We report on the anisotropy of the London penetration depth of ${\mathrm{Ca}\mathrm{K}\mathrm{Fe}}_4{\mathrm{As}}_4$, discussing how it relates to its electronic structure and how it modifies under introduction of disorder, both chemically induced (by $\mathrm{Ni}$ substitution) and irradiation induced (by 3.5-MeV protons). Indeed, ${\mathrm{Ca}\mathrm{K}\mathrm{Fe}}_4{\mathrm{As}}_4$ is particularly suitable for the study of fundamental superconducting properties due to its stoichiometric composition, exhibiting clean-limit behavior in the pristine samples and having a fairly high critical temperature, $T_c\approx 35$ K. The London penetration depth ${\lambda }_L$ is measured with a microwave-coplanar-resonator technique that allows us to deconvolve the anisotropic contributions ${\lambda }_{L,ab}$ and ${\lambda }_{L,c}$ and obtain the anisotropy parameter ${\gamma }_{\lambda }={\lambda }_{L,c}/{\lambda }_{L,ab}$. The ${\gamma }_{\lambda }(T)$ found for the undoped pristine sample is in good agreement with previous literature and is here compared to ab initio density-functional-theory and Eliashberg calculations. The dependence of ${\gamma }_{\lambda }(T)$ on both chemical and irradiation-induced disorder is discussed to highlight which method is more suitable to decrease the direction dependence of the electromagnetic properties while maintaining a high critical temperature. Lastly, the relevance of an intrinsic anisotropy such as ${\gamma }_{\lambda }$ on application-related anisotropic parameters (critical current, pinning) is discussed in light of the recent employment of ${\mathrm{Ca}\mathrm{K}\mathrm{Fe}}_4{\mathrm{As}}_4$ in the production of wires.

Interest in atomic scale structures of individual specimens has invigorated developments of high-resolution probes, which include single-particle imaging using x-ray free-electron lasers (XFELs). The demonstrated spatial resolution, however, remains at tens of nanometers with difficulty in collecting diffraction signals at high frequency distinguished from noises. As such, various resolution-enhancement methods have been introduced, but few experimental verifications are available. Here, by carrying out XFEL single-pulse diffraction experiments, we explicitly unveil the dependence of SNRs on incident x-ray flux, data averaging, or multiparticle interference. We further propose a data-accumulation method of resolution-shell averaging as a robust scheme to improve the SNR. This study establishes a roadmap with which high-resolution XFEL single-pulse experiments can be contrived.

Ghost imaging (GI) enables reconstruction of the image of an object by measuring the intensity correlation of two beams, neither of which independently carries the spatial characteristics of the object. By exploiting the space-time duality, GI has been extended from the spatial domain to the time domain, and has been successfully demonstrated in the optical regime. This article reports on the experimental demonstration of acoustic GI in the time domain. Using ultrasound instead of photons, we created high-quality ghost images of a temporal object, even in the presence of strong ambient noise. Our work offers a fresh approach for imaging disturbance-sensitive acoustic signals, and could open the gateway to identifying new approaches in acoustic sensing, dynamic imaging, and communications.

The spin-current efficiency in ferromagnet (FM)/nonmagnetic metal (NM) heterostructures is a key issue for many subcategories of spintronics, and it is determined by both the spin Hall angle $({\theta }_{\mathrm{SH}})$ of the NM and the spin-mixing conductance $(g_{\uparrow \downarrow })$ related to the constituents and, in particular, to the interface. Here, we study spin transport in the ${\mathrm{Y}}_3{\mathrm{Fe}}_5{\mathrm{O}}_{12}(\mathrm{YIG})$/${\mathrm{W}}_{1\text{−}x}{\mathrm{Cr}}_x$ heterostructure over the full composition by means of the spin Seebeck effect and spin pumping. An appreciably enhanced inverse spin Hall voltage is observed for the substitutional alloy of bcc-$\mathrm{W}$ and $\mathrm{Cr}$, with the maximum obtained at an equiatomic composition. We find that the ${\theta }_{\mathrm{SH}}$ of bcc-${\mathrm{W}}_{0.5}{\mathrm{Cr}}_{0.5}$ is 1.3 times as large as that of β-$\mathrm{W}$, which mainly originates from intensively disordered atomic scattering. More significantly, $\mathrm{YIG}$/bcc-${\mathrm{W}}_{0.5}{\mathrm{Cr}}_{0.5}$ has a $g_{\uparrow \downarrow }$ value of $1.42\times {10}^{18}{\mathrm{m}}^{-2}$, which is more than twice that for $\mathrm{YIG}$/β-$\mathrm{W}$ ($5.98\times {10}^{17}{\mathrm{m}}^{-2}$). Meanwhile, a sizable interfacial electric field is identified in $\mathrm{YIG}$/β-$\mathrm{W}$, but not in $\mathrm{YIG}$/bcc-${\mathrm{W}}_{0.5}{\mathrm{Cr}}_{0.5}$, showing strong correlation between $g_{\uparrow \downarrow }$ and the interfacial electric field. This work not only provides a readily achievable alloy with ${\theta }_{\mathrm{SH}}$ exceeding that of the metastable β-$\mathrm{W}$, but also implies the possibility of manipulating $g_{\uparrow \downarrow }$ via Rashba-type spin-orbit coupling by engineering an interfacial electric field in insulating FM/NM heterostructures.

Reciprocity is a fundamental property of wave propagation in linear time-invariant media. However, it is substantially harder to break reciprocity for elastic waves in a linear elastic system with the help of a magnetostatic field, since elastic waves are spinless in nature. Here, we realize nonreciprocal elastic waves in one-dimensional linear magnetoelastic phononic crystal slabs by an external magnetostatic field. The asymmetric dispersion of spin waves, which is induced by the simultaneous breaking of the time-reversal and spatial-inversion symmetries, results in an efficient elastic wave diode effect in a periodic system due to magnetoelastic interactions. Remarkably, a bidirectional nonreciprocity of elastic waves is achieved based on the folding-back effect of the periodic structure. Our proposed scheme opens up avenues for the design of nonreciprocal devices, such as on-chip tunable diodes, rectifiers, and topological insulators.

Spatial-polarization-independent (SPI) parametric conversion is the basis of many optical applications, such as SPI frequency interfaces for communication channels carried by vector modes and up-conversion detection for polarization-resolved imaging. However, realizing such conversion remains a challenge. In the proof-of-principle study presented here, we demonstrate SPI parametric up-conversion using a polarization Sagnac nonlinear interferometer based on type-II second-harmonic generation. Our results show that the vector profile (including both polarization and intensity) and associated spin-orbit-coupling state of the vector signal beam can be transferred to the frequency-doubled beam with high fidelity. The principle lays the foundation for a SPI frequency interface for quantum and classical channels based on vector modes and also paves the way for up-conversion detection of polarization-resolved images in the mid- and far-infrared regions.

Bessel-Gauss beams have mainly been proposed in optics as a solution for reducing the on-axis intensity oscillations typical of Bessel beams. Previous investigations on Bessel-Gauss beams are based on a scalar theory in the paraxial approximation, and thus cannot be extended to the microwave range where a fully vectorial approach is needed. Here, the generation of Bessel-Gauss beams through leaky waves is investigated. First, the nondiffractive and focusing properties of Bessel-Gauss beams generated through leaky waves are extensively examined in the frame of a vectorial approach. Useful design criteria are derived to optimize both the radiation and the focusing efficiency of such beams. On this basis, leaky-wave radiators synthesized to support the generation of a Bessel-Gauss beam over a given frequency band in the microwave range are presented. Full-wave results corroborate the concept.

Soft tissues, such as skin, myocardium, and chordae tendineae, typically display anisotropic mechanical behavior due to their fibrous nature. In constitutive modeling, fiber families frequently are assumed to be unidirectional. Recent numerical results, however, display the need to incorporate dispersion of fiber orientation. This evidence gets supplemented by new experimental results based on high-resolution second-harmonic imaging microscopy. Generalized structure-tensor (GST) models are frequently utilized to model fiber dispersion, as they are mathematically easy to treat and demand only a little effort to implement. They can be regarded as Taylor-series expansions of the numerically more challenging angular-integration (AI) method, which encompasses a distribution of fiber orientations together with the associated fiber stress. In this work, we show how low-order GST models give rise to numerical instabilities as they show strong sensitivity with regards to the mean fiber orientation. To overcome these instabilities, we propose a different class of GST models, termed squared GST (SGST), which computes faster, is easier to implement, and converges to the AI faster than previous GST models of similar order. The SGST models promise to be adaptable to generalized problems, such as functional decomposition of fiber density as well as coupling between different fiber families. Advanced simulations with the proposed models will shed light on the complex behavior of fiber reinforced soft materials.

A laser diode subjected to optoelectronic feedback in which some light is converted to photocurrent that is fed back into the laser injection terminals can display periodic oscillations in its optical intensity. We demonstrate experimental and numerical evidence that, as the feedback level is varied, a stepwise change in the oscillation frequency manifests itself in the output optical intensity. These transitions in the dynamics can either correspond to an abrupt jump between two limit cycles, associated with a subcritical torus bifurcation of a limit cycle, or to a progressive switching mediated by an intermediate quasiperiodic state. Finally, when ramping the feedback level down, hysteresis is observed in the sequence of switching events between the limit cycles. Such devices are of interest for photonic microwave sources for wavelength-division multiplexed radio-over-fiber systems.

Power dissipation is a great challenge for the continuous scaling down and performance improvement of CMOS technology, due to the thermionic-current switching limit of conventional MOSFETs. In this paper, we show that this problem can be overcome by using a “cold” metal as the injection source of a transistor; these metals are different from conventional metals and can filter out high-energy electrons to break the “Boltzmann tyranny.” It is proved that the subthreshold swing of the thermionic current of a transistor using a “cold”-metal contact can be much smaller than 60 mV/decade at room temperature. Specifically, the two-dimensional transition-metal-dichalcogenide (TMD) “cold” metals $\mathrm{Nb}{X}_2$ and $\mathrm{Ta}{X}_2$ ($X=\mathrm{S},\mathrm{Se},\mathrm{Te}$) are proposed as injection sources for FETs. Quantum transport simulations indicate that a promising switching efficiency and on-state current can be achieved using TMD “cold”-metal injection sources, which is beneficial for energy efficiency.

Aluminum nitride ($\mathrm{Al}\mathrm{N}$) is a promising candidate for the manufacture of vacuum ultraviolet (VUV) photodetectors. However, its poor electrical conductivity limits its applications. Herein, a high-performance $\mathrm{Al}\mathrm{N}$ nanowire (NW)-$\mathrm{Ni}\mathrm{O}$ quantum dot (QD) based VUV photodetector is constructed via a mixed-dimensional strategy. The formation of isolated nanoscale p-n heterojunctions greatly increases the concentration of photogenerated electrons, resulting in an ultrahigh photoconductive gain (9.96) in the $\mathrm{Al}\mathrm{N}$ NW-$\mathrm{Ni}\mathrm{O}$ QD material, which is about 27-fold higher than that of $\mathrm{Al}\mathrm{N}$ NW (0.368). In addition, the significant improvement in both photocurrent and response speed convincingly suggest the great potential of $\mathrm{Al}\mathrm{N}$ NW-$\mathrm{Ni}\mathrm{O}$ QD based VUV photodetectors. Furthermore, the properties of these photodetectors in an irradiation environment are also evaluated here. It is found that $\mathrm{Al}\mathrm{N}$ NW exhibits excellent anti-irradiation characteristics towards both electron and proton irradiation, while the $\mathrm{Al}\mathrm{N}$ NW-$\mathrm{Ni}\mathrm{O}$ QD material also presents promising potential under low-dose electron irradiation. This study can be used as a guideline to design and fabricate high-performance VUV photodetectors based on wide-band-gap semiconductors coupled with other nanostructured systems.

The ultimate target for silicon photonics is to merge together all electronic and photonic functions on the same CMOS platform. This requires efficient light emitters and absorbers directly integrated on silicon chips, the lack of which presently forms one of the major bottlenecks for further progress. In this work, a possible solution is presented where diffusion-driven charge transport (DDCT), lateral doping of silicon, and III–V nanowire growth are combined to create fully integrated near-surface light emitters and absorbers controlled solely by biasing the underlying silicon wafer. According to the three-dimensional full-device simulations carried out in this paper, DDCT enables high efficiencies for both light emission and photodiode operation in technically feasible silicon-integrated free-standing nanowire structures. Moreover, the results indicate that DDCT is especially well suited for the commonly used 1.55 $\mu \mathrm{m}$ wavelength due to the optimal band-gap difference with silicon, which promotes both a high injection efficiency and low voltage losses.

Achieving optical operation of logic elements, especially those that involve two-dimensional (2D) layers, could kick-start the long-awaited era of optical computing. However, efficient optical modulation of the electronic properties of 2D materials, including the rewritable memory effect, is currently lacking. Here we report all-optical control of the conductivity of graphene with write-erase operation yet under ultralow optical fluence. The competition between light-induced charge generation in a ferroelectric-photovoltaic substrate and relaxation processes provides the selective photocarrier-trapping control affecting the doping of the 2D overlayer. These findings open the way to photonic control of 2D devices for all-optical modulators, a variety of all-optical logic circuits, memories, and field-effect transistors.

Commutated transmission lines have been recently explored as an interesting way to break Lorentz reciprocity, avoiding any resonant structure, and enabling broad bandwidths with giant isolation combined with lenient requirements on the modulation frequency. The scheme relies on precise synchronization among different switches connected through transmission lines, offering in principle infinite bandwidth. Here, we investigate the effects of realistic switching parameters and synchronization on the device performance, providing interesting physical insights on the operation of these devices. Our research shows that the nonreciprocal response of these systems experiences a linear regression of insertion loss and isolation with respect to the timing error among switches. Remarkably, impedance matching and nonreciprocal phase shifts are immune from synchronization issues, and reasonable levels of synchronization errors still guarantee low insertion loss and good isolation. Our study also provides practical guidelines to envision nonreciprocal devices based on commutated modulation of conductivity, opening interesting opportunities for several fields of technology, including wireless communications, quantum technologies, and photonic circuits.

On-chip channelless diffraction-free beam guiding enables dense integration of optical circuits in a reconfigurable manner, where total internal reflection, which is considered the cornerstone of guided-wave optics, is utilized to confine light in the out-of-plane direction. Here, we theoretically propose a physical mechanism to achieve on-chip channelless diffraction-free beam guiding beyond the light cone, utilizing the physics of bound states in the continuum. A bound state in the continuum with a tailored spatial dispersion plays an important role in cancelling both the in-plane diffraction and the out-of-plane scattering when the condition for total internal reflection is not satisfied. As a proof-of-concept verification, we experimentally demonstrate such an effect based on an all-dielectric platform using microwaves. The on-chip channelless diffraction-free beam guiding beyond the light cone also allows direct free-space coupling to such self-collimation modes. Our results may open up an avenue for exploring the physics and applications of guided-wave optics.

We explore an implementation of a higher-order Chern insulator in a topolectrical circuit as a platform to implement robust signal multiplexing. By utilizing basic circuit layouts that realize the required complex hopping between neighboring lattice sites, we demonstrate different topological phases in a three-dimensional topolectrical circuit, including a Wannier-based Chern insulator and a crystalline topological phase. The topological chiral hinge states supported by the circuit are nonreciprocal and avoid propagation to the hinge opposite the excitation. Such an unusual response can be explained in terms of a synthetic gauge field of the circuit lattice, enabling multiplexing inherently robust to defects.

Hydrodynamic and thermal cloaking effects have attracted interest among researchers using metamaterials. This paper explores an analytical solution using only isotropic materials to achieve cloaking under forced convective heat transfer. It is shown that, under a background of uniform flow and linearly varying temperature, a circular bilayer cloak with an inner layer made of impenetrable and insulated material and an outer layer made of a porous medium having a constant permeability and effective conductivity does not disturb the external flow and temperature fields. Moreover, the cloaked region is shielded from the external flow and temperature fields. Analytical temperature and velocity distributions within the porous outer layer are presented to demonstrate both the hydrodynamic and thermal cloaking effects of such a bilayer. Finally, numerical results, based on practically attainable material properties, are given to confirm the analytical model.

Electric field tuning of single $\mathrm{In}\mathrm{As}$ quantum dots (QDs) provides an important tool for physics experiments and for the development of scalable and in situ–tunable devices. Control of a single QD with either a growth direction or an in-plane electric field is well established, but a device that can apply the two-dimensional (2D) vector electric fields necessary to simultaneously and independently tune more than one parameter has not yet been demonstrated. We use comsol multiphysics simulations to illustrate the device-design challenges in applying a uniform 2D electric field in a $\mathrm{Ga}\mathrm{As}$ solid-state system and to systematically explore the effects of electrode coverage, mesa size, and doping on the device performance. We develop and present a design that can apply 2D electric fields to a single QD while remaining compatible with optical experiments. We then fabricate and characterize a proof-of-concept device that validates the design presented here. We discuss the applications for this device and the potential for full 2D-electric-field control of a single QD or a quantum-dot molecule.

In rare-earth permanent magnets (REPMs), trade-offs between intrinsic magnetic properties are often encountered. A recent example is ${\mathrm{Sm}\mathrm{Fe}}_{12}$ where excellent magnetic properties can be achieved at the sacrifice of bulk structure stability. Bulk structure stability is sustained by the presence of a third substitute element as is the case with ${\mathrm{Sm}\mathrm{Fe}}_{11}\mathrm{Ti}$, where $\mathrm{Ti}$ degrades magnetic properties. It is now in high demand to find out with which chemical composition a good compromise in the trade-off between structure stability and strong ferromagnetism is reached. We inspect the effects of representative substitute elements, $\mathrm{Zr}$, $\mathrm{Ti}$, and $\mathrm{Co}$ in ${\mathrm{Sm}\mathrm{Fe}}_{12}$ by combining ab initio data with experimental data from neutron diffraction. The trend in the intrinsic properties with respect to the concentration of substitute elements are monitored and a systematic way to search the best compromise is constructed. A certain minimum amount of $\mathrm{Ti}$ is identified with respect to the added amount of $\mathrm{Co}$ and $\mathrm{Zr}$. It is found that $\mathrm{Zr}$ brings about a positive effect on magnetization, in line with recent experimental developments, and we argue that this can be understood as an effective doping of extra electrons.