Small spin ensembles play an important role in many areas of condensed-matter physics. Here we present a method to measure spin densities in very dilute spin systems. We report on surface and bulk spin-density measurements of diamond using ultrasensitive magnetic force microscopy with magnetic field gradients up to 0.5 $\mathrm{T}/\mu \mathrm{m}$. At temperatures between 25 and 800 mK, we measure the shifts in the resonance frequency and quality factor of a cantilever with a micromagnet attached to it. A recently developed theoretical analysis allows us to extract a surface spin density of 0.072 ${\mathrm{spins}/\mathrm{nm}}^2$ and a bulk spin density of 0.4 ppm from these data. In addition, we find an increase of the $T_1$ time of the surface spins in high magnetic field gradients due to the suppression of spin diffusion. Our technique is applicable to a variety of samples other than diamond, and could be of interest for several research fields where surface, interface or impurity bulk spin densities are an important factor.

Active control of spin-wave dynamics is demonstrated using broadband ferromagnetic resonance in two-dimensional ${\mathrm{Ni}}_{80}{\mathrm{Fe}}_{20}$ antidot lattices arranged in a hexagonal lattice with fixed lattice constant, but varying antidot diameter. A strong modification in the spin-wave spectra is obtained with the variation in the antidot diameter as well as with the strength and orientation of the bias magnetic field. A broad band of modes is observed for the lattice with the higher antidot diameter, which decreases systematically as the antidot diameter is reduced. A crossover between the higher-frequency branches is achieved in lattices with higher antidot diameters. In addition, the spin-wave modes in all lattices show a strong six-fold anisotropic behavior due to the variation of internal field distribution as a function of the bias-field orientation. A mode-hopping-like behavior is observed in the angular dispersions of spin-wave spectra for samples having intermediate hole diameters. Micromagnetic simulations qualitatively reproduce the experimentally observed spin-wave modes and the simulated mode profiles reveal the presence of extended and quantized standing spin-wave modes in these lattices. These observations are significant for large tunability and anisotropic propagation of spin waves in GHz-frequency magnetic devices.

Reflective metasurfaces (or meta-mirrors) composed of artificially engineered subwavelength structures provide an alternate paradigm for arbitrarily controlling wave reflection with high efficiency. We introduce the concept of frequency-selective coding of a meta-mirror to perform desired scattering properties in different frequency bands by tailoring the phase dispersion of the metasurface. As a particular example, broadband backward-scattering reduction and high-efficient specular reflection are combined by a single meta-mirror in a frequency-selective manner. Two meta-atoms with optimized metallic patterns are used as the binary coding elements to realize an out-of-phase function in two side-bands, while the in-phase function is in the center frequency window. Then, we experimentally demonstrate that polarization-insensitive diffusion-like scatterings can be achieved by the proposed meta-mirror with randomly distributed meta-atoms in two side-bands from 7.5 to 9.5 GHz and 11.6 to 15 GHz, while in the selected center frequency window around 10.7 GHz, a high-efficient specular reflection property is realized. The experimental results are in good agreement with the full-wave simulations. Compared with the conventional metasurface designs for scattering reduction, the proposed concept exhibits a frequency-selective effect and hence can be used in many practical applications, for example, working as the ground plane of antennas to enable a high-efficient in-band radiation with out-of-band low-backward scattering.

We present a comprehensive micromagnetic model of isolated axisymmetric skyrmions in magnetic multilayers with perpendicular anisotropy. Most notably, the essential role of the internal dipolar field is extensively considered with a minimum amount of assumptions on the magnetization profiles. The tridimensional structure of the multilayered skyrmions is modeled by their radial profiles in each layer. We first compare the results of the model against a full micromagnetic description in Cartesian coordinates. Our model combines information on both layer-dependent size and chirality of the skyrmions. We also provide a convenient criterion in order to characterize the stability of skyrmions against anisotropic elongations that would break their cylindrical symmetry, which allows us to confirm the stability of the determined solutions. Because this model is able to treat magnetization configurations twisted through the thickness of multilayered skyrmions, it can provide predictions on any potential hybrid chirality in skyrmions due to the interplay of Dzyaloshinskii-Moriya and dipolar interactions in multilayers. We finally apply the results of our model to the description of the current-driven dynamics of hybrid chiral skyrmions. Using the Thiele formalism, we show that we can predict the forces exerted on the multilayered skyrmions by vertical spin-polarized currents, which provides a method to conform hybrid skyrmion chiralities and spin-current injection geometries in order to optimize skyrmion motion in multilayers, to the aim of maximizing the current-induced velocity, or canceling the skyrmion Hall angle.

The lack of coherent room-temperature sources in the whole terahertz spectral window (0.3–10 THz) has significantly hampered the growth of scientific and technological applications in this range. Among them, high-precision frequency measurements of molecular transitions play a central role but remain an open challenge. Here, room-temperature generation and detection of continuous-wave, broadly tunable, narrow-linewidth THz radiation are presented, and their application to high-resolution spectroscopy in the broad 1–7.5 THz spectral range is demonstrated. This result has been achieved by implementing a Cherenkov phase-matching scheme into a channel waveguide in a nonlinear crystal. This simple approach, entirely based on robust telecom technology, unprecedently merges in a single source an ultra-broad continuous-wave spectral coverage and a state-of-the-art accuracy (approximately ${10}^{-9}$) in molecular-transition-center determination.

Molecular-dynamics simulations are used to compute the threshold displacement energy ($E_d$) in a series of progressively damaged graphite structures. The $E_d$ values are obtained by a statistically robust probabilistic method using a large number of primary-knock-on-atom events at energies up to 100 eV. No sharp threshold for $E_d$ is observed, and a number of possible definitions are considered. For pristine graphite, the best estimate of $E_d$ is 24 eV. $E_d$ decreases with increasing irradiation damage, dropping by nearly a factor of 2 at a dose of one displacement per atom. For a fully disordered amorphous-carbon structure, $E_d$ is around 5 eV. This evolution of $E_d$ is an important missing ingredient in current estimates of radiation doses in nuclear reactors, which assume $E_d$ is constant over the reactor lifetime, despite substantial structural evolution.

We design two-dimensional mechanical metamaterials that may be deformed substantially at little or no energy cost. Examples of such deformable structures include assemblies of rigid isosceles triangles hinged at their corners on the macroscale and polymerized phenanthrene molecules forming porous graphene on the nanoscale. In these and in a large class of related structures, the Poisson ratio $\nu$ diverges for particular strain values. It also changes its magnitude and sign, and displays a shape-memory effect.

Active polarization rotators are desirable for many practical applications such as liquid crystal displays, fiber communication, etc. Ultrathin metasurfaces incorporating phase-change or magneto-optical materials provide active tuning of polarization, but suffer from a long response time or the requirement for a strong magnetic field. In this study, a tunable plasmonic polarization rotating system is proposed, which consists of two coupled metal screens perforated with subwavelength apertures. The polarization rotating mechanism can be attributed to the near-field coupling of metal screens and its dependence on the screen’s lateral displacement. The theory and experiment in the microwave band demonstrate that, by using the x polarized incident wave and displacing one metal screen at the millimeter scale, a continuous and efficient polarization rotation of the outgoing wave (with a nearly linear polarization state) from −90° to 90° can be achieved. This arbitrarily directional and tunable polarization rotating effect may be extended to the THz regime as well and find potential applications in constructing versatile polarization rotators and signal modulators.

We propose nonreciprocal phonon lasing in a coupled cavity system composed of an optomechanical resonator and a spinning resonator. We show that the optical Sagnac effect leads to significant modifications in both the mechanical gain and the power threshold for phonon lasing. More importantly, the phonon lasing in this system is unidirectional; that is, the phonon lasing occurs when the coupled system is driven in one direction but not the other. Our work establishes the potential of spinning optomechanical devices for low-power mechanical isolation and unidirectional amplification. This provides a new route, well within the reach of current experimental abilities, to operate cavity optomechanical devices for a wide range of applications such as directional phonon switches, invisible sound sensing, and topological or chiral acoustics.

Acoustic illusion devices are usually designed using transformation optics. An alternate method is proposed to achieve acoustic illusions without external devices by elaborately manipulating the acoustic scattering potential of an object. The proposed method is more of a “cosmetic operation” for an object, which modifies the scattered acoustic pressure distribution of the object to mimic another object by exchanging their scattering potentials in two symmetrical areas in the wave vector domain. The advantage of this method is the simplicity of material parameters: only positive isotropic mass density and bulk modulus are required, which is impossible with the conventional method of using transformation optics due to the complex material requirements (anisotropic and negative index parameters).

Finding the shortest path in a graph has applications in a wide range of optimization problems. However, algorithmic methods scale with the size of the graph in terms of time and energy. We propose a method to solve the shortest-path problem using circuits of nanodevices called memristors and validate it on graphs of different sizes and topologies. It is both valid for an experimentally derived memristor model and robust to device variability. The time and energy of the computation scale with the length of the shortest path rather than with the size of the graph, making this method particularly attractive for solving large graphs with small path lengths.

Bragg-edge strain imaging from energy-resolved neutron-transmission measurements poses an interesting tomography problem. The solution to this problem will allow the reconstruction of detailed triaxial stress and strain distributions within polycrystalline solids from sets of Bragg-edge strain images. Work over the last decade has provided some solutions for a limited number of special cases. In this paper we provide a general approach to reconstruction of an arbitrary system based on a least-squares process constrained by equilibrium. This approach is developed in two dimensions before it is demonstrated experimentally on two samples with use of the RADEN instrument at the Japan Proton Accelerator Research Complex spallation neutron source. Validation of the resulting reconstructions is provided through a comparison with conventional constant-wavelength strain measurements performed with the KOWARI engineering diffractometer of the Australian Nuclear Science and Technology Organisation. The paper concludes with a discussion on the range of problems to be addressed in a three-dimensional implementation.

Alloys with adjustable mechanical performance are of fundamental interest in material designs. Here, we investigate the magnetic- and chemical-ordering behavior of the ferromagnetic $\mathrm{Fe}\text{−}\mathrm{Cr}\text{−}\mathrm{Co}\text{−}\mathrm{Ni}\text{−}{\mathrm{Al}}_x$ $(1\le x\le 2.5)$ high-entropy alloys with the help of first-principle alloy theory. The lattice constants and the single- and polycrystalline elastic parameters for partially ordered and random structures are considered. In contrast to the trend found for the completely disordered phase, we demonstrate that ordering driven primarily by $\mathrm{Al}$ results in an enhanced Young’s modulus, especially at high-$\mathrm{Al}$ concentrations, which is in line with the observed increase of the hardness for systems with a body-centered-cubic underlying lattice. The results suggest that outstanding strength and ductility can be realized by proper control of the ordering level in single- and multiphase high-entropy alloys.

The afterpulse effect is an intrinsic characteristic of the single-photon avalanche photodiode that has been widely used in quantum key distribution (QKD). As QKD systems move into the gigahertz regime, the afterpulse effect is no longer ignorable, which will lead to a great deviation compared with the existing analytical model. Here we develop an analytical model to make QKD systems more afterpulse compatible. In addition, we obtain the secure key rate for our model with the analysis of statistical fluctuation using Hoeffding’s inequality and Azuma’s inequality. Our results show that the optimized parameters of the afterpulse-compatible model can provide a much higher key rate than the optimized parameters of the previous afterpulse-omitted model in the same situation.

We study the dynamics of a magnetic vortex in a permalloy disk, as it is switched between bistable pinning sites in the presence of room-temperature thermal noise. We use static and time-resolved magneto-optical Kerr-effect microscopy to characterize the equilibrium and nonequilibrium motion of the vortex in response to a train of magnetic field pulses. The limitations of this approach due to inhomogeneous averaging are overcome by using a proximal nitrogen-vacancy spin in diamond to probe the final position of the vortex core following a single magnetic field pulse. Repeating this single-shot measurement, we characterize the probability of switching between two bistable pinning sites as a function of pulse time. We develop a simple model of the stochastic vortex trajectory through the pinning sites’ basins of attraction to understand how the separate contributions of damping and dephasing result in the observed switching behavior.

We invoke the Huygens principle to derive dispersion characteristics of acoustic transformational metasurfaces that can deflect parallel wavefronts in a desired direction. We also propose a dual-Lorentz resonator whose aperture fields can be tuned by geometrical changes to implement a particular phase with unity reflection coefficient. The proposed metascreen is designed by our arranging slightly detuned Lorentz cavities that generate the necessary interference to compensate for the incident wavefronts. Since a complete 0–$2\pi$ range of the reflection phase is achieved, the metascreen can steer a beam across the full horizon. Moreover, since the proposed dual resonator has subwavelength dimensions, the metascreen is capable of high-resolution imaging, which is an essential trait in cloaking and deceptive acoustics. Successful demonstration of acoustic beam steering and carpet cloaking is done for a wavelength of 790 mm with the help of full-wave simulations.

Two-Dimensional Transition Metal Dichalcogenides (2D TMDCs) such as ${\mathrm{Mo}\mathrm{S}}_2$, ${\mathrm{Mo}\mathrm{Se}}_2$, ${\mathrm{WS}}_2$, and ${\mathrm{W}\mathrm{Se}}_2$ with van der Waal's type interlayer coupling are being widely explored as channel materials in a Schottky Barrier Field Effect Transistor (SB FET) configuration. While their excellent electrostatic control and high on/off ratios have been identified, a clear correlation between electronic transport and the low-frequency noise with different atomic-layer thickness is missing. For multilayer channels in ${\mathrm{Mo}\mathrm{S}}_2$ FETs, the effects of interlayer-coupling resistance on device conductance and mobility have been studied, but no systematic study has included interlayer effects in consideration of the intrinsic (channel) and extrinsic (total device) noise behavior. Here, we report the 1/f noise properties in ${\mathrm{Mo}\mathrm{Se}}_2$ FETs with varying channel thicknesses (3–40 atomic layers). Contributions of channel vs access/contact regions are extracted from current-voltage (transport) and 1/f noise measurements. The measured noise amplitude shows a direct crossover from channel- to contact-dominated noise as the gate voltage is increased. The results can be interpreted in terms of a Hooge relationship associated with the channel noise, a transition region, and a saturated high-gate-voltage regime whose characteristics are determined by a voltage-independent conductance and noise source associated with the metallurgical contact and the interlayer resistance. Both the channel Hooge coefficient and the channel/access noise amplitude decrease with increasing channel thickness over the range of 3–15 atomic layers, with the former remaining approximately constant and the latter increasing over a range of 20–40 atomic layers. The analysis can be extended to devices based on other TMDCs.

We experimentally demonstrate long-distance continuous-variable quantum key distribution over a 50-km standard optical fiber based on continuous-variable Einstein-Podolsky-Rosen entangled states. The entanglement survives despite being distributed over a high-loss optical fiber channel. At a channel excess noise level of 0.01 shot-noise units, we achieve an asymptotic secret key rate of 0.03 bit per sample, which is superior to the optimized coherent-state protocol. The superiority is even more evident at a high-channel excess noise level of 0.1 shot-noise units. Our work paves the way toward practical applications of continuous-variable quantum key distribution under high amounts of excess channel noise.

Dominant recombination paths in $\mathrm{Al}\mathrm{N}$ and ${\mathrm{Al}}_x{\mathrm{Ga}}_{1-x}\mathrm{N}$-related structures are investigated using cathodoluminescence (CL) mapping measurements and photoluminescence (PL) spectroscopy. The dark spot contrasts originating from nonradiative recombination at threading dislocations (TDs), which are observed in CL intensity maps, drastically decrease upon elevating the temperature. This is because carriers can reach TDs at low temperatures (9–60 K), but are captured by point defects (PDs) even in the vicinity of TDs near RT. Calculations based on the experimental results indicate that in the current $\mathrm{Al}\mathrm{N}$ and $\mathrm{Al}$-rich ${\mathrm{Al}}_x{\mathrm{Ga}}_{1-x}\mathrm{N}$ crystals, TDs scarcely affect the internal quantum efficiency (IQE) at RT as long as the TD density is less than $2.6\times {10}^{10}{\mathrm{cm}}^{-2}$. Because a TD density less than $2.6\times {10}^{10}{\mathrm{cm}}^{-2}$ has already been achieved even for heteroepitaxially grown $\mathrm{Al}\mathrm{N}$ films on sapphire substrates, it is evident that the most effective method to further improve the IQE of ${\mathrm{Al}}_x{\mathrm{Ga}}_{1-x}\mathrm{N}$-related materials is to reduce PDs not TDs. Moreover, we clarify the existence of two types of PD states, which mainly degrade the emission efficiency, using temperature-dependent PL measurements. Combining the CL and PL results allows the activation energies of these PDs and TDs to be evaluated. Furthermore, we highlight the probability that PDs, which predominantly act as nonradiative recombination centers at room temperature, are complexes formed by $\mathrm{Al}$ vacancies and oxygen impurities that enhance the deep-level emissions at 3.2 and 3.5–3.7 eV near room temperature. Such a large impact of PDs on the efficiency degradation may be attributed to the high density of $\mathrm{Al}$-vacancy–related PDs in $\mathrm{Al}\mathrm{N}$ and $\mathrm{Al}$-rich ${\mathrm{Al}}_x{\mathrm{Ga}}_{1-x}\mathrm{N}$ compared with that of $\mathrm{Ga}$-vacancy–related PDs in $\mathrm{Ga}\mathrm{N}$ due to the small formation energy.

In conventional materials, Cherenkov radiation (CR) due to a moving charged particle is associated with a broad frequency range and velocity threshold. Here, we show that using a periodic grating structure, which possesses photonic bound states in the continuum (BICs) and quasi-BICs, unusual CR can be generated in a very narrow frequency band at a particle velocity below the common threshold. This effect arises from an interesting light-amplification process in the BIC structure, which can dramatically improve the efficiencies of evanescent-to-propagating wave conversion and CR generation. Our results present a mechanism for realizing Cherenkov lasing at low electron velocity and may find applications in compact free-electron lasers.