The efficiency of ($\mathrm{In}$,$\mathrm{Ga}$)$\mathrm{N}$-based light-emitting diodes (LEDs) is limited by the failure of holes to evenly distribute across the ($\mathrm{In}$,$\mathrm{Ga}$)$\mathrm{N}$/$\mathrm{Ga}\mathrm{N}$ multiquantum well stack that forms the active region. To tackle this problem, it is important to understand carrier transport in these alloys. In this work, we study the impact that random alloy fluctuations have on the distribution of electrons and holes in such devices. To do so, an atomistic tight-binding model is employed to account for alloy fluctuations on a microscopic level and the resulting tight-binding energy landscape forms input to a quantum corrected drift-diffusion model. Here, quantum corrections are introduced via localization-landscape theory. Similar to experimental studies in the literature, we have focused on a multiquantum well system in which two of the three wells have the same $\mathrm{In}$ content, while the third well differs in $\mathrm{In}$ content. By changing the order of wells in this “multicolor” quantum well structure and looking at the relative radiative-recombination rates of the different emitted wavelengths, we (i) gain insight into the distribution of carriers in such a system and (ii) can compare our findings to trends observed in experiment. We focus on three factors and evaluate the impact that each have on carrier distribution: an electron blocking layer, quantum corrections, and random alloy fluctuations. We find that the electron blocking layer is of secondary importance. However, in order to recover experimentally observed features—namely, that the $p$-side quantum well dominates the light emission—both quantum corrections and random alloy fluctuations should be considered. The widely assumed homogeneous virtual-crystal approximation fails to capture the characteristic light emission distribution across a multiquantum well stack.

Nonlinear microwave circuits are key elements for many groundbreaking research directions and technologies, such as quantum computation and quantum sensing. The majority of microwave circuits with Josephson nonlinearities to date are based on aluminum thin films; therefore, they are severely restricted in their operation range regarding temperatures and external magnetic fields. Here, we present the realization of superconducting niobium microwave resonators with integrated, three-dimensional (3D) nanobridge-based superconducting quantum interference devices. The 3D nanobridges (constriction weak links) are monolithically patterned into prefabricated microwave $LC$ circuits using neon focused-ion-beam milling, and the resulting quantum interference circuits show frequency tunabilities, flux responsivities, and Kerr nonlinearities on par with comparable aluminum nanobridge devices, but with the perspective of a much larger operation parameter regime. Our results demonstrate that neon focused-ion-beam milling is a promising method for fabricating 3D constriction junctions with flexible parameters and reveal great potential for application of the resulting microwave circuits in hybrid systems with, e.g., magnons and spin ensembles or in flux-mediated optomechanics.

The Dzyaloshinskii-Moriya interaction (DMI) is known to play a central role in stabilizing chiral spin textures such as skyrmions and domain walls (DWs). Electrical manipulation of DW and skyrmion motion offers possibilities for next-generation, scalable and energy-efficient spintronic devices. However, achieving the full potential of these nanoscale devices requires overcoming several challenges, including reliable electrical write and read techniques for these magnetic objects, and addressing pinning and Joule-heating concerns. Here, through micromagnetic simulations and analytical modeling, we show that DMI can directly induce magnetization switching of a nanomagnet with perpendicular magnetic anisotropy (PMA). We find that the switching is driven by the interplay between the DMI-induced magnetic frustration and the PMA. By introducing magnetic tunnel junctions to electrically access and control the magnetization direction of the PMA nanomagnet, we first show the potential of this concept to enable high-density field-free spin-orbit torque magnetic random-access memory. Ultimately, we demonstrate that it offers a way of transferring and processing spin information for logic operation without relying on current-driven DW or skyrmion motion.

One-dimensional topological acoustic systems offer robust signal transmission and localization in compact spaces. Here, we demonstrate the ability to control both longitudinal and transverse topological waves in a quasi-one-dimensional elastic lattice, composed of an array of repelling magnets connected to elastic cantilevers. The two different topological modes can be separately or simultaneously excited at selected frequencies. Our system enables controlled multimodal propagation and edge localization, owing to its finite size and to the design of longitudinal and transverse topological band gaps within similar frequency ranges. In addition to the robustness of the topological modes to unintentional disorder, we show that disorder can couple degrees of freedom and tailor the energy distribution between longitudinal and transverse waves. This system may enable other studies on the interactions of topological modes useful in sensing and information-processing applications.

The inelastic energy deposition of energetic ions is a decisive quantity for numerous industrial-scale applications, such as sputtering and ion implantation, yet the underlying physics being governed by dynamic many-particle processes is commonly only qualitatively understood. Recently, transmission experiments on single-crystalline targets (Phys. Rev. Lett. 124, 096601 & Phys. Rev. A 102, 062803) revealed a complex energy scaling of the inelastic energy loss of low-energy ions heavier than protons along different trajectories. We use a Monte Carlo like binary collision approximation code equipped with an impact-parameter-dependent modeling of the inelastic energy losses to assess the role of local contributions to electronic excitations in these cases. We compare angular intensity distributions of calculated trajectories with experimental results for 50-keV ${}^4\mathrm{He}$ and 100-keV ${}^{29}\mathrm{Si}$ ions transmitted in a time-of-flight setup through single-crystalline silicon (001) foils with nominal thicknesses of 200 and 50 nm, respectively. In these calculations, we employ different models of electronic energy loss, i.e., local and nonlocal forms for light and heavy projectiles. We find that the vast number of projectiles are eventually channeled along their trajectories, regardless of the alignment of the crystal with respect to the incident beam. It is, however, only when local electronic energy loss is considered that the simulated two-dimensional maps and energy distributions show excellent agreement with the experimental results, where channeling leads to significantly reduced stopping, especially for heavier projectiles. We demonstrate the relevance of these effects for ion implantations by assessing the nonlinear and nonmonotonic scaling of the ion range with the thickness of a random surface layer.

Niobium offers the benefit of increased operating temperatures and frequencies for Josephson junctions, which are the core component of superconducting devices. However, existing niobium processes are limited by more complicated fabrication methods and higher losses than now-standard aluminum junctions. Combining recent trilayer fabrication advancements, methods to remove lossy dielectrics and modern superconducting qubit design, we revisit niobium trilayer junctions and fabricate all-niobium transmons using only optical lithography. We characterize devices in the microwave domain, measuring coherence times up to $62\text{μ}\mathrm{s}$ and an average qubit quality factor above ${10}^5$: much closer to state-of-the-art aluminum-junction devices. We find the higher superconducting gap energy also results in reduced quasiparticle sensitivity above 0.16 K, where aluminum-junction performance deteriorates. Our low-loss junction process is readily applied to standard optical-based foundry processes, opening alternative avenues for direct integration and scalability, and paves the way for higher-temperature and higher-frequency quantum devices.

Mid-infrared superconducting-nanowire single-photon detectors require higher detector sensitivity because of the weaker photon energy. Previous experiments demonstrated that the sensitivity of superconducting-nanowire single-photon detectors (SNSPDs) could be improved by optimizing the stoichiometry of superconducting films. However, the physical mechanism has not been completely studied. In this work, we attempt to explain the optimization mechanism from the hotspot dynamics point of view of thin films and detectors. We obtained the film’s electron-transport parameters using magnetoconductance and Hall coefficient measurements and characterized the dynamic process of the hotspot based on the hotspot-relaxation time. Based on our experimental results, it is proposed that the stoichiometry of the $\mathrm{W}$-$\mathrm{Si}$ film plays a role in the specific heat and boundary thermal conductivity, and therefore, affects the thermal relaxation process and the detector’s sensitivity. The research results may lay the foundation for the development of highly efficient mid-infrared SNSPD.

Smart switchable windows for buildings and vehicles become more and more important because they can reduce energy consumption. They exhibit two optically contrasting states: a transparent state and an optically scattering (or light-absorbing) state. Most of them are, however, monostable, namely, only one of the optical states is stable in the absence of applied voltage, and the other state must be sustained under continuously applied voltage, which would consume energy. Here, we report a bistable smart window based on polymer-stabilized cholesteric liquid crystals. The window exhibits two stable states at 0 V: one of them is the transparent homeotropic state with a high transmittance and the other state is the scattering focal conic state with a low transmittance. The bistable states are achieved by using an anisotropic polymer network, known as polymer stabilization. The window is switched from the transparent state to the scattering state by a low-frequency ac voltage pulse under the flexoelectric effect and is switched from the scattering state back to the transparent state by a high-frequency ac voltage pulse under the dielectric interaction. No voltage is needed to sustain the two states of the window. Therefore, the window almost does not consume energy. The optical contrast between the two states is high, and the window is very efficient at privacy control. Due to its superior performance and very low energy consumption, this technology is expected to have a significant impact on switchable architectural and automobile windows.

The second-order nonlinear Hall effect (NLHE), driven by the Berry-curvature dipole (BCD), quantum metrics, and disorder-related mechanisms, have potential in energy harvesting and signal doubling. However, for a large class of known materials, in which inversion symmetry is preserved, the BCD and NLHE are strictly zero. Here, we report that, by interface modulation through ${\mathrm{Al}}_x{\mathrm{O}}_3$-assisted exfoliation, a strong NLHE is generated in few-layer ${\mathrm{Fe}}_5{\mathrm{Ge}\mathrm{Te}}_2$, in which both the BCD and disorder-related contributions are in principle prohibited due to its $R\overline{3}m$ lattice symmetry. Scaling analysis indicates the existence of a large BCD reaching $\mathrm{\Lambda }\approx 150\text{nm}$. The negative results from ${\mathrm{Fe}}_5{\mathrm{Ge}\mathrm{Te}}_2$ devices on ${\mathrm{Si}\mathrm{O}}_2$ substrates confirm that the interface between ${\mathrm{Fe}}_5{\mathrm{Ge}\mathrm{Te}}_2$ and ${\mathrm{Al}}_x{\mathrm{O}}_3$ is responsible for the observed NLHE, revealing an interface-induced BCD, which has never been reported before. Our study provides a feasible way to generate NLHE in materials with inversion symmetry, expanding the material family for NLHE research and inspiring potential applications due to the advantages in mass production.

Tin selenide ($\mathrm{Sn}\mathrm{Se}$) has recently been widely studied due to its ultralow lattice thermal conductivity and excellent thermoelectric properties. However, the thermal conductivity of the low-temperature phase of $\mathrm{Sn}\mathrm{Se}$ is usually overestimated with consideration of only three-phonon scattering processes and the effect of the four-phonon interaction has not been investigated. Meanwhile, it is still unclear how the widely distributed intrinsic vacancy defects, $V_{\mathrm{Sn}}$ and $V_{\mathrm{Se}}$, affect the lattice thermal conductivity. In this work, using the accurate first-principles-based deep-neural-network potential combined with the phonon Boltzmann transport equation and molecular-dynamics simulations, the lattice thermal conductivity of the low-temperature phase of $\mathrm{Sn}\mathrm{Se}$ is calculated with high-order phonon-scattering processes being considered, and the results match well with the experimental values. In addition, the effect of intrinsic vacancy defects on the thermal conductivity is also investigated using nonequilibrium molecular-dynamics simulations. The results show that the $\mathrm{Se}$ vacancy has a more significant effect on the reduction of the thermal conductivity than the $\mathrm{Sn}$ vacancy, due to the enhanced phonon anharmonicity attributed to the more significant vibration of the weakly bonded $\mathrm{Sn}$ atom, as well as the appreciable inhibition of the low-frequency-phonon propagation speed. The weakening of interactions for the transition of $\mathrm{Sn}$ atoms around vacancies from bonding with $\mathrm{Se}$ atoms to bonding with $\mathrm{Sn}$ atoms is the reason for the decline of the phonon group velocity. This study deepens our understanding of phonon thermal transport in the low-temperature phase of $\mathrm{Sn}\mathrm{Se}$ and provides a good example of using the deep-neural-network potential to explore the effects of the four-phonon interaction and intrinsic point defects on the thermal conductivity of solids.

We report a measurement of the radiative lifetime of the $5s5p{}^3{P}_0^{\mathrm{o}}$ metastable state in ${}^{87}\mathrm{Sr}$, which is coupled to the $5s^2{}^1{S}_0$ ground state via a hyperfine-induced electric dipole transition. The radiative lifetime is determined to be 151.4(48) s, in good agreement with theoretical results. Our approach relies on accurate measurements of laser intensity and free-space Rabi frequency, enabling lifetime measurements of any excited state and particularly suitable for long-lived states.

High-quality cavities are crucial for various fundamental physical studies and applications. Here we find that by coupling two cavities directly or via a phase-tunable coupling channel, the photon lifetime of the local field can exceed that of the bare cavities. The cavity photon lifetime is modified by the phases of the initial states and the phase accumulation on the coupling channel, which affect the interference between cavities. In experiments, by coupling superconducting radio-frequency cavities via phase-tunable cables, we realize a factor of 2 improvement in the cavity photon lifetime. The results can bring rich revenue to quantum information science, sensing, and high-energy physics.

Two-dimensional (2D) vanadium disulfide (${\mathrm{VS}}_2$) can serve as a universal host for reversible intercalation and deintercalation of alkali and alkaline earth metal ions. However, its practical application in rechargeable metal-ion batteries is limited by its low energy density (559 Wh ${\mathrm{g}}^{-1}$). Herein, the effects of $\mathrm{O}$ doping and $\mathrm{C},\mathrm{O}$ codoping on the electrochemical performance of 2D ${\mathrm{VS}}_2$ used as anode for magnesium-ion batteries (MIBs) are investigated by first-principles calculations. Values of both the energy density and specific capacity increase with increasing $\mathrm{O}$-doping concentration, and those of $\mathrm{VSO}$ are 2.17 times and 1.16 times higher than those of ${\mathrm{VS}}_2$, respectively. However, due to the strong bond between $\mathrm{O}$ and $\mathrm{Mg}$, the diffusion barrier of $\mathrm{Mg}$ atoms on 2D $\mathrm{VSO}$ is relatively high (1.02 eV). Further introduction of $\mathrm{C}$ (${\mathrm{VSO}}_{0.75}{\mathrm{C}}_{0.25}$) can reduce the diffusion barrier of $\mathrm{Mg}$ atoms (0.80 eV) to a level comparable to that of ${\mathrm{VS}}_2$. Meanwhile, the values of energy density and specific capacity of ${\mathrm{VSO}}_{0.75}{\mathrm{C}}_{0.25}$ are 1.53 times and 1.17 times those of ${\mathrm{VS}}_2$. Our results suggest that $\mathrm{O}$ doping and $\mathrm{C},\mathrm{O}$ codoping of 2D ${\mathrm{VS}}_2$ are effective strategies to improve the overall performance of MIBs and it should be possible to generalize such doping strategies to other rechargeable MIBs based on 2D transition metal dichalcogenides.

We show that an oscillator array prefers to operate at an exceptional point of degeneracy (EPD) occurring in a waveguide periodically loaded with discrete nonlinear gain and radiating elements. The concept of the EPD is employed to conceptualize an exceptional synchronization regime, which leads to enhanced radiating power efficiency. The system maintains a steady-state degenerate mode of oscillation at a frequency of 3 GHz, even when the small-signal nonlinear gain values are nonuniform along the array. We designed the system using small-signal gain to work at the EPD of zero phase shift in consecutive unit cells. Contrarily to the original expectation of zero phase shift, after reaching saturation, the time-domain signal in consecutive unit cells displays a $\pi$ phase shift. Hence, we demonstrate that the saturated system tends to oscillate at a distinct EPD, associated to a $\pi$ phase shift between consecutive cells, than the one at which the system was originally designed using small-signal gain. This alternative EPD at which the nonlinear system is landing is associated to higher radiating power efficiency with respect to power provided by nonlinear gains. Finally, we demonstrate that the oscillation frequency is independent of the length of the array, contrarily to what happens ordinary oscillating systems based on one-dimensional cavity resonances. These findings may have a high impact on high-power radiating arrays with distributed active elements.

In the search for stable lead-free perovskites, vacancy-ordered double perovskites (VODPs), ${\mathrm{A}}_2{\mathrm{BX}}_6$, have emerged as a promising class of materials for solar harvesting owing to their nontoxicity, better stability, and unique optoelectronic properties. Recently, this class has been explored for a wide range of applications, such as photovoltaics, photodetectors, photocatalysis, and light-emitting diodes. Here, we present the stability and the key physical attributes of a few selected compounds in a systematic manner using state-of-the-art first-principles calculations. A careful structural and stability analysis via simulation of convex hulls and compositional phase diagrams for different structural prototypes discloses 14 stable compounds and one metastable compound in this class. Electronic structure calculations using hybrid functionals reveals that six compounds possess band gaps in the ideal visible region. These six compounds, namely ${\mathrm{Cs}}_2{\mathrm{Sn}\mathrm{I}}_6$, ${\mathrm{Cs}}_2{\mathrm{Pd}\mathrm{I}}_6$, ${\mathrm{Cs}}_2{\mathrm{Te}\mathrm{I}}_6$, ${\mathrm{Cs}}_2{\mathrm{Ti}\mathrm{I}}_6$, ${\mathrm{Cs}}_2{\mathrm{Pt}\mathrm{I}}_6$, and ${\mathrm{Cs}}_2{\mathrm{Pd}\mathrm{Br}}_6$, show high optical absorption ($\approx {10}^5{\mathrm{cm}}^{-1}$), giving rise to high spectroscopic limited maximum efficiency (15–23%) in the thin-film thickness range. Close inspection of the transport properties reveals polar optical phonon scattering to be the dominant mechanism limiting overall mobility. Further analysis of the polaron excitations discloses the possibility of large polaron formation at low to moderate defect concentrations. At high defect concentrations, ionized impurity scattering takes over. Such analysis can be extremely useful for choosing the optimal growth conditions for a given material intended to be used for a desired application. Additionally, a few selected compounds show moderate to high electron mobility values ($\sim 13–63{\mathrm{cm}}^2{\mathrm{V}}^{-1}{\mathrm{s}}^{-1}$) at room temperature. Overall, the present study paves an important path to help design VODPs as lead-free potential candidates for future optoelectronic applications.

Implementation of high-fidelity gate operations on integrated-qubit systems is vital for fault-tolerant quantum computation. Qubit-frequency allocation is an essential part of improving control fidelity. A metric for qubit-frequency allocation, frequency collision, has been proposed on simple systems of only a few qubits driven by a monomodal microwave drive. However, frequency allocation for quantum processors for more advanced purposes, such as quantum error correction, needs further investigation. In this study, we propose a Floquet analysis of frequency collisions. The key to our proposed method is a reinterpretation of frequency collisions as an unintended degeneracy of Floquet states, which allows a collision analysis on more complex systems with many qubits driven by multimodal microwave drives. Although the Floquet state is defined in an infinite-dimensional Hilbert space, we develop algorithms, based on operation perturbation theory, to truncate the Hilbert space down to the optimal computational complexity. In particular, we show that the computational complexity of the collision analysis for a sparse qubit lattice is linear with the number of qubits. Finally, we demonstrate our proposed method on cross-resonance-based experimental protocols. We first study the cross-resonance gate in an isolated three-qubit system, where the effectiveness of our method is verified by comparing it with previous studies. We next consider the more complex problem of syndrome extraction in the heavy-hexagon code [Christopher et al., Phys. Rev. X 10, 011022 (2020). doi:10.1103/PhysRevX.10.011022]. Our proposed method advances our understanding of quantum control for quantum processors and contributes to their improved design and control.

Surface acoustic waves (SAWs) are a reliable solution to the transport of single electrons with precision in piezoelectric semiconductor devices. Recently, highly efficient single-electron transport with a strongly compressed single-cycle acoustic pulse has been demonstrated. This approach, however, requires surface gates constituting the quantum dots, their wiring, and multiple gate movements to load and unload the electrons, which is very time-consuming. Here, on the contrary, we employ such a single-cycle acoustic pulse in a much simpler way—without any quantum dot at the entrance or exit of a transport channel—to perform single-electron transport between distant electron reservoirs. We observe the transport of a solitary electron in a single-cycle acoustic pulse via the appearance of the quantized acoustoelectric current. The simplicity of our approach allows for on-demand electron emission with arbitrary delays on a nanosecond time scale. We anticipate that enhanced synthesis of the SAWs will facilitate electron quantum optics experiments with multiple-electron flying qubits.

On-chip efficient nonlinear functions are instrumental in escalating the utilities and performance of photonic integrated circuits, especially for a wide range of classical and quantum applications, such as tunable coherent radiation, optical frequency conversion, spectroscopy, quantum science, etc. Here, we fabricated a $Z$-cut lithium tantalate on insulator (LTOI) microdisk with high quality ($Q\sim {10}^6$) factors. Cascaded three- and four-wave mixing generation simultaneously were achieved in the LTOI microdisk cavity, due to the high $Q$ factor with a strongly confined light field, relatively large nonlinear coefficient, and optical damage threshold of the LTOI microdisk. The strict mode phase-matching condition and the second-harmonic efficiency of the whole three-wave mixing processes were analyzed. Our work shows that the LTOI microresonators with the high $Q$ factor and its high optical damage threshold and wide transparency range can support a variety of on-chip optical nonlinear processes, which heralds its application potential in integrated nonlinear photonics.