We demonstrate a simple, robust, and contactless method for nondestructive testing of magnetic materials such as steel. This uses a fiber-coupled magnetic sensor based on nitrogen-vacancy centers (NVCs) in diamond without magnetic shielding. Previous NVC magnetometry has sought a homogeneous bias magnetic field on the diamond to improve the sensitivity. In contrast, here we show that the spatial resolution for imaging is improved by applying an inhomogeneous magnetic field to the steel even though this leads to an inhomogeneous magnetic field on the diamond. Structural damage in the steel distorts the inhomogeneous magnetic field and by detecting this distortion we reconstruct the damage profile through quantifying the shifts in the NVC Zeeman splitting. With a 1-mm magnet as the source of our inhomogeneous magnetic field, we achieve a high spatial resolution of 1 mm in the plane parallel and 0.1 mm in the direction perpendicular to the surface of the steel. This works even when the steel is covered by a nonmagnetic material. The lift-off distance of our sensor head from the surface of 316 stainless steel is up to 3 mm.

The possibility of measuring in situ operando photoluminescence spectroscopy within a photonic atom probe allows for the real-time study of the mechanical stress state within a field emitter either statically, as a function of the field-induced tensile stress, or dynamically, as a result of the evolution of the shape of the emitter upon its evaporation. Dynamic evolution results from the relaxation of strain induced by lattice mismatch and by the propagation of stress from the apex, while the morphology of the field emitter changes. Optomechanical information can be interpreted through the three-dimensional atomic scale images of the chemical composition of the emitter obtained through standard atom probe analysis. Here, the photoluminescence signal of a $\mathrm{Zn}\mathrm{O}/(\mathrm{Mg},\mathrm{Zn})\mathrm{O}$ quantum well allows for the local measurement of strain within the well and of the electrostatic field applied to the apex of the nanoscale field emitter.

Increasing the signal-to-noise ratio in dynamic atomic force microscopy plays a key role in nanomechanical mapping of materials with atomic resolution. In this work, we develop an experimental procedure for increasing the sensitivity of higher harmonics of an atomic-force-microscope cantilever without modifying the cantilever geometry but instead by utilizing dynamical mode coupling between its flexural modes of vibration. We perform experiments on different cantilevers and samples and observe that via nonlinear resonance frequency tuning we can obtain a frequency range where strong modal interactions lead to 7-fold and 16-fold increases in the sensitivity of the 6th and 17th harmonics while reducing sample indentation. We derive a numerical model that captures the observed physics and confirms that nonlinear mode coupling is the reason for the increase of the amplitude of higher harmonics during tip-sample interactions.

Permanent magnets generally require a favorable, but difficult-to-achieve combination of high magnetization, Curie point, and magnetic anisotropy. Thus there have been few, if any, viable permanent magnets developed since the 1982 discovery of ${\mathrm{Nd}}_2{\mathrm{Fe}}_{14}\mathrm{B}$ [M. Sagawa, S. Fujimura, H. Yamamoto, Y. Matsuura, and S. Hirosawa, J. Appl. Phys. 57, 4094 (1985)]. Here we point out, both by direct first-principles calculations on the iron carbides and silicides ${\mathrm{Fe}}_5{\mathrm{C}}_2$, ${\mathrm{Fe}}_5\mathrm{Si}\mathrm{C},$ and ${\mathrm{Fe}}_7{\mathrm{C}}_3$ as well as a discussion of recent experimental findings, that there are numerous rare-earth-free iron-rich potential permanent-magnet materials with sufficient intrinsic magnetic properties to reasonably achieve room-temperature energy products of 20–25 MG Oe. This is substantially better than the performance of the best available rare-earth-free magnets based on ferrite, as well as shape-anisotropy-employing alnico. These magnets could plausibly fill, at low cost, the present performance “gap” [J. M. D. Coey, Scr. Mater. 67, 524 (2012)] between the best rare-earth-free magnets and rare-earth magnets such as ${\mathrm{Nd}}_2{\mathrm{Fe}}_{14}\mathrm{B}$ and $\mathrm{Sm}\text{−}\mathrm{Co}$.

Exciton delocalization, a phenomenon in which spatial extension of the excitons spreads over aggregates, has attracted significant attention in both academia and industry. Studies on exciton delocalization focus mainly on dye molecules and organic conjugated solids because of their high binding energies. Here, we report on exciton delocalization from inorganic ${\mathrm{Mo}\mathrm{S}}_2$ quantum disks (QDs) functionalized with diethylenetriamine (DETA). By increasing the QD concentration, a giant Davydov splitting with an energy of up to 850 meV and an exchange narrowing with an exciton delocalization length up to several tens of QDs are observed. Notably, exciton delocalization in amino-functionalized ${\mathrm{Mo}\mathrm{S}}_2$ QDs can be controlled by the DETA concentration. Amino-related bonding is suggested to modulate the Davydov splitting. Understanding the mechanism of exciton delocalization in two-dimensional QDs is expected to be useful and open up a route for the discovery of unique physical phenomena and the development of as-yet unrealized devices.

This study proposes an optimization method for the design of planar arbitrary-geometry thermal cloaks using natural materials based on the bilayer theory in thermal-cloaking studies. Our objective is to provide a practical design method for bilayer thermal cloaks of arbitrary shapes by determining the optimal outer-layer conductivity of the bilayer yielding the best thermal-cloaking performance. The method accounts for a potentially conducting inner layer, which is deemed more realistic from a practical standpoint as opposed to an ideal bilayer having a perfectly insulated inner layer. The proposed method is applied to solve thermal-cloaking problems involving not only conventional circular and elliptical geometries but also arbitrary-geometry thermal cloaks, as well as nonlinear background temperature distributions. For the ideal circular and elliptical bilayer cloaks, the proposed method yields the same analytical results as reported in the literature. For those cases where an analytical solution is not known to exist, the results demonstrate that excellent thermal-cloaking performance can be achieved on the basis of the bilayer design using the optimal outer-layer conductivity determined from the optimization method for a given set of background, inner-layer, and cloaked-region conductivities.

Constriction-based Josephson weak links display a thermal bistability between two states exhibiting zero and finite voltages. This manifests in experiments either as hysteresis in the current-voltage characteristics of weak links or as a random telegraphic signal in voltage. In the latter case, a noise-driven amplification of a sinusoidal excitation of the device is observed, at frequencies matching the characteristic switching frequency in the telegraphic signal, a phenomenon known as stochastic resonance. The observed behavior is understood using a two-state model of stochastic resonance and is exploited to illustrate an enhanced signal-to-noise-ratio in a $\mu$-SQUID as a magnetic field sensor.

Despite enormous efforts being invested in the elastic wave’s band gap, achieving a broadband low-frequency band gap is still a great challenge. Previous attempts to realize a broadband low-frequency band gap have focused on specific cases, such as linkage connections, the piezoelectric effect, or elastic foundations, which cannot be extended to other meaningful advances. Herein, we propose a way to achieve a broadband low-frequency band gap without any specific conditions, but with only the continuum metamaterial itself. Our proposed idea consists of a hollow cylindrical configuration and a bow-tie-shaped part, which can be easily extended to any other vibrational systems. To explain the idea, the extended mass-spring system is analytically investigated, and the idea is explained in detail based on the mass, inertia, bending, and shear-stiffness values. The proposed idea is supported by various numerical simulations. In addition, experimental realization of the broadband low-frequency band gap is carried out. It is expected that the proposed idea could be utilized to realize various vibration systems, such as a low-frequency cavity, vibration shielding, or filtering devices, which can extend vibration physics at the low-frequency regime.

Precise control of doping in single nanowires (NWs) is essential for the development of NW-based devices. Here, we investigate a series of MBE-grown $\mathrm{Ga}\mathrm{As}$ NWs with $\mathrm{Be}$ ($p$-type) and $\mathrm{Si}$ ($n$-type) doping using high-resolution cathodoluminescence (CL) mapping at low and room temperatures. CL spectra are analyzed selectively in different regions of the NWs. Room-temperature luminescence is fitted with the generalized Planck law and an absorption model, and the bandgap and band tail width are extracted. For $\mathrm{Be}$-doped $\mathrm{Ga}\mathrm{As}$ NWs, the bandgap narrowing provides a quantitative determination of the hole concentration ranging from about $1\times {10}^{18}$ to $2\times {10}^{19}{\mathrm{cm}}^{-3}$, in good agreement with the targeted doping levels. High-resolution maps of the hole concentration demonstrate the homogeneous doping in the pure zinc-blende segment. For $\mathrm{Si}$-doped $\mathrm{Ga}\mathrm{As}$ NWs, the electron Fermi level and the full width at half maximum of low-temperature CL spectra are used to assess the electron concentration to approximately $3\times {10}^{17}$ to $6\times {10}^{17}{\mathrm{cm}}^{-3}$. These findings confirm the difficulty in obtaining highly doped $n$-type $\mathrm{Ga}\mathrm{As}$ NWs, maybe due to doping compensation. Notably, signatures of high concentration [(5–9)$\times {10}^{18}{\mathrm{cm}}^{-3}$] at the very top of NWs are unveiled.

Doping is a fundamental property of semiconductors and constitutes the basis of modern microelectronic and optoelectronic devices. Their miniaturization requires contactless characterization of doping with nanometer-scale resolution. Here, we use low- and room-temperature cathodoluminescence (CL) measurements to analyze $p$-type and $n$-type $\mathrm{Ga}\mathrm{As}$ thin films over a wide range of carrier densities ($2\times {10}^{17}$ to $1\times {10}^{19}{\mathrm{cm}}^{-3}$). The spectral shift and broadening of CL spectra induced by shallow dopant states and band filling are the signature of doping. We fit the whole spectral lineshapes with the generalized Planck law and refined absorption models to extract the bandgap narrowing and the band tail for both doping types, and the electron Fermi level for $n$ doping. This work provides a rigorous method for the quantitative assessment of $p$-type and $n$-type carrier densities using CL. Taking advantage of the high spatial resolution of CL, it can be used to map the doping in $\mathrm{Ga}\mathrm{As}$ nanostructures, and it could be extended to other semiconductor materials.

Metasurfaces are planar metamaterials with a flat surface and a subwavelength thickness that are able to shape arbitrary wave fronts such as focusing or imaging. There is a broad interest in the literature about subwavelength focusing and imaging based on bulk metamaterials while the utilization of metasurfaces for elastic waves has rarely been reported. Here, we present a type of elastic metasurface consisting of a line of gradient resonant pillars for robust deep subwavelength focusing and imaging of elastic waves in a plate. Numerical approaches supported by analytic Huygens-Fresnel demonstrations show that the subwavelength full width at half maximum (FWHM) behaves linearly as a function of the ratio F/D where F is the measured focal length and D the metasurface length. We discuss the range of F/D where FWHM remains smaller than half a wavelength in the near field. The focal length F and the FWHM exhibit stable performances when submitted to disorder perturbations in the geometrical parameters and to frequency fluctuations. We show that the enhanced focusing resolution with smaller FWHM can be very beneficial for energy harvesting since the output electric power can be increased by more than one order of magnitude. The proposed elastic metasurfaces bring a way for high resolution focusing and imaging which is useful for applications in various domains such as energy harvesting, wave sensing, communication, nondestructive evaluation.

Glide symmetry offers new degrees of freedom to engineer the properties of periodic structures, and thus it has been exploited in various electromagnetic structures. However, so far there has been little exploration on the impact that glide symmetry can offer in the field of acoustics. In this paper, we explore glide-symmetric acoustic waveguides, highlighting their dispersion characteristics and guiding properties and demonstrating opportunities in the context of acoustic devices. Here we analytically derive their dispersive features applying a semianalytical mode-matching technique. We then demonstrate how the unusual dispersion properties of glide-symmetric acoustic waveguides can be used to achieve very sharp frequency responses. Based on these results, we propose a sensing platform for liquid analytes that exhibits large sensitivity and linearity. Furthermore, by introducing fluid motion, we leverage these responses to design an acoustic isolator based on acoustic Mach-Zehnder interferometry, whose design is more favorable in terms of footprint and complexity in comparison to other acoustic nonreciprocal devices that do not rely on glide symmetry.

We investigate the screening effects of excess electrons in the doped layer on the mobility of a $\mathrm{Ga}\mathrm{As}$ two-dimensional electron system (2DES) with a modern architecture using short-period superlattice (SL) doping. By controlling the density of excess electrons in the SL with a top gate while keeping the 2DES density constant with a back gate, we are able to compare 2DESs with the same density but different degrees of screening using one sample. Using a field-penetration technique and circuit-model analysis, we determine the density of states and the excess-electron density in the SL, quantities directly linked to the screening capability. The obtained relation between the mobility and the excess-electron density is consistent with theory that takes the screening by the excess electrons in the SL into account. The quantum lifetime determined from Shubnikov–de Haas oscillations is much lower than expected from theory and does not show a discernible change with the excess-electron density.

A valley photonic crystal is one type of photonic topological insulator, the realization of which needs only P-symmetry breaking. The domain wall between two valley-contrasting photonic crystals supports robust edge states, which can wrap around sharp corners without backscattering. Using the robust edge states, one can achieve pulse transmission. Here, using time-domain measurements in the microwave regime, we show distortionless pulse transmission in a sharply bended waveguide. An Ω-shaped waveguide with four 120° bends is constructed with the domain wall between two valley photonic crystal slabs. Experimental results show the progress of Gaussian pulse transmission without distortion, and the full width at half maximum of the output signal changes slightly in the Ω-shaped waveguide. By measuring the steady-state electric field distribution, we also confirm the confined edge states without out-of-plane radiation, which benefits from dispersion below the light line. Our work provides a way for high-fidelity optical pulse signal transmission and the development of high-performance optical elements, such as photonic circuits or optical delay lines.

Perpendicular magnetic anisotropy (PMA) has recently been shown to emerge at interfaces of 3d and 5d transition-metal oxides (TMOs). However, strategies to systematically stabilize such interface-driven PMA still remains elusive, hindering further applications of this design approach. Here, tuning crystal symmetry is shown to be an effective means to engineer this interfacial phenomenon. The evolution of PMA strength as a function of ferromagnetic oxide thickness quantitatively reveals the competition between volume- and interface-specific contributions that determine the magnetic anisotropy. By applying different degrees of epitaxial strain, the relative contributions to PMA are modulated, clearly revealing their correlations with crystal symmetries. To be more specific, the volume anisotropy energy is found to be correlated with the tetragonal distortion of the ferromagnetic layer, while the interface anisotropy energy is mainly modulated by the octahedral tilting at the interface. With these insights, superlattices with enhanced interface-driven PMA and higher Curie temperature are realized. These findings reveal a route to engineering interface-driven PMA and associated magnetic phenomena in TMO heterostructures for future spintronic applications.

Accurate measurements of the thermal resistance between micro-objects made of insulating materials are complex because of their small size, low conductivity, and the presence of various ill-defined gaps. We address this issue using a modified scanning thermal microscope operating in vacuum and in air. The sphere-plate geometry is considered. Under controlled heating power, we measure the temperature on top of a glass microsphere glued to the probe as it approaches a glass plate at room temperature with nanometer accuracy. In vacuum, a jump is observed at contact. From this jump in temperature and the modeling of the thermal resistance of a sphere, the sphere-plate contact resistance $R_K=(1.4\pm 0.18)\times {10}^7{\mathrm{K}\mathrm{W}}^{-1}$ and effective radius $r=36\pm 4$ nm are obtained. In air, the temperature on top of the sphere shows a decrease starting from a sphere-plate distance of $200\mu \mathrm{m}$. A jump is also observed at contact, with a reduced amplitude. The sphere-plate coupling out of contact can be described by the resistance shape factor of a sphere in front of a plate in air, placed in a circuit involving a series and a parallel resistance that are determined by fitting the approach curve. The contact resistance in air $R_K^{\ast }=(1.2\pm 0.46)\times {10}^7{\mathrm{K}\mathrm{W}}^{-1}$ is then estimated from the temperature jump. The method is quantitative without requiring any tedious multiple-scale numerical simulation, and is versatile to describe the coupling between micro-objects from large distances to contact in various environments.

The main challenge in the antenna or laser array design is to find the element distribution that best meets the optimal performance for broadband emission and large angle beam steering. In the past, the design strategy was restricted to arrays with periodic, aperiodic, and random distributions, which are characterized by several fundamental limitations related to the operating frequency, the power consumption that arises from interelement interference, and the computation time required during the random optimization process. Furthermore, the interelement spacing has a lower or upper bound due to the elements’ physical dimensions and the former prohibits the use of the aforementioned element distributions for small operating wavelengths, whereas the latter induces high-order grating lobes. We prove that hyperuniform disorder is an array element distribution evolving through natural selection processes that warrants a disordered solution to the array design when this is treated as a packing problem. We theoretically and experimentally report that the array with hyperuniform disorder exhibits extraordinary directive emission and scanning features, while being scalable for extra-large arrays without any additional computational effort.

Exploring the third dimension in magnonic systems is essential for the investigation of alternative physical phenomena and for the control of spin-wave propagation at the nanoscale. Here, the characteristics of spin waves in vertical meander-shaped ${\mathrm{Co}}_{40}{\mathrm{Fe}}_{40}{\mathrm{B}}_{20}$ thin films consisting of nanosegments located at 90° angles with respect to each other are investigated by Brillouin-light-scattering spectroscopy over four Brillouin zones in reciprocal space. We reveal the dispersion relations and the periodic character of several dispersive branches as well as alternating frequency bands, where spin waves are allowed or forbidden to propagate. Between each couple of successive modes, frequency band gaps exist only for wave numbers k = 2mπ/a, where m is an integer number and a is the size of the meander unit cell, whereas the spectra show propagating modes in the orthogonal film segments for all the other wave numbers. Micromagnetic simulations and analytical calculations are used to understand and explain the results in terms of the mode spatial localization and symmetry. We show that the width and the center frequency of the magnonic band gaps can be controlled by changing the geometrical parameters of the meander-shaped film. The investigated samples behave as three-dimensional waveguides where spin waves propagate in the film segments located at 90° angles with respect to each other, thus making possible vertical spin-wave transport for multilayer magnonic architectures and signal processing.

Quantum networks require robust quantum channels for fast and reliable entanglement distribution over long distances. As quantum communication technology matures, it moves towards utilizing actual fibers and free-space optical channels; hence, there is a growing need for physical models describing decoherence. The primary challenge is to concisely account for numerous elements distributed along a lengthy optical path. We approach this by starting with an analytical model of a channel with just two lumped elements, one representing decoherence and the other representing mode filtering. Interestingly, we find that, while the order and relative orientation of the two elements produce a wealth of different biphoton states, the amount of entanglement in all those states is exactly the same. Then, we conduct experiments that implement this channel and verify our analytical findings. Finally, we expand our analysis to the most general fiber polarization channel, comprising a statistically significant number of arbitrarily oriented elements. We show that, over an ample range of parameters, our two-element analytical model is quite accurate in describing the fiber channel, which makes it an effective tool for gaining insights into channel decoherence.

A Bessel beam has the properties of propagation invariance and a self-healing effect, leading to a variety of interesting phenomena and applications. Recently, as a planar diffractive element with miniaturized size, metasurfaces are widely employed to manipulate light in the subwavelength region, including generating a Bessel beam. However, such a metasurface-generated Bessel beam allows output light with no tunable functions. Here, with the interplay of the geometric phase and the dynamic phase, we propose a method to generate and allow conversion from any orthogonal polarizations to independent Bessel beams with a single-layer dielectric metasurface. The simulation results indicate that the arbitrary conversion between different Bessel beams is related to the spin-dependent orbit motion caused by the tight-focusing effect, leading to the singularity of the spot. This physical mechanism is well studied and the theoretical model for revealing the dependence of different incident polarization on the conversion dynamics is presented. Our approach paves a way for efficient generation and multifunctional applications, ranging from high-numerical-aperture devices to compact nanophotonic platforms for spin-dependent structured beams.