The performance of photoinjectors is limited by the lowest value of the mean transverse energy of the electrons obtained from photocathodes. The factors that influence the mean transverse energy are poorly understood. In this paper, we develop models to calculate the effect of spatial work-function variations and subnanometer-scale roughness and surface defects on the mean transverse energy. We show that these can limit the lowest value of mean transverse energy achieved and that atomically perfect surfaces will be required to further reduce the mean transverse energy obtained from photocathodes.

A traveling-wave parametric amplifier (TWPA) composed of a transmission line made up of a chain of coupled asymmetric superconducting quantum interference devices (SQUIDs) is proposed. The unique nature of this transmission line is that its nonlinearity can be tuned with an external magnetic flux and can even change sign. This feature of the transmission line can be used to perform phase matching in a degenerate four-wave mixing process which can be utilized for the parametric amplification of a weak signal in the presence of a strong pump. Numerical simulations of the TWPA design show that, with tuning, phase matching can be achieved and an exponential gain as a function of the transmission-line length can be realized. The flexibility of the proposed design can realize: compact TWPAs with fewer than 211 unit cells, signal gains greater than 20 dB, 3-dB bandwidth greater than 5.4 GHz, and saturation powers up to $-98\mathrm{dBm}$. This amplifier design is well suited for the multiplexed readout of quantum circuits or astronomical detectors in a compact configuration which can foster on-chip implementations.

We propose a metamaterial to realize true electromagnetically induced transparency (EIT), where the incidence of an auxiliary electromagnetic wave called the control wave induces transparency for a probe wave. The analogy to the original EIT effect in an atomic medium is shown through analytical and numerical calculations derived from a circuit model for the metamaterial. We perform experiments to demonstrate the EIT effect of the metamaterial in the microwave region. The width and position of the transparent region can be controlled by the power and frequency of the control wave. We also observe asymmetric transmission spectra unique to the Fano resonance.

High-fidelity coherent control of quantum systems is critical to building quantum devices and quantum computers. We provide a general optimal control framework for designing control sequences that account for hardware control distortions while maintaining robustness to environmental noise. We demonstrate the utility of our algorithm by presenting examples of robust quantum gates optimized in the presence of nonlinear distortions. We show that nonlinear classical controllers do not necessarily incur additional computational cost to pulse-optimization, enabling more powerful quantum devices.

Here we demonstrate quantum interference of photons on a silicon chip produced from a single-ring-resonator photon source. The source is seamlessly integrated with a Mach-Zehnder interferometer, which path entangles degenerate biphotons produced via spontaneous four-wave mixing in the silicon ring resonator. The resulting entangled N00N state is controlled by varying the relative phase of the integrated Mach-Zehnder interferometer, resulting in high two-photon interference visibilities of $V\sim 96\%$. Furthermore, we show that the interference can be produced using pump wavelengths tuned to all of the ring resonances accessible with our tunable lasers ($C+L$ band). This work is a key demonstration towards the simplified integration of multiple photon sources and quantum circuits together on a monolithic chip, in turn, enabling quantum-information chips with much greater complexity and functionality.

Nanoscale materials display enhanced strength and toughness but also larger fluctuations and more pronounced size effects with respect to their macroscopic counterparts. Here we study the system size dependence of the failure strength distribution of a monolayer graphene sheet with a small concentration of vacancies by molecular dynamics simulations. We simulate sheets of varying size encompassing more than three decades and systematically study their deformation as a function of disorder, temperature, and loading rate. We generalize the weakest-link theory of fracture size effects to rate- and temperature-dependent failure and find quantitative agreement with the simulations. Our numerical and theoretical results explain the crossover of the fracture strength distribution between a thermal and rate-dependent regime and a disorder-dominated regime described by the extreme-value theory.

The scaling of the energy barrier to magnetization reversal in thin-film nanomagnets with perpendicular magnetization as a function of their lateral size is of great current interest for high-density magnetic random-access memory devices. Here we determine the micromagnetic states that set the energy barrier to thermally activated magnetization reversal of circular thin-film nanomagnets with large perpendicular magnetic anisotropy. We find a critical length in the problem that is set by the exchange and effective perpendicular magnetic anisotropy energies, with the latter including the size dependence of the demagnetization energy. For diameters smaller than this critical length, the reversal occurs by nearly coherent magnetization rotation and the energy barrier scales with the square of the diameter normalized to the critical length (for fixed film thickness), while for larger diameters, the transition state has a domain wall, and the energy barrier depends linearly on the normalized diameter. Simple analytic expressions are derived for these two limiting cases and verified using full micromagnetic simulations with the string method. Further, the effect of an applied field is considered and shown to lead to a plateau in the energy barrier versus diameter dependence at large diameters.

Modern applications of scintillator materials demand cutting-edge performances and require often a response speed in the nanosecond time scale. Slow light emission causing an “afterglow” is, therefore, of considerable concern in the development of fast scintillators. The mechanism of afterglow emission in mixed Ce-doped oxyorthosilicate scintillators is investigated by means of time-resolved scintillation, thermally stimulated luminescence (TSL), and radio-luminescence measurements. Various Ce-doped ${\mathrm{Lu}}_{2x}{\mathrm{Gd}}_{2-2x}{\mathrm{SiO}}_5$ oxyorthosilicate crystals (with $x$ ranging from 0 to1) and ${\mathrm{Lu}}_{1.8}{\mathrm{Y}}_{0.2}{\mathrm{SiO}}_5$ grown by the Czochralski technique are considered. The detailed TSL analysis reveals that thermally assisted tunneling recombination of electrons trapped by oxygen vacancies with holes trapped by Ce luminescence centers occurs for all compositions. The reduction of the afterglow intensity by adding gadolinium or yttrium into the host is accompanied by a lowering of the traps concentration, as deduced by the TSL intensity. Such lowering of the oxygen vacancy concentrations is found to be correlated with the decrease of the melting temperature induced by gadolinium or yttrium content increase, which governs the oxygen vapor pressure. The occurrence of a similar mechanism also in other scintillators and its influence on carrier trapping is discussed.

The suitability of the spin dynamics of paramagnetic silicon dangling bonds ($E^{\prime }$ centers) in high-$E^{\prime }$-density amorphous silicon dioxide (${\mathrm{SiO}}_2$) as probe spins for single-spin tunneling force microscopy (SSTFM) is studied. SSTFM is a spin-selection-rule-based scanning-probe single-spin readout concept. Following the synthesis of ${\mathrm{SiO}}_2$ thin films on (111)-oriented crystalline-silicon substrates with room-temperature stable densities of $[E^{\prime }]>5\times {10}^{18}{\mathrm{cm}}^{-3}$ throughout the 60-nm thin film, pulsed electron paramagnetic resonance spectroscopy is conducted on the $E^{\prime }$ centers at temperatures between $T=5\mathrm{K}$ and $T=70\mathrm{K}$. The measurements reveal that the spin coherence (the transverse spin-relaxation time $T_2$) of these centers is significantly shortened compared to low-$E^{\prime }$-density ${\mathrm{SiO}}_2$ films and within error margins not dependent on temperature. In contrast, the spin-flip times (the longitudinal relaxation times $T_1$) are dependent on the temperature but with much weaker dependence than low-density ${\mathrm{SiO}}_2$, with the greatest deviations from low-density ${\mathrm{SiO}}_2$ seen for $T=5\mathrm{K}$. These results, discussed in the context of the spin-relaxation dynamics of dangling-bond states of other silicon-based disordered solids, indicate the suitability of $E^{\prime }$ centers in high-density ${\mathrm{SiO}}_2$ as probe spins for SSTFM.

Graphene applications in chemical sensing are based on the chemical doping of graphene. In this process, molecules adsorbed on graphene serve as charge-carrier donors or acceptors, thus changing the graphene conductivity. While the previous studies have been focused on chemical sensors with electrical detection, we theoretically investigate chemical sensing based on photonic structures covered with graphene. By considering chemical doping of graphene as a small perturbation, we show that optimal photonic structures operate at low-terahertz frequencies, with the reflectance intensity as the output signal. In order to achieve an efficient chemical sensing, photonic structures should provide the electric-field enhancement within the graphene plane. As a result, the proposed structure consists of the metallic mirror and quarter-wavelength-thick dielectric spacer with graphene on the top of it. The sensitivity is maximized when the Fermi energy in the graphene not exposed to the environment is around 30 meV. By taking the resolution for the reflectance measurement of 1%, we show that the proposed sensing structure can detect graphene doping by 150 electrons or holes per square micrometer in the dynamic range of around 3000 charge carriers.

In this paper, we demonstrate a general-purpose macroscopic refrigerator based on the transport of electrons through superconducting tunnel junctions. Our refrigerator is intended to provide access to temperatures below those achievable using pumped ${}^3\mathrm{He}$. The refrigerator is cooled by 96 normal-metal–insulator–superconductor ($N\text{−}I\text{−}S$) junctions divided among three separate silicon substrates. The use of thin-film devices on different substrates shows the potential to achieve higher cooling powers by connecting $N\text{−}I\text{−}S$ devices in parallel. Improving on previous work by Lowell et al. [Appl. Phys. Lett. 102, 082601 (2013)], we demonstrate a larger temperature reduction, a more robust mechanical suspension, and a new electromechanical heat switch that will make it easier to integrate our refrigerator into other cryostats. The electromechanical heat switch has a measured thermal conductance in the on state of $1.2\pm 0.3\mu \mathrm{W}/\mathrm{K}$ at 300 mK and no measurable thermal conductance in the off state. We observe a temperature reduction from 291 to 233 mK and infer cooling to 228 mK on longer time scales. The cooled payload is a metal stage whose mass exceeds 150 g and with $28{\mathrm{cm}}^2$ of area for attaching user-supplied devices. Using the product of the cooled mass and the temperature reduction as a performance metric, this work is a more than tenfold advance over previous efforts.

The deformation and interaction of a droplet pair in an electric field determine the success of droplet coalescence. Electric intensity and initial droplet separation are crucial parameters in this process. In this work, a combined theoretical and numerical analysis is performed to study the electrohydrodynamics of confined droplet pairs in a rectangular microchannel under ac electric fields. We develop a theoretical model to predict the relationship between critical electric intensity and droplet separation. A geometrical model relating the initial droplet separation to the cone angle is also established to determine the critical separation for partial coalescence. These models are validated by comparisons with existing experimental observations. According to the initial separation and electric intensity, five regimes of droplet interactions are classified by direct numerical simulations, namely noncoalescence, coalescence, partial coalescence, ejection after coalescence, and ejection with partial coalescence. According to their controlling mechanisms, the five regimes are distinguished by three well-defined boundaries. The detailed dynamics of the partial coalescence phenomenon is resolved when the droplet separation exceeds the critical value. A dynamic liquid bridge between the droplets is sustained by the competition between surface tension and electric stress. The dynamics of ejected microjets at the exterior ends are also addressed to show their responses to the oscillating electric field. The full understanding of the droplet dynamics under electric fields can be used to predict the droplet fusion behaviors and thus to facilitate the design of droplet-based microfluidic devices.

Nuclear magnetic resonance spectroscopy has approached the limit of single-molecule sensitivity; however, the spectral resolution is currently insufficient to obtain detailed information on chemical structure and molecular interactions. Here we demonstrate more than 2 orders of magnitude improvement in spectral resolution by performing correlation spectroscopy with shallow nitrogen-vacancy magnetic sensors in diamond. In principle, the resolution is sufficient to observe chemical shifts in approximately $1\mathrm{T}$ magnetic fields and is currently limited by molecular diffusion at the surface. We measure oil diffusion rates of $D=0.15–0.2{\mathrm{nm}}^2/\mu \mathrm{s}$ within $(5\mathrm{nm}{)}^3$ volumes at the diamond surface.

Conventional phased arrays require a large number of sources in forming a complex wave front, resulting in complexity and a high cost to operate the individual sources. We present a passive phased array using an acoustic metascreen that transmits sound energy from a single source and steers the transmitted wave front to form the desired fields. The metascreen is composed of elements that have a discrete resolution along the screen at an order smaller than the wavelength, allowing for fine wave-front shaping beyond the paraxial approximation. The performance is verified in experiment by forming a self-bending beam. Our metascreen-based passive array with its simplicity and capability has applications in places where conventional active arrays are complex and have limitations.

Josephson junctions and superconducting nanowires, when biased close to superconducting critical current, can switch to a nonzero voltage state by thermal or quantum fluctuations. The process is understood as an escape of a Brownian particle from a metastable state. Since this effect is fully stochastic, we propose to use it for generating random numbers. We present protocol for obtaining random numbers and test the experimentally harvested data for their fidelity. Our work is prerequisite for using the Josephson junction as a tool for stochastic (probabilistic) determination of physical parameters such as magnetic flux, temperature, and current.

Optical or electrical stimulation of neural circuits in mice during natural behavior is an important paradigm for studying brain function. Conventional systems for optogenetics and electrical microstimulation require tethers or large head-mounted devices that disrupt animal behavior. We report a method for wireless powering of small-scale implanted devices based on the strong localization of energy that occurs during resonant interaction between a radio-frequency cavity and intrinsic modes in mice. The system features self-tracking over a wide (16-cm diameter) operational area, and is used to demonstrate wireless activation of cortical neurons with miniaturized stimulators ($10{\mathrm{mm}}^3$, 20 mg) fully implanted under the skin.

Incandescent filaments and membranes are often used as infrared sources despite their low efficiency, broad angular emission, and lack of spectral selectivity. Here, we introduce a metasurface to control simultaneously the spectrum and the directivity of blackbody radiation. The plasmonic metasurface operates reliably at $600°\mathrm{C}$ with an emissivity higher than 0.85 in a narrow frequency band and in a narrow solid angle. This emitter paves the way for the development of compact, efficient, and cheap IR sources and gas detection systems.

The advances of nanotechnologies applied to high-critical-temperature superconductors (HTSs) have recently given a huge boost to the field, opening new prospectives for their integration in hybrid devices. The feasibility of this research goes through the realization of HTS nanogaps with superconductive properties close to the as-grown bulk material at the nanoscale. Here we present a fabrication approach allowing the realization of ${\mathrm{YBa}}_2{\mathrm{Cu}}_3{\mathrm{O}}_{7-\delta }$ (YBCO) nanogaps with dimensions as small as 35 nm. To assess the quality of the nanogaps, we measure, before and after an ozone treatment, the current-voltage characteristics and the resistance versus temperature of YBCO nanowires with various widths and lengths, fabricated by using different lithographic processes. The analysis of the superconducting transition with a thermally activated vortex-entry model allows us to determine the maximum damage the nanowires undergo during the patterning which relates to the upper bound for the dimension of the nanogap. We find that the effective width of the nanogap is of the order of 100 nm at the superconducting transition temperature while retaining the geometrical value of about 35 nm at lower temperatures. The feasibility of the nanogaps for hybrid Josephson devices is demonstrated by bridging them with thin Au films. We detect a Josephson coupling up to 85 K with an almost ideal magnetic-field response of the Josephson current. These results pave the way for the realization of complex hybrid devices, where tiny HTS nanogaps can be instrumental to study the Josephson effect through barriers such as topological insulators or graphene.

The article addresses the possibility of alloy elements in MnBi which may modify the thermodynamic stability of the NiAs-type structure without significantly degrading the magnetic properties. The addition of small amounts of Rh and Mn provides an improvement in the thermal stability with some degradation of the magnetic properties. The small amounts of Rh and Mn additions in MnBi stabilize an orthorhombic phase whose structural and magnetic properties are closely related to the ones of the previously reported high-temperature phase of MnBi (HT MnBi). To date, the properties of the HT MnBi, which is stable between 613 and 719 K, have not been studied in detail because of its transformation to the stable low-temperature MnBi (LT MnBi), making measurements near and below its Curie temperature difficult. The Rh-stabilized MnBi with chemical formula ${\mathrm{Mn}}_{1.0625-x}{\mathrm{Rh}}_x\mathrm{Bi}$ [$x=0.02(1)$] adopts a new superstructure of the $\mathrm{NiAs}/{\mathrm{Ni}}_2\mathrm{In}$ structure family. It is ferromagnetic below a Curie temperature of 416 K. The critical exponents of the ferromagnetic transition are not of the mean-field type but are closer to those associated with the Ising model in three dimensions. The magnetic anisotropy is uniaxial; the anisotropy energy is rather large, and it does not increase when raising the temperature, contrary to what happens in LT MnBi. The saturation magnetization is approximately $3{\mu }_B/\mathrm{f}.\mathrm{u}.$ at low temperatures. While this exact composition may not be application ready, it does show that alloying is a viable route to modifying the stability of this class of rare-earth-free magnet alloys.

The maximum open-circuit voltage of a solar cell can be evaluated in terms of its ability to emit light. We herein verify the reciprocity relation between the electroluminescence spectrum and subband-gap quantum efficiency spectrum for several photovoltaic technologies at different stages of commercial development, including inorganic, organic, and a type of methyl-ammonium lead- halide ${\mathrm{CH}}_3{\mathrm{NH}}_3{\mathrm{PbI}}_{3-x}{\mathrm{Cl}}_x$ perovskite solar cells. Based on the detailed balance theory and reciprocity relations between light emission and light absorption, voltage losses at open circuit are quantified and assigned to specific mechanisms, namely, absorption edge broadening and nonradiative recombination. The voltage loss due to nonradiative recombination is low for inorganic solar cells (0.04–0.21 V), while for organic solar cell devices it is larger but surprisingly uniform, with values of 0.34–0.44 V for a range of material combinations. We show that, in ${\mathrm{CH}}_3{\mathrm{NH}}_3{\mathrm{PbI}}_{3-x}{\mathrm{Cl}}_x$ perovskite solar cells that exhibit hysteresis, the loss to nonradiative recombination varies substantially with voltage scan conditions. We then show that for different solar cell technologies there is a roughly linear relation between the power conversion efficiency and the voltage loss due to nonradiative recombination.