Selective filtering of spectral and angular optical transmission has recently attracted a great deal of interest. While optical passband and stop-band spectral filters are already widely used, angle-selective transmission and reflection filtering represents a less than fully explored alternative. Nonetheless, this approach can be promising for several applications, including stray radiation minimization and background emission exclusion. In this work, a concept for angle-selective reflection filtering using guided-mode resonance coupling is proposed. Although guided-mode resonance structures are already used for spectral filtering, in this work, a variation of angle-selective reflection filtering using guided-mode resonance coupling is proposed. We investigate angle-dependent properties of such structures for potential use as angle-selective reflective filters. We utilize interference between diffraction modes to provide tunable selectivity with a sufficient angular width. Combining these structures with thermal emitters can exclude selected emission angles for spatially selective thermal emissivity reduction toward sensitive targets, as well as directionally selective emissivity exclusion for suppression of solar heating. We show a very large selective reduction of heat exchange by 99.77% between an engineered emitter and a distant receiver using just a single-groove grating and an emitting substrate in the emitter’s side. Also, we show a selective reduction of heat exchange by approximately 77% between an emitter covered by engineered sets of angle-selective reflective filters and a nearby sensitive target. The suggested angle-selective structure may have applications in excluding background thermal radiation, in particular, thermal emission reduction for daytime radiative cooling, sensitive IR telescope detectors, and high-fidelity thermoluminescent spectroscopy.

The theories of electron emission from planar surfaces are well understood, but they are not suitable for describing emission from spherical surfaces; their incorrect application to highly curved, nanometer-scale surfaces can overestimate the emitted current by several orders of magnitude. This inaccuracy is of particular concern for describing modern nanoscale electron sources, which continue to be modeled using the planar equations. In this paper, the field-induced and thermal currents are treated in a unified way to produce Fowler-Nordheim-type and Richardson-Schottky-type equations for the emitted current density from earthed nanoscale spherical surfaces. The limits of applicability of these derived expressions are considered along with the energy spectra of the emitted electrons. Within the relevant limits of validity, these equations are shown to reproduce the results of precise numerical calculations of the emitted current densities. The methods used here are adaptable to other one-dimensional emission problems.

We measure the contribution of loss of charge balance and exciton confinement in the emission zone to the operational lifetime of blue phosphorescent organic light-emitting diodes (PHOLEDs). Charge balance and exciton confinement are monitored as functions of time by measuring the emission intensity of either phosphorescent or fluorescent red-emitting “sensing” layers embedded within the charge-transport layers outside of the emission zone. We find no significant change in charge balance over the lifetime of the device, while loss of exciton confinement accounts for $<5\%$ of luminance loss, confirming that degradation is primarily due to decomposition of molecular constituents within the emission layer of the PHOLED.

We present a tuneup protocol for qubit gates with tenfold speedup over traditional methods reliant on qubit initialization by energy relaxation. This speedup is achieved by constructing a cost function for Nelder-Mead optimization from real-time correlation of nondemolition measurements interleaving gate operations without pause. Applying the protocol on a transmon qubit achieves 0.999 average Clifford fidelity in one minute, as independently verified using randomized benchmarking and gate-set tomography. The adjustable sensitivity of the cost function allows the detection of fractional changes in the gate error with a nearly constant signal-to-noise ratio. The restless concept demonstrated can be readily extended to the tuneup of two-qubit gates and measurement operations.

We present a device demonstrating a lithographically patterned transmon integrated with a micromachined cavity resonator. Our two-cavity, one-qubit device is a multilayer microwave-integrated quantum circuit (MMIQC), comprising a basic unit capable of performing circuit-QED operations. We describe the qubit-cavity coupling mechanism of a specialized geometry using an electric-field picture and a circuit model, and obtain specific system parameters using simulations. Fabrication of the MMIQC includes lithography, etching, and metallic bonding of silicon wafers. Superconducting wafer bonding is a critical capability that is demonstrated by a micromachined storage-cavity lifetime of $34.3\mu \mathrm{s}$, corresponding to a quality factor of $2\times {10}^6$ at single-photon energies. The transmon coherence times are $T_1=6.4\mu \mathrm{s}$, and $T_2^{\mathrm{echo}}=11.7\mu \mathrm{s}$. We measure qubit-cavity dispersive coupling with a rate ${\chi }_{q\mu }/2\pi =-1.17\mathrm{MHz}$, constituting a Jaynes-Cummings system with an interaction strength $g/2\pi =49\mathrm{MHz}$. With these parameters we are able to demonstrate circuit-QED operations in the strong dispersive regime with ease. Finally, we highlight several improvements and anticipated extensions of the technology to complex MMIQCs.

The open-circuit voltage ($V_{\mathrm{oc}}$) of organic solar cells generally approaches its maximum obtainable values as the temperature decreases. However, recent experiments have revealed that the $V_{\mathrm{oc}}$ may suffer from an ultrahigh loss at low temperatures. In order to verify this explanation and investigate the impacts of energetic disorder on the temperature-dependent behaviors of the $V_{\mathrm{oc}}$ in general, we calculate the $V_{\mathrm{oc}}-T$ plots with the drift-diffusion method under various device working parameters. With the disorder being incorporated into the device model by considering the disorder-suppressed (temperature-dependent) charge-carrier mobilities, it is found that the ultrahigh $V_{\mathrm{oc}}$ losses cannot be reproduced under the Onsager-Braun–type charge generation rate. With the charge generation rate being constant or weakly dependent on temperature, for nonselective contacts, the $V_{\mathrm{oc}}$ reduces drastically at low temperatures, while for selective contacts, the $V_{\mathrm{oc}}$ increases monotonically with decreasing temperature. With higher carrier mobilities or smaller device thicknesses, the ultrahigh loss occurs at lower temperatures. The mechanism is that, since the disorder-suppressed charge mobilities give rise to both low charge-extraction efficiency and small bimolecular recombination rate, plenty of charge carriers can be extracted from the wrong electrode and can form a large leakage current, which counteracts the majority-carrier current and reduces the $V_{\mathrm{oc}}$ at low temperatures. Our results thus highlight the essential role of charge-carrier kinetics, except for the charge-filling effect, on dominating the disorder-induced $V_{\mathrm{oc}}$ losses.

A consistent mathematical approach is presented that connects the Shockley-Queisser (SQ) theory to the analysis of real-world devices. We demonstrate that the external photovoltaic quantum efficiency $Q_e^{\mathrm{PV}}$ of a solar cell results from a distribution of SQ-type band-gap energies and how this distribution is derived from experimental data. This leads us to the definition of a photovoltaic band-gap energy $E_g^{\mathrm{PV}}$ as a reference value for the analysis of the device performance. For a variety of solar-cell devices, we show that the combination of $Q_e^{\mathrm{PV}}$ and electroluminescence measurements allows for a detailed loss analysis that is fully compatible with the principle of detailed balance.

In this work, nonvolatile magnetoelectric tunable inductors are developed based on Metglas/(011) $\mathrm{Pb}({\mathrm{Mg}}_{1/3}{\mathrm{Nb}}_{2/3}){\mathrm{O}}_3\text{−}{\mathrm{PbTiO}}_3$ multiferroic composites. They exhibit a large nonvolatile tunability up to 250% at 10 kHz and 120% at 1 MHz, in which the voltage control of inductance is achieved through strain-mediated magnetoelastic anisotropy. Such high nonvolatile tunability is attributed to a dramatic change of the in-plane lattice strain arising from non-180° ferroelastic domain switching in $\mathrm{Pb}({\mathrm{Mg}}_{1/3}{\mathrm{Nb}}_{2/3}){\mathrm{O}}_3\text{−}{\mathrm{PbTiO}}_3$. Electric field dependent inductance is then calculated from the strain-induced effective magnetic field and effective permeability change, and it is consistent with our experimental results. Engineering of ferroelastic domain states in multiferroic composites provides a pathway to realize nonvolatile electrically tunable inductors for lightweight, compact, power-efficient integrated power electronics, rf devices, and systems.

In the experimental electroluminescence (EL) spectra of light-emitting diodes (LEDs) based on N-polar (In,Ga)N/GaN nanowires (NWs), we observe a double-peak structure. The relative intensity of the two peaks evolves in a peculiar way with injected current. Spatially and spectrally resolved EL maps confirm the presence of two main transitions in the spectra and suggest that they are emitted by a majority of the single nano LEDs. In order to elucidate the physical origin of this effect, we perform theoretical calculations of the strain, electric field, and charge-density distributions for both planar LEDs and NW LEDs. On this basis, we simulate also the EL spectra of these devices, which exhibit a double-peak structure for N-polar heterostructures, in both the NW and the planar case. By contrast, this feature is not observed when Ga-polar planar LEDs are simulated. We find that the physical origin of the double-peak structure is a stronger quantum-confined Stark effect occurring in the first and last quantum well of the N-polar heterostructures. The peculiar evolution of the relative peak intensities with injected current, seen only in the case of the NW LED, is attributed to the three-dimensional strain variation resulting from elastic relaxation at the free sidewalls of the NWs. Therefore, this study provides important insights on the working principle of N-polar LEDs based on both planar and NW heterostructures.

We propose a method to trap liquid (both oil and water) in a microchannel by sequentially injecting oil and water (or vice versa) over photopatterned obstacles with controlled wetting properties. We present a simple geometrical model to understand the liquid-entrapment process and predict the evolution of the water/oil interface over an obstacle. Our analysis provides an analytic solution that can successfully predict useful properties such as angular position of pinch-off and the amount of captured liquid. We show that we are able to obtain a liquid bridge over two circular obstacles, and we are also able to predict the condition for the formation of a bridge. We further demonstrate the effect of the obstacle shape and how a sudden change in gradient results in larger liquid entrapment. We also demonstrate the ability to entrap isolated liquid chambers for parallel experimentation.

Dynamic failure of impact-loaded structures is often caused by dynamic shear localization, that is also known as adiabatic shear banding (ASB). While ASB has long been thought to be triggered by thermal softening, another potent softening mechanism has been recently identified in which islands of dynamically recrystallized nanograins nucleate and coalesce, ultimately leading to fracture. However, the exact nature and extent of the softening has not yet been characterized experimentally. Ti-6Al-4V is chosen as a model material to study the influence of impact-induced dynamic recrystallization (DRX) on the subsequent quasistatic flow properties through a systematic combination of dynamic tests up to a predefined level of strain followed by static testing to fracture. With the dynamic prestrain, the subsequent quasistatic yield strength of the material increases, while the strain-hardening capacity decreases noticeably once the relative dynamic prestrain level exceeds 0.5. Those observations, which are supported by transmission-electron-microstructural characterization, confirm not only the early formation of dynamically recrystallized islands reported by D. Rittel et al. [Phys. Rev. Lett. 101, 165501 (2008)], but mostly the influence this sparse phase has on the bulk mechanical response. In that respect, the present experiments confirm previously reported trends for other bulk nanograined materials, namely, elevation of the yield stress, significant drop in the strain hardening, and enhanced tendency for shear localization. The first two effects are clearly observed for the sparse islands of DRX that form in the bulk impacted material and allow for future modeling of the response of such hierarchical microstructures composed of both ultrafine and coarse grains.

The fabrication of hybrid systems in which specific physical properties of materials are coupled provides a setting to explore alternative functionality in condensed matter physics. Here, we realize the formation of spin-polarized electronic states, arising from a strong spin-orbit interaction, on the prototype ferroelectric ${\mathrm{BaTiO}}_3(001)$ by epitaxial growth of ultrathin bismuth layers. Low-energy electron diffraction shows that the films grow along the Bi(0001) orientation and form two twinned and 90°-rotated domains. Using angle-resolved photoelectron spectroscopy, we directly confirm the presence of metallic electronic states with large spin splitting in the bulk band gap of the substrate. At a low film thickness of approximately 4 monolayers, we observe the emergence of an interface state close to the Fermi energy that features a holelike dispersion near the $\overline{X}$ point of ${\mathrm{BaTiO}}_3(001)$. Our results demonstrate the formation of low-dimensional electronic states with large spin splitting coupled to a ${\mathrm{BaTiO}}_3$ substrate, with importance for the exploration of spin manipulation at interfaces based on ferroelectricity.

We experimentally investigate a cascade of temperature-compensated unequal-path interferometers that can be used to measure frequency states in a high-dimensional quantum distribution system. In particular, we demonstrate that commercially available interferometers have sufficient environmental isolation so that they maintain an interference visibility greater than 98.5% at a wavelength of 1550 nm over extended periods with only moderate passive control of the interferometer temperature ($<\pm 0.50°\mathrm{C}$). Specifically, we characterize two interferometers that have matched delays: one with a free spectral range of 2.5 GHz and the other with 1.25 GHz. We find that the relative path of these interferometers drifts less than 3 nm over a period of 1 h during which the temperature fluctuates by $<\pm 0.10°\mathrm{C}$. When we purposely heat the interferometers over a temperature range of $20–50°\mathrm{C}$, we measure a path-length shift of $26\pm 9\mathrm{nm}/°\mathrm{C}$ for the 2.5-GHz interferometer. For the 1.25-GHz interferometer, the path-length shift is nonlinear and is locally equal to zero at a temperature of $37.1°\mathrm{C}$ and is $50\pm 17\mathrm{nm}/°\mathrm{C}$ at $22°\mathrm{C}$. With these devices, we realize a proof-of-concept quantum key distribution experiment and achieve quantum bit error rates of 1.94% and 3.69% in time and frequency basis, respectively, at a quantum channel loss of 14 dB.

Spintronic applications based on magnetic domain-wall (DW) motion, such as magnetic data storage, sensors, and logic devices, require approaches to reliably manipulate the magnetization in nanowires. In this paper, we report the direct dynamic experimental visualization of reliable switching from the onion to the vortex state by DW automotion at zero field in asymmetric ferromagnetic rings using a uniaxial field pulse. Employing time-resolved x-ray microscopy, we demonstrate that depending on the detailed spin structure of the DWs and the size and geometry of the rings, the automotive propagation can be tailored during the DW relaxation from the higher-energy onion state to the energetically favored vortex state, where both DWs annihilate. Our measurements show DW automotion with an average velocity of about $60\mathrm{m}/\mathrm{s}$, which is a significant speed for spintronic devices. Such motion is mostly governed by local forces resulting from the geometry variations in the device. A closer study of the annihilation process via micromagnetic simulations reveals that a new vortex is nucleated in between the two initial walls. We demonstrate that the annihilation of DWs through automotion in our scheme always occurs with the detailed topological nature of the walls influencing only the DW dynamics on a local scale. The simulations show good quantitative agreement with our experimental results. These findings shed light on a robust and reliable switching process of the onion state in ferromagnetic rings, which paves the way for further optimization of these devices.

Micro- and nanomechanical resonators with ultralow dissipation have great potential as useful quantum resources. The superfluid micromechanical resonators presented here possess several advantageous characteristics: straightforward thermalization, dissipationless flow, and in situ tunability. We identify and quantitatively model the various dissipation mechanisms in two resonators, one fabricated from borosilicate glass and one from single-crystal quartz. As the resonators are cryogenically cooled into the superfluid state, the damping from thermal effects and from the normal-fluid component are strongly suppressed. At our lowest temperatures, damping is limited solely by internal dissipation in the substrate materials, and the resonators reach quality factors of up to 913 000 at 13 mK. By lifting this limitation through substrate-material choice and resonator design, modeling suggests that the resonators could reach quality factors as high as ${10}^8$ at 100 mK, putting this architecture in an ideal position to harness mechanical quantum effects and to facilitate the study of superfluids in confined geometries.

The microstructural evolution of a porous electrode consisting of a metal-ceramic matrix, consisting of nickel and yttria-stabilized zirconia ($YSZ$), is one of the main degradation mechanisms in a solid-oxide cell (SOC), in either fuel cell or electrolyzer mode. In that respect, the agglomeration of nickel particles in a SOC electrode leads to a decrease in the electronic conductivity as well as in the active catalytic area for the oxidation-reduction reaction of the fuel-water steam. An analytical model of the agglomeration behavior of a $\mathrm{Ni}/YSZ$ electrode is proposed that allows for a quantitative description of the nickel agglomeration. The accuracy of the model is validated in terms of a comparison with experimental degradation measurements. The model is based on contact probabilities of nickel clusters in a porous network of nickel and $YSZ$, derived from an algorithm of the agglomeration process. The iterative algorithm is converted into an analytical function, which involves structural parameters of the electrode, such as the porosity and the nickel content. Furthermore, to describe the agglomeration mechanism, the influence of the steam content and the flux rate are taken into account via reactions on the nickel surface. In the next step, the developed agglomeration model is combined with the mechanism of the Ostwald ripening. The calculated grain-size growth is compared to measurements at different temperatures and under low flux rates and low steam content, as well as under high flux rates and high steam content. The results confirm the necessity of connecting the two mechanisms and clarify the circumstances in which the single processes occur and how they contribute to the total agglomeration of the particles in the electrode.

We propose an architecture to implement multi-input one-output Boolean functions using chaos computing in hybrid digital-analog systems consisting of a digital block of conventional and gates and a nonlinear circuit. This architecture efficiently utilizes the superstable initial conditions of a nonlinear circuit and enables us to implement all possible $2^{2^m}$ Boolean functions of $m$ data inputs in just $m$ iterations of the nonlinear circuit, resulting in better operating speed and noise tolerance. In an ideal nonlinear map, this architecture eliminates the need for a decoder, as the outputs are mapped to maxima and minima of the map and can be fed directly to the next stage, enabling multilayer concatenation. We demonstrate the utility of this architecture in a three-transistor circuit.

The efficiency of spin-transfer-torque (STT) switching and the thermal-stability factor are important figures of merit in STT-based magnetoresistive random-access memory. We derive analytical expressions of the STT-switching efficiency and the thermal-stability factor for a perpendicularly magnetized spin-valve nanopillar with the first- and the second-order uniaxial magnetic anisotropy. It is shown that the STT-switching efficiency is maximized when the effective first-order anisotropy constant ($K_{u1,\mathrm{eff}}$) is equal to the second-order anisotropy constant ($K_{u2}$). It is also shown that the thermal-stability factor is most (least) sensitive to a variation of the applied current when $K_{u2}=-0.41$ (0.70) $K_{u1,\mathrm{eff}}$.

We demonstrate an instrument for time-resolved magnetic imaging that is highly sensitive to the in-plane magnetization state and dynamics of thin-film bilayers of yttrium iron garnet ${[\mathrm{Y}}_3{\mathrm{Fe}}_5{\mathrm{O}}_{12}(\mathrm{YIG})]/\mathrm{Pt}$: the time-resolved longitudinal spin Seebeck (TRLSSE) effect microscope. We detect the local in-plane magnetic orientation within the YIG by focusing a picosecond laser to generate thermally driven spin current from the YIG into the Pt by the spin Seebeck effect and then use the inverse spin Hall effect in the Pt to transduce this spin current to an output voltage. To establish the time resolution of TRLSSE, we show that pulsed optical heating of patterned YIG $(20\mathrm{nm})/\mathrm{Pt}(6\mathrm{nm})/\mathrm{Ru}(2\mathrm{nm})$ wires generates a magnetization-dependent voltage pulse of less than 100 ps. We demonstrate TRLSSE microscopy to image both static magnetic structure and gigahertz-frequency magnetic resonance dynamics with submicron spatial resolution and a sensitivity to magnetic orientation below $0.3°/\sqrt{Hz}$ in ultrathin YIG.

By using the ${\mathrm{Y}}_3{\mathrm{Fe}}_5{\mathrm{O}}_{12}$ $(\mathrm{YIG})/\mathrm{Cu}/{\mathrm{Co}}_{0.65}{\mathrm{Pt}}_{0.35}$ pseudo-spin-valve (SV) heterostructure, we identify the large inverse spin Hall effect (ISHE) from Co-Pt together with the conventional anomalous Nernst effect and anomalous Righi-Leduc effect. When thin ferromagnetic (FM) Co-Pt layers are exchanged coupled by an additional antiferromagnetic (AFM) Ir-Mn layer, the well-separated ISHE signal generated in the Co-Pt layer is significantly enhanced because the spin current coming from the YIG and produced by the Co-Pt itself is mostly reflected at the Co-Pt surface for the pseudo-SV heterostructure but absorbed by the AFM Ir-Mn layer in the exchange-bias SV samples. The SV heterostructure with a soft Co layer also gives a distinguishable ISHE signal but with much smaller magnitude. The present results show that the metallic FM-AFM structure is an efficient spin-current detector. In addition, FM metals alloyed with heavy elements like Pt are beneficial to spin-orbit coupling-correlated phenomena, including ISHE and the anomalous Hall effect.