Elastic valley metamaterials offer an excellent platform to manipulate elastic waves and have potential applications in energy harvesting and elastography. Here we introduce a series of strategies to realize a topological transition in spiral elastic valley metamaterials by parameter modulations. We show the evolution of the Berry curvature and valley Chern number as a function of inherent parameters of a spiral, which further results in a general scheme to achieve topological valley edge states. Our strategy leverages multiple degrees of freedom in spiral elastic valley metamaterials to provide enhanced opportunities for desired topological states.

We combine two different fields, topological physics and graded metamaterials, to design a topological metasurface to control and redirect elastic waves. We strategically design a two-dimensional crystalline perforated elastic plate, using a square lattice, that hosts symmetry-induced topological edge states. By concurrently allowing the elastic substrate to spatially vary in depth, we are able to convert the incident slow wave into a series of robust modes, with differing envelope modulations. This adiabatic transition localizes the incoming energy into a concentrated region where it can then be damped or extracted. For larger transitions, different behavior is observed; the incoming energy propagates along the interface before being partitioned into two disparate chiral beams. This “topological rainbow” effect leverages two main concepts, namely the quantum valley Hall effect and the rainbow effect usually associated with electromagnetic metamaterials. The topological rainbow effect transcends specific physical systems; hence, the phenomena we describe can be transposed to other wave physics. Because of the directional tunability of the elastic energy by geometry, our results have potential for applications such as switches, filters, and energy harvesters.

The generation of topological magnetic vortex-domain structures in iron-oxide nanomaterials has promising applications in biomedical scenarios, such as heat generators for hyperthermia treatments. In this report we describe alternative kinds of magnetic-vortex nanoparticles, circular ${\mathrm{Fe}}_3{\mathrm{O}}_4$ nanodiscs (NDs), and dissect their heating properties by in-depth investigation of their shape and size, stoichiometry, orientations, and switching field “$H_S$” behaviors, through experiments and theoretical simulation. We find that the stoichiometric NDs show better heating performance than nonstoichiometric materials because of the significant electron hopping between ${\mathrm{Fe}}^{3+}$ and ${\mathrm{Fe}}^{2+}$ ion. The higher heating efficiency (in terms of specific absorption rate, SAR) is observed only for the higher switching field regime, an effect that is associated with the parallel and perpendicular alignment of nanodiscs with respect to low and high ac magnetic field, respectively. A higher SAR of approximately 270 W/g is observed at a higher switching field (approximately 700 Oe) for NDs of diameter 770 nm, which increases by a factor of 4 at a switching field of approximately 360 Oe for NDs of diameter 200 nm. The reported results suggest that the heating efficiency in these systems can be enhanced by controlling the switching field, which is, in turn, tuned by size, shape, and orientation of circular magnetic vortex nanodiscs.

One of the main objectives of spintronics is to provide power-efficient switching of magnetic layers through electrical means, and in order to achieve this goal, alternate material systems with enhanced spin-orbit torque (SOT) must be engineered. In this work we provide evidence of anisotropy in the SOT and spin Hall effect (SHE) in epitaxial $\mathrm{Pt}$(110) grown on $\mathrm{Mg}\mathrm{O}$(110) single-crystal substrates, and find that the spin Hall angle and the dampinglike torque are 20% larger when current is applied along the [001] crystallographic direction as compared to [$1\overline{1}0$], leading to an equivalent reduction in switching current density along [001]. The anisotropy in SOT is attributed to the bulk contributions of the SHE in the $\mathrm{Pt}$ layer through its anisotropic resistance in this specific orientation. Measurements additionally suggest that the Rashba-Edelstein effect at the $\mathrm{Pt}$/$\mathrm{Ti}$ interface due to the $\mathrm{Pt}$(110) surface has a non-negligible effect on the spin diffusion length and SOT. By providing experimental evidence of the crystal orientation dependence of SOT-induced magnetization switching, this work helps to establish a path for energy-efficient magnetization switching through the alignment of devices with crystallographic directions of enhanced SOT generation.

Inspired by Savart’s pioneering work, we study the self-stimulated dynamics of a capillary jet. The feedback loop is realized by extracting surface perturbations from a section of the jet itself via a laser-photodiode pair, whose amplified signal drives an electromechanical actuator that, in turn, produces pressure perturbations at the exit chamber. Under specific conditions, this loop establishes phase-locked stimulation regimes that overcome the otherwise random natural breakup. For each laser position along the jet, the gain of the amplifier acts as a selector across a discrete set of observable frequencies. The main observed features are explained by a linear theory that combines the transfer function of each stage in the loop. Our findings are relevant to continuous inkjet technologies for the production of equally sized droplets.

We demonstrate an atomic rf receiver and spectrum analyzer based on thermal Rydberg atoms coupled to a planar microwave waveguide. We use an off-resonant rf heterodyne technique to achieve continuous operation for carrier frequencies ranging from dc to 20 GHz. The system achieves an intrinsic sensitivity of up to $-120(2)$, dc coupling, 4-MHz instantaneous bandwidth, and over 80 dB of linear dynamic range. By connecting through a low-noise preamplifier, we demonstrate high-performance spectrum analysis with peak sensitivity of better than $-145\mathrm{dBm/Hz}$. Attaching a standard rabbit-ears antenna, the spectrum analyzer detects weak ambient signals including FM radio, AM radio, WiFi, and bluetooth. We also demonstrate waveguide-readout of the thermal Rydberg ensemble by nondestructively probing waveguide-atom interactions. The system opens the door for small, room-temperature, ensemble-based Rydberg sensors that surpass the sensitivity, bandwidth, and precision limitations of standard rf sensors, receivers, and analyzers.

Detection of phonons is critical for research on the interconversion between resonantly coupled magnons and phonons. Here we report the direct visualization of the resonant coupling of magnons and phonons by microfocused Brillouin light scattering on the $\mathrm{Ni}/{\mathrm{Li}\mathrm{Nb}\mathrm{O}}_3$ hybrid heterostructure. The static patterns of surface acoustic wave phonons, originating from the interference between the incident wave ${\psi }_0(A_0,\mathbf{k},{\phi }_0)$ and reflected wave ${\psi }_1(A_1,-\mathbf{k},{\phi }_1)$, can be modulated by magnetic field due to the magnon-phonon coupling. By analyzing the information of phonons obtained from Brillouin spectroscopy, the properties of the magnon system (Ni film), e.g., ferromagnetic resonance field and resonance linewidth, can be determined. The results provide spatially resolved information about phonon manipulation and detection in a coupled magnon-phonon system.

We present a method for the control of waves based on inverse multiple scattering theory. Conceived as a generalization of the concept of metagrating, we call metaclusters to a finite set of scatterers whose position and properties are obtained by inverse design once we have defined their response to some external incident field. The particular focus is on designing passive metaclusters that do not require an external source of energy. The method is applied to the propagation of flexural waves in thin plates, and to the design of far-field patterns, although its generalization to acoustic or electromagnetic waves is straightforward. Numerical examples are presented to the design of uni- and bidirectional “anomalous scatterers,” which will bend the scattering energy along a specific direction, “odd pole” scatterers, whose radiation pattern presents an odd number of poles, and to the generation of vortical patterns. Finally, some considerations about the optimal design of these metaclusters are discussed.

We show that a magnetically levitated microsphere in high vacuum can be used as an accelerometer by comparing its response to that of a commercially available geophone. This system shows great promise for ultrahigh acceleration sensitivities without the need for large masses or cryogenics. With feedback cooling, the transient decay time is reduced and the center-of-mass motion is cooled to 9 K or less. Remarkably, the levitated-particle accelerometer has a sensitivity down to $3.6\times {10}^{-8}\mathrm{g}/\sqrt{\mathrm{Hz}}$ and gives measurements similar to those of the commercial geophone at frequencies up to 14 Hz despite a test mass that is four billion times smaller. With no free parameters in the calibration, the responses of the accelerometers match within 3% at 5 Hz. The system reaches this sensitivity due to a relatively large particle mass of $0.25\mu \mathrm{g}$, a low center-of-mass oscillation frequency of 1.75 Hz, and an image-analysis method that can measure the displacement with an uncertainty of 1.6 nm in a single image.

A quantum router is an elementary building block for quantum-information processing. It directs the incoming quantum signal to different routes with phase coherence. To realize scalable quantum routing, a single particle needs to deterministically and coherently route the paths of a flying qubit, which requires nonlinear interaction at the single quantum level and complicated path-data qubit control so far beyond the experimental reach. Here we demonstrate a deterministic quantum router working at microwave frequencies based on superconducting circuits, which can naturally integrate with a leading platform for quantum computing. We encode the quantum signal into a weak coherent pulse and use an entanglement witness to rigorously verify the generated entanglement between the control qubit and the path of the data qubit, thus proving quantum coherence of the routing operation. Our experiment lays the foundation for useful applications of quantum routers in quantum networks and quantum random-access memories.

Scanning probes with functional optical responses are key components of scanning near-field optical microscopes. For nanospectroscopy performed at IR and terahertz (THz) frequencies, one major challenge is that the commonly used metal-coated silicon tips yield nonadjustable coupling efficiency across the spectrum, which greatly limits the signal-to-noise ratio. Here, we test the possibility of a generic design scheme for wavelength-selective tip enhancement via finite-element numerical modeling. We employ a $\mathrm{Si}$-based tip with various gold-coating lengths on the top, yielding a customizable near-field field strength at the tip apex. Calculations show a wavelength-dependent enhancement factor of the metal-coated tip due to the geometrical antenna resonances, which can be precisely tuned throughout a broad spectral range from visible to terahertz frequencies by adjusting the length of the metal coating. By changing the coating pattern into a chiral helical structure on an achiral tip, we also demonstrate the usefulness of coating-length effect in designing high-performance enantiomeric near-field scanning. Our methods and findings offer interesting perspectives for developing near-field optical probes, pushing the detection and resolution limits of tip-enhanced near-field detections, such as fluorescence, Raman, IR, and THz nanospectroscopies.

Electric fields are commonly used for manipulating particles and liquids in microfluidic systems. In this work, we report stationary electro-osmotic flow vortices around dielectric micropillars induced by ac electric fields in electrolytes. The flow characteristics are theoretically predicted based on the well-known phenomena of surface conductance and concentration polarization around a charged object. The stationary flows arise from two distinct contributions working together: an oscillating nonuniform zeta potential induced around the pillar and a rectified electric field induced by the ion concentration gradients. We refer to this fluid flow as concentration-polarization electro-osmosis (CPEO). We present experimental data in support of the theoretical predictions. The magnitude and frequency dependence of the electro-osmotic velocity are in agreement with the theoretical estimates and are significantly different from predictions based on the standard theory for induced-charge electro-osmosis, which has previously been postulated as the origin of the stationary flow around dielectric objects. In addition to furthering our understanding of the influence of ac fields on fluid flows, we anticipate that this work will also expand the use of ac fields for flow control in microfluidic systems.

The steady-state, space-charge-limited piezoresistance (PZR) of defect-engineered, silicon-on-insulator device layers containing silicon divacancy defects changes sign as a function of applied bias. Above a punch-through voltage ($V_t$) corresponding to the onset of a space-charge-limited hole current, the longitudinal $⟨110⟩$ PZR $\pi$ coefficient is $\pi \approx 65\times {10}^{-11}$ $\mathrm{Pa}$${}^{-1}$, similar to the value obtained in charge-neutral, $p$-type silicon. Below $V_t$, the mechanical stress dependence of the Shockley-Read-Hall (SRH) recombination parameters, specifically the divacancy trap energy $E_T$ that is estimated to vary by approximately $30\mu \mathrm{V}/$MPa, yields $\pi \approx -25\times {10}^{-11}$ $\mathrm{Pa}$${}^{-1}$. The combination of space-charge-limited transport and defect engineering that significantly reduces SRH recombination lifetimes makes this work directly relevant to discussions of giant or anomalous PZR at small strains in nanosilicon whose characteristic dimension is larger than a few nanometers. In this limit the reduced electrostatic dimensionality lowers $V_t$ and amplifies space-charge-limited currents and efficient SRH recombination occurs via surface defects. The results reinforce the growing evidence that in steady state, electromechanically active defects can result in anomalous, but not giant, PZR.

Extracting quantitative microstructure information of living tissue by noninvasive imaging is an outstanding challenge for understanding disease mechanisms and allowing early stage diagnosis of pathologies. Magnetic resonance imaging (MRI) is a promising and widely used technique to pursue this goal, but still provides low resolution to reveal microstructure details. We here report on a method to produce images of filtered microstructure sizes based on selectively probing the nuclear-spin dephasing induced by the molecular diffusion within specific tissue compartments. The microstructure-size filter relies on suitable dynamical control of nuclear spins that sense magnetization “decay shifts” rather than the commonly used spin-echo decay rates. The feasibility and performance of the method are illustrated with proof-of-principle experiments and simulations on typical size distributions of white matter in the mouse brain. These results position spin-echo decay shifts as a promising MRI tool as they could offer the ability to perform noninvasive histology without assuming a microstructure distribution model. This sets a step towards unraveling diagnostic information based on microscopic parameters of biological tissue.

We previously reported highly monochromatic electron emission from the planar-type electron emission devices based on a graphene/hexagonal boron nitride (h-BN) heterostructure. In this paper, the electron energy distribution (EED) of these devices is examined to clarify the mechanism of monochromatic electron emission. We find that the monochromaticity of the electron beam depends significantly on the electronic structure of the substrate material; for the devices with an n-type silicon substrate, the narrowest FWHM of the electron beam is 0.18 eV, whereas that of devices with a metallic (Nb) substrate is 0.33 eV. At the same time, simulations considering the electron scattering by phonons acceptably reproduced the shape of each EED spectrum considering the small energy loss due to out-of-plane acoustic phonon modes in h-BN. Thus, the monochromatic electron emission from the $\mathrm{graphene}/h$-$\mathrm{BN}/n$-$\mathrm{Si}$ device is ascribed to a combination of the narrow energy distribution of electrons at the conduction band of the n-$\mathrm{Si}$ substrate and small phonon energy of the h-BN insulating layer. These features also realize the excellent emission properties in addition to the monochromaticity of the beam, such as a high emission current density of $9.3{\mathrm{A}/\mathrm{cm}}^2$, insensitivity to environmental pressure up to 10 Pa, and long lifetime of more than 7 days with little decay.

We theoretically and experimentally demonstrate a thermally tunable anapole metasurface. By thermally changing the dielectric function, our studies unveil the tolerance of the nonradiating anapole source to the presence of dissipation losses in systems of all-dielectric meta-atoms. The so-called anapole state is an ac current distribution of electric and toroidal dipoles in such a relation that their exclusive destructive interference leads to the absence of far-field radiation. It has been shown to exist in variable dielectric configurations that involve assemblies of infinite dielectric rods, yet a systematic study of its tolerance to losses is still open. Here we examine two designs focusing on their tolerance to dissipation losses: one combining the toroidal mode of a peripheral four-cylinder system with the mixed toroidal mode of a single central cylinder of adjusted size, and another consisting of five rods in a regular pentagon arrangement. The first design exhibits a particularly enhanced toroidal dipole moment and high anapole precision. The second design exhibits anapole states with enhanced loss tolerance and, thus, it is the target of our experimental investigation. For the experiment, we use water-based rod metasurfaces designed for anapole operation in the microwave regime and we employ a characterization in a standard rectangular waveguide. The dielectric properties of water exhibit significant temperature dependence that allows for controlled permittivity and dissipation, leading to the dynamical modification of Mie resonances in meta-atoms. Thus, we experimentally prove the emergence of an anapole state and its tolerance to the losses in a well-controlled system; furthermore, we demonstrate and expose the potential of aqueous schemes for tunable electromagnetic applications.

We study the magneto-transport and magnetoelectroluminescence properties of purely $n$-doped ${\mathrm{Ga}\mathrm{As}/\mathrm{Al}}_{0.6}{\mathrm{Ga}}_{0.4}\mathrm{As}$ resonant-tunneling diodes with an ${\mathrm{In}}_{0.15}{\mathrm{Ga}}_{0.85}\mathrm{As}$ quantum well and an emitter prewell. Before the resonant-current condition, magneto-transport measurements reveal charge-carrier densities comparable for diodes with and without the emitter prewell. Landau-level splitting is observed in the electroluminescence emission from the emitter prewell, enabling the determination of the charge-carrier buildup. Our findings show that magnetoelectroluminescence spectroscopy techniques provide useful insights into the charge-carrier dynamics in resonant-tunneling diodes and comprise a versatile tool to complement magneto-transport techniques. This approach might pave the way for the development of potentially more efficient optoelectronic resonant-tunneling devices by, e.g., monitoring voltage-dependent charge accumulation for the improvement of built-in fields and hence the maximization of the photodetector efficiency and/or the minimization of optical losses.