The combination of superconducting and magnetic materials to create superconducting devices has been motivated by the discovery of Josephson critical current ($I_{cs}$) oscillations as a function of magnetic layer thickness and the demonstration of devices with switchable critical currents. However, none of the hybrid devices has shown any spintronic effects, such as spin-transfer torque, which are currently used in room-temperature magnetic devices, including spin-transfer torque random-access memory and spin-torque nano-oscillators. We develop nanopillar Josephson junctions with a minimum feature size of 50 nm and magnetic barriers exhibiting magnetic pseudo-spin-valve behavior at 4 K. With a bias current higher than $I_{cs}$, these devices allow current-induced magnetization switching that results in tenfold changes in $I_{cs}$. The current-induced magnetic switching is consistent with spin-transfer torque models for room-temperature magnetic devices. Our work demonstrates that devices that combine superconducting and spintronic functions show promise for the development of a nanoscale, nonvolatile, cryogenic memory technology.

We fabricate plasmonic nanohole arrays in gold thin films by focused-ion-beam lithography. Subsequent heat treatment of the lithographic patterns induces the growth of sub-100-nm structures including tips, rods, and flakes, all localized in the nanohole-array region. The coupled nanohole-array–nanostructure system comprises an efficient photoemitter. High-brightness photoemission is observed from this construct, following 780-nm femtosecond laser irradiation, using photoemission electron microscopy (PEEM). By comparing our PEEM observables to finite-difference time-domain simulations, we demonstrate that photoemission from the sub-100-nm structures is enhanced in the region of propagating surface plasmons launched from the nanohole arrays.

We report the creation of a low-loss broadband optical antenna giving highly directed output from a coherent single spin in the solid state. The device, a crystalline solid-state realization of a dielectric antenna, is engineered for individual nitrogen-vacancy electronic spins in diamond. We demonstrate a directionality close to 10. The photonic structure preserves the high spin coherence of single-crystal diamond ($T_2\gtrsim 100\mu \mathrm{s}$). The single-photon count rate approaches a megahertz facilitating efficient spin readout. We thus demonstrate a key enabling technology for quantum applications such as high-sensitivity magnetometry and long-distance spin entanglement.

The electrical, chemical, and structural interactions between Ni films and ${\mathrm{I}\mathrm{n}}_{0.53}{\mathrm{G}\mathrm{a}}_{0.47}\mathrm{A}\mathrm{s}$ for source-drain applications in transistor structures have been investigated. It was found that for thick ($>10\mathrm{nm}$) Ni films, a steady decrease in sheet resistance occurs with increasing anneal temperatures, however, this trend reverses at $450°\mathrm{C}$ for 5 nm thick Ni layers, primarily due to the agglomeration or phase separation of the Ni-(In,Ga)As layer. A combined hard-x-ray photoelectron spectroscopy (HAXPES) and x-ray absorption spectroscopy (XAS) analysis of the chemical structure of the Ni-(In,Ga)As alloy system shows: (1) that Ni readily interacts with ${\mathrm{I}\mathrm{n}}_{0.53}{\mathrm{G}\mathrm{a}}_{0.47}\mathrm{A}\mathrm{s}$ upon deposition at room temperature resulting in significant interdiffusion and the formation of NiIn, NiGa, and NiAs alloys, and (2) the steady diffusion of Ga through the Ni layer with annealing, resulting in the formation of a ${\mathrm{Ga}}_2{\mathrm{O}}_3$ film at the surface. The need for the combined application of HAXPES and XAS measurements to fully determine chemical speciation and sample structure is highlighted and this approach is used to develop a structural and chemical compositional model of the Ni-(In,Ga)As system as it evolves over a thermal annealing range of 250 to $500°\mathrm{C}$.

Cloaking of nanoscopic objects via the scattering-cancellation technique is theoretically demonstrated in integrated plasmonic circuits with deep subwavelength confinement in the infrared. By proper design of the cloaking shell, transparency of the cloaked objects can be achieved, even when these objects occupy the whole cross section of a metal-insulator-metal waveguide. The effects of material losses, possible implementations, and potential applications are discussed.

Manipulation of submicron-sized particles using second-order acoustic radiation forces at ultrasonic frequencies is hindered by the time-independent streaming flows. A similar phenomenon occurs when open fluid volumes are vibrated at low frequencies in the range of 100 Hz. The streaming phenomenon, in this lower-frequency range, is studied here by using horizontally actuated liquid-filled rectangular chambers. The formation of capillary waves at the liquid-air interface generates spatially varying flow fields in the bulk fluid, which can be used to collect particles at stable locations. However, the same spatial variation is the source of the streaming fields, which, under some conditions, can drag particles away from these stable locations. The governing equations for the second-order flow are derived and simulated, after which a particle-tracing algorithm is executed in the obtained flow field. Critical particle parameters are determined in multiple simulated chambers of different dimensions, with the aim of reducing the effect of the streaming field on the particle’s movement. The simulation results are then applied experimentally to demonstrate the ability of this system to collect particles as small as 50 nm in diameter.

In dilute-nitride semiconductors, the possibility to selectively passivate N atoms by spatially controlled hydrogen irradiation allows for tailoring the effective N concentration of the host—and, therefore, its electronic and structural properties—with a precision of a few nanometers. In the present work, this technique is applied to the realization of ordered arrays of ${\mathrm{GaAs}}_{1-x}{\mathrm{N}}_x/{\mathrm{GaAs}}_{1-x}{\mathrm{N}}_x:\mathrm{H}$ wires oriented at different angles with respect to the crystallographic axes of the material. The creation of a strongly anisotropic strain field in the plane of the sample, due to the lattice expansion of the fully hydrogenated regions surrounding the ${\mathrm{GaAs}}_{1-x}{\mathrm{N}}_x$ wires, is directly responsible for the peculiar polarization properties observed for the wire emission. Temperature-dependent polarization-resolved microphotoluminescence measurements, indeed, reveal a nontrivial dependence of the degree of linear polarization on the wire orientation, with maxima for wires parallel to the [110] and $[1\overline{1}0]$ directions and a pronounced minimum for wires oriented along the [100] axis. In addition, the polarization direction is found to be precisely perpendicular to the wire when the latter is oriented along high-symmetry crystal directions, whereas significant deviations from a perfect orthogonality are measured for all other wire orientations. These findings, which are well reproduced by a theoretical model based on finite-element calculations of the strain profile of our ${\mathrm{GaAs}}_{1-x}{\mathrm{N}}_x/{\mathrm{GaAs}}_{1-x}{\mathrm{N}}_x:\mathrm{H}$ heterostructures, demonstrate our ability to control the polarization properties of dilute-nitride micro- and nanostructures via H-assisted strain engineering. This additional degree of freedom may prove very useful in the design and optimization of innovative photonic structures relying on the integration of dilute-nitride-based light emitters with photonic crystal microcavities.

Since the introduction of the decoy-state technique, phase-randomized weak coherent light pulses have been the key to increase the practicality of quantum-based communications. Their ultrafast generation was accomplished via compact gain-switched (GS) lasers, leading to high key rates in quantum key distribution (QKD). Recently, the question arose whether the same laser could be employed to achieve high-speed measurement-device-independent QKD, a scheme that promises long-haul quantum communications immune to all detector attacks. For that, a challenging high-visibility interference between independent picosecond optical pulses is required. Here, we answer the above question in the affirmative by demonstrating high-visibility interference from two independent GS lasers triggered at 1 GHz. The result is obtained through a careful characterization of the laser frequency chirp and time jitter. By relating these quantities to the interference visibility, we obtain a parameter-free verification of the experimental data and a numerical simulation of the achievable key rates. These findings are beneficial to other applications making use of GS lasers, including random number generation and standard QKD.

Carbon is a common unintentional impurity in oxide semiconductors. We use hybrid density functional theory to calculate the electronic and structural properties of carbon impurities in ZnO, ${\mathrm{In}}_2{\mathrm{O}}_3$, and ${\mathrm{Ga}}_2{\mathrm{O}}_3$—materials that are used as transparent conductors. In each of these semiconducting oxides, we find that carbon is most likely to occupy the cation site under most electronic and chemical potential conditions. In ZnO, ${\mathrm{C}}_{\mathrm{Zn}}$ acts as a shallow double donor and exhibits large local breathing-mode relaxations. In ${\mathrm{In}}_2{\mathrm{O}}_3$ and ${\mathrm{Ga}}_2{\mathrm{O}}_3$, C acts as a shallow donor and moves off the cation site to become threefold oxygen coordinated. In all three oxides, ${\mathrm{C}}_{\text{cation}}$ exhibits modest formation energies, indicating that these species will be likely to incorporate. Our results indicate that C impurities are suitable donor dopants in these oxides and will contribute to background $n$-type conductivity if unintentionally present.

By investigating thoroughly the tunable behavior of coupled modes, we highlight how it provides a means to tune the properties of spin-transfer nano-oscillators. We first demonstrate that the main features of the microwave signal associated with coupled vortex dynamics, i.e., frequency, spectral coherence, critical current, and mode localization, depend drastically on the relative vortex core polarities. Second, we report a large reduction of the nonlinear linewidth broadening obtained by changing the effective damping through the control of the core configuration. Such a level of control on the nonlinear behavior reinforces our choice to exploit the microwave properties of collective modes for applications in advanced spintronics devices for integrated telecommunication concerns.

Wear and removal of material from polycrystalline metal surfaces is inherently connected to plastic flow. Here, plowing-induced unconstrained surface plastic flow on a nanocrystalline copper surface has been studied by massive molecular dynamics simulations and atomic force microscopy scratch experiments. In agreement with experimental findings, bulges in front of a model asperity develop into vortexlike fold patterns that mark the disruption of laminar flow. We identify dislocation-mediated plastic flow in grains with suitably oriented slip systems as the basic mechanism of bulging and fold formation. The observed folding can be fundamentally explained by the inhomogeneity of plasticity on polycrystalline surfaces which favors bulge formation on grains with suitably oriented slip system. This process is clearly distinct from Kelvin-Helmholtz instabilities in fluids, which have been previously suggested to resemble the formed surface fold patterns. The generated prow grows into a rough chip with stratified lamellae that are identified as the precursors of wear debris. Our findings demonstrate the importance of surface texture and grain structure engineering to achieve ultralow wear in metals.

Strongly correlated electron systems such as the rare-earth nickelates ($R{\mathrm{NiO}}_3$, $R$ denotes a rare-earth element) can exhibit synapselike continuous long-term potentiation and depression when gated with ionic liquids; exploiting the extreme sensitivity of coupled charge, spin, orbital, and lattice degrees of freedom to stoichiometry. We present experimental real-time, device-level classical conditioning and unlearning using nickelate-based synaptic devices in an electronic circuit compatible with both excitatory and inhibitory neurons. We establish a physical model for the device behavior based on electric-field-driven coupled ionic-electronic diffusion that can be utilized for design of more complex systems. We use the model to simulate a variety of associate and nonassociative learning mechanisms, as well as a feedforward recurrent network for storing memory. Our circuit intuitively parallels biological neural architectures, and it can be readily generalized to other forms of cellular learning and extinction. The simulation of neural function with electronic device analogs may provide insight into biological processes such as decision making, learning, and adaptation, while facilitating advanced parallel information processing in hardware.

Metasurfaces with subwavelength thicknesses have exhibited unconventional phenomena in ways that could not be mimicked by traditional materials. Here we report on the analytical design and experimental realizations of acoustic metasurfaces with hitherto inaccessible functionality of manipulating the reflected waves arbitrarily. By suitably designing the phase-shift profile with a $2\pi$ span induced by labyrinthine units, the metasurface can reflect acoustic waves in an unusual yet controllable manner. Anomalous reflection and ultrathin planar lenses are both demonstrated with carefully designed metasurfaces. Remarkably, the free manipulation of phase shifts offers great flexibility in the design of nonparaxial acoustic self-accelerating beams with arbitrary trajectories. With extraordinary wave-steering ability, the metasurface should open exciting possibilities for designing compact acoustic components with versatile potential and may find a variety of applications ranging from ultrasound imaging to caustic engineering where designing the shape of a focused trajectory of sound is required.

Several technologies and biotechnologies employing small-sized particles in microfluidics and nanofluidics rely on the ability of hampering thermal motion for progress. We experimentally demonstrate that nanoparticles suspended in a dilute polymer solution in Poiseuille flow can be trapped in the central region of a microtube, with a trapping efficiency that depends on the squared flow rate. The trap force is caused by the viscoelasticity of the suspending fluid, and can be modulated by selecting liquids with specific rheology. We also propose a simple theoretical argument that supports the experimental evidence, and links the trapping force to a dimensionless parameter comparing viscoelastic normal forces and Brownian forces. The theoretical argument distills into a simple equation, which could be used to downscale flow cytometers, or to design microfluidic devices for counting, coding, or separating nanoparticles.

The practicality of a newly proposed infrared-terahertz (IR-THz) double-resonance (DR) spectroscopic technique for remote trace-gas identification is explored. The strength of the DR signatures depends on known molecular parameters from which a combination of pump-probe transitions may be identified to recognize a specific analyte. Atmospheric pressure broadening of the IR and THz trace-gas spectra relaxes the stringent pump coincidence requirement, allowing many DR signatures to be excited, some of which occur in the favorable atmospheric transmission windows below 500 GHz. By designing the DR spectrometer and performing a detailed signal analysis, the pump-probe power requirements for detecting trace amounts of methyl fluoride, methyl chloride, or methyl bromide may be estimated for distances up to 1 km. The strength of the DR signature increases linearly with pump intensity but only as the square root of the probe power because the received signal is in the Townes noise limit. The concept of a specificity matrix is introduced and used to quantify the recognition specificity and calculate the probability of false positive detection of an interferent.

Pt-Al and Pt-Y-Al thin-film electrodes on yttria-stabilized zirconia electrolytes were prepared by dealloying of cosputtered Pt-Al or Pt-Y-Al films. The selective dissolution of Al from the Pt-alloy compound causes the formation of a highly porous nanowire network with a mean branch thickness below 25 nm and a pore intercept length below 35 nm. The oxygen-reduction capability of the resulting electrodes is analyzed in a micro-solid-oxide fuel-cell setup at elevated temperatures (598–873 K). Here, we demonstrate that these nanoporous thin films excel “state-of-the-art” fuel-cell electrodes in terms of catalytic activity and thermal stability. The nanoporous Pt electrodes exhibit exchange-current densities that are up to 13 times higher than conventional Pt electrodes, measured at 648 K. It is shown that the enhanced intrinsic electrocatalytic activity of these Pt electrodes is achieved by the addition of yttrium as a ternary constituent, which allows for an engineering of the material’s band structure.

Spin labeling of molecules with paramagnetic ions is an important approach for determining molecular structure; however, current ensemble techniques lack the sensitivity to detect few isolated spins. In this paper, we demonstrate two-dimensional nanoscale imaging of paramagnetic gadolinium compounds using scanning relaxometry of a single nitrogen-vacancy (NV) center in diamond. Gadopentetate dimeglumine attached to an atomic-force microscope tip is controllably interacted with and detected by the NV center by virtue of the fact that the NV exhibits fast relaxation in the fluctuating magnetic field generated by electron spin flips in the gadolinium. Using this technique, we demonstrate a reduction in the $T_1$ relaxation time of the NV center by over 2 orders of magnitude, probed with a spatial resolution of 20 nm. Our result exhibits the viability of the technique for imaging individual spins attached to complex nanostructures or biomolecules, along with studying the magnetic dynamics of isolated spins.

Currently, a major nanotechnological challenge is to design thermoelectric devices that have a high figure of merit. To that end, we propose to use bilayer excitons in two-dimensional nanostructures. Bilayer-exciton systems are shown to have an improved thermopower and an enhanced electric counterflow and thermal conductivity, with respect to regular semiconductor-based thermoelectrics. We suggest an experimental realization of a bilayer-exciton thermocouple. Based on current experimental parameters, a bilayer-exciton heterostructure of $p$- and $n$-doped ${\mathrm{Bi}}_2{\mathrm{Te}}_3$ can enhance the figure of merit an order of magnitude compared to bulk ${\mathrm{Bi}}_2{\mathrm{Te}}_3$. Another material suggestion is to make a bilayer out of electron-doped ${\mathrm{SrTiO}}_3$ and hole-doped ${\mathrm{Ca}}_3{\mathrm{Co}}_4{\mathrm{O}}_9$.

We demonstrate a dual-axis accelerometer and gyroscope atom interferometer, which can form the building blocks of a six-axis inertial measurement unit. By recapturing the atoms after the interferometer sequence, we maintain a large atom number at high data rates of 50 to 100 measurements per second. Two cold ensembles are formed in trap zones located a few centimeters apart and are launched toward one another. During their ballistic trajectory, they are interrogated with a stimulated Raman sequence, detected, and recaptured in the opposing trap zone. We achieve sensitivities at $\mu \mathit{g}/\sqrt{\mathrm{Hz}}$ and $(\mu \mathrm{rad}/\mathrm{s})/\sqrt{\mathrm{Hz}}$ levels, making this a compelling prospect for expanding the use of atom interferometer inertial sensors beyond benign laboratory environments.

The effects of a driven fast-oscillating spinneret on the bending instability of electrified jets, leading to the formation of spiral structures in electrospinning experiments with charged polymers, are explored by means of extensive computer simulations. It is found that the morphology of the spirals can be placed in direct correspondence with the oscillation frequency and amplitude. In particular, by increasing the oscillation amplitude and frequency, thinner fibers can be extracted by the same polymer material, thereby opening design scenarios in electrospinning experiments.