Designing a permanent magnet with reduced critical rare-earth content is of paramount importance in the development of cost-effective modern technologies. By performing comprehensive first-principles calculations, we investigate the potential avenues for reducing the critical rare-earth content in ${\mathrm{Sm}}_2{\mathrm{Fe}}_{17}{\mathrm{N}}_3$ and ${\mathrm{Sm}}_2{\mathrm{Fe}}_{17}{\mathrm{C}}_3$ by making a La or Ce substitution for Sm. The calculated magnetic properties of base compounds are in good agreement with the previous low-temperature (4.2-K) experimental measurements, and they show a large axial anisotropy. Although La or Ce substitution results in a slight reduction of magnetic anisotropy, the magnetic moments of Fe atoms mostly remain unchanged. Specifically, large axial anisotropies of 7.2 and $4.1\mathrm{MJ}/{\mathrm{m}}^3$ are obtained for ${\mathrm{SmCeFe}}_{17}{\mathrm{N}}_3$ and ${\mathrm{SmLaFe}}_{17}{\mathrm{N}}_3$, respectively. These values of anisotropies are comparable to the state-of-the-art permanent magnet ${\mathrm{Nd}}_2{\mathrm{Fe}}_{14}\mathrm{B}$. The foremost limitation of ${\mathrm{Sm}}_2{\mathrm{Fe}}_{17}{X}_3$ magnets for practical application is the formation nitrogen or carbon vacancies at high temperatures. By calculating the N- (C)- vacancy formation energy, we show that La or Ce substitution enhances the vacancy formation energy. This enhanced vacancy formation energy will likely improve the thermodynamic stability of these alloys at high temperatures. Therefore, La- or Ce-substituted ${\mathrm{Sm}}_2{\mathrm{Fe}}_{17}{\mathrm{C}}_3$ and ${\mathrm{Sm}}_2{\mathrm{Fe}}_{17}{\mathrm{N}}_3$ compounds are promising candidates for high-performance permanent magnets with substantially reduced rare-earth content.

Defects play an important role in the performance of organic solar cells. The investigation of trap states and their origin can provide ways to further improve their performance. Here, we investigate defects in a system composed of the small-molecule oligothiophene derivative DCV5T-Me blended with ${\mathrm{C}}_{60}$, which shows power conversion efficiencies above 8% when used in a solar cell. From a reconstruction of the density of trap states by impedance spectroscopy, we obtain a Gaussian distribution of trap states with $E_t=470\mathrm{meV}$ below the electron transport level, $N_t=8\times {10}^{14}{\mathrm{cm}}^{-3}$, and ${\sigma }_t=41\mathrm{meV}$. From $V_{\mathrm{oc}}$ vs illumination intensity and open-circuit corrected charge carrier extraction measurements, we find that these defects lead to trap-assisted recombination. Moreover, drift-diffusion simulations show that the trap states decrease the fill factor by 10%. By conducting degradation measurements and varying the blend ratio, we find that the observed trap states are structural defects in the ${\mathrm{C}}_{60}$ phase due to the distortion of the natural morphology induced by the mixing.

In this work, we propose a multiband linear-polarization (LP) conversion and circular polarization (CP) maintaining reflector using a symmetric anisotropic metasurface. The anisotropic metasurface is composed of a square array of a two-corner-cut square multiring disk printed on a grounded dielectric substrate, which is a symmetric structure with a pair of mutually perpendicular symmetric axes $u$ and $v$ along the $\pm 45°$ directions with respect to the $y$-axis direction. The simulated results show that the reflector can realize LP conversion in five frequency bands at both $x$- and $y$-polarized incidence, the first four bands all have a certain bandwidth, and the fourth one, especially, is an ultrawideband. In addition, because of the symmetry of the reflector structure, the polarization state of a CP wave can be maintained after reflection, and the magnitude of the copolarized reflection coefficient at the CP incidence is just equal to that of the cross-polarized reflection coefficient at the $x$- and $y$-polarized incidence. We analyze the root cause of the multiband LP conversion and CP maintaining reflection, and carry out one experiment to verify the proposed reflector.

Charge carrier transport in thin hydrogen (H)-terminated Si(001) sheets is explored via a four-probe device fabricated on silicon-on-insulator (SOI) using the bulk host Si as a back gate. The method enables electrical measurements without the need to contact the sample surface proper. Sheet conductance measurements as a function of back-gate voltage demonstrate the presence of acceptor- and donorlike surface states. These states are distributed throughout the gap and can be removed or transformed with low-temperature annealing. The density of donorlike states is just under ${10}^{12}{\mathrm{cm}}^{-2}{\mathrm{eV}}^{-1}$ and 3 times higher than that of the acceptorlike states. We discuss the possible origins of these states. The conductance through the surface layer is too small to measure.

We investigate a passive noise-control approach to engineering the two-phase flow in a microfluidic coflow system. The presence or absence of the jet breakup is studied for two immiscible oil phases, in a straight microchannel (referred to as the $J$ device in the main text), an expansion microchannel (the $W$ device) and a microchannel with the expansion-contraction geometry (the $S$ device), respectively. We show that the jet breaks into droplets, in the jetting regime and the dripping regime (also referred to as the widening-jetting regime) for the straight channel and expansion channel, respectively, while a stable long jet does not break for the expansion-contraction geometry. As the inner phase passes the expansion-contraction functional unit, the random noise on the interface is significantly reduced and the hydrodynamic instability is suppressed, for a range of experimental parameters including flow rates, device geometry, liquid viscosity, and interfacial tension. We further present scale-up devices with multiple noise-control units and achieve decimeter-long yet stable jets. Our simple, effective, and robust noise-control approach can benefit microfluidic applications such as microfiber fabrication, interface chemical reaction, and on-chip distance transportation.

Thermophoresis is the motion of micro- or nanoparticles driven by the temperature gradient of the surrounding fluid. We develop an on-chip microfluidic device to induce the thermophoresis of microparticles. It is clarified that both the bulk material and the chemical modification to the surface affect the thermophoresis of particles in an aqueous solution, even regarding the direction of particle motion. Polystyrene (PS) beads with a diameter of $1\mu \mathrm{m}$ show a thermophobic response to a temperature gradient induced on the developed microfluidic device and move toward the colder region. The interplay between a uniform flow in the microchannel and thermophoresis leads to a dynamic pattern formation of the PS beads in the upstream region. Thus, a purified solution can be obtained in the downstream region, demonstrating the potential of thermophoresis as a microparticle filter. Furthermore, silica beads with a diameter of $1\mu \mathrm{m}$ move toward the hotter region on the device. Therefore, the PS and silica beads show the opposite response to the temperature gradient despite having the same diameter and surface modification. The thermophoretic separation of a mixture of the PS and silica beads is demonstrated.

We present a theory for ion and water transport through reverse-osmosis (RO) membranes based on a Maxwell-Stefan framework combined with hydrodynamic theory for the reduced motion of particles in thin pores. We take into account all driving forces and frictions both on the fluid (water) and on the ions including ion-fluid friction and ion-wall friction. By including the acid-base characteristic of the carbonic acid system, the boric acid system, ${\mathrm{H}}_3{\mathrm{O}}^{+}/{\mathrm{OH}}^{-}$, and the membrane charge, we locally determine $p\mathrm{H}$, the effective charge of the membrane, and the dissociation degree of carbonic acid and boric acid. We present calculation results for an experiment with fixed feed concentration, where effluent composition is a self-consistent function of fluxes through the membrane. A comparison with experimental results from literature for fluid flow vs pressure, and for salt and boron rejection, shows that our theory agrees very well with the available data. Our model is based on realistic assumptions for the effective size of the ions and makes use of a typical pore size of a commercial RO membrane.

Autonomous wireless body-implanted devices for biotelemetry, telemedicine, and neural interfacing constitute an emerging technology providing powerful capabilities for medicine and clinical research. We study the through-tissue electromagnetic propagation mechanisms, derive the optimal frequency range, and obtain the maximum achievable efficiency for radiative energy transfer from inside a body to free space. We analyze how polarization affects the efficiency by exciting TM and TE modes using a magnetic dipole and a magnetic current source, respectively. Four problem formulations are considered with increasing complexity and realism of anatomy. The results indicate that the optimal operating frequency $f$ for deep implantation (with a depth $d\gtrsim 3\mathrm{cm}$) lies in the (${10}^8–{10}^9$)-Hz range and can be approximated as $f=2.2\times {10}^7/d$. For a subcutaneous case ($d\lesssim 3\mathrm{cm}$), the surface-wave-induced interference is significant: within the range of peak radiation efficiency (about $2\times {10}^8$ to $3\times {10}^9\mathrm{Hz}$), the max-to-min ratio can reach a value of 6.5. For the studied frequency range, 80%–99% of radiation efficiency is lost due to the tissue-air wave-impedance mismatch. Parallel polarization reduces the losses by a few percent; this effect is inversely proportional to the frequency and depth. Considering the implantation depth, the operating frequency, the polarization, and the directivity, we show that about an order-of-magnitude efficiency improvement is achievable compared to existing devices.

The surface energy of a magnetic domain wall (DW) strongly affects its static and dynamic behaviors. However, this effect is seldom directly observed, and some of the related phenomena are not well understood. Moreover, a reliable method to quantify the DW surface energy is still absent. Here, we report a series of experiments in which the DW surface energy becomes a dominant parameter. We observe that a semicircular magnetic domain bubble can spontaneously collapse under the Laplace pressure induced by DW surface energy. We further demonstrate that the surface energy can lead to a geometrically induced pinning when the DW propagates in a Hall cross or from a nanowire into a nucleation pad. Based on these observations, we develop two methods to quantify the DW surface energy, which can be very helpful in the estimation of intrinsic parameters such as Dzyaloshinskii-Moriya interactions or exchange stiffness in magnetic ultrathin films.

In photoinduced force microscopy (PIFM), amplitude modulation techniques, such as direct mode, and heterodyne amplitude modulation techniques have been used to detect the photoinduced force. These amplitude modulation techniques are affected by other forces because the resonance frequency shifts and nonconservative force damp the cantilever motion. Here, we investigate and propose the heterodyne-frequency-modulation (heterodyne-FM) technique for inhibiting the influence of the other forces and photothermal force. Heterodyne-FM PIFM enables the acquisition of PIFM images and force spectra without those artifacts.

Dissipative solitons formed in Kerr microresonators may enable chip-scale frequency combs for precision optical metrology. Here, we explore the creation of an octave-spanning, 15-GHz repetition-rate microcomb suitable for both $f\text{−}2f$ self-referencing and optical-frequency comparisons across the near infrared. This is achieved through a simple and reliable approach to deterministically generate, and subsequently frequency stabilize, soliton pulse trains in a silica-disk resonator. Efficient silicon-nitride waveguides provide a supercontinuum spanning 700 to 2100 nm, enabling both offset-frequency stabilization and optical-frequency measurements with $>100\mathrm{nW}$ per mode. We demonstrate the stabilized comb by performing a microcomb-mediated comparison of two ultrastable optical-reference cavities.

Conventional magnonic devices use magnetostatic waves whose properties are sensitive to device geometry and the details of magnetization structure, so the design and the scalability of the device or circuitry are difficult. We propose topological magnonics, in which topological exchange spin waves are used as information carriers, that do not suffer from conventional problems of magnonic devices with additional nice features of nanoscale wavelength and high frequency. We show that a perpendicularly magnetized ferromagnet on a honeycomb lattice is generically a topological magnetic material in the sense that topologically protected chiral edge spin waves exist in the band gap as long as a spin-orbit-induced nearest-neighbor pseudodipolar interaction (and/or a next-nearest-neighbor Dzyaloshinskii-Moriya interaction) is present. The edge spin waves propagate unidirectionally along sample edges and domain walls regardless of the system geometry and defects. As a proof of concept, spin-wave diodes, spin-wave beam splitters, and spin-wave interferometers are designed by using sample edges and domain walls to manipulate the propagation of topologically protected chiral spin waves. Since magnetic domain walls can be controlled by magnetic fields or electric current or fields, one can essentially draw, erase, and redraw different spin-wave devices and circuitry on the same magnetic plate so that the proposed devices are reconfigurable and tunable. The topological magnonics opens up an alternative direction towards a robust, reconfigurable and scalable spin-wave circuitry.

We study experimentally the displacement of one fluid by another in a granular pack to uncover relationships between fluid invasion and medium deformation. We develop an experimental setup that allows us to reconstruct the coupled invasion-deformation dynamics in 3D. We simultaneously characterize the fluid invasion pattern and document a transition from fluid-fluid displacement in pores to the formation of conduits by grain motion. We rationalize the findings in terms of a simple poromechanics model that indeed captures this transition as a result of the balance between viscous and frictional forces. These results contribute to elucidating the role of three dimensionality in the timing, mode, and morphology of fluid-fluid displacement and injection-induced deformation in porous media.

We report an experimental observation of the midinfrared surface plasmon excited in a carbon nanotube plasmonic metasurface. The absorption of a 400-nm-thick single-walled carbon nanotube film perforated with laser-drilled subwavelength holes arranged in a 2D lattice is resonantly enhanced by 75% as compared with the unstructured film. The enhancement of absorption has a resonant behavior associated with the excitation of the surface plasmon and occurs at the wavelengths around $15\mu \mathrm{m}$ for the lattice period of $10\mu \mathrm{m}$. The spectral position and the magnitude of the resonance are controlled entirely by the structure geometry and can be tuned in a broad range. We demonstrate that periodic patterning can be applied to tailor the bolometric performance of carbon nanotube thin films. Namely, the voltage response of the metasurface is enhanced by 100% at the wavelength of the plasmon resonance as compared with the unstructured film. We discuss mechanisms of the enhancement and compare experimental results with the finite-difference time-domain and scattering-matrix method simulations.

We develop a time-resolved microscopic gate-modulation ($\mu \mathrm{GM}$) imaging technique to investigate the temporal evolution of the channel current and accumulated charges in polycrystalline pentacene thin-film transistors (TFTs). A time resolution of as high as 50 ns is achieved by using a fast image-intensifier system that could amplify a series of instantaneous optical microscopic images acquired at various time intervals after the stepped gate bias is switched on. The differential images obtained by subtracting the gate-off image allows us to acquire a series of temporal $\mu \mathrm{GM}$ images that clearly show the gradual propagation of both channel charges and leaked gate fields within the polycrystalline channel layers. The frontal positions for the propagations of both channel charges and leaked gate fields coincide at all the time intervals, demonstrating that the layered gate dielectric capacitors are successively transversely charged up along the direction of current propagation. The initial $\mu \mathrm{GM}$ images also indicate that the electric field effect is originally concentrated around a limited area with a width of a few micrometers bordering the channel-electrode interface, and that the field intensity reaches a maximum after 200 ns and then decays. The time required for charge propagation over the whole channel region with a length of $100\mu \mathrm{m}$ is estimated at about 900 ns, which is consistent with the measured field-effect mobility and the temporal-response model for organic TFTs. The effect of grain boundaries can be also visualized by comparison of the $\mu \mathrm{GM}$ images for the transient and the steady states, which confirms that the potential barriers at the grain boundaries cause the transient shift in the accumulated charges or the transient accumulation of additional charges around the grain boundaries.

Sequencing DNA modifications and lesions, such as methylation of cytosine and oxidation of guanine, is even more important and challenging than sequencing the genome itself. The traditional methods for detecting DNA modifications are either insensitive to these modifications or require additional processing steps to identify a particular type of modification. Transverse-current sequencing in nanopores can potentially identify the canonical bases and base modifications in the same run. In this work, we demonstrate that the most common DNA epigenetic modifications and lesions can be detected with any predefined accuracy based on their tunneling current signature. Our results are based on simulations of the nanopore tunneling current through DNA molecules, calculated using nonequilibrium electron-transport methodology within an effective multiorbital model derived from first-principles calculations, followed by a base-calling algorithm accounting for neighbor current-current correlations. This methodology can be integrated with existing experimental techniques to improve base-calling fidelity.

We report on the synthesis of single-crystal and polycrystalline samples of ${\mathrm{Ce}}_{3-x}{\mathrm{Mg}}_x{\mathrm{Co}}_9$ solid solution ($0\le x\lesssim 1.4$) and characterization of their structural and magnetic properties. The crystal structure remains rhombohedral in the whole composition range and Mg partially replaces Ce in the $6c$ site of the ${\mathrm{CeCo}}_3$ structure. Ferromagnetism is induced by Mg substitutions starting as low as $x=0.18$ and reaching a Curie temperature as high as 450 K for $x=1.35$. Measurements on single crystals with $x=1.34$ and $T_C=440\mathrm{K}$ indicate an axial magnetic anisotropy with an anisotropy field of 6 T and a magnetization of $6{\mu }_B/\mathrm{f}.\mathrm{u}.$ at 300 K. Coercicity is observed in the polycrystalline samples consistent with the observed axial magnetic anisotropy. Our discovery of ferromagnetism with large axial magnetic anisotropy induced by substituting a rare-earth element by Mg is a very promising result in the search of inexpensive permanent-magnet materials and suggests that other nonmagnetic phases, similar to ${\mathrm{CeCo}}_3$, may also conceal nearby ferromagnetic phases.

A metal-dielectric topological-insulator capacitor device based on hexagonal-boron-nitrate- ($h$-BN) encapsulated CVD-grown ${\mathrm{Bi}}_2{\mathrm{Se}}_3$ is realized and investigated in the radio-frequency regime. The rf quantum capacitance and device resistance are extracted for frequencies as high as 10 GHz and studied as a function of the applied gate voltage. The superior quality $h$-BN gate dielectric combined with the optimized transport characteristics of CVD-grown ${\mathrm{Bi}}_2{\mathrm{Se}}_3$ ($n\sim {10}^{18}{\mathrm{cm}}^{-3}$ in 8 nm) on $h$-BN allow us to attain a bulk depleted regime by dielectric gating. A quantum-capacitance minimum and a linear variation of the capacitance with the chemical potential are observed revealing a Dirac regime. The topological surface state in proximity to the gate is seen to reach charge neutrality, but the bottom surface state remains charged and capacitively coupled to the top via the insulating bulk. Our work paves the way toward implementation of topological materials in rf devices.

We present a method to design an initial state in a quantum antenna in order to shape the emitted field higher-order correlation functions at will. This method is based on quantum state reconstruction techniques and relies on an entanglement of the emitters. We show that even the simplest antenna arrangements such as linear dipole arrays can exhibit a large variability in the emitted field-correlation function patterns, including, e.g., the generation of highly codirectional and contradirectional correlated twin photons, as well as multiphoton entangled states. Moreover, we identify a class of initial states that lead to a complete suppression of the field in the far-field zone. We also demonstrate the possibility to use a modified semiclassical approach for designing quantum antennas, simplifying the antenna state inference task. Our approach can find applications in the development of future quantum optics devices and methods, such as quantum sources for superresolution quantum imaging, high-precision sensing, as well as emitter-field interfaces for quantum information processing systems.

Kirigami, a modified form of origami which includes cutting, has been used to improve material stretchability and compliance. However, this technique is, so far, underexplored in patterning piezoelectric materials towards developing efficient and mechanically flexible thin-film energy generators. Motivated by existing kirigami-based applications, we introduce interdigitated cuts to polyvinylidene fluoride (PVDF) films to evaluate the effect on voltage generation and stretchability. Our results from theoretical analysis, numerical simulations, and experimental tests show that kirigami PVDF films exhibit an extended strain range while still maintaining significant voltage generation compared to films without cuts. Various cutting patterns are studied, and it is found that films with denser cuts have a larger voltage output. This kirigami design can enhance the properties of existing piezoelectric materials and help to integrate tunable PVDF generators into biomedical devices.