Magnesium-aluminum (Mg-Al) alloys are important metal alloys with a wide range of engineering applications. We investigate the elastic and thermodynamic properties of Mg, Al, and four stoichiometric Mg-Al compounds including ${\mathrm{Mg}}_{17}{\mathrm{Al}}_{12}$, ${\mathrm{Mg}}_{13}{\mathrm{Al}}_{14}$, and ${\mathrm{Mg}}_{23}{\mathrm{Al}}_{30}$, and ${\mathrm{MgAl}}_2$ with orbital-free density-functional theory (OFDFT). We first calculate the lattice constants, zero-temperature formation energy, and independent elastic constants of these six materials and compare the results to those computed via Kohn-Sham DFT (KSDFT) benchmarks. We obtain excellent agreement between these two methods. Our calculated elastic constants of hexagonal close-packed Mg and face-centered-cubic Al are also consistent with available experimental data. We next compute their phonon spectra using the force constants extracted from the very fast OFDFT calculations, because such calculations are computationally challenging using KSDFT. This is especially the case for the ${\mathrm{Mg}}_{23}{\mathrm{Al}}_{30}$ compound, whose $3\times 3\times 3$ supercell consists of 1431 atoms. We finally employ the quasiharmonic approximation to investigate temperature-dependent thermodynamic properties, including formation energies, heat capacities, and thermal expansion of the four Mg-Al intermetallic compounds. The calculated heat capacity and thermal expansion of both Mg and Al agree well with experimental data. We additionally find that ${\mathrm{Mg}}_{13}{\mathrm{Al}}_{14}$ and ${\mathrm{MgAl}}_2$ are both unstable, consistent with their absence from the equilibrium Mg-Al phase diagram. Our work demonstrates that OFDFT is an efficient and accurate quantum-mechanical computational tool for predicting elastic and thermodynamic properties of complicated Mg-Al alloys and also should be applicable to many other engineering alloys.

We propose the concept of a quantized single-electron source based on the interplay between Coulomb blockade and magnetic flux-controllable superconducting proximity effect. We show that flux dependence of the induced energy gap in the density of states of a nanosized metallic wire can be exploited as an efficient tunable energy barrier which enables charge-pumping configurations with enhanced functionalities. This control parameter strongly affects the charging landscape of a normal metal island with non-negligible Coulombic energy. Under a suitable evolution of a time-dependent magnetic flux the structure behaves like a turnstile for single electrons in a fully electrostatic regime.

One of the aspirations for artificial microswimmers is their application in noninvasive medicine. For any practical use, adequate mechanisms enabling control of multiple artificial swimmers will be of paramount importance. Here we theoretically propose a multihelical, freely jointed motor as a selective control mechanism. We show that the nonlinear step-out behavior of a magnetized helix driven by a rotating magnetic field can be exploited when used in conjunction with other helices to obtain a velocity profile that is non-negligible only within a chosen interval of operating frequencies. Specifically, the force balance between the competing opposite-handed helices is tuned to give no net motion at low frequencies (tug-of-war), while in the middle-frequency range, the magnitude and, potentially, the sign of the swimming velocity can be adjusted by varying the driving frequency. We illustrate this idea on a two-helix system and demonstrate how to generalize to $N$ helices, both numerically and theoretically. We then explain how to solve the inverse problem and design an artificial swimmer with an arbitrarily complex velocity vs frequency relationship. We finish by discussing potential experimental implementation.

We introduce here a parity-time-($\mathcal{P}\mathcal{T}$) symmetric system using an optically pumped, active graphene metasurface paired with a resistive metallic filament, realizing a unidirectional reflectionless propagation of terahertz (THz) waves. The amplified THz stimulated emission and tailored plasmon resonances in the graphene metasurface may achieve an equivalent negative-resistance converter at THz frequencies. We theoretically demonstrate that the combination of the spectral singularity in a $\mathcal{P}\mathcal{T}$-symmetric system and the chemical sensitivity of graphene may give rise to exotic scattering responses, strongly influenced by the presence of charged impurities in graphene at the spontaneous $\mathcal{P}\mathcal{T}$-symmetry-breaking point. This graphene-based $\mathcal{P}\mathcal{T}$-symmetric device may have broad relevance beyond the extraordinary manipulation of THz waves, as it may also open exciting prospects for detecting gas, chemical, and biological agents with high sensitivities.

Transistors based on single-crystal films of 2-decyl-7-phenyl-[1]benzothieno[3,2-b][1]benzothiophene (Ph-BTBT-10) fabricated using the blade-coating method are investigated by the four-probe method down to low temperatures. The four-probe mobility is as large as $18{\mathrm{cm}}^2/\mathrm{V}\mathrm{s}$ at room temperature, and increases to $45{\mathrm{cm}}^2/\mathrm{V}\mathrm{s}$ at 80 K. At 60 K the two-probe mobility drops abruptly by about 50%, but the mobility drop is mostly attributed to the increase of the source resistance. The carrier transport in the present single-crystal film is regarded as essentially bandlike down to 30 K.

Recent advances in micro- and nanotechnology have enabled the development of ultrasensitive sensors capable of detecting small numbers of species. In general, however, the response induced by the random adsorption of a small number of objects onto the surface of such sensors results in significant fluctuations due to the heterogeneous sensitivity inherent to many such sensors coupled to statistical fluctuations in the particle number. At present, this issue is addressed by considering either the limit of very large numbers of analytes, where fluctuations vanish, or the converse limit, where the sensor response is governed by individual analytes. Many cases of practical interest, however, fall between these two limits and remain challenging to analyze. Here, we address this limitation by deriving a general theoretical framework for quantifying measurement variations on mechanical resonators resulting from statistical-number fluctuations of analyte species. Our results provide insights into the stochastic processes in the sensing environment and offer opportunities to improve the performance of mechanical-resonator-based sensors. This metric can be used, among others, to aid in the design of robust sensor platforms to reach ultrahigh-resolution measurements using an array of sensors. These concepts, illustrated here in the context of biosensing, are general and can therefore be adapted and extended to other sensors with heterogeneous sensitivity.

The state of polarization (SOP) is an inherent property of the vectorial nature of light and a crucial parameter in a wide range of remote sensing applications. Nevertheless, the SOP is rather cumbersome to probe experimentally, as conventional detectors respond only to the intensity of the light, hence losing the phase information between orthogonal vector components. In this work, we propose a type of polarimeter that is compact and well suited for in-plane optical circuitry while allowing for immediate determination of the SOP through simultaneous retrieval of the associated Stokes parameters. The polarimeter is based on plasmonic phase-gradient birefringent metasurfaces that facilitate normal incident light to launch in-plane photonic-waveguide modes propagating in six predefined directions with the coupling efficiencies providing a direct measure of the incident SOP. The functionality and accuracy of the polarimeter, which essentially is an all-polarization-sensitive waveguide metacoupler, is confirmed through full-wave simulations at the operation wavelength of $1.55\mu \mathrm{m}$.

Permanent-magnet systems are widely used for generation of magnetic fields with specific properties. The reciprocity theorem, an energy-equivalence principle in magnetostatics, can be employed to calculate the optimal remanent flux density of the permanent-magnet system, given any objective functional that is linear in the magnetic field. This approach, however, yields a continuously varying remanent flux density, while in practical applications, magnetic assemblies are realized by combining uniformly magnetized segments. The problem of determining the optimal shape of each of these segments remains unsolved. We show that the problem of optimal segmentation of a two-dimensional permanent-magnet assembly with respect to a linear objective functional can be reduced to the problem of piecewise linear approximation of a plane curve by perimeter maximization. Once the problem has been cast into this form, the globally optimal solution can be easily computed employing dynamic programming.

We analytically derive a decoherence model for orbital angular momentum states of a photon in a multimode optical fiber and show that the rate of decoherence scales approximately exponentially with $l^2$, where $l$ is the azimuthal mode order. We also show numerically that for large values of $l$ the orbital angular momentum photon state completely dephases. However, for lower values of $l$ the decoherence can be minimized by using dynamical decoupling to allow for qudit high-bandwidth quantum communication and similar applications.

When an acoustic wave passes through short narrow channels in a regenerator having an axial temperature difference, the acoustic power is amplified for the waves going from cold to hot, whereas it is damped for the waves going in the opposite direction. This study applies such asymmetric wave propagation to demonstrate a thermoacoustic diode, which plays the role of the acoustic counterpart to an optical isolator. Four regenerators having the same longitudinal temperature difference are aligned in series to make four-stage amplification and damping of the acoustic power possible. This alignment leads to the enlarged difference between the acoustic power gains in the forward and backward propagation directions, even with a moderate temperature difference. Furthermore, by introducing the acoustical impedance-matching unit, the power-reflection coefficient is kept as low as 0.017 in forward propagation. The results show that the power-transmission coefficients in the forward and backward directions, respectively, reach 0.98 and 0.023, which means that the power-transmission ratio is 16 dB.

Electrorheological fluids offer potential for developing rapidly actuated hydraulic devices where shear forces or pressure-driven flow are present. In this study, the Bingham yield stress of electrorheological fluids with different particle volume fractions is investigated experimentally in wall-driven and pressure-driven flow modes using measurements in a parallel-plate rheometer and a microfluidic channel, respectively. A modified Krieger-Dougherty model can be used to describe the effects of the particle volume fraction on the yield stress and is in good agreement with the viscometric data. However, significant yield hardening in pressure-driven channel flow is observed and attributed to an increase and eventual saturation of the particle volume fraction in the channel. A phenomenological physical model linking the densification and consequent microstructure to the ratio of the particle aggregation time scale compared to the convective time scale is presented and used to predict the enhancement in yield stress in channel flow, enabling us to reconcile discrepancies in the literature between wall-driven and pressure-driven flows.

Optical limiters transmit low-intensity light, while blocking laser radiation with excessively high irradiance or fluence. A typical optical limiter involves a nonlinear material which is transparent at low light intensity and becomes opaque when the light intensity exceeds a certain level. Most of the high-level radiation is absorbed by the nonlinear material causing irreversible damage. This fundamental problem could be solved if the state of the nonlinear material changed from transparent to highly reflective (not absorptive) when the intensity becomes too high. None of the known nonlinear optical materials display such a property. A solution can be provided by a nonlinear photonic structure. In this communication, we report the experimental realization of a reflective optical limiter. The design is based on a planar microcavity composed of alternating ${\mathrm{SiO}}_2$ and ${\mathrm{Si}}_3{\mathrm{N}}_4$ layers with a single GaAs defect layer in the middle. At low intensity, the planar microcavity displays a strong resonant transmission via a cavity mode. As the intensity increases, two-photon absorption in GaAs kicks in, initially resulting in the microcavity-enhanced light absorption. A further increase in light intensity, though, suppresses the cavity mode along with the resonant transmission; the entire planar microcavity turns highly reflective within a broad frequency range covering the entire photonic band gap. This seemingly counterintuitive behavior is a general feature of resonant transmission via a cavity mode with purely nonlinear absorption.

The external field control of antiferromagnetism is a significant subject both for basic science and technological applications. As a useful macroscopic response to detect magnetic states, the anomalous Hall effect (AHE) is known for ferromagnets, but it has never been observed in antiferromagnets until the recent discovery in ${\mathrm{Mn}}_3\mathrm{Sn}$. Here we report another example of the AHE in a related antiferromagnet, namely, in the hexagonal chiral antiferromagnet ${\mathrm{Mn}}_3\mathrm{Ge}$. Our single-crystal study reveals that ${\mathrm{Mn}}_3\mathrm{Ge}$ exhibits a giant anomalous Hall conductivity $|{\sigma }_{xz}|\sim 60{\mathrm{\Omega }}^{-1}{\mathrm{cm}}^{-1}$ at room temperature and approximately $380{\mathrm{\Omega }}^{-1}{\mathrm{cm}}^{-1}$ at 5 K in zero field, reaching nearly half of the value expected for the quantum Hall effect per atomic layer with Chern number of unity. Our detailed analyses on the anisotropic Hall conductivity indicate that in comparison with the in-plane-field components $|{\sigma }_{xz}|$ and $|{\sigma }_{zy}|$, which are very large and nearly comparable in size, we find $|{\sigma }_{yx}|$ obtained in the field along the $c$ axis to be much smaller. The anomalous Hall effect shows a sign reversal with the rotation of a small magnetic field less than 0.1 T. The soft response of the AHE to magnetic field should be useful for applications, for example, to develop switching and memory devices based on antiferromagnets.

We present an alternative version of transformation optics that allows us to mold the flow of light without rotating or scaling the electromagnetic fields. The resulting media experience unusual force densities, are nonreciprocal, and exhibit loss or gain. Because of these singular features, a variety of effects and devices unreachable by standard transformation optics can be achieved, including reflectionless light compression, optical modes with arbitrary in-plane polarization, and special isolators.

Increasing the magnetic data recording density requires reducing the size of the individual memory elements of a recording layer as well as employing magnetic materials with temperature-dependent functionalities. Therefore, we predict that the near future of magnetic data storage technology involves a combination of energy-assisted recording on nanometer-scale magnetic media. We present the potential of heat-assisted magnetic recording on a patterned sample; a ferrimagnetic alloy composed of a rare-earth and a transition metal ${\text{DyCo}}_5$, which is grown on a hexagonal-ordered nanohole array membrane. The magnetization of the antidot array sample is out-of-plane oriented at room temperature and rotates towards in plane upon heating above its magnetic anisotropy reorientation temperature ($T_R$) of 350 K, just above room temperature. Upon cooling back to room temperature (below $T_R$), we observe a well-defined and unexpected in-plane magnetic domain configuration modulating with 45 nm. We discuss the underlying mechanisms giving rise to this behavior by comparing the magnetic properties of the patterned sample with the ones of its extended thin-film counterpart. Our results pave the way for future applications of ferrimagnetic antidot arrays of superior functionality in magnetic nanodevices near room temperature.

Based on model calculations, we predict a magnetoelectric tunneling electroresistance effect in multiferroic tunnel junctions consisting of ferromagnetic electrodes and magnetoelectric antiferromagnetic barriers. Switching of the antiferromagnetic order parameter in the barrier in applied electric field by means of the magnetoelectric coupling leads to a substantial change of the resistance of the junction. The effect is explained in terms of the switching of the orientations of local magnetizations at the barrier interfaces affecting the spin-dependent interface transmission probabilities. Magnetoelectric multiferroic materials with finite ferroelectric polarization exhibit an enhanced resistive change due to polarization-induced spin-dependent screening. These results suggest that devices with active barriers based on single-phase magnetoelectric antiferromagnets represent an alternative nonvolatile memory concept.

We develop a theoretical formalism for the description of the interaction of microwave photons with a thin (compared to the photon wavelength) magnetic metasurface comprised of dipolarly interacting nanoscale magnetic elements. We derive a scattering matrix describing the processes of photon transmission and reflection at the metasurface boundary. As an example of the use of the developed formalism, we demonstrate that the introduction of a magnetic metasurface inside a microstrip electromagnetic waveguide quantitatively changes the dispersion relation of the fundamental waveguide mode, opening a nonpropagation frequency band gap in the waveguide spectrum. The frequency position and the width of the band gap are dependent on the waveguide thickness and can be controlled dynamically by switching the magnetic ground state of the metasurface. For sufficiently thin waveguides, the position of the band gap is shifted from the resonance absorption frequency of the metasurface. In such a case, the magnetic metasurface inside a waveguide works as an efficient reflector, as the energy absorption in the metasurface is small, and most of the electromagnetic energy inside the nonpropagation band gap is reflected.

The layered semiconductor black phosphorus has attracted attention as a 2D atomic crystal that can be prepared in ultrathin layers for operation as field-effect transistors. Despite the susceptibility of black phosphorus to photo-oxidation, improvements to the electronic quality of black phosphorus devices has culminated in the observation of the quantum Hall effect. In this paper, we demonstrate the room-temperature operation of a dual-gated black phosphorus transistor operating as a velocity-modulated transistor, whereby modification of hole density distribution within a black phosphorus quantum well leads to a twofold modulation of hole mobility. Simultaneous modulation of Schottky-barrier resistance leads to a fourfold modulation of transconductance at a fixed hole density. Our work explicitly demonstrates the critical role of charge-density distribution upon charge carrier transport within 2D atomic crystals.

The propagation of domain walls in a ferromagnetic film is largely determined by domain-wall pinning at defects in the material. In this article, we map the effective potential landscape for domain-wall pinning in permalloy films by raster scanning a single ferromagnetic vortex and monitoring the hysteretic vortex displacement vs applied magnetic field. The measurement is carried out using a differential magneto-optical microscopy technique which yields spatial sensitivity of approximately 10 nm. We present a simple algorithm for extracting an effective pinning potential from the measurement of vortex displacement vs applied field. The resulting maps of the pinning potential reveal distinct types of pinning sites, which we attribute to quasi-zero-, one-, and two-dimensional defects in the permalloy film.