We theoretically and numerically investigate the switching of perpendicular magnetization by ac spin-orbit torque. We find that the threshold switching current for ac spin-orbit torque is much smaller than that for dc spin-orbit torque. Moreover, the ac spin-orbit torque acts like an effective out-of-plane field in the rotating frame and thus allows field-free deterministic switching of perpendicular magnetization. The ac spin-orbit torque scheme will be useful for energy-efficient operation of spin-orbit torque devices.

Materials with tunable thermal conductivity play a critical role in advanced thermal management, including building and vehicle heating and cooling, solar energy harvesting, and energy storage. 2D materials provide a platform for controlling heat transport via surface modification. Using density functional theory and the Boltzmann transport equation for phonons, we demonstrate the ability to tune thermal conductivity in a 2D material ${\mathrm{C}}_3\mathrm{N}$ by adding and removing $\mathrm{C}$ atoms: (1) although ${\mathrm{C}}_3\mathrm{N}$ is even stiffer than graphene, its lattice thermal conductivity at 300 K is nearly ten times lower (482 W/mK); (2) by adding $\mathrm{C}$ atoms, the formation of a more complex dumbbell structure results in more tortuous paths for phonons, but counterintuitively, the room-temperature thermal conductivity increases; (3) by removing $\mathrm{C}$ atoms, the thermal conductivity at 300 K further reduces to 34 W/mK. Moreover, ${\mathrm{C}}_3\mathrm{N}$ shows much lower thermal conductivity than the dumbbell structures at low temperature but much higher thermal conductivity at high temperature, rendering ${\mathrm{C}}_3\mathrm{N}$ and its derivatives highly promising for various applications at ambient temperature. All these distinct behaviors are related to the localized out-of-plane phonon modes that rely on chemical bonding. Our work provides an intuitive understanding of how structure influences heat transport and demonstrates the possibility to control heat transport precisely via structural modification.

Exceptional points (EPs) are non-Hermitian degeneracies that can enable various intriguing applications such as enhanced sensing and asymmetric mode switching. Here, we theoretically propose the design of a 1 × 2 terahertz switch by employing the topological structure of the energy surfaces (i.e., eigenvalue distributions in a two-variable parameter space) near an EP. The switch consists of a pair of terahertz waveguides, one of which has a monolayer graphene laid on the bottom. The wave transmission through the system can be understood by considering an adiabatic trajectory on self-intersecting Riemann sheets due to the existence of an EP. The trajectory can be tuned by changing the chemical potential of graphene, and as a result, we can choose in which waveguide the power exits to achieve the switching functionality. The physics behind the switch lies in the fact that the trajectories on different sides of the EP lead to different final states due to the unique topological structure of energy surfaces near the EP. We perform numerical simulations to optimize the performance of the terahertz switch.

We demonstrate a vector magnetometer that simultaneously measures all Cartesian components of a dynamic magnetic field using an ensemble of nitrogen-vacancy (NV) centers in a single-crystal diamond. Optical NV-diamond measurements provide high-sensitivity, broadband magnetometry under ambient or extreme physical conditions and the fixed crystallographic axes inherent to this solid-state system enable vector sensing free from heading errors. In the present device, multichannel lock-in detection extracts the magnetic-field-dependent spin-resonance shifts of NVs oriented along all four tetrahedral diamond axes from the optical signal measured on a single detector. The sensor operates from near dc (5 Hz) up to a 12.5-kHz measurement bandwidth and simultaneously achieves approximately $50\mathrm{pT}/\sqrt{\mathrm{Hz}}$ magnetic-field sensitivity for each Cartesian component, which is, to date, the highest demonstrated sensitivity of a full vector magnetometer employing solid-state spins. Compared to optimized devices interrogating the four NV orientations sequentially, the simultaneous vector magnetometer enables a $4\times$ measurement speedup. This technique can be extended to pulsed-type sensing protocols and parallel wide-field magnetic imaging.

The dependence of the asymmetric magnetoimpedance (MI) response on the directions of both the magnetic field and the exchange bias is studied for an $[\mathrm{Ni}\text{−}\mathrm{Fe}(60\mathrm{nm})/\mathrm{Ir}\text{−}\mathrm{Mn}(35\mathrm{nm})]\times 5$ multilayer system. The antiferromagnetic (AFM) layers create an exchange bias that shifts both the hysteresis loop and the MI response of $\mathrm{Ni}\text{−}\mathrm{Fe}$; the strength of this coupling depends on the thicknesses of both the ferromagnetic layer and the AFM layer. Tuning the exchange-bias angle and the applied-magnetic-field direction provides a practical method to control the symmetry and the magnitude of the MI response. The observed asymmetric response can be attributed to the coexistence of two anisotropies, the induced exchange-bias anisotropy and the magnetocrystalline anisotropy. Both are distinct components of the exchange-coupled Stoner-Wohlfarth energy density that forms a basis for simulations within the conventional Landau-Lifshitz-Gilbert framework. The model reproduces the main features of the experimental results, providing a deeper understanding of the effect of the anisotropy components on the MI characteristic curve and ratio. The results can be used for the development of an asymmetric MI exchange-biased thin-film element in an autobiased linear-magnetic-field sensor.

Thin films of lead salt nanocrystals (NCs) offer attractive opportunities as active media for near-infrared optoelectronics but suffer from limiting trade-offs between optical and electrical properties. While NCs separated by nanometer-long ligands are good light emitters, NCs capped with shorter molecules provide a high carrier mobility but degrade the photo- and electroluminescence and broaden the narrow emission spectrum. Here we show that this severe quenching and spectral broadening can be averted with an unconventional use of metallic antennas. The resulting NC-antenna hybridization not only provides a strong boost in luminescence, but also makes it possible to remodel the emission spectrum in radical ways, even at wavelengths where the NC assembly does not emit light. These results cannot be explained with the standard theory of single-emitter luminescence assisted by optical antennas. We propose an alternative model based on a statistical description of light emission by an ensemble of emitters and discuss important consequences of our findings for nano-optics and solution-processed optoelectronics.

We present a study of the fundamental electrodynamics of an electronic ink pixel based on frustrated total internal reflection, which is relevant for the ongoing development of video-speed-capable electronic ink displays. Devices are fabricated with plane-parallel electrodes covered with a dielectric coating of the poly(p-xylylene) polymer Parylene C and filled with ink proprietary fluorochemical fluid containing absorbing particles. Reflectivity measurements of frustrated total internal reflection and transient current measurements provide information on the concentration of particles near the electrodes, conductivity of the ink, and screening of the electric field. By comparing the measurements with an electro-optical model, it is found that the electrical properties of the ink are dominated by positively charged particles and negative countercharges. Screening of the electric field depends as expected on the dielectric coating thickness. The observation of an asymmetry in the steady-state gray level response to an applied voltage is explained by fixed positive charges adsorbed at the interface between the dielectric coating and the dispersion. The overall switching dynamics of the device for different voltage sequences is in good agreement with simulations.

The duration and fidelity of qubit readout are critical factors for applications in quantum-information processing as they limit the fidelity of algorithms which reuse qubits after measurement or apply feedback based on the measurement result. Here we present fast multiplexed readout of five qubits using a single 1.2-GHz-wide readout channel. Using a readout pulse length of 80 ns and populating readout resonators for less than 250 ns, we find an average probability of correct assignment for the five measured qubits to be $97\mathrm{\%}$. The differences between the individual readout errors and those found when measuring the qubits simultaneously are within $1\mathrm{\%}$. We employ individual Purcell filters for each readout resonator to suppress off-resonant driving, which we characterize by measuring the dephasing imposed on unintentionally measured qubits. We expect the readout scheme presented here to become particularly useful for the selective readout of individual qubits in multiqubit quantum processors.

Our examination of vortex fields of scalar waves illuminating an off-axis-located axisymmetric object shows that the ratio of orbital angular momentum (OAM) transfer to energy transfer is not quantized by the ratio of topological charge $l$ (integer) to wave angular frequency $\omega$. We find that the transfer ratio can even be opposite to the ratio $l/\omega$. The findings reveal how a radiation torque associated with energy absorption can spin an axisymmetric object around its center of mass in a direction reversed with respect to the wave vortex’s handedness. We understand the reversal by OAM superposition. The absorption can be either in the absorbing object or in the viscous surrounding fluid. Features of reversed spinning motion of a small particle in a slightly viscous fluid are revealed by use of Bessel-function vortex fields. The results are of interest for the development of acoustic and optical vortex-based tweezers as well as for the field of acoustofluidics.

A recent experimental study [Phys. Rev. Appl. 9, 024023 (2018)] on paramagnetic $\mathrm{Ce}{\mathrm{Co}}_3$ finds that magnesium alloying induces a ferromagnetic transition with intrinsic properties large enough for permanent magnet applications. Here we explain these surprising results via a first-principles study of the electronic structure and magnetism of magnesium-alloyed $\mathrm{Ce}{\mathrm{Co}}_3$. We find the origin of this magnesium-induced ferromagnetic transition to be Stoner physics—the substantial increase in the Fermi-level density of states $N(E_F)$ with $\mathrm{Mg}$ alloying. Our calculations suggest that both $\mathrm{Ce}$ and $\mathrm{Co}$ atoms are likely to be important for generating large magnetic anisotropy suggesting the viability of $\mathrm{Co}$-3d and $\mathrm{Ce}$-4f interaction for the generation of magnetic anisotropy in magnetic materials. These results offer a route to the discovery of ferromagnetic materials and provide fundamental insight into the magnetic properties of these alloys.

Blood plasma separation in microchannels is a key element in microfluidics-based biodetection and lab-on-chip technology. In microfluidics, acoustophoresis-based blood plasma separation is considered one of the most efficient approaches for high-throughput separation. However, the physics of the separation of plasma from whole blood (dense suspension) by using acoustophoresis is not well understood. In this paper, we provide improved understanding of blood plasma separation from whole blood using acoustophoresis based on experiments and simulations. The interplay between the acoustophoretic timescale ${\tau }_{\mathrm{ACP}}$ and shear-induced diffusion timescale ${\tau }_{\mathrm{SID}}$ that represents the underlying mechanism of the focusing of cells and separation of plasma is illustrated. The results show that, for whole blood, ${\tau }_{\mathrm{SID}}\ll {\tau }_{\mathrm{ACP}}$; thus, acoustophoresis fails to concentrate the cells and form a cell-free plasma layer. Serial extraction of concentrated cells from the center of the channel gives rise to ${\tau }_{\mathrm{SID}}\gg {\tau }_{\mathrm{ACP}}$, thus enabling the focusing of cells and formation of the plasma layer. Blood plasma separation in the acoustophoretic device is simulated using a simple 2D model that employs the mixture theory. Based on the improved understanding, we develop an acoustophoretic device that is capable of separating $22\mu \mathrm{l}/\min$ plasma from $100\mu \mathrm{l}/\min$ of whole blood (at 40% hematocrit), offering significantly higher plasma throughput and recovery (36.67%) compared to devices reported previously by Lenshoff et al. [1] and Tajudin et al. [2]. Finally, we present a rule of thumb for the design of acoustophoretic devices for different hematocrit concentrations to offer the desired plasma throughput and recovery.

The resonant interaction of electrically excited traveling surface acoustic waves and magnetization has hitherto been probed through the acoustic component. In this work we investigated it using time-resolved magneto-optical detection of magnetization dynamics. To that end, we develop an experimental scheme where laser pulses are used both to generate the acoustic wave frequency and to probe magnetization dynamics, thus ensuring perfect phase locking. The light-polarization dependence of the signal enables us to disentangle elasto-optical and magneto-optical contributions and to obtain the in-plane and out-of-plane components of the magnetization dynamics. Magnetization precession is proven to be driven solely by the acoustic wave. Its amplitude is shown to resonate at the same field at which we detect piezoelectrically the resonant attenuation of the acoustic wave, clearly evidencing the magnetoacoustic resonance with high sensitivity.

Double-resonance optically pumped magnetometers are an attractive instrument for unshielded magnetic-field measurements due to their wide dynamic range and high sensitivity. The use of linearly polarized pump light creates alignment in the atomic sample, which evolves in the local static magnetic field, and is driven by a resonant applied field perturbation, modulating the polarization of transmitted light. We demonstrate experimentally that the amplitude and phase of observed first- and second-harmonic components in the transmitted polarization signal contain sufficient information to measure the static-magnetic-field magnitude and orientation. We describe a laboratory system for experimental measurements of these effects and verify a theoretical derivation of the observed signal. We demonstrate vector-field tracking under varying static-field orientations and show that the static-field magnitude and orientation may be observed simultaneously, with an experimentally realized resolution of 1.7 pT and 0.63 mrad in the most sensitive field orientation.

We explore the nature of occupation and relaxation within the charge transfer density of states (CT DOS) for bulk heterojunction organic solar cells consisting of the donor boron subphthalocyanine chloride and the acceptor ${\mathrm{C}}_{60}$. We observe relaxation of geminate CT states on a sub-ns timescale via an approximately 70-meV dynamic redshift in their photoluminescence, whereas free carrier relaxation at longer times leads to the formation of nongeminate CT states at even lower energy. In steady state, we find that thermalization within the DOS is incomplete, resulting in a Boltzmann-like CT state distribution characterized by an effective temperature above that of the ambient. These results confirm that electron and hole populations can be far from equilibrium in organic solar cells and may prompt a reassessment of analyses that assume the same temperature for their charge-carrier distributions in the dark and under illumination.

Quantum digital signatures (QDSs) offer the information-theoretic security verified by quantum mechanics to send a message that is robust against forging and repudiation. We report a proof-of-principle demonstration of a passive decoy-state QDS system that can avoid leaking the modulation information of decoy states. To realize this, four local detection events are generated from the idler light of a parametric down-conversion process, and are then used as decoy states to estimate channel parameters more accurately. As a result, we successfully sign a more-than-one-bit message within 7 s at 100 km. Besides, we achieve a transmission distance of 200 km, which represents a further step toward the practical application of QDSs.

Understanding the mechanisms behind high-$T_c$ type-II superconductors (SCs) is still an open task in condensed-matter physics. One way to gain further insight into the microscopic mechanisms leading to superconductivity is to study the magnetic properties of the SCs in detail, for example, by studying the properties of vortices and their dynamics. In this work, we describe a method of wide-field imaging magnetometry using nitrogen-vacancy (NV) centers in diamond to image vortices in an yttrium-barium-copper-oxide (YBCO) thin film. We demonstrate quantitative determination of the magnetic-field strength of the vortex stray field, the observation of vortex patterns for different cooling fields, and direct observation of vortex pinning in our disordered YBCO film. This method opens prospects for imaging of the magnetic-stray fields of vortices at frequencies from dc to several megahertz within a wide range of temperatures, which allows for the study of both high-$T_C$ and low-$T_C$ SCs. The wide temperature range allowed by NV center magnetometry also makes our approach applicable for the study of phenomena like island superconductivity at elevated temperatures (e.g., in metal nanoclusters).

We propose a free-electron-beam (FEB)-driven frequency mixer for generating terahertz-wave radiation. It employs an initial steady-current FEB to drive two cascaded gratings. On the first grating, the FEB is macrobunched by interacting with the self-stimulated backward slow waves. On the second grating, which operates at high harmonics of the first grating, it is further microbunched at high frequencies, producing mixed bunching components within the FEB. The multibunched FEB then generates a series of superradiant Smith-Purcell radiations from the second grating, achieving multicolor radiations in the terahertz region. The radiation can be tuned by adjusting the beam energy, covering the frequency range from 0.8 to 1.8 THz. It is a promising broad-tunable and multicolor terahertz source.

Long-term inertial navigation is currently limited by the bias drifts of gyroscopes and accelerometers and ultrastable cold-atom interferometers offer a promising alternative for the next generation of high-end navigation systems. Here, we present an experimental setup and an algorithm hybridizing a stable matter-wave interferometer with a classical accelerometer. We use correlations between the quantum and classical devices to track the bias drift of the latter and form a hybrid sensor. We apply the Kalman-filter formalism to obtain an optimal estimate of the bias and simulate experimentally a harsh environment representative of that encountered in mobile sensing applications. We show that our method is more precise and robust than traditional sine-fitting methods. The resulting sensor exhibits a 400-Hz bandwidth and reaches a stability of 10 ng after 11 h of integration.

Nanostructures have exhibited unprecedented optical and thermal properties including super-Planckian thermal radiation. Nevertheless, nanostructures are usually made of isotropic materials while anisotropic counterparts are rarely considered despite their wide applications. This paper investigates near-field thermal radiation between gratings consisting of natural anisotropic graphite. Graphite gratings with optical axes lying out of plane are found to possess sevenfold larger radiative heat flux than counterpart plane plates at nanoscale gap distances, outperforming blackbodies by over four orders of magnitude. The dominant radiative heat transfer channel changes from nonresonant hyperbolic modes to anisotropic surface resonant modes after patterning. The strong coupling of anisotropic surface modes with high k waves in a broad frequency range results in the extremely efficient radiative heat transfer of graphite gratings. This work helps deepen the understanding of thermal radiation of anisotropic nanostructures and opens routes for more efficient thermal radiative transport.

Static magnetic configurations of thin, circular, soft (permalloy) magnetic nanodots, coupled to a hard antidot matrix with perpendicular magnetization, are studied by micromagnetic simulations. It is demonstrated that dipolar fields of the antidot matrix promote the formation of a magnetic-vortex state in nanodots. The vortex is the dot ground state at zero external field in ultrathin nanodots with diameters as low as 60 nm, which is far beyond the vortex stability range in an isolated permalloy nanodot. Depending on the geometry and antidot-matrix material, it is possible to stabilize either a radial vortex state or unconventional vortices with the angle between in-plane magnetization and radial direction $\psi \ne 0,\pi /2$.