=1.0\begin{array}{c}+0.0\\ -0.1\end{array}$ for two qubits over multiple cooldowns. Through noise-induced dephasing measurement, we obtained an upper bound of $2\times {10}^{-4}$ on the residual photon population in the fundamental mode of the readout cavity, which to our knowledge is the lowest value reported so far. These results validate an effective method for protecting qubits against photon noise, which can be developed into a standard technology for quantum circuit experiments.

Josephson circuits have been ideal systems with which to study complex nonlinear dynamics that can lead to chaotic behavior and instabilities. More recently, Josephson circuits in the quantum regime, particularly in the presence of microwave drives, have demonstrated their ability to emulate a variety of Hamiltonians that are useful for the processing of quantum information. In this paper, we show that these drives lead to an instability that results in the escape of the circuit mode into states that are not confined by the Josephson cosine potential. We observe this escape in a ubiquitous circuit: a transmon embedded in a 3D cavity. When the transmon occupies these free-particle-like states, the circuit behaves as though the junction had been removed and all nonlinearities are lost. This work deepens our understanding of strongly driven Josephson circuits, which is important for fundamental and application perspectives, such as the engineering of Hamiltonians by parametric pumping.

We present a method developed to actively compensate common-mode magnetic disturbances on a multisensor device devoted to differential measurements. The system uses a field-programmable-gated-array card, and operates in conjunction with a high-sensitivity magnetometer: compensating the common mode of magnetic disturbances results in a relevant reduction of the difference-mode noise. The digital nature of the compensation system allows for the use of a numerical approach aimed at automatically adapting the feedback-loop filter response. A common-mode-disturbance attenuation exceeding 50 dB is achieved, resulting in a final improvement of the differential noise floor by a factor of 10 over the whole spectral interval of interest.

The possibility of switching the sign of the tunneling electrons’ spin polarization by an electric field could introduce dramatic and technologically promising changes in the way spin transport is controlled in spintronic devices. Recently, such switching was observed experimentally in ${\mathrm{Co}\text{/}\mathrm{Pb}\mathrm{Zr}}_{0.2}{\mathrm{Ti}}_{0.8}{\mathrm{O}}_3$ (PZT)/${\mathrm{La}}_{0.7}{\mathrm{Sr}}_{0.3}{\mathrm{Mn}\mathrm{O}}_3$ multiferroic tunnel junctions. Using ab initio calculations, we identify a microscopic mechanism that can explain this effect. Following an extensive search, we single out a $\mathrm{Co}$/PZT-interface atomic configuration, including an oxygen-related defect complex, which can account for the experimental spin-polarization switching trend. For this configuration, we find that inversion of the PZT ferroelectric polarization induces drastic reversible changes in the reactivity and chemical binding of the interfacial $\mathrm{O}$ with $\mathrm{Co}$. This ferroelectric-controllable reactivity of $\mathrm{O}$ leads virtually to an on-off switchable $\mathrm{O}$-$\mathrm{Co}$ hybridization and a swappable spin polarization in the PZT barrier.

We uncover the origin of the blue luminescence (BL) peak in a $\mathrm{Mg}$-doped $\mathrm{Ga}\mathrm{N}$ thin film using a combination of experimental x-ray absorption near-edge spectroscopy (XANES), first-principles calculations based on density functional theory (DFT), and full multiple scattering (FMS) theoretical analysis of various possible defect complexes and their XANES signatures. We demonstrate that a defect complex composed of $\mathrm{Mg}$ substituted at a $\mathrm{Ga}$ site (${\mathrm{Mg}}_{\mathrm{Ga}}$) and $\mathrm{Mg}$ at an interstitial site (${\mathrm{Mg}}_i$) is primarily responsible for the observed BL by a donor–acceptor pair transition (DAP) associated with a deep donor state in the gap. It correlates with a higher (lower) oxidation state of $\mathrm{N}$ ($\mathrm{Ga}$) in heavily $\mathrm{Mg}$-doped $\mathrm{Ga}\mathrm{N}$ than in its pristine structure, evident in our experiments as well as calculations. Physical and chemical mechanisms identified here point out a route to achieving efficient $p$-type $\mathrm{Ga}\mathrm{N}$, which has been one of the major limiting factors in obtaining highly efficient $\mathrm{Ga}\mathrm{N}$-based optoelectronic devices.

Accurate characterization of thermal emitters can be challenging due to the presence of background thermal emission from components of the experimental setup and the surrounding environment. This is especially true for an emitter operating close to room temperature. Here, we explore the characterization of near-room-temperature thermal emitters using Fourier-transform infrared (FTIR) spectroscopy. We find that the thermal background arising from optical components placed between the beam splitter and the detector in an FTIR spectrometer appears as a “negative” contribution to the Fourier-transformed signal, leading to errors in thermal-emission measurements near room temperature. Awareness of this contribution will help properly calibrate low-temperature thermal-emission measurements.

Laser-based additive manufacturing offers a promising route for 3D printing of metallic parts. We evidence experimentally a particular columnar solidification microstructure in a laser-powder-bed-fusion-processed Inconel 718 nickel-based alloy, which we interpret using phase-field simulations and classical dendritic growth theories. Owing to the large temperature gradient and cooling rate, solidification takes places through dendritic arrays wherein the characteristic length scales, i.e., tip radius, diffusion length, and primary spacing, are of the same order. This leads to a weak mutual interaction between dendrite tips and a drastic reduction of side branching. The resulting irregular cellularlike solidification pattern then remains stable on time scales comparable to the complete melt pool solidification, as observed in the as-built material.

Accumulation of energy by reactive elements is limited by the amplitude of time-harmonic external sources. In the steady-state regime, all incident power is fully reflected back to the external source, and the stored energy does not increase in time, although the external source continuously supplies energy. Here we show that this limitation can be removed if the reactive element is varying in time, properly modulated by an additional active but lossless device. We show that such time-varying lossless loads of a transmission line or lossless metasurfaces can accumulate electromagnetic energy supplied by a time-harmonic source continuously in time without any theoretical limit. We analytically derive the required time dependence of the load reactance and show that it can, in principle, be realized as a series connection of mixers and filters. Furthermore, we prove that by properly designing time-varying $LC$ circuits, one can arbitrarily engineer the time dependence of the electric current in the circuit fed by a given time-harmonic external source. As an example, we theoretically demonstrate a circuit with a linearly increasing electric current through the inductor. Calculating the energy delivered to the $LC$ circuit and the energy released from it, we show that a modulated $LC$ circuit can accumulate huge energy from both the time-harmonic external source and the modulating device. Finally, we discuss how this stored energy can be released in the form of a time-compressed pulse.

We present an extensive formulation of the inverse grating problem for flexural waves in which the energy of each diffracted mode is selected and the grating configuration is then obtained by our solving a linear system of equations. The grating is designed as a lineal periodic repetition of a unit cell comprising a cluster of resonators attached at points whose physical properties are directly derived by inversion of a given matrix. Although both active and passive attachments can be required in the most-general case, it is possible to find configurations with only passive (i.e., damped) solutions. This inverse design approach is an alternative to the design of metasurfaces for flexural waves, overcoming the limitations of gradient phase metasurfaces, which require a continuous variation of the surface’s impedance. When the grating is designed in such a way that all the energy is channeled into a single diffracted mode, it behaves as an anomalous refractor or reflector. The negative refractor is analyzed in depth, and it is shown that with only three scatterers per unit cell is it possible to build such a device with unitary efficiency.

We have studied superconducting coplanar-waveguide (CPW) resonators fabricated from disordered films of aluminum. The very high kinetic inductance of these films, inherent to disordered materials, allows us to implement ultrashort (200 $\mu \mathrm{m}$ at 5 GHz resonance frequency) and high-impedance (up to 5 $\mathrm{k}\mathrm{\Omega }$) half-wavelength resonators. We have shown that the intrinsic losses in these resonators at temperatures $\lesssim 250$ mK are limited by resonator coupling to two-level systems (TLSs) in the environment. The demonstrated internal quality factors are comparable with those for CPW resonators made of conventional superconductors. The high kinetic inductance and well-understood losses make these disordered aluminum resonators promising for a wide range of microwave applications, which include kinetic-inductance photon detectors and superconducting quantum circuits.

Nonlinear dynamics has been the key ingredient to improve the performance, in terms of sensitivity, of biased resonant spintronic diodes beyond their semiconductor counterparts. We experimentally demonstrate a nonlinear regime broadband detection for nanoscale spintronic diodes (NSD) where the rectification properties are independent of the input microwave frequency, and compare the device performance with the state of the art Schottky diode for low-power rectification. This regime is achieved in magnetic tunnel junctions with a canted magnetization of the free layer. We further show that the developed NSD provides sufficient dc voltage to supply a low-power nanodevice − a black phosphorus photosensor. Our results could pave the way for using spintronic detectors as building blocks for self-powered nanosystems, such as implantable biomedical devices, wireless sensors, and portable electronics.

We demonstrate a link between surface plastic flow and the ambient chemical environment—a mechanochemical effect—in large-strain deformation of metals, using high-speed in situ observations. This link, which is different from known mechanochemical effects, is studied using aluminum and an alcohol environment. Three distinct flow modes—sinuous, laminar, and segmented—occur, depending on the action of alcohol on the metal surface. Two transitions, one from sinuous to laminar and the other from sinuous to segmented flow, are demonstrated. In both cases, the final flow modes are characterized by smaller deformation forces (an order of magnitude) as well as a much improved quality of the final surface. The action of the chemical medium itself is coupled to the flow mode, distinguishing it from other mechanochemical effects that have previously been reported. The effect appears to be replicable to different degrees in other metal systems such as copper, iron, stainless steels, and nickel. Based on the observations, a schematic stability phase diagram for plastic flow is proposed. Implications of the results for enhancing the performance of cutting and surface-deformation processes for soft and highly strain-hardening metals are discussed.

Neuro-inspired computing architectures are one of the leading candidates to solve complex, large-scale associative learning problems. In addition to the widely studied spiking neural network architecture, other neuro-inspired architectures, such as the oscillatory neural network, can be used to solve such special purpose computing problems. In both cases, voltage- or current-controlled relaxation oscillators are considered a minimal representation of neurons for hardware implementations. For an oscillatory neural-network architecture, such oscillators should demonstrate synchronization and coupling dynamics to achieve collective learning behavior, in addition to desirable individual characteristics such as scaling, power, and performance. To this end, we propose the use of nanoscale, epitaxial heterostructures of phase-change oxides and oxides with metallic conductivity as the fundamental unit of a low-power, tunable electrical oscillator capable of operating in the microwave regime. We perform simulations to show that an optimized heterostructure design with low thermal boundary resistance can result in an operation frequency of up to 3 GHz and a power consumption as low as 15 fJ/cycle with rich coupling dynamics between the oscillators on a 100-nm channel. This study provides an alternative device design of tunable relaxation oscillators for neuromorphic computing applications.

Wide-gap oxides exhibiting both n- and p-type bipolar conductivity are extremely desirable for the development of next generation transparent optoelectronics. While most metal oxides show a propensity for n-type conductivity, nickel oxide ($\mathrm{Ni}\mathrm{O}$) is one of the few that exhibits p-type conductivity without intentional doping. We synthesize nanocrystalline-${\mathrm{Ni}}_x{\mathrm{Cd}}_{1-x}{\mathrm{O}}_{1+\delta }$-alloy thin films over the entire composition range with variable crystal stoichiometry using RT radio-frequency magnetron sputtering. Optical and electronic properties of the films are investigated with a variety of experimental techniques, including spectroscopic ellipsometry, the Seebeck effect, and the variable-temperature Hall effect as well as x-ray photoelectron spectroscopy. We find that the conduction type of ${\mathrm{Ni}}_x{\mathrm{Cd}}_{1-\mathrm{x}}{\mathrm{O}}_{1+\delta }$-alloy thin films depends on the alloy composition and oxygen stoichiometry. Stoichiometric ${\mathrm{Ni}}_x{\mathrm{Cd}}_{1-x}O$ alloys are n type in the composition range of 0 ≤ x ≤ 0.52 and insulating for higher x. On the other hand, $\mathrm{O}$-rich alloys are p-type conducting for x ≥ 0.38. This demonstrates that in the alloy composition range of 0.38 < x < 0.52, n-type and p-type ${\mathrm{Ni}}_x{\mathrm{Cd}}_{1-x}\mathrm{O}$ alloys can be synthesized by controlling the oxygen stoichiometry. The unusual electrical and optical properties of ${\mathrm{Ni}}_x{\mathrm{Cd}}_{1-\mathrm{x}}{\mathrm{O}}_{1+\delta }$-alloy thin films can be explained by the modifications of the electronic band structure due to anticrossing interactions between localized $\mathrm{Ni}$ d levels and extended valence and conduction band states of the alloy. The results offer an interesting opportunity for using ${\mathrm{Ni}}_x{\mathrm{Cd}}_{1-x}{\mathrm{O}}_{1+\delta }$ alloys for transparent devices that require bipolar conductivity.

Fourier-domain mode locking has been a popular laser design for high-speed optical-frequency-domain imaging (OFDI), but achieving long coherence lengths, and therefore imaging range, has been challenging. The narrow linewidth of a Fourier-domain mode-locked (FDML) frequency-comb (FC) laser could provide an attractive platform for high-speed as well as long-range OFDI. Unfortunately, aliasing artifacts arising from signals beyond the principal measurement depth of the free spectral range have prohibited the use of an FDML FC laser for imaging so far. To make the increased coherence length of an FDML FC laser available, methods to manage such artifacts are required. Recently, coherent circular ranging that uses frequency combs for imaging in much-reduced rf bandwidths has been demonstrated. Here we revisit circular ranging as a tool for making the long coherence length of an FDML FC laser and its use for tissue imaging accessible. Using an acousto-optic frequency shifter (AOFS), we describe an active method to mitigate signal aliasing that is both stable and wavelength independent. We show that an FDML FC laser increases the coherence length by an order of magnitude compared with traditional FDML-laser designs without requiring precise dispersion engineering. We discuss design parameters of a frequency-stepping laser resonator as well as aliasing from a frequency comb and AOFS in OFDI with numerical simulations. The use of circular ranging additionally reduces acquisition bandwidths 15-fold compared with traditional OFDI methods. The FDML FC–AOFS design offers a convenient platform for long-range and high-speed imaging as well as for exploring signal- and image-processing methods in circular ranging.

We investigate a nonadiabatic holonomic operation that enables us to entangle two fixed-frequency superconducting transmon qubits attached to a common bus resonator. Two coherent microwave tones are applied simultaneously to the two qubits and drive transitions between the first excited resonator state and the second excited state of each qubit. The cyclic evolution within this effective three-level $\mathrm{\Lambda }$-type system gives rise to a holonomic operation entangling the two qubits. Two-qubit states with 95% fidelity, limited mainly by charge noise of the current device, are created within 213 ns. This scheme is a step toward implementation of a swap-type gate directly in an all-microwave controlled hardware platform. By extending the available set of two-qubit operations in the fixed-frequency qubit architecture, the proposed scheme may find applications in near-term quantum applications using variational algorithms to efficiently create problem-specific trial states. We illustrate this point by computing the ground state of molecular hydrogen using the holonomic operation.

Using a two-temperature model coupled with the modified time-dependent Ginzburg-Landau equation, we calculate the delay time ${\tau }_d$ in the appearance of a growing normal domain in the current-biased superconducting strip after absorption of the single photon. We demonstrate that ${\tau }_d$ depends on the place in the strip where the photon is absorbed and monotonically decreases with an increasing current. We argue that the variation of ${\tau }_d$ (timing jitter), connected either with position-dependent response or Fano fluctuations, could be as small as the lowest relaxation time of the superconducting order parameter, approximately $\hslash /k_B{T}_c$ ($T_c$ is the critical temperature of the superconductor), when the current approaches the depairing current.