Hyperdoping germanium with gold is a potential method to produce room-temperature short-wavelength-infrared radiation (SWIR; $1.4–3.0\mu \mathrm{m}$) photodetection. We investigate the charge carrier dynamics, light absorption, and structural properties of gold-hyperdoped germanium ($\mathrm{Ge}$:$\mathrm{Au}$) fabricated with varying ion implantation and nanosecond pulsed laser melting conditions. Time-resolved terahertz spectroscopy (TRTS) measurements show that $\mathrm{Ge}$:$\mathrm{Au}$ carrier lifetime is significantly higher than that in previously studied hyperdoped silicon systems. Furthermore, we find that lattice composition, sub-band-gap optical absorption, and carrier dynamics depend greatly on hyperdoping conditions. We use density functional theory (DFT) to model dopant distribution, electronic band structure, and optical absorption. These simulations help explain experimentally observed differences in optical and optoelectronic behavior across different samples. DFT modeling reveals that substitutional dopant incorporation has the lowest formation energy and leads to deep energy levels. In contrast, interstitial or dopant-vacancy complex incorporation yields shallower energy levels that do not contribute to sub-band-gap light absorption and have a small effect on charge carrier lifetimes. These results suggest that it is promising to tailor dopant incorporation sites of $\mathrm{Ge}$:$\mathrm{Au}$ for SWIR photodetection applications.

The acoustic analog of the quantum black hole for airborne sound in two dimensions was denominated as an omnidirectional acoustic absorber by Climente et al. [see Appl. Phys. Lett., 100, 144103 (2012)], who characterized its absorbing properties without providing any theoretical support. The viscothermal losses of the underlying structure, which consists of an absorbing core and a surrounding gradient-index (GRIN) lens both made of periodic distributions of cylindrical rods, are here comprehensively studied by using the boundary element method (BEM) in two dimensions. It is shown that the numerical simulations in two dimensions reproduce fairly well the increase in absorption of the core when the GRIN lens is added and reveal that the discrepancy between measured and calculated values of absorbance is an artifact of the experimental setup. The possibility of independent calculation of viscous and thermal losses contributions in the two-dimensional (2D) BEM algorithm is employed for the comparison with a homogenization theory in which the cluster of cylinders is represented by a single fluidlike viscous cylinder with effective parameters. We conclude that viscous losses represent about 90% of the total energy dissipated in the core. The homogenization approach results are only 2% below the results calculated with 2D BEM, indicating that the effective parameters obtained by the homogenization are very accurate.

Acoustic systems that are without limitations imposed by the Fermi level have been demonstrated as a significant platform for the exploration of fruitful topological phases. By surrounding the nontrivial domain with trivial “environment,” the domain-wall topological states have been theoretically and experimentally demonstrated. In this work, based on the topological crystalline insulator with a kagome lattice, we rigorously derive the corresponding Hamiltonian from the traditional acoustics perspective, and exactly reveal the correspondences of the hoppings and on-site terms within acoustic systems. Crucially, these results directly indicate that instead of applying the trivial domain, the soft-boundary condition precisely corresponds to the theoretical models, which always require generalized chiral symmetry. These results may provide a platform to construct desired acoustic topological devices hosting desired topological phenomena for versatile applications.

In ${\mathrm{Ta/(Co,Fe)B/HfO}}_2$ stacks, a gate voltage drives, in a nonvolatile way, the system from an underoxidized state exhibiting in-plane anisotropy (IPA) to an optimum oxidation level resulting in perpendicular anisotropy (PMA) and further into an overoxidized state with IPA. The $\mathrm{IPA}\to \mathrm{PMA}$ regime is found to be significantly faster than the $\mathrm{PMA}\to \mathrm{IPA}$ regime, whereas only the latter shows full reversibility under the same gate voltages. The effective damping parameter also shows a marked dependence with gate voltage in the $\mathrm{IPA}\to \mathrm{PMA}$ regime, going from 0.029 to 0.012, and only a modest increase to 0.014 in the $\mathrm{PMA}\to \mathrm{IPA}$ regime. The existence of two magnetoionic regimes has been linked to a difference in the chemical environment of the anchoring points of oxygen species added to underoxidized or overoxidized layers. Our results show that multiple magnetoionic regimes can exist in a single device and that their characterization is of great importance for the design of high-performance spintronics devices.

We describe an experimental effort designing and deploying error-robust single-qubit operations using a cloud-based quantum computer and analog-layer programming access. We design numerically optimized pulses that implement target operations and exhibit robustness to various error processes including dephasing noise, instabilities in control amplitudes, and crosstalk. Pulse optimization is performed using a flexible optimization package incorporating a device model and physically relevant constraints (e.g., bandwidth limits on the transmission lines of the dilution refrigerator housing IBM Quantum hardware). We present techniques for conversion and calibration of physical Hamiltonian definitions to pulse waveforms programmed via Qiskit Pulse and compare performance against hardware default derivative removal by adiabatic gate (DRAG) pulses on a five-qubit device. Experimental measurements reveal default DRAG pulses exhibit coherent errors an order of magnitude larger than tabulated randomized-benchmarking measurements; solutions designed to be robust against these errors outperform hardware-default pulses for all qubits across multiple metrics. Experimental measurements demonstrate performance enhancements up to: about a $10$ times single-qubit gate coherent-error reduction; about a $5$ times average coherent-error reduction across a five-qubit system; about a $10$ times increase in calibration window to one week of valid pulse calibration; about a $12$ times reduction gate-error variability across qubits and over time; and up to about a $9$ times reduction in single-qubit gate error (including crosstalk) in the presence of fully parallelized operations. Randomized benchmarking reveals error rates for Clifford gates constructed from optimized pulses consistent with tabulated $T_1$ limits, and demonstrates a narrowing of the distribution of outcomes over randomizations associated with suppression of coherent errors.

We present a density-functional-theory-based study of a two-dimensional phase of chromium bismuthate ($\mathrm{Cr}\mathrm{Bi}$), a previously unknown material with exceptional magnetic and magneto-optical characteristics. Monolayer $\mathrm{Cr}\mathrm{Bi}$ is a ferromagnetic metal with strong spin-orbit coupling induced by the heavy bismuth atoms, resulting in a strongly anisotropic Ising-type magnetic ordering with the Curie temperature estimated to be higher than 300 K. The electronic structure of the system is topologically nontrivial, giving rise to a nonzero Berry curvature in the ground magnetic state, leading to the anomalous Hall effect with a conductivity plateau of about $1.5e^2/h$ at the Fermi level. Remarkably, the Hall conductivity and the magneto-optical constant are found to be strongly dependent on the direction of magnetization. Besides, monolayer $\mathrm{Cr}\mathrm{Bi}$ demonstrates the polar magneto-optical Kerr effect in the visible and near-ultraviolet spectral ranges with the maximum rotation angles of up to 10 mrad. Our findings suggest that monolayer $\mathrm{Cr}\mathrm{Bi}$ is a promising system for practical applications in magneto-optical and spintronic devices.

Topologically nontrivial nanowhirls, called magnetic skyrmions, are often considered attractive for spintronic applications like racetrack data storage devices. However, skyrmions do not move parallel to applied currents and typically do not coexist with other nano-objects, making the realization of such storage concepts difficult. Herein we consider materials with an anisotropic Dzyaloshinskii-Moriya interaction, like tetragonal Heusler materials, and show that four distinct types of topologically trivial bubbles can coexist. We show that each of them can be written by spin torques and that they can be distinguished using a single magnetic tunneling junction. Due to their trivial topology, the four types of bubbles move parallel to applied electrical currents and remain equidistant under motion. Still, the bubbles have a small size and high stability comparable to that of topologically nontrivial spin textures. This allows the construction of a quaternary-digit-based racetrack storage device that overcomes the two mentioned drawbacks of skyrmion-based racetracks.

Magnetic fields are recently being applied to metal additive manufacturing to control the molten pool dynamics and dendritic morphology. To study the molten pool dynamics under an external magnetic field, a thermoelectric magnetohydrodynamic (TEMHD) model is developed by incorporating the electrodynamic model with the Seebeck effect into the multiphysics thermal-fluid flow model. The Seebeck effect in the molten pool is analyzed with the simulation of stationary laser melting on a bare plate. Furthermore, different external magnetic fields are assigned in the TEMHD simulations with the stationary laser melting and the results show that the Lorentz force can damp the keyhole fluctuations. Finally, the laser scanning simulations on a bare plate under external magnetic fields indicate that the Lorentz force can smooth the fluid flow fields in the molten pool and increase the ratio of equiaxed grains in the solidification front.

Hybrid circuit QED involves the study of coherent quantum physics in solid-state systems via their interactions with superconducting microwave circuits. Here we present a crucial step in the implementation of a hybrid superconducting qubit that employs a carbon nanotube as a Josephson junction. We realize the junction by contacting a carbon nanotube with a superconducting Pd/Al bilayer, and implement voltage tunability of the quantum circuit’s frequency using a local electrostatic gate. We demonstrate a strong dispersive coupling to a coplanar waveguide resonator by investigating the gate-tunable resonator frequency. We extract qubit parameters from spectroscopy using dispersive readout and find qubit relaxation and coherence times in the range of $10$–$200\mathrm{ns}$.

We report on experimental detection of 10 nm $\mathrm{Si}$ nanoparticles on $\mathrm{Si}$ surface using spatial modulation microscopy. We show that a simple optical layout with a galvo-mirror allows reliable detection of $9\times 10$ nm $\mathrm{Si}$ nanocylinders with signal-to-noise ratio 4.6 when the wavelength is tuned to the spectral range of direct-band optical transitions in silicon. We also show the possibility to use the spatial modulation microscopy principles for nonabsorbing particles. We demonstrate successful detection of $\mathrm{Si}$ nanocylinders down to $20\times 40\mathrm{nm}$ by scattered light ($\lambda =680$ nm) analysis. Our results pave the way to an inspection method for semiconductor metrology.

Highly conductive nanoscale electrical contacts suffer from strong current crowding at the contact edges, which can lead to nonuniform heat deposition; the formation of local hot spots, aggravation of electromigration; and, in the worst-case scenario, lead to thermal runaway and breakdown of the device. These effects severely affect the overall device properties, reliability, and lifetime. Devices based on thin-film junctions, nanotubes or nanowires, and two-dimensional (2D) materials are especially sensitive to current transport at electrical contacts, due to their reduced dimensions and increased geometrical confinement for current flow. Here, we demonstrate a method to mitigate current crowding, by engineering the interface layer properties and geometry. Based on a self-consistent transmission-line model, we show that the distribution of the contact current greatly depends on the properties of the interfacial layer between two contacting members. Current steering and redistribution can be realized by strategically designing the specific contact resistivity, ${\rho }_c$, along the contact length. For similar contact members, parabolically varying ${\rho }_c$ along the contact interface significantly reduces the edge-current crowding in ohmic contacts. Similarly, the nonuniform current distribution of 2D semiconductor-3D metal contacts can be decreased, and the current-transfer length can be increased by varying the Schottky barrier height along the interface. It is also found that introducing a nanometer- or subnanometer-scale thin insulating tunneling gap between contact members can greatly reduce current crowding, while maintaining a similar total contact resistance.

We develop an ultracompact high-resolution imaging system for cold atoms. Its only in-vacuum element is a multimode optical fiber with a diameter of 230 $\mu \mathrm{m}$, which simultaneously collects light and guides it out of the vacuum chamber. External adaptive optics allow us to image cold $\mathrm{Rb}$ atoms with approximately 1-$\mu \mathrm{m}$ resolution over a $100\times 100\mu {\mathrm{m}}^2$ field of view. These optics can be easily rearranged to switch between fast absorption imaging and high-sensitivity fluorescence imaging. This system is particularly suited for hybrid quantum-engineering platforms where cold atoms are combined with optical cavities, superconducting circuits, or optomechanical devices restricting optical access.

Recently, there has been increasing activity to build dedicated Ising machines to accelerate the solution of combinatorial optimization problems by expressing these problems as a ground-state search of the Ising model. A common theme of such Ising machines is to tailor the physics of the underlying hardware to the mathematics of the Ising model to improve some aspect of performance that is measured in speed to solution, energy consumption per solution, or area footprint of the adopted hardware. One such approach to build an Ising spin, or a binary stochastic neuron (BSN), is a compact mixed-signal unit based on a low-barrier nanomagnet-based design that uses a single magnetic tunnel junction (MTJ) and three transistors (3T-1MTJ design) where the MTJ functions as a stochastic resistor (1SR). Such a compact unit can drastically reduce the area footprint of BSNs while promising massive scalability by leveraging the existing magnetic RAM technology that has integrated 1T-1MTJ cells in approximate Gbit densities. The 3T-1SR design however can be realized using different materials or devices that provide naturally fluctuating resistances. Extending previous work, we evaluate hardware BSNs from this general perspective by classifying necessary and sufficient conditions to design a fast and energy-efficient BSN that can be used in scaled Ising machine implementations. We connect our device analysis to systems-level metrics by emphasizing hardware-independent figures of merit such as flips per second and dissipated energy per random bit that can be used to classify any Ising machine.

This study presents a method of producing thick (i.e., in the $\mu \mathrm{m}$ range) polycrystalline $\mathrm{Gd}$ free-standing flexible films. Preparation is carried out by sputtering on silicon conventional substrates using tantalum as a buffer and capping layer. Magnetic and magnetocaloric properties show good agreement with data from high-purity bulk $\mathrm{Gd}$, and are not altered by substrate removal. Moreover, the free-standing film is flexible and all the relevant magnetic properties (i.e., Curie temperature $T_c$, saturation magnetization $M_s$, and isothermal entropy change $\mathrm{\Delta }S$) are preserved under bending (up to a $\epsilon =\pm 0.78\mathrm{\%}$ strain over the two film sides). The technological opportunities heralded by availability of magnetocaloric flexible self-sustaining films are discussed in the conclusions with particular focus on energy-conversion applications (i.e., cooling and thermal energy harvesting). More precisely, the output-power upper bound of an thermal-energy harvester deploying a $\mathrm{Gd}$ flexible film with the reported properties is worked-out using a thermal switch model presented elsewhere. The calculations show a potential output able to supply the new generation of IoT wireless devices as well as small medical implants. The result moot $\mathrm{Gd}$ free-standing flexible films as a benchmark for a new generation of small high-throughput magnetocaloric energy-conversion devices.

We discuss the generation and measurement of a Bessel vortex beam carrying multiple orbital-angular-momentum (OAM) modes in the radio-frequency domain. Considering that a high-order Bessel beam inherently has high-order OAM modes, in this paper, we propose to superpose these high-order Bessel vortex beams together, which can combine the advantages of the orthogonality of the vortex beam’s multiple OAM modes and the Bessel beam’s nondiffraction property. A theoretical formula and the basic design method are presented. A reflective metasurface is designed as the beam launcher to generate a deflected Bessel vortex beam that carries two OAM modes of $\ell =1$ and $\ell =2$ simultaneously. The corresponding reflective metasurface is designed, fabricated, and measured. The full-wave electromagnetic simulations and experimental measurements both verify the effectiveness of the design. The proposed method could further promote the application of a Bessel vortex beam in related near-field scenarios such as near-field wireless communication and radar detection and imaging.

The micro- and/or nanostructures of electrode materials play an essential role in their electrochemical performance, and the further understanding of their electrochemical behavior is vitally important for the rational design of high-performance electrochemical energy storage devices (EESDs). Herein, four types of ${\mathrm{Ni}\mathrm{Co}}_2{\mathrm{S}}_4$ hierarchical microstructures (HMSs; i.e., sphere-, hat-, brush-, and flowerlike HMSs) assembled from similar hollow nanoneedle building blocks are specially synthesized via a multistep strategy. A series of electrochemical analyses are performed to investigate the structure-activity relationship of these ${\mathrm{Ni}\mathrm{Co}}_2{\mathrm{S}}_4$ electrodes. The four ${\mathrm{Ni}\mathrm{Co}}_2{\mathrm{S}}_4$ electrodes all exhibit hybrid electrochemical characteristics combined with the adsorption and diffusion-controlled process of Faradaic redox reaction kinetics. Notably, the brushlike ${\mathrm{Ni}\mathrm{Co}}_2{\mathrm{S}}_4$ electrode displays the best specific capacitance (capacity) of 1684 F g (234 mAh g) at 1 A g, and retains 91% of its initial performance after 10 000 cycles, which is mainly due to its well-ordered three-dimensional microstructure with sufficient surface reaction sites, fast charge transport, and efficient ion diffusion properties. The resulting hybrid supercapacitor delivers a high energy density of 2667 mWh kg with a power density of 240 W kg and is able to light up light-emitting diodes. Furthermore, the comparative electrochemical analysis of dynamic responses in different electrodes is helpful to further understand their charge-discharge mechanism and provide basic guidance to fabricate efficient electrodes for EESDs.

The control of friction at the atomic scale is fundamental to optimize the exfoliation of layered materials. To this aim, we report a density-functional investigation of how intercalated molecules affect the nanoscale friction of van der Waals transition-metal dichalcogenides. We find that the molecule does not interact with the electronic density of the layers directly; nonetheless it determines the features of the valence band of the system. In particular, the valence-band width appears to be a promising parameter to correlate the electronic properties with the nanofrictional response; it then constitutes a guide for the automatic search of intercalation molecules suitable for layer exfoliation. The present outcomes also constitute a theoretical tool for future investigations of the effect that intercalated species have on the nanoscale friction in layered materials.

We propose and numerically simulate an optoelectronic compact circular polarimeter. It allows for electrical measurement of the degree of circular polarization and light intensity at room temperature for a wide range of incidence angles in a single shot. The device, being based on ${\mathrm{Ga}\mathrm{As}}_{1-x}{\mathrm{N}}_x$, is easy to integrate into standard electronics and does not require bulky movable parts nor extra detectors. Its operation hinges mainly on two phenomena: the spin-dependent capture of electrons and the hyperfine interaction between bound electrons and nuclei on ${\mathrm{Ga}}^{2+}$ paramagnetic centers in ${\mathrm{Ga}\mathrm{As}}_{1-x}{\mathrm{N}}_x$. The first phenomenon confers the device with sensitivity to the degree of circular polarization and the latter phenomenon allows one to discriminate the handedness of the incident light.

Light-emission phenomena of storage phosphors have received widespread attention, provoking the continuous developments of persistent luminescence, photostimulated luminescence, and mechanoluminescence. In the study and application of storage phosphors, as a matter of experience, it is always necessary to avoid or eliminate ambient light. It is the requirement for ambient darkness that is hindering the further development of storage phosphors under different ambient conditions. Here, we report an interesting glow phenomenon of storage phosphors in bright ambient light by introducing a concept termed ambient stimulated emission, in which the ambient temperature and ambient lighting can simultaneously release energy stored in the storage phosphors and lead to long-lasting emissions beyond the visible spectrum. As a proof of concept, we focus our discussion on an x-ray-charged ${\mathrm{La}\mathrm{Mg}\mathrm{Al}}_{11}{\mathrm{O}}_{19}:{\mathrm{Gd}}^{3+}$ phosphor, which exhibits ambient stimulated emission in the ultraviolet-B region, with increased initial light emission when the phosphor is exposed to ambient illumination instead of dark conditions. The ambient-stimulated-emission performances are found to depend on the illuminance and spectral distribution of ambient light. Moreover, our study indicates that the glow phenomenon in bright ambient light commonly appears in various storage phosphors, involving ultraviolet and infrared phosphors. The introduction of ambient stimulated emission brings different insights to luminescence research and offers approaches to further learn about and utilize storage phosphors.