Graphene is a promising substrate for future spintronic devices owing to its remarkable electronic mobility and low spin-orbit coupling. Hanle precession in spin-valve devices is commonly used to evaluate spin diffusion and spin lifetime. In this work, we demonstrate that this method is no longer accurate when the distance between the inner and outer electrodes is smaller than 6 times the spin diffusion length, leading to errors as large as 50% for the calculation of the spin figures of merit of graphene-based devices. We suggest simple but efficient approaches to circumvent this limitation by addressing a revised version of the Hanle fit function. Complementarily, we provide clear guidelines for the design of four-terminal nonlocal spin valves suitable for the flawless determination of the spin lifetime and the spin diffusion coefficient.

A frequency-tunable low-phase-noise magnetoacoustic resonator is developed on the base of a parallel-plate straight-edge bilayer consisting of an yttrium-iron-garnet (YIG) layer grown on a substrate of a gallium gadolinium garnet (GGG). When a YIG-GGG sample forms an ideal parallel plate, it supports a series of high-quality-factor acoustic modes standing along the plate thickness. Due to the magnetostriction of the YIG layer the ferromagnetic resonance (FMR) mode of the YIG layer can strongly interact with the acoustic thickness modes of the YIG-GGG structure, when the modes’ frequencies match. A particular acoustic thickness mode used for the resonance excitations of the hybrid magnetoacoustic oscillations in a YIG-GGG bilayer is chosen by the YIG-layer FMR frequency, which can be tuned by the variation of the external bias magnetic field. A composite scheme of a magnetoacoustic oscillator, which includes a FMR-based resonance preselector, is developed to guarantee satisfaction of the Barkhausen criteria for a single-acoustic-mode oscillation regime. The developed low-phase-noise composite magnetoacoustic oscillator can be tuned from 0.84 to 1 GHz with an increment of about 4.773 MHz (frequency distance between the adjacent acoustic thickness modes in a YIG-GGG parallel plate), and demonstrates the phase noise of $-116$ dBc/Hz at the offset frequency of 10 kHz.

Traditional electronic frequency demodulation of a microwave frequency voltage is challenging because it requires complicated phase-locked loops, narrowband filters with fixed passbands, or large footprint local oscillators and mixers. Herein, a different frequency demodulation concept is proposed based on refractive index modulation of two coupled microcavities excited by an optical wave. A frequency-modulated microwave frequency voltage is applied to two photonic crystal microcavities in a spatially odd configuration. The spatially odd perturbation causes coupling between the even and odd supermodes of the coupled-cavity system. It is shown theoretically and verified by finite-difference time-domain simulations how careful choice of the modulation amplitude and frequency can switch the optical output from on to off. As the modulating frequency is detuned from its off value, the optical output switches from off to on. Ultimately, the optical output amplitude is proportional to the frequency deviation of the applied voltage making this device a frequency-modulated-voltage to amplitude-modulated-optical-wave converter. The optical output can be immediately detected and converted to a voltage that would result in a frequency-demodulated voltage signal. Or the optical output can be fed into a larger radio-over-fiber optical network. In this case the device presents a compact, low power, and tunable route for multiplexing frequency-modulated voltages with amplitude-modulated optical communication systems. The resulting system requires modest modulation amplitudes and operates at frequencies relevant for modern communication systems. The cavity designs have realistic quality factors that are well within the range of experimental implementation. The role of modulation sidebands in reducing switching contrast is explored, and two methods for mitigating these effects are demonstrated.

Quantum platforms based on trapped ions are the main candidates to build a quantum hardware with computational capacities that largely surpass those of classical devices. Among the available control techniques in these setups, pulsed dynamical decoupling (pulsed DD) has been revealed as a useful method to process the information encoded in ion registers, whilst minimizing the environmental noise over them. In this work, we incorporate a pulsed DD technique that uses random pulse phases, or correlated pulse phases, to significantly enhance the robustness of entangling spin-spin dynamics in trapped ions. This procedure was originally conceived in the context of nuclear magnetic resonance for nuclear spin detection purposes, and here we demonstrate that the same principles apply for robust quantum-information processing in trapped-ion settings.

Under the passive and lossless requirement, acoustic metasurfaces are mostly designed based on the general Snell’s law (GSL), under which the power flow-conservation condition is locally satisfied along the whole surface. Recent investigations on metagrating show, however, that such kinds of structures have an intrinsic problem in their efficiency. To overcome this problem, the restriction of the local power flow-conservation condition has to be loosened and replaced by a nonlocal one. Here, we show that a similar problem also exists in the GSL-based acoustic focusing metasurface and the nonlocal idea needs to be introduced into the design. Based on this understanding, a two-dimensional nonlocal metasurface for acoustic focusing is designed by first customizing a desired reflective field and then searching for a power flow-conformal surface profile. Compared with a focusing metasurface based on the GSL theory, our design has an increased focusing effect, especially when the numerical aperture of the metasurface becomes large. The suggested method provides a general technique for designs with a complicated target field.

Second-order photonic topological insulators that host highly localized corner states resilient to defects, are opening alternative routes towards developing fascinating photonic devices. However, the existing works on second-order photonic topological insulators have mainly focused on either transverse-magnetic or transverse-electric modes. In this paper, we propose a dual-polarization topological photonic crystal structure for both transverse magnetic and transverse electric modes through topology optimization. Simple tight-binding lattice models are constructed to reveal the topological features of the optimized photonic crystal structure in a transparent way. The optimized dual-polarization second-order photonic topological insulator hosts four groups of corner states with different profiles and eigenfrequencies for both the transverse-magnetic and transverse-electric modes. Moreover, the robustness of these corner states against defects is explicitly demonstrated. Our results offer opportunities for developing polarization-independent topological photonic devices.

Detector blinding attacks have been proposed in the last few years, and they could potentially threaten the security of quantum key distribution systems. Even though such attacks are technically challenging to implement, it is important to consider countermeasures to avoid information leakage. In this paper, we present a countermeasure against these kinds of attacks based on the use of multipixel detectors. We show that with this method, we are able to estimate an upper bound on the information an eavesdropper could have on the key exchanged. Finally, we test a multipixel detector based on superconducting nanowire single-photon detectors to show it can fulfill all the requirements for our countermeasure to be effective.

Quantum key distribution (QKD) can generate secure key bits between remote users with quantum mechanics. However, the gap between the theoretical model and practical realizations gives eavesdroppers opportunities to intercept the secret key. The most insidious attacks, known as quantum hacking, are the ones with no significant discrepancy of the measurement results using side-channel loopholes of QKD systems. Depicting full-time-scale characteristics of the quantum signals, the quantum channel, and the QKD system can provide legitimate users extra capabilities to defeat malicious attacks. Here, we propose the method exploring a temporal ghost-imaging (TGI) scheme to perceive quantum hacking with temporal fingerprints and experimentally verify its validity. The scheme presents a common approach to promote QKD’s practical security from a unique perspective of signals and systems.

Controlling the low-energy excitations in magnetically active materials is the key to numerous phenomena in magnonics and spintronics. This study demonstrates a generic scheme for highly efficient control of magnonic signals in a stack of magnetic slabs separated by current-carrying metallic nanoscale spacer layers. We demonstrate analytically and confirm full numerically that the proposed structure has a magnonic spectrum that is highly susceptible to external perturbations and is governed by a parity-time ($\mathcal{P}\mathcal{T}$) symmetric Hamiltonian. The enhanced sensitivity can be tuned by the dc charge currents, the number of stacking layers or/and by the intrinsic properties of the stacking layers. Physically, the currents in the spacer layers cause spin-orbit torques acting on the adjacent magnetic layers. Effectively, these torques damp or antidamp magnonic excitations. Depending on the spacer-charge current density and the number of stacking layers, a point can be reached where damping and antidamping are balanced. Beyond this exceptional point (EP) the magnonic system enters a $\mathcal{P}\mathcal{T}$-symmetry-broken phase. Near EP we show analytically and numerically that the system exhibits a nonlinear response even to weak perturbing fields. Scanning the external fields in a loop to enclose the EP in the dispersion manifold, we identify a nonreciprocal topological energy transfer between different magnon modes. The results point to a promising route in magnonics and spintronics as well as to a versatile testing ground for $\mathcal{P}\mathcal{T}$-symmetry-driven phenomena.

In this paper we combine the aperture field estimation technique and adiabatic Floquet-wave expansion method for the analysis of leaky-wave holograms capable of generating orbital angular momentum vortex waves. In these formulations the propagation and leakage constants are theoretically estimated and, as a result, the aperture field and far-zone field are calculated. Thus, the conditions required to achieve the desired topological charges for the far-field region are obtained. This theoretical approach significantly reduces the computational complexity in such a way that the analysis of holograms with large dimensions is possible with low memory requirements. Isotropic and anisotropic unit cells are used to realize the holograms. It has been shown that the anisotropic cells give an additional degree of freedom, leading to improved performance in the control of wave polarization. The holograms are designed to generate vortex beams with topological charge of $m=2$ and circular polarization states in the microwave regime. To evaluate the accuracy of the proposed method, the results of the theoretical model and full-wave simulations are compared, with good agreement.

The negative capacitance (NC) stabilization of a ferroelectric (FE) material can potentially provide an alternative way to further reduce the power consumption in ultrascaled devices and thus has been of great interest in technology and science in the past decade. In this paper, we present a physical picture for a better understanding of the hysteresis-free charge-boost effect observed experimentally in metal-ferroelectric-insulator-metal ($M$-$F$-$I$-$M$) capacitors. By introducing the dielectric (DE) leakage and interfacial trapped charges, our simulations of the hysteresis loops are in a strong agreement with the experimental measurements, suggesting the existence of an interfacial oxide layer at the FE-metal interface in metal-ferroelectric-metal ($M$-$F$-$M$) capacitors. Based on the pulse-switching measurements, we find that the charge enhancement and hysteresis are dominated by the FE domain viscosity and DE leakage, respectively. Our simulation results show that the underlying mechanisms for the observed hysteresis-free charge enhancement in $M$-$F$-$I$-$M$ may be physically different from the alleged NC stabilization and capacitance matching. Moreover, the link between Merz’s law and the phenomenological kinetic coefficient is discussed, and the possible cause of the residual charges observed after pulse switching is explained by the trapped charge dynamics at the FE-DE interface. The physical interpretation presented in this work can provide useful insights into the NC effect in $M$-$F$-$I$-$M$ capacitors and future studies of low-power logic devices.

Understanding the electrical manipulation of the antiferromagnetic order is a crucial aspect to enable the design of antiferromagnetic devices working at THz frequencies. Focusing on collinear insulating antiferromagnetic $\mathrm{Ni}\mathrm{O}/\mathrm{Pt}$ thin films as a materials platform, we identify the crystallographic orientation of the domains that can be switched by currents and quantify the Néel-vector direction changes. We demonstrate electrical switching between different T domains by current pulses, finding that the Néel-vector orientation in these domains is along [$\pm 5$ $\pm 5$ 19], different compared to the bulk $⟨112⟩$ directions. The final state of the in-plane component of the Néel vector ${\mathbf{n}}_{\mathrm{IP}}$ after switching by current pulses $\mathbf{j}$ along the $[1\pm 10]$ directions is ${\mathbf{n}}_{\mathrm{IP}}\parallel \mathbf{j}$. By comparing the observed Néel-vector orientation and the strain in the thin films, assuming that this variation arises solely from magnetoelastic effects, we quantify the order of magnitude of the magnetoelastic coupling coefficient as $b_0+2b_1=3\times {10}^7\mathrm{J}/{\mathrm{m}}^3$. This information is key for the understanding of current-induced switching in antiferromagnets and for the design and use of such devices as active elements in spintronic devices.

We study the interaction of Rayleigh and shear horizontal surface acoustic waves (SAWs) with spin waves in thin $\mathrm{Ni}$ films on a piezoelectric ${\mathrm{Li}\mathrm{Ta}\mathrm{O}}_3$ substrate, which supports both SAW modes simultaneously. Because Rayleigh and shear horizontal modes induce different strain components in the $\mathrm{Ni}$ thin films, the symmetries of the magnetoelastic driving fields, of the magnetoelastic response, and of the transmission nonreciprocity differ for both SAW modes. Our experimental findings are well explained by a theoretical model based on a modified Landau-Lifshitz-Gilbert approach. We show that the symmetries of the magnetoelastic response driven by Rayleigh and shear horizontal SAWs complement each other, which makes it possible to excite spin waves for any relative orientation of magnetization and SAW propagation direction and, moreover, can be utilized to characterize surface strain components of unknown acoustic wave modes.

We develop a time-dependent density-matrix model to study photon-assisted (PA) electron transport in quantum cascade lasers. The Markovian equation of motion for the density matrix in the presence of an optical field is solved for an arbitrary field amplitude. Level-broadening terms emerge from microscopic Hamiltonians and supplant the need for empirical parameters that are often employed in related approaches. We show that, in quantum cascade lasers with diagonal design, photon resonances have a pronounced impact on electron dynamics around and above the lasing threshold, an effect that stems from the large spatial separation between the upper and lower lasing states. With the inclusion of PA tunneling, the calculated current density and output power are in good agreement with experiment.

The achievement of the laser regime upon the excitation of weak plasmon modes in structures based on active graphene has been theoretically investigated. The weak “acoustic” plasmon modes in two-layer structures and weak “nonradiative” gated modes in noncentrosymmetric grating-gated structures are investigated. The radiative damping and gain rate of strong and weak plasmon modes in graphene structures are calculated. The possibilities of changing the radiative damping of plasmon modes by changing the geometry of the structure are considered. It is proposed to use weak plasmon modes with low radiative damping for decreasing the threshold for the laser plasmon regime in periodic graphene structures.

Spinel-type magnetite ${\mathrm{Fe}}_3{\mathrm{O}}_4$ is predicted to be a half-metal material with negative spin polarization. However, magnetic tunnel junctions (MTJs) using an ${\mathrm{Fe}}_3{\mathrm{O}}_4$ electrode exhibit a small tunnel magnetoresistance (TMR) effect, the sign of which has not been established experimentally. The development of ${\mathrm{Fe}}_3{\mathrm{O}}_4$ as an excellent TMR material requires a better understanding of the characteristics of the interface and the phase transition of ${\mathrm{Fe}}_3{\mathrm{O}}_4$ called the Verwey transition. We fabricate MTJs using epitaxial ${\mathrm{Fe}}_3{\mathrm{O}}_4$-based stacks on $\mathrm{Mg}\mathrm{O}(001)$ substrates and find a large inverse TMR ratio of −55.8% at 80 K, which corresponds to 126% by the optimistic definition of the TMR ratio. The temperature dependence of the TMR ratio is significantly affected by the Verwey transition. Moreover, we investigate the dependence of TMR on oxygen partial pressure during ${\mathrm{Fe}}_3{\mathrm{O}}_4$ deposition. It is found that the magneto-transport properties of the MTJs show different behaviors depending on the oxygen partial pressure because the Verwey transition is sensitive to the oxygen concentration. Furthermore, the electronic and magnetic properties at the interfacial regions are investigated by x-ray magnetic spectroscopy and first-principles calculation. These findings greatly support the use of ${\mathrm{Fe}}_3{\mathrm{O}}_4$ in spintronic devices and should lead to further developments in oxide spintronics.

Being a general wave phenomenon, bound states in the continuum (BICs) appear in acoustic, hydrodynamic, and photonic systems of various dimensionalities. Here, we report the first experimental observation of an accidental electromagnetic BIC in a one-dimensional periodic chain of coaxial ceramic disks. We show that the accidental BIC manifests itself as a narrow peak in the transmission spectra of the chain placed between two loop antennas. We demonstrate a linear growth of the radiative quality factor of the BICs with the number of disks that is well described with the developed tight-binding model. We estimate the number of disks when the radiation losses become negligible in comparison to material absorption and, therefore, the chain can be considered as practically infinite. The presented analysis is supported by near-field measurements of the BIC profile. The obtained results provide useful guidelines for practical implementations of structures with BICs opening up horizons for the development of radio-frequency and optical metadevices.

Bismuth chalcohalides of ${\mathrm{Bi}}_{13}{\mathrm{S}}_{18}{X}_2$ (X = $\mathrm{Cl},\mathrm{Br},\mathrm{I}$) are solvothermally synthesized and systematically characterized as a light absorber for photovoltaics. Rietveld refinements reveal that ${\mathrm{Bi}}_{13}{\mathrm{S}}_{18}{X}_2$ compounds adopt the same hexagonal structure with space group P3 and a chemical composition of $({\mathrm{Bi}}^{3+}{)}_{12}({\mathrm{Bi}}_2^{4+}{)}_{0.5}({\mathrm{S}}^{2-}{)}_{18}(X^{-}{)}_2$, where a triangular tunnel is constructed from [${\mathrm{Bi}}_4{\mathrm{S}}_6$]-ribbon spokes in the c-axial direction, $X^{-}$ is located in the tunnel sites, and Bi(II) dimers of (${\mathrm{Bi}}_2^{4+}$) are located at the hexagonal center. The photoelectric behavior is investigated using ultraviolet-to-visible-to-near-infrared (UV-vis-NIR) absorption spectroscopy, ultraviolet photoelectron spectroscopy, x-ray photoelectron spectroscopy, Mott-Schottky plots, and conductivity measurements. The ${\mathrm{Bi}}_{13}{\mathrm{S}}_{18}{X}_2$ compounds are indirect n-type semiconductors with narrow band gaps of 0.76, 0.80, and 0.81 eV for ${\mathrm{Bi}}_{13}{\mathrm{S}}_{18}{\mathrm{Cl}}_2$, ${\mathrm{Bi}}_{13}{\mathrm{S}}_{18}{\mathrm{Br}}_2$, and ${\mathrm{Bi}}_{13}{\mathrm{S}}_{18}{\mathrm{I}}_2$, respectively, and exhibit strong light absorbance over a wide wavelength range from UV to NIR. A facile physical vapor deposition approach is developed for the fabrication of a homogeneous ${\mathrm{Bi}}_{13}{\mathrm{S}}_{18}{X}_2$ thin film to study the photovoltaic performances. Solar cells with the architecture fluorine-doped tin oxide/${\mathrm{Ti}\mathrm{O}}_2$/${\mathrm{Bi}}_{13}{\mathrm{S}}_{18}{X}_2$/(${\mathrm{I}}_3^{-}/{\mathrm{I}}^{-}$ redox couple)/$\mathrm{Pt}$ demonstrate power conversion efficiencies of 0.91, 1.12, and 0.75% for ${\mathrm{Bi}}_{13}{\mathrm{S}}_{18}{\mathrm{Cl}}_2$, ${\mathrm{Bi}}_{13}{\mathrm{S}}_{18}{\mathrm{Br}}_2$, and ${\mathrm{Bi}}_{13}{\mathrm{S}}_{18}{\mathrm{I}}_2$, respectively. The highest cell performance of ${\mathrm{Bi}}_{13}{\mathrm{S}}_{18}{\mathrm{Br}}_2$ can be attributed to its higher light-absorption ability.

Transformation optics, which is generically applicable to other classical waves such as acoustic and elastic waves, provides an emerging design paradigm to manipulate waves. However, some lenses and optical-transformation devices require a singular refractive index; meeting this requirement is a significant challenge. A method called transmutation can relax some types of index singularity into finite anisotropy around the singularity. Here, we show that such lenses with a singularity for flexural waves can be obtained by approaching a near-zero thickness of the plate precisely at the location of the singularity. As examples, we demonstrate a series of Eaton lenses theoretically and experimentally by projecting the refractive index in space onto the thickness in plates and by working in a broad frequency range in which impedance mismatch is negligible. This framework offers an insight into feasible methods that can be used to develop singular devices such as cloaking devices on thin flexible curved plates and can be further extended to a general methodology for shaping elastic waves. We hope that this elastic platform can also be a test bed to indirectly study unprecedented phenomena enabled by gravitational and quantum fields in terms of analog models.