We theoretically propose an $X\mathrm{ene}$ ($X=\mathrm{Si}$, $\mathrm{Ge}$, or $\mathrm{Sn}$) transistor that can be operated with high and low threshold light parameters. The results reveal that a spin-dependent nonconductive path in the $X\mathrm{ene}$ superlattice can be formed by utilizing an off-resonant light-induced topological phase transition and the band mismatch between illuminated and unilluminated regions. This topological transistor can be switched between an on state with a $100\mathrm{\%}$ spin-polarized weak current, an on state with a nonpolarized strong current, and an off state with a controllable breakdown voltage, just by adjusting the polarization state of circularly polarized light. With the assistance of an electric field, the $X\mathrm{ene}$ transistor can be operated at low light parameters, the threshold parameter of the transistor can be reduced to much lower than the spin-orbit coupling strength, and the breakdown voltage can be larger than the bulk band gap of the unilluminated $X\mathrm{ene}$. All the results indicate that the proposed $X\mathrm{ene}$ nanosystems are promising candidates for topological electronic devices.

Characterizing the radiated or received acoustic field of ultrasonic transducers using the spatial impulse response (SIR) represents an important step in testing, design, and optimization of ultrasonic transducers. However, for the ultrahigh-frequency acoustic field, conventional methods, such as the hydrophone method and the small-ball reflection method, are limited by narrow bandwidth and poor spatial resolution. Here, we propose a method to obtain the transducer’s SIR through its response to photoacoustic waves, which allows high-precision acoustic field measurements, with spatial resolution as fine as 1.7 µm. We subsequently measure the SIRs of two focused ultrasonic transducers, with 20 and 93 MHz center frequencies, and confirm that the three-dimensional acoustic fields can be accurately reconstructed using the angular spectrum approach. More importantly, this method is unique to receive-only ultrasonic detectors, the SIR of which could not be measured previously with conventional methods, and it could facilitate ultrasonic transducer design, as well as other related fields, such as nondestructive evaluation, biomedical imaging, and particle manipulation.

We employ a combination of machine learning and first-principles calculations to predict magnetic properties of rare-earth lean magnets. For this purpose, based on a training set constructed out of experimental data, the machine is trained to make predictions on magnetic transition temperature ($T_c$), largeness of saturation magnetization (${\mu }_0{M}_s$), and the nature of the magnetocrystalline anisotropy ($K_u$). Subsequently, the quantitative values of ${\mu }_0{M}_s$ and $K_u$ of the yet-to-be synthesized compounds, screened by machine learning, are calculated by first-principles density-functional theory. The applicability of the proposed technique of combined machine learning and first-principles calculations is demonstrated on 2-17-X magnets, ${\mathrm{Ce}}_2{\mathrm{Fe}}_{17-x}{\mathrm{Co}}_x\mathrm{CN}$. Further to this study, we explore the stability of the proposed compounds by calculating vacancy formation energy of small atom interstitials ($\mathrm{N}$/$\mathrm{C}$). Our study indicates a number of compounds in the proposed family and offers the possibility to become a solution for cheap and efficient permanent magnets.

Thermal rectification of far-field heat currents is experimentally demonstrated by exploiting the metal-insulator transition of ${\mathrm{VO}}_2$ driving the significant temperature variations of its emissivity, within a narrow interval of temperatures. This is achieved by measuring remarkable differences on the radiative heat flux between a ${\mathrm{VO}}_2$ film placed in vacuum and in front of a heat fluxmeter, when their temperature difference is reversed. By testing three ${\mathrm{VO}}_2$ films deposited on a substrate of $r$-sapphire, $c$-sapphire, and silicon, the highest rectification factor of $61\mathrm{\%}$ is obtained for the first film operating with a temperature difference of ${40}^{\circ }\mathrm{C}$ with respect to the fluxmeter. This rectification factor is higher than or comparable to the respective ones reported in the literature for near- or far-field radiative diodes subjected to a temperature difference of ${70}^{\circ }\mathrm{C}$ between their terminals. This experimental value is consistent with the theoretical one predicted by an analytical expression derived for the maximum rectification factor, as a function of the ${\mathrm{VO}}_2$ emissivity in the metallic and insulating phases, sensor emissivity, and geometrical parameters. The obtained results thus show that the rectification factor of these diodes can be enhanced, while reducing the temperature difference of their terminals, by increasing not only the emissivity variations between the insulating and metallic phases of ${\mathrm{VO}}_2$ films deposited on $r$-sapphire, but also decreasing their emissivity in the metallic phase.

We propose an electrically driven spin injector into normal metals and semiconductors, which is based on a magnetic tunnel junction (MTJ) subjected to a microwave voltage. Efficient functioning of such an injector is provided by electrically induced magnetization precession in the “free” layer of MTJ, which generates the spin pumping into a metallic or semiconducting overlayer. To validate the feasibility of the proposed device, we theoretically describe the spin and charge dynamics in the ${\mathrm{Co}}_{20}{\mathrm{Fe}}_{60}{\mathrm{B}}_{20}$/$\mathrm{Mg}\mathrm{O}$/${\mathrm{Co}}_{20}{\mathrm{Fe}}_{60}{\mathrm{B}}_{20}$/$\mathrm{Au}$ and ${\mathrm{Co}}_{20}{\mathrm{Fe}}_{60}{\mathrm{B}}_{20}$/$\mathrm{Mg}\mathrm{O}$/${\mathrm{Co}}_{20}{\mathrm{Fe}}_{60}{\mathrm{B}}_{20}$/$\mathrm{Ga}\mathrm{As}$ tunneling heterostructures. First, the magnetization dynamics in the free ${\mathrm{Co}}_{20}{\mathrm{Fe}}_{60}{\mathrm{B}}_{20}$ layer is quantified with the account of a spin-transfer torque generated by the spin-polarized current flowing through the MTJ and a voltage-controlled magnetic anisotropy associated with the ${\mathrm{Co}}_{20}{\mathrm{Fe}}_{60}{\mathrm{B}}_{20}$–$\mathrm{Mg}\mathrm{O}$ interface. The calculations are performed in the macrospin approximation for an ultrathin ${\mathrm{Co}}_{20}{\mathrm{Fe}}_{60}{\mathrm{B}}_{20}$ layer with perpendicular anisotropy and nanoscale in-plane dimensions. By numerically solving the Landau-Lifshitz-Gilbert-Slonczewski equation, we determine dependences of the precession amplitude on the frequency $f$ and magnitude $V_{\max }$ of the ac voltage applied to the MTJ. It is found that the frequency dependence changes drastically above the threshold amplitude $V_{\max }\approx 200$ mV, exhibiting a break at the resonance frequency $f_{\mathrm{res}}$ due to nonlinear effects. The results obtained for the magnetization dynamics are then used to describe the spin injection and pumping into the $\mathrm{Au}$ and $\mathrm{Ga}\mathrm{As}$ overlayers. The total spin-current density near the interface is calculated as a function of time at different excitation frequencies and voltage amplitudes. Since the generated spin current creates additional charge current owing to the inverse spin Hall effect, we also calculate distributions of the charge-current density and electric potential in the thick $\mathrm{Au}$ overlayer. The calculations show that the arising transverse voltage, which can be used to probe the efficiency of spin generation electrically, becomes experimentally measurable at $f=f_{\mathrm{res}}$. Finally, we evaluate the spin accumulation in a long $n^{+}$-$\mathrm{Ga}\mathrm{As}$ bar coupled to the MTJ and determine its temporal variation and spatial distribution along the bar. It is found that the ac spin accumulation under resonant excitation is large enough for experimental detection via a voltage between two ferromagnetic nanocontacts even at micrometer distances from the MTJ. This result demonstrates high efficiency of the described nanoscale spin injector driven by microwave voltage.

Nanostructured and bulk silicon carbide ($\mathrm{Si}\mathrm{C}$) materials are relevant for electronics, nano- and micromechanical systems, and biosensing applications. $\mathrm{Si}\mathrm{C}$ has recently emerged as an alternative platform for nanophotonics and quantum applications due to its intra-band-gap point defects, emitting from the visible to the near-infrared, which are ideal for photoluminescent probes. Here, we use a single-molecule localization microscope to study the photoluminescence (PL) properties of $\mathrm{Si}\mathrm{C}$ point defects in bulk, quantum dots, and nanoparticles of different sizes in the 3C and $4H\text{−}\mathrm{Si}\mathrm{C}$ polytypes using intra-band-gap excitation. We study the PL dynamics by using different excitation wavelengths, and we use the point-defect PL intermittency to achieve superresolved images, with a resolution of 20 nm and a minimum distance between emitters of 40 nm. We observe that, while 561 nm is an ideal excitation wavelength to obtain a sufficient blinking behavior, 638 nm is mostly quenching the PL. We further incubate $4H\text{−}\mathrm{Si}\mathrm{C}$ nanoparticles with MCF10A cells in vitro and observe superresolved images of the nanoparticles in cells by combining 561 and 638 nm excitation. This approach is very promising for the application of $\mathrm{Si}\mathrm{C}$ fluorescent nanocrystals and their quantum dots, hosting a combination of intra-band-gap color centers and surface defects, for quantum nanophotonics, magnetic sensing, and biomedical imaging, paving the way for single-particle tracking combined with spin sensing within a cellular environment using near-infrared emission.

We measure the thermoelectric response of Corbino structures in the quantum Hall effect regime and compare it with a theoretical analysis. The measured thermoelectric voltages are qualitatively and quantitatively simulated based upon the independent measurement of the conductivity, indicating that they originate predominantly from the electron diffusion. In contrast to earlier Hall-bar experiments, electron-phonon interaction does not lead to a phonon-drag contribution. This implies a description of the Onsager coefficients on the basis of a single transmission function, from which both thermovoltage and conductivity can be predicted with a single fitting parameter. Furthermore, it lets us predict a figure of merit for the efficiency of thermoelectric cooling, which becomes very large for partially filled Landau levels and high magnetic fields.

Irradiation of an ultrathin foil target by a high-intensity laser pulse drives collective electron motion and the generation of strong electrostatic fields, resulting in ultrabright sources of high-order harmonics and energetic ions. The ion energies can be significantly enhanced if the foil undergoes relativistic self-induced transparency during the interaction, with the degree of enhancement depending in part on the onset time of transparency. We report on a simple and effective approach to diagnose the time during the interaction at which the foil becomes transparent to the laser light, providing a route to optically controlling and optimizing ion acceleration and radiation generation. The scheme involves a self-referencing approach to spectral interferometry, in which coherent transition radiation produced at the foil rear interferes with laser light transmitted through the foil. The relative timing of the onset of transmission with respect to the transition-radiation generation is determined from spectral fringe spacing and compared to simultaneous frequency-resolved optical-gating measurements. The results are in excellent agreement, and are discussed with reference to particle-in-cell simulations of the interaction physics and an analytical model for the onset time of transparency in ultrathin foils.

We report on efficient spin injection in p-doped $(\mathrm{In},\mathrm{Ga})\mathrm{As}/\mathrm{Ga}\mathrm{As}$ quantum-dot (QD) spin light-emitting diodes (spin LEDs) under zero applied magnetic field. A high degree of electroluminescence circular polarization ($P_c$) ∼19% is measured in remanence up to 100 K. This result is obtained thanks to the combination of a perpendicularly magnetized $\mathrm{Co}\text{−}\mathrm{Fe}\text{−}\mathrm{B}/\mathrm{Mg}\mathrm{O}$ spin injector allowing efficient spin injection and an appropriate p-doped $(\mathrm{In},\mathrm{Ga})\mathrm{As}/\mathrm{Ga}\mathrm{As}$ QD layer in the active region. By analyzing the bias and temperature dependence of the electroluminescence circular polarization, we evidence a two-step spin-relaxation process. The first step occurs when electrons tunnel through the $\mathrm{Mg}\mathrm{O}$ barrier and travel across the $\mathrm{Ga}\mathrm{As}$ depletion layer. The spin relaxation is dominated by the Dyakonov-Perel mechanism related to the kinetic energy of electrons, which is characterized by a bias-dependent $P_c$. The second step occurs when electrons are captured into QDs prior to their radiative recombination with holes. The temperature dependence of $P_c$ reflects the temperature-induced modification of the QD doping, together with the variation of the ratio between the charge-carrier lifetime and the spin-relaxation time inside the QDs. The understanding of these spin-relaxation mechanisms is essential to improve the performance of spin LEDs for future spin optoelectronic applications at room temperature under zero applied magnetic field.

Resistive and capacitive switching in capacitor metal/nanocomposite/metal ($\mathrm{M}/\mathrm{NC}/\mathrm{M}$) structures based on $(\mathrm{Co}\text{−}\mathrm{Fe}\text{−}\mathrm{B}{)}_x({\mathrm{Li}\mathrm{Nb}\mathrm{O}}_3{)}_{100-x}$ NC fabricated by ion-beam sputtering with metal content x ≈ 8–20 at. % is studied. The peculiarity of the structure synthesis was the use of increased oxygen content (approximately equal to 2 × ${10}^{-5}$ Torr) at the initial stage of the NC growth. The NC films, along with metal nanogranules of 3–6 nm in size, contained a large number of dispersed Co (Fe) atoms (up to approximately ${10}^{22}$ ${\mathrm{cm}}^{-3}$). Measurements are performed both in dc and ac (frequency range 5–13 MHz) regimes. When switching structures from high-resistance $(R_{\mathrm{off}})$ to low-resistance $(R_{\mathrm{on}})$ state, the effect of a strong increase in their capacity is found, which reaches 8 times at x ≈ 15 at. % and the resistance ratio $R_{\mathrm{off}}$:$R_{\mathrm{on}}$ ≈ 40. The effect is explained by the synergetic combination of the multifilamentary character of resistive switching (RS) and structural features of the samples associated, in particular, with the formation of a high-resistance and strongly polarizable ${\mathrm{Li}\mathrm{Nb}\mathrm{O}}_3$ layer near the bottom electrode of the structures. The proposed model is confirmed by investigations of RS of two-layer nanoscale $\mathrm{M}/\mathrm{NC}/{\mathrm{Li}\mathrm{Nb}\mathrm{O}}_3/\mathrm{M}$ structures as well as by studies of the magnetization of $\mathrm{M}/\mathrm{NC}/\mathrm{M}$ structures in the pristine state and after RS.

Light shifts are known to be an important limitation to the mid- and long-term fractional frequency stability of different types of atomic clocks. In this article, we demonstrate the experimental implementation of an anti-light-shift interrogation protocol onto a continuous-wave (cw) microcell atomic clock based on coherent population trapping (CPT). The method, inspired by the autobalanced Ramsey spectroscopy technique demonstrated in pulsed atomic clocks, consists in the extraction of atomic based information from two successive light-shifted clock frequencies obtained at two different laser-power values. Two error signals, computed from the linear combination of signals acquired along a symmetric sequence, are managed in a dual-loop configuration to generate a clock frequency free from light shift. Using this method, the sensitivity of the clock frequency to both laser-power and microwave-power variations can be reduced by more than an order of magnitude compared to normal operation. In the present experiment, the consideration of the nonlinear light-shift dependence allows enhancement of the light-shift mitigation. The implemented technique allows an improvement of the clock Allan deviation for time scales higher than 1000 s. This method can be applied in various kinds of atomic clocks such as CPT-based atomic clocks, double-resonance $\mathrm{Rb}$ clocks, or cell-stabilized lasers.

Orbital angular momentum (OAM) carried by acoustic vortex attracts extensive research interests due to the possible application such as acoustic tweezer and communications. The OAM in the conventional symmetric acoustic vortex is generally determined by topological charge l. Here, we provide the beam symmetry as an alternative factor to manipulate the OAM beyond the topological charge. We propose a nondiffractive asymmetric acoustic vortex beam, which can be generated by an acoustic metasurface and modulate the OAM to arbitrary values. Rather than the radially symmetric distribution, the intensity of the asymmetric acoustic vortex is characterized with a crescent shape controlled by the asymmetry parameter $\alpha$. The ratio between axial OAM and sound energy on the transverse plane depends on the asymmetry parameter and thus is not quantized by the ratio of topological charge l to wave angular frequency $\omega$ as in conventional vortex beam. Moreover, we present the realization of the proposed vortex beam by designed metasurface, and reveal the nondiffractive nature of the asymmetric acoustic vortex beam. Our work expands the family of acoustic vortex and demonstrates an alternative route to control acoustic OAM.

Topological physics requires stable methods to measure polarization singularities in optical vector fields. Here, the use of a periodic plasmonic metasurface is proposed for computation of the divergence of vectorial paraxial beams. We design such an optical device to perform spatial differentiation in two directions, parallel and perpendicular to the incidence plane, simultaneously. The divergence operation is achieved by creating constructive interference between two derivative results. We demonstrate that such optical computations provide a direct pathway to identifying specific polarization singularities in vector fields.

We study the nature and dynamics of spin-dependent charge-carrier recombination in tris(8-hydroxyquinolinato) aluminum (${\mathrm{Alq}}_3$) films in light-emitting diodes at room temperature using continuous-wave and pulsed electrically detected magnetic resonance (EDMR) spectroscopy. We find that the EDMR signal is dominated by an electron-hole recombination process and another weaker EDMR signal, the fundamental nature of which is investigated. From the pulsed EDMR measurements, we obtain a carrier-spin-relaxation time, $T_2$= 45 ns ± 25 ns, which is much shorter than that of $T_2$ in conjugated polymers, but relatively long for a molecule containing elements with a high atomic number. Using multifrequency continuous-wave EDMR spectroscopy, we obtain the local hyperfine field distributions for electrons and holes, as well as their respective spin-orbit-coupling-induced g-factor and g-strain values.

Solid-state quantum emitters are excellent sources of on-demand indistinguishable or entangled photons and can host long-lived spin memories, crucial resources for photonic quantum-information applications. However, their scalability remains an outstanding challenge. Here, we present a scalable technique to multiplex streams of photons from multiple independent quantum dots, on chip, into a fiber network for use “off chip.” Multiplexing is achieved by incorporating a multicore fiber into a confocal microscope and spatially matching the multiple foci, seven in this case, to quantum dots in an array of deterministically positioned nanowires. As a proof-of-principle demonstration, we perform parallel spectroscopy on the nanowire array to identify two nearly identical quantum dots at different positions, which are subsequently tuned into resonance with an external magnetic field. Multiplexing of background-free single photons from these two quantum dots is then achieved. Our approach, applicable to all types of quantum emitters, can readily be integrated into scalable photonic based quantum technologies.

Present-day, noisy, small or intermediate-scale quantum processors—although far from fault tolerant—support the execution of heuristic quantum algorithms, which might enable a quantum advantage, for example, when applied to combinatorial optimization problems. On small-scale quantum processors, validations of such algorithms serve as important technology demonstrators. We implement the quantum approximate optimization algorithm on our hardware platform, consisting of two superconducting transmon qubits and one parametrically modulated coupler. We solve small instances of the NP (nondeterministic polynomial time)-complete exact-cover problem, with 96.6% success probability, by iterating the algorithm up to level two.

Airlines today are faced with a number of large-scale scheduling problems. One such problem is the tail-assignment problem, which is the task of assigning individual aircraft to a given set of flights, minimizing the overall cost. Each aircraft is identified by the registration number on its tail fin. In this paper, we simulate the quantum approximate optimization algorithm (QAOA) applied to instances of this problem derived from real-world data. The QAOA is a variational hybrid quantum-classical algorithm recently introduced and likely to run on near-term quantum devices. The instances are reduced to fit on quantum devices with 8, 15, and 25 qubits. The reduction procedure leaves only one feasible solution per instance, which allows us to map the tail-assignment problem onto the exact-cover problem. We find that repeated runs of the QAOA identify the feasible solution with close to unit probability for all instances. Furthermore, we observe patterns in the variational parameters such that an interpolation strategy can be employed, which significantly simplifies the classical optimization part of the QAOA. Finally, we empirically find a relation between the connectivity of the problem graph and the single-shot success probability of the algorithm.