Nanomagnets are the building blocks of many existing and emergent spintronic technologies. The magnetization dynamics of nanomagnets is often dominated by nonlinear processes, which have been recently shown to have many surprising features and far-reaching implications for applications. Here we develop a theoretical framework uncovering the selection rules for multimagnon processes and discuss their underlying mechanisms. For its technological relevance, we focus on the degenerate three-magnon process in thin elliptical nanodisks to illustrate our findings. We parameterize the selection rules through a set of magnon interaction coefficients which we calculate using micromagnetic simulations. We postulate the selection rules and investigate how they are altered by perturbations that break the symmetry of static magnetization configuration and spatial spin-wave profiles and that can be realized by applying off-symmetry-axis or nonuniform magnetic fields. Our work provides the phenomenological understanding of the mechanics of magnon interaction as well as the formalism for determining the interaction coefficients from simulations and experimental data. Our results serve as a guide to analyze the magnon processes inherently present in spin-torque devices in order to boost their performance or to engineer a specific nonlinear response in a nanomagnet used in a neuromorphic or quantum magnonic application.

Point defects in solid-state materials are promising systems in the fields of quantum information, communications, and sensing. Particular to sensing, defects with coherent spin states are used for high-sensitivity room-temperature sensing under ambient conditions. Applications involving magnetic sensing with point defects are mostly dominated by the nitrogen-vacancy center in diamond, which possesses an attractive combination of spin coherence at room temperature as well as spin initialization and read-out. However, spin defects in other materials are explored as alternatives, especially in industrially mature materials, such as silicon carbide. Here, we report on the improved sensitivity of a magnetic sensor system utilizing an ensemble of silicon vacancies in silicon carbide due to isotopic purification of the host crystal. A maximum sensitivity of 4.0 nT/√Hz is reported, representing an order of magnitude improvement from the best previously reported sensitivity. Additional sensing modalities, such as angle-resolved magnetic imaging and highly broadband ac field sensing, are also demonstrated.

Our ability to efficiently detect and generate far-infrared (i.e., terahertz) radiation is vital in areas spanning from biomedical imaging to interstellar spectroscopy. Despite decades of intense research, bridging the terahertz gap between electronics and optics remains a major challenge due to the lack of robust materials that can efficiently operate in this frequency range, and two-dimensional (2D) type-II heterostructures may be ideal candidates to fill this gap. Herein, using highly accurate many-body perturbation theory within the GW plus Bethe-Salpeter equation approach, we predict that a type-II heterostructure consisting of an electron-rich ${\mathrm{C}}_3\mathrm{N}$ and an electron deficient ${\mathrm{C}}_3\mathrm{B}$ monolayers can give rise to extraordinary optical activities in the mid- to far-infrared range. ${\mathrm{C}}_3\mathrm{N}$ and ${\mathrm{C}}_3\mathrm{B}$ are two graphene-derived 2D materials that have attracted increasing research attention. Although both ${\mathrm{C}}_3\mathrm{N}$ and ${\mathrm{C}}_3\mathrm{B}$ monolayers are moderate-gap 2D materials, and they couple only through the rather weak van der Waals interactions, the bilayer heterostructure surprisingly supports extremely bright, low-energy interlayer excitons with large binding energies of 0.2–0.4 eV, offering an ideal material with interlayer excitonic states for mid- to far-infrared applications at room temperature. We also investigate in detail the properties and formation mechanism of the inter- and intralayer excitons.

Valence-change memory (VCM) cells are promising candidates for future nonvolatile memory devices. A special setup of VCM devices consists of bilayer cells where two thin oxide layers are placed in between two metal electrodes. One oxide layer serves as a tunnel barrier, whereas the second oxide layer is a highly doped conductive semiconductor. Experiments show that an exchange of oxygen between the two layers changes the resistance of the cell. However, the exchange process and how it influences the resistance is not well understood yet. With a drift-diffusion model for electrons and oxygen vacancies, we investigate the movement and exchange of oxygen vacancies and their influence on the band structure as well as on the shape of the tunnel barrier. The simulation results show that a high oxygen-vacancy concentration lowers the height of the tunnel barrier; thus it increases the conductivity of the bilayer cell. The effect of the band lowering is stronger in materials with low permittivity. Hence, two different resistance states evolve if there is an exchange of oxygen between the two oxide layers with different permittivities. Thereby, the switching polarity depends on the relation of the permittivities of the two oxide layers. Furthermore, it is revealed that resistance switching can be induced by the movement of vacancies only inside the conductive oxide, without any oxygen exchange between the layers.

In this work, stable ${\mathrm{Cs}}_2\mathrm{Ag}M{\mathrm{Cl}}_6(M$ = $\mathrm{Bi},\mathrm{In},\mathrm{Sb}$) double perovskite materials are successfully employed as photoanodes for solar water oxidation. These materials show an extraordinary oxidative stability, where ${\mathrm{Cs}}_2{\mathrm{Ag}\mathrm{In}\mathrm{Cl}}_6$ and ${\mathrm{Cs}}_2{\mathrm{Ag}\mathrm{Bi}\mathrm{Cl}}_6$ are stable between 0 and 1.2 V (vs $\mathrm{Ag}\text{−}\mathrm{Ag}\mathrm{Cl}$) and ${\mathrm{Cs}}_2{\mathrm{Ag}\mathrm{Sb}\mathrm{Cl}}_6$ between 0 and 0.75 V (vs $\mathrm{Ag}\text{−}\mathrm{Ag}\mathrm{Cl}$) over 100 cycles of electrochemical cycling. This enables us to employ these materials for photoelectrochemical (PEC) water oxidation in ${\mathrm{CH}}_3\mathrm{CN}$ and ${\mathrm{H}}_2\mathrm{O}$, with and without ${\mathrm{Ir}\mathrm{O}}_x$ cocatalyst. All three materials show PEC activity, with ${\mathrm{Cs}}_2{\mathrm{Ag}\mathrm{In}\mathrm{Cl}}_6$ showing the highest performance. At 1.23 V (vs a reversible hydrogen electrode), a photocurrent density of 0.5 mA ${\mathrm{cm}}^{-2}$ is observed, and with an applied overpotential of 600 mV, the photocurrent increases to about 0.75 mA ${\mathrm{cm}}^{-2}$. From the band gap of ${\mathrm{Cs}}_2{\mathrm{Ag}\mathrm{In}\mathrm{Cl}}_6$, the estimated theoretical maximum current is around 0.82 mA ${\mathrm{cm}}^{-2}$. So, ${\mathrm{Cs}}_2{\mathrm{Ag}\mathrm{In}\mathrm{Cl}}_6$-coated ${\mathrm{Ir}\mathrm{O}}_x$ gives nearly 60% and about 90% of the theoretical maximum photocurrent at zero bias and at an applied overpotential, respectively. The device is shown to be stable and reproducibility is confirmed.

The electroviscous effect deals with the change in the viscosity of fluids due to an external electric field. Here, we report experimental studies on the electroviscous effects in a ferroelectric nematic liquid crystal. It is synthesized by accomplishing an optimized synthetic route, which provides higher yield than the conventional one. We measure electric-field-dependent viscosity under a steady shear at different temperatures. In the low-field range, the increase in viscosity [$\mathrm{\Delta }\eta =\eta (E)-{\eta }_0$] is proportional to $E^2$ and the corresponding viscoelectric coefficient [$f\approx {10}^{-9}{\mathrm{m}}^2/{\mathrm{V}}^2$] of the ferroelectric nematic is 2 orders of magnitude larger than the apolar nematic liquid crystals and the largest ever measured for a fluid. The apparent viscosity measured under a high electric field shows a power-law divergence $\eta \sim (T-T_c{)}^{-0.7\pm 0.05}$, followed by nearly an order of magnitude drop below the $N$-$N_F$ phase transition. Experimental results within the dynamical scaling approximation demonstrate rapid growth of polar domains under a strong electric field as the $N$-$N_F$ phase transition is approached. The gigantic electroviscous effects demonstrated here are useful for emerging applications and understanding striking electrohydromechanical effects in ferroelectric nematic liquid crystals.

An ideal third-order nonlinear optical material, which can efficiently absorb and refract light over a broad spectral range, is of both fundamental and technological significance as it enables many helpful functionalities in photonics. However, the optical nonlinearities are rather weak due to their perturbative nature and are limited to electronic resonances, thus restricting the response to a narrow spectral range. A charge-coupled donor-acceptor material pair can enhance the nonlinear optical response and circumvent the narrow spectral range limitation. However, such studies on potential material pairs remain largely unexplored. Here, we report the experimental observation of ultrafast third-order nonlinear optical response spanning the entire visible-to-near-infrared (400–900 nm) region in single-wall-carbon-nanotube (SWCNT)–${\mathrm{VSe}}_2$ nanohybrid in the strong coupling regime, enabled by efficient charge transfer. Compared to control systems, the measured nonlinear absorption and refraction of the nanohybrid show unprecedented or many-fold enhancements. Further, our density-functional theory and Bader-charge analysis show the strong electronic coupling of the nanohybrid in which the electrons are transferred from ${\mathrm{VSe}}_2$ to SWCNT, verified by steady-state and time-resolved photoluminescence measurements. The physics of the ultrafast nonlinear optical response is well captured by our five-level rate-equation model both qualitatively and quantitatively. Using the nanohybrid, we design a liquid cell-based optical limiter with an order of magnitude better device performance parameters, such as the optical limiting onset ($2.5–8.0\mathrm{mJ}{\mathrm{cm}}^{-2}$) and the differential transmittance ($0.42–0.62$), compared to several other benchmark optical limiters in the femtosecond regime.

We study permeability ($\mu$) variation of a lithographically prepared magnetic meta-atom consisting of $\mathrm{Ta}$/Py/$\mathrm{Pt}$ trilayers by means of spin-torque ferromagnetic resonance (ST FMR). With an injection of a direct current up to $\pm 20\mathrm{mA}$ together with an alternating current in GHz frequencies, the meta-atom under external magnetic fields shows significant changes in position and width of ST FMR signals due to a massive spin-current injection. This leads to the spin-current-driven $\mu$ variation, which is verified by analytical calculation based upon the experimentally obtained resonance field and Gilbert-damping parameter. The present study paves a way to time-varying spintronic metamaterials with a high modulation frequency for realizing microwave sources toward the post-fifth-generation mobile-communication system.

Due to the exaggerated properties of highly excited atomic states, a Rydberg-atom-based microwave receiver holds the promise to achieve superior performances in microwave metrology and communication when compared to conventional antennas. However, developing long-distance communication applications based on Rydberg atomic receiver remains challenging, because it requires the simultaneous achievement of low bit error rate (BER), high symbol rate, and weak carrier amplitude. The key to tackle this problem is to encode the information with a proper scheme that maintains high sensitivity and robustness of the Rydberg atomic receiver. Here, we use Rydberg atom-based heterodyne detection for the reception of phase-modulated microwave field, and demonstrate more than one order of magnitude improvement in reception sensitivity compared to the previously investigated amplitude-modulation scheme. Therefore, we achieve low BER communication with high symbol rate of 12 k bit/s and weak carrier amplitude of $13\text{μ}\mathrm{V}{\mathrm{cm}}^{-1}$ simultaneously. Furthermore, we also demonstrate that our scheme is compatible with frequency-division-multiplexing communication.

The strongly correlated material ${\mathrm{La}}_{0.7}{\mathrm{Sr}}_{0.3}{\mathrm{Mn}\mathrm{O}}_3$ ($\mathrm{LSMO}$) exhibits metal-to-insulator and magnetic transition near room temperature. Although the physical properties of $\mathrm{LSMO}$ can be manipulated by strain, chemical doping, temperature, or magnetic field, they often require large external stimuli. To include additional flexibility and tunability, we developed a hybrid optoelectronic heterostructure that uses photocarrier injection from cadmium sulfide ($\mathrm{Cd}\mathrm{S}$) to an $\mathrm{LSMO}$ layer to change its electrical conductivity. $\mathrm{LSMO}$ exhibits no significant optical response; however, the $\mathrm{Cd}\mathrm{S}/\mathrm{LSMO}$ heterostructures show an enhanced conductivity, with a resistance drop of about 37%, at the transition temperature under light stimuli. This enhanced conductivity in response to light is comparable to the effect of a 9 T magnetic field in pure $\mathrm{LSMO}$. Surprisingly, the optical and magnetic responses of $\mathrm{Cd}\mathrm{S}/\mathrm{LSMO}$ heterostructures are decoupled and exhibit different effects when both stimuli are applied. This unexpected behavior shows that heterostructuring strongly correlated oxides may require a new understanding of the coupling of physical properties across the transitions and provide the means to implement new functionalities.

Nonadiabatic geometric quantum computation (NGQC) has attracted a lot of attention for noise-resilient quantum control. However, previous implementations of NGQC require long evolution paths that make them more vulnerable to incoherent errors than their dynamical counterparts. In this work, we experimentally realize a universal short-path nonadiabatic geometric gate set (SP NGQC) with a 2-times shorter evolution path on a superconducting quantum processor. Characterizing with both quantum process tomography and randomized benchmarking methods, we report an average single-qubit gate fidelity of $99.86\mathrm{\%}$ and a two-qubit gate fidelity of $97.9\mathrm{\%}$. Additionally, we demonstrate superior robustness of single-qubit SP NGQC gate to Rabi frequency error in some certain parameter space by comparing their performance to those of the dynamical gates and the former NGQC gates.

Tightly focusing cylindrical vector beams (CVBs) are widely used in diverse applications from particle trapping to high-resolution microscopy. Recently, planar metasurfaces have been proposed as promising alternatives to traditional bulky optical devices for tightly focused CVB generation. However, few metasurfaces allow the generation and independent phase control of different CVBs. In this paper, cascaded dual-channel transmissive metasurfaces that can be used to simultaneously generate different CVBs and independently control their phases are designed. The polarization and phase of the transmitted waves are controlled independently via only two geometric parameters. Under $x$- and $y$-polarized wave incidences, the radially polarized beam (RPB) and azimuthally polarized beam (APB) are generated by a first metasurface. In addition, a second metasurface is used to produce different focal fields in different focal planes under RPB and APB excitations. In such a proposed approach, the phase responses of the RPB and APB are distinct, which is significantly different from existing designs. A regular polygonlike focus and a lateral displacement of the focal spot are, respectively, realized in different focal planes at 7.8 GHz through different channels. Our approach improves the flexibility of focusing, allowing the generation of focal arrays at different locations within the same focal plane. Since the electromagnetic energy can be confined to a small specific zone in the microwave band, the proposed method can provide the basis for a number of important applications in the medical domain, such as the accurate hyperthermia treatment of large area tumors.

We realize a superconducting nanodevice in which vortex traps in the form of an aluminum square are integrated with a Dayem nanobridge. We perform field cooling of the traps arriving to different vortex configurations, dependent on the applied magnetic field, to demonstrate that the switching current of the bridge is highly sensitive to the presence and location of vortices in the trap. Our measurements exhibit unprecedented precision and ability to detect the first and successive vortex entries into all fabricated traps, from few hundred nm to $2\text{μ}\mathrm{m}$ in size. The experimental results are corroborated by Ginzburg-Landau simulations, which reveal the subtle yet crucial changes in the density of the superconducting condensate in the vicinity of the bridge with every additional vortex entry and relocation inside the trap. An ease of integration and simplicity make our design a convenient platform for studying dynamics of vortices in strongly confining geometries, involving a promise to manipulate vortex states electronically with simultaneous in situ control and monitoring.

Frequency interface, namely, converting the colors of photons while retaining their other features, is indispensable for numerous applications ranging from up-conversion imaging to the multimode quantum network. However, limited by the nonuniform nonlinear frequency-conversion efficiency, constructing the frequency interface for high-dimensional spatial modes is still challenging. Here, to break this fundamental barrier, we construct a high-dimensional (HD) orbital-angular-momentum (OAM) frequency interface via conducting sum-frequency generation between HD OAM states to be converted and a radial structured pump. Specifically, by replacing the conventional Gaussian pump with an optimized coherent superposition of HD Laguerre-Gaussian radial modes (i.e., its azimuthal number is zero), we experimentally achieve the OAM frequency interface in nine dimensions with a record fidelity up to 90.38 %. Additionally, by increasing the number of radial modes, our scheme can be flexibly extended to higher dimension. Such a high-dimensional frequency interface could find potential applications in the high-capacity free-space optical communication and would also pave the way for constructing high-dimensional quantum networks.

We report a combined theoretical and experimental study of the topological semimetal CrFeVGa with an emphasis on the role of atomic disorder on the magnetoelectronic properties. CrFeVGa belongs to the quaternary Heusler alloy family and crystallizes in the cubic structure with B2 disorder. It is found that the disorder plays a crucial role in quenching the magnetization (net moment $\sim 5\times {10}^{-2}{\mu }_B$ per formula unit) and other anomalies. Ac and dc magnetization data reveal the occurrence of Griffith’s-phase-like behavior in the presence of small magnetic clusters with a weak antiferromagnetic or ferrimagnetic ordering. A nonsaturating, linear positive magnetoresistance is observed even at 70 kOe, in a wide temperature range, which is attributed to the quantum linear magnetoresistance arising due to the zero- or small-gap band structure. Hall measurements show some anomalous behavior (including an anomalous Hall conductivity ${\sigma }_{xy0}=270\mathrm{S}{\mathrm{cm}}^{-1}$ and an anomalous Hall angle of 0.07 at 2 K) with a significant contribution from the semimetallic bands. Hall data analysis also reveals the presence of some non-negligible topological Hall contribution, which is significant at low temperatures. Ab initio calculations confirm the topological Weyl behavior of CrFeVGa, which originates from a unique combination of broken time-reversal symmetry and noncentrosymmetry. The nontrivial band topology stems from the $p$ and $d$ states of vanadium, which overlap near the Fermi level. The presence of multi-Weyl points (24 pairs) near the Fermi level causes a large Berry curvature and hence reasonably high anomalous Hall conductivity. The coexistence of so many emerging features in a single material is rather rare and thus opens up new avenues for future topological and spintronics-based research.

A wide variety of practical applications utilize three-dimensional (3D) hollow spots. To improve their performance, it is critical to independently control the transverse and longitudinal size of a 3D hollow spot. Here, we propose a method of creating a superoscillation 3D hollow spot by properly engineering the spatial polarization distribution. A cylindrical vector wave can be generated and the transverse and longitudinal sizes of the 3D hollow spot can be independently controlled by individually optimizing the wave fronts of the radial and azimuthal components in the cylindrical vector wave. A cylindrical vector metalens integrated with the functions of polarization conversion and wave-front manipulation is optimized and fabricated. The generated 3D hollow spot is experimentally demonstrated with a transverse size of (0.336 ± 0.005)λ and a longitudinal size of (1.170 ± 0.005)λ. Our method provides an easy and practical approach to generate a 3D hollow spot with a subwavelength transverse size, which can be intriguing for nanoparticle manipulation and in nanomicroscopy.