We present a quantum-limited Josephson-junction-based three-wave-mixing parametric amplifier, the superconducting nonlinear asymmetric inductive element (SNAIL) parametric amplifier (SPA), which uses an array of SNAILs as the source of tunable nonlinearity. We show how to engineer the nonlinearity over multiple orders of magnitude by varying the physical design of the device. As a function of design parameters, we systematically explore two important amplifier nonidealities that limit dynamic range: the phenomena of gain compression and intermodulation distortion, whose minimization are crucial for high-fidelity multiqubit readout. Through a comparison with first-principles theory across multiple devices, we demonstrate how to optimize both the nonlinearity and the input-output port coupling of these SNAIL-based parametric amplifiers to achieve higher saturation power, without sacrificing any other desirable characteristics. The method elaborated in our work can be extended to improve all forms of parametrically induced mixing that can be employed for quantum-information applications.

We have extended the Charge Extraction by a Linearly Increasing Voltage (CELIV) technique for determination of the built-in potential and the charge-carrier mobility in thin-film metal-insulator-metal (M-I-M) diodes. The validity of the presented analytical theory is verified by drift-diffusion simulations and experimentally demonstrated on organic solar cells. In contrast to the original CELIV theory, which assumes a uniform charge-carrier distribution in the active layer of the device, here we derive an analytical expression for determining the built-in potential and mobility in the case of a nonuniform charge-carrier distribution where charges have diffused into the active layer from the contacts. The extended CELIV theory is applicable on all thin-film M-I-M diodes, e.g., organic solar cells. Drift-diffusion simulations show that the error for mobility estimation can be an order of magnitude if not correcting for the carrier profile.

The Dzyaloshinskii-Moriya interaction (DMI) has attracted considerable recent attention owing to the intriguing physics behind it and the fundamental role it plays in stabilizing magnetic solitons, such as magnetic skyrmions and chiral domain walls. A number of experimental efforts have been devoted to probe the DMI, among which the most popular method is the Brillouin light scattering spectroscopy (BLS) to measure the frequency difference of spin waves with opposite wave vectors $\pm \mathbf{k}$ perpendicular to the in-plane magnetization $\mathbf{m}$. Such a technique, however, is not applicable to the cases of $\mathbf{k}\parallel \mathbf{m}$, since the spin-wave reciprocity is recovered then. For a narrow magnetic strip, it is also difficult to measure the DMI strength using BLS because of the spatial resolution limit of lights. To fill these gaps, we propose to probe the DMI via the propagation of spin waves in ferromagnetic films. We show that the DMI can cause the noncollinearity of the group velocities of spin waves with $\pm \mathbf{k}\parallel \mathbf{m}$. In heterogeneous magnetic thin films with different DMIs, negative refractions of spin waves emerge at the interface under proper conditions. These findings enable us to quantify the DMI strength by measuring the angle between the two spin-wave beams with $\pm \mathbf{k}\parallel \mathbf{m}$ in homogeneous film and by measuring the incident and negative refraction angles in heterogeneous films. For a narrow magnetic strip, we propose a nonlocal scheme to determine the DMI strength via nonlinear three-magnon processes. We implement theoretical calculations and micromagnetic simulations to verify our ideas. The results presented here are helpful for future measurement of the DMI and for designing novel spin-wave spintronic devices.

We experimentally study the auto-oscillating spin-wave modes in ${\text{Ni}}_{80}{\text{Fe}}_{20}/\beta \text{−}\mathrm{W}$ constriction-based spin Hall nano-oscillators as a function of bias current, strength of the in-plane applied field, and azimuthal field angle in the low-field range of 40–80 mT. We observe two different spin-wave modes: (i) a linearlike mode confined to the internal field minima near the edges of the nanoconstriction, and only weakly dependent on the bias current and the applied-field angle, and (ii) a second, lower-frequency mode with significantly higher threshold current and stronger dependence on both the bias current and the external-field angle. Micromagnetic modeling qualitatively reproduces the experimental data and reveals that the second mode is a spin-wave bullet and that the spin Hall nano-oscillator mode hops between the two modes, resulting in a substantial increase in linewidths. In contrast to the linearlike mode, the bullet is localized in the middle of the constriction and shrinks with increasing bias current. Using intrinsic frequency doubling at zero field angle, we can reach frequencies above 9 GHz in fields as low as 40 mT, which is important for the development of low-field spintronic oscillators with applications in microwave-signal generation and neuromorphic computing.

Lead-halide perovskites attract attention as materials for high-efficiency solar cells and light-emitting applications. Among their attributes are solution processability, high absorbance in the visible spectral range, and defect tolerance, as manifested in long photocarrier lifetimes and diffusion lengths. The microscopic origin of photophysical properties of perovskites is, however, still unclear and under debate. Here, we observe an interesting universal scaling behavior in a series of (hybrid and all-inorganic) perovskite single crystals investigated via simultaneous measurements of the Hall effect, photoconductivity, and photoluminescence. A clear correlation between photoconductivity and photoluminescence as functions of the incident photon flux is observed. While photoconductivity exhibits a crossover in the power-law dependence between power exponents 1 and 1/2, photoluminescence exhibits a crossover between power exponents 2 and 3/2. This correlation is found in all the studied compounds regardless of the cation type (organic or inorganic) or crystallographic phases. We propose phenomenological microscopic mechanisms that explain these interesting nontrivial power exponents and crossovers between them in this broad class of lead-halide perovskites.

Equiatomic ${\mathrm{Fe}}_{50}{\mathrm{Rh}}_{50}$, with its highly unusual antiferromagnetic to ferromagnetic phase transition at approximately 370 K, provides the ability to create artificial multifunctional materials when combined with high-anisotropy magnetic thin films in an exchange-mediated structure. We investigate the temperature dependence of switching in $\mathrm{L}{1}_0$ ${\mathrm{Fe}}_{55}{\mathrm{Pt}}_{45}/{\mathrm{Fe}}_{49}{\mathrm{Rh}}_{51}$ bilayer thin films, where the ${\mathrm{Fe}}_{49}{\mathrm{Rh}}_{51}$ assists the switching of the ${\mathrm{Fe}}_{55}{\mathrm{Pt}}_{45}$ at modestly increased temperatures. A simple layered macrospin model is able to capture the switching characteristics of these films and shows good agreement with experimental results. Patterned $\mathrm{L}{1}_0$ ${\mathrm{Fe}}_{55}{\mathrm{Pt}}_{45}/{\mathrm{Fe}}_{49}{\mathrm{Rh}}_{51}$structures, measured using the anomalous Hall effect, show a similar temperature-dependent switching behavior, paving the way for reduced switching fields in future applications including heat-assisted bit-patterned-media recording and spintronic devices.

We demonstrate how to extract the material-dependent spin-torque efficiency ($ϵ$) and asymmetry ($\lambda$) from the field-current nucleation boundaries of magnetic droplet solitons in orthogonal nano-contact spin-torque oscillators with ${\mathrm{Co}}_x{\mathrm{(Ni}}_{80}{\mathrm{Fe}}_{20}{)}_{1-x}$, ($x=0-1$), fixed layers. As the perpendicular component of the fixed-layer magnetization plays a central role in governing droplet nucleation, the nucleation boundaries exhibit monotonic shifts towards higher perpendicular magnetic fields when the fixed-layer magnetization ${\mu }_0{M}_{s,p}$ is tuned from 1.04 to 1.7 T. We then extract $ϵ$ and $\lambda$ from fits to the nucleation boundaries and find that while $ϵ$ does not vary with composition, $\lambda$ increases from 1.5 to 3 with increasing $\mathrm{Co}$ content. The analysis of droplet nucleation boundaries is hence a useful tool for the systematic study of both $ϵ$ and $\lambda$ as functions of material composition.

Nonvolatile magnetic random-access memory is one of the most promising memory candidates to meet the requirements of high-density and low-power data storage. Downsize scalability and energy efficiency of conventional memory-unit cells with elliptic shape, however, remain matters of great concern. The development of alternative memory-unit-cell architecture that would potentially enhance the performance of practical devices is thus particularly interesting for applications. Here the magnetic configurations for nanoring magnetic tunnel junctions with an in-plane magnetic storage layer are studied by micromagnetic simulation, revealing that, for an appropriate ring width, the outer diameter can be scaled down to less than 20 nm, where the magnetic onion configuration becomes energetically favored. We also study the spin-transfer-torque switching process and dynamic resistance state diagram with respect to the applied magnetic field and spin-polarized current. A low switching-current density is demonstrated. The results indicate the advantage of using nanoring magnetic tunnel junctions as memory units, which may provide an alternative solution for high-storage-density and low-power-consumption nonvolatile memory.

In this work the polarizability of a subwavelength core-shell sphere is considered, where the shell exhibits a radially inhomogeneous permittivity profile. The mathematical treatment of the electrostatic polarizability is formulated in terms of the scattering potentials and the corresponding scattering amplitudes. As a result, a generalized expression of the polarizability is presented to be dependent of the radial inhomogeneity function. The extracted general model is applied for two particular cases, i.e., a power-law profile and a new class of permittivity profiles that exhibit exponential radial dependence. The proposed analysis quantifies in a simple manner the inhomogeneity effects, allowing the direct implementation of naturally or artificially occurring permittivity inhomogeneities for a wide range of applications within and beyond the metamaterial paradigm. Specifically, a special case of symmetric-antisymmetric resonant plasmonic degeneracy is identified and shown for the case of a core-shell sphere with a power-law permittivity profile. This degeneracy could be used for the experimental identification of inhomogeneity-induced effects or for applications where a strong coupling resonant regime is required. Furthermore, the described analysis opens avenues towards the phenomenological and first-principles modeling of the electrodynamic scattering effects for graded-index plasmonic particles at the nanoscale. Finally, such a description can be readily used either for the benchmarking of novel computational methods incorporating inhomogeneous materials or for inverse scattering purposes.

Subnatural-linewidth single photons are generated from atomic ensembles and temporally reshaped as desired so that their temporal modes are eligible for constructing qubits. However, the photon-counting measurement scheme most commonly used in the time-bin basis does not directly give the complex coefficients between time-bins. On the other hand, a mode-sensitive measurement technique, such as homodyne tomography, is hard to implement since a mode-restricting cavity is not allowed in this reshaped photon source. Here, we utilize a cavity-free homodyne detection scheme for reconstructing the temporal-density matrix for the subnatural-linewidth single photons generated from a cold atomic cloud. The characterization of the pure temporal-spectral state of subnatural-linewidth single photons paves the way for exploiting the temporal-spectral degree of freedom to develop photonic-quantum information processing.

Cubic silicon carbide is an excellent platform for integration of defect qubits into established wafer-scale device architectures for quantum information and sensing applications, where a divacancy qubit, which is similar to the negatively charged nitrogen-vacancy (NV) center in diamond, has favorable coherence properties. We demonstrate by means of density-functional-theory calculations that for most types of distortion the $3C$ divacancy exhibits slightly smaller spin-strain coupling parameters but greater spin-stress coupling parameters in comparison with the diamond NV. We predict that high-quality $3C$-$\mathrm{Si}\mathrm{C}$ thin films hosting divacancy qubits are prospective platforms for quantum-enhanced pressure-sensor devices.

Faithfully transferring the quantum state is essential for quantum information processing. Here we demonstrate a fast (in 84 ns) and high-fidelity (99.2%) transfer of arbitrary quantum states in a chain of four superconducting qubits with nearest-neighbor coupling. This transfer relies on full control of the effective couplings between neighboring qubits, which is realized only by our parametrically modulating the qubits without increasing circuit complexity. Once the couplings between qubits fulfill a specific ratio, perfect quantum state transfer can be achieved in a single step, and is therefore robust to noise and accumulation of experimental errors. This quantum state transfer can be extended to a larger qubit chain and thus adds a desirable tool for future quantum information processing. The demonstrated flexibility of the coupling tunability is suitable for quantum simulation of many-body physics, which requires different configurations of qubit couplings.

We report a one-dimensional planar optical waveguide with a transverse distribution of the inhomogeneous loss profile, which exhibits an exceptional point (EP). The waveguide hosts two leaky resonant modes, where the interaction between them in the vicinity of the EP is controlled by proper adjustment of the inhomogeneity in the attenuation profile only. We study the adiabatic dynamics of propagation constants of the coupled modes by quasistatic encirclement of control parameters around the EP. Realizing such an encirclement with the inhomogeneous loss distribution along the direction of light propagation, we report the breakdown of adiabatic evolution of two coupled modes through the waveguide in the presence of an EP. During conversion the output mode is irrespective of the choice of the input excited mode but depends on the direction of light transportation. This topologically controlled, robust scheme of asymmetric mode conversion in the platform of the proposed all-lossy waveguide structure may provide a means for implementation of state-transfer applications in chirality-driven waveguide-based devices.

The electroluminescence from organic light-emitting diodes can be predicted with molecular-scale resolution using three-dimensional kinetic Monte Carlo (3D KMC) simulations [M. Mesta et al., Nat. Mater. 12, 652 (2013)]. However, around and below the built-in voltage KMC simulations are computationally inefficient. 3D master-equation (3D ME) simulation methods, which are fastest for low voltages, are so far mainly available for describing unipolar charge transport. In such simulations, the charge-carrier interactions are treated within a mean-field approach. It is not $\mathrm{a\; priori}$ evident whether such simulations, when applied to bipolar devices, can be extended to include the Coulomb attraction between the individual electrons and holes, so that charge-carrier recombination is sufficiently well described. In this work, we develop a systematic method for extending 3D ME simulations to bipolar devices. The method is applied to devices containing materials with Gaussian energetic disorder, and validated by a comparison with the results of 3D KMC simulations. The comparison shows that the 3D nonuniformity of the molecular-site-resolved carrier concentration and the one-dimensional layer-averaged profile of the recombination rate are fully retained, and that the 3D nonuniformity of the molecular-site-resolved recombination rate is fairly well retained.

Plasma processing is a key technology for fabrication of state-of-the-art semiconductor devices. The device performance is often limited by electronic defects generated during plasma processing, although most of those defects are annihilated by postannealing. Here we experimentally study the defect kinetics in hydrogenated amorphous silicon ($a\text{−}\mathrm{Si}:\mathrm{H}$) during hydrogen- (${\mathrm{H}}_2$-) and argon- ($\mathrm{Ar}$-) plasma treatment, and also consecutive postannealing. The generation and annihilation of electronic defects such as silicon dangling bonds are monitored via in situ measurements of the photocurrent. From the measurements, the following results are found: (i) The defects are generated dominantly by radical species such as $\mathrm{H}$ atoms for a ${\mathrm{H}}_2$ plasma and metastable $\mathrm{Ar}$ atoms for an $\mathrm{Ar}$ plasma. (ii) The ${\mathrm{Ar}}^{+}$-ion bombardment creates residual defects, whereas the ${\mathrm{H}}_3^{+}$-ion bombardment does not create such defects. The residual defects are mostly recovered by an additional ${\mathrm{H}}_2$-plasma and postannealing treatment. (iii) The annihilation of defects exhibits stretched exponential behavior, revealing the dispersive nature of $a\text{−}\mathrm{Si}:\mathrm{H}$. (iv) The activation energy for the annihilation of defects depends on the origins of the defect generation. The activation energy is smaller for defects generated by plasma treatments compared with that for defects induced by photon irradiation. A rate equation for the generation and annihilation of defects is described, and the defect kinetics is discussed in terms of microstructural changes associated with hydrogen diffusion.

The cavitation-driven expansion dynamics of liquid tin microdroplets set in motion by the ablative impact of a 15-ps laser pulse is investigated. We combine high-resolution stroboscopic shadowgraphy with an intuitive fluid-dynamic model that includes the onset of fragmentation, and find good agreement between model and experimental data for two different droplet sizes over a wide range of laser pulse energies. The dependence of the initial expansion velocity on these experimental parameters is heuristically captured in a single power law. Further, the late-time mass distributions obtained are shown to be governed by a single parameter. These studies are performed under conditions relevant for plasma light sources for extreme-ultraviolet nanolithography.

Many factors influence the design of quantum annealing processing units. Here we address the issue of improving quantum-annealing-processing-unit designs from the point of view of the critical behavior of spin glasses. It has been argued [Katzgraber et al., Phys. Rev. X 4, 021008 (2014)] that among the most-difficult Ising spin-glass ground-state problems are those related to lattices that exhibit a finite-temperature spin-glass transition. Here we show that addition of small-world couplers between qubits (spins) to the native quasiplanar quantum-processing-unit graph results in a topology where a disordered Ising system can undergo a finite-temperature spin-glass transition, even when an Ising spin glass on the quasiplanar native graph does not display a transition to a glassy phase at any finite temperature. To ensure that these systems can be engineered with current fabrication techniques, using large-scale Monte Carlo simulations, we demonstrate that highly constrained systems restricted to a few fabrication layers and with fixed coupler angles can also exhibit a finite-temperature spin-glass transition. This indicates that these systems might be mean-field-like, which also means that embedding highly nonplanar problems might be simplified when compared with the underlying native topology. Our results are illustrated with use of the quasiplanar Chimera topology currently used in the quantum annealing machines from D-Wave Systems Inc. as well as standard two-dimensional square lattices. The approach presented can be generalized to other topologies.

Colloidal crystals exhibit structural color without any color pigment due to the crystals’ periodic nanostructure, which can interfere with visible light. This crystal structure is iridescent as the resulting color changes with the viewing or illumination angle, which limits its use for printing or displays. To eliminate the iridescent property, it is important to make the packing of the colloidal nanoparticles disordered. Here, we introduce a drop-casting method where a droplet of a water-ethanol mixture containing monodisperse polymer-coated silica nanoparticles creates a relatively uniform and noniridescent deposit after the droplet evaporates completely on a heated substrate. The uniformity is caused by a thermal Marangoni flow and fast evaporation effects due to the heated substrate, whereas noniridescence is the outcome of short-range-ordered packing of nanoparticles by depletion attraction and friction effects produced by polymer brushes. We show that the colors of the final deposits from individual droplets remain unchanged while the viewing angle is varied under ambient light. We expect the coating method to be compatible with ink-jet printing and the uniformly coated self-assembled noniridescent nanostructures have potential for color displays using reflection mode and other optical devices.

Pure spin currents are considered as key processes for future low-dissipation electronics. Current detection schemes use the inverse spin Hall effect for converting the spin current into an electrical voltage. Here we present an alternative, contact-free detection scheme based on a paramagnetic molecular probe layer that in principle allows the integration of spin currents into molecular electronics. In a measurement without the requirement of lithographically tailored samples and electrical contacts, we detect changes of the electron spin resonance in the probe layer when a spin current is injected from a microwave-driven ferromagnetic layer. Tuning the microwave resonance conditions of the ferromagnetic and molecular layer to the same resonance condition, we observe the flow of spin momentum at the interface through the change of the microwave-power-dependent absorption of the adsorbed molecular layer. We use oleic acid as the molecular probe layer on metallic and oxidized iron films.

We consider the phenomenon of electric Mott transition (EMT), which is an electrically induced insulator-to-metal transition. Experimentally, it is observed that depending on the magnitude of the electric excitation, the final state may show a short-lived or a long-lived resistance change. We extend a previous model for the EMT to include the effect of local structural distortions through an elastic energy term. We find that by strong electric pulsing, the induced metastable phase may become further stabilized by the electroelastic effect. We present a systematic study of the model by numerical simulations and compare the results to experiments in Mott insulators of the ${\mathrm{AM}}_4{\mathrm{Q}}_8$ family. Our work significantly extends the scope of our recently introduced leaky-integrate-and-fire Mott neuron [P. Stoliar et al., Adv. Funct. Mat. 27, 1604740 (2017)] to provide a better insight into the physical mechanism of its relaxation. This is a key feature for future implementations of neuromorphic circuits.