The hybridization of magnons and phonons is playing a critical role in the emerging field of hybrid magnonics because it combines the high tunability of magnetism with the long lifetime of mechanics for comprehensive coherent information processing. Recently there has been increasing interest in thin-film bulk acoustic waves because of their long lifetimes at high frequencies. However, the unique multimode nature of such phonon modes has not been exploited as an important resource for coherent information processing. In this Letter we study the simultaneous hybridization of multiple high-overtone bulk acoustic resonances with a magnon and a microwave mode. The demonstrated multimode hybridization allows us to observe coherent pulse echoes, opening opportunities for both fundamental studies and practical applications of hybrid magnonics.

Rydberg electromagnetically induced transparency (EIT) enables the realization of atom-based Systeme-International-traceable microwave sensing, imaging, and communication devices by exploiting the strong microwave electric dipole coupling of highly excited Rydberg states. Essential to the development of robust devices is a careful characterization of the sensor performance and systematic uncertainties. In this work, we present a comparison of microwave-induced EIT splitting in a cesium atomic vapor for four possible Rydberg couplings, $65S_{1/2}\to 65P_{1/2}$, $66S_{1/2}\to 66P_{3/2}$, $79D_{5/2}\to 81P_{3/2}$, and $62D_{5/2}\to 60F_{7/2}$, at microwave transition frequencies around 13 GHz. Our work highlights the impact of multiphoton couplings on neighboring Rydberg states in breaking both the symmetry and linearity of the observed splitting, with excellent agreement between experimental observations and a theoretical model accounting for multiphoton couplings. We identify an optimal angular-state choice for robust microwave measurements, as well as demonstrating an alternative regime in which microwave polarization can be measured.

The current due to cold tunneling of electrons from a metallic surface exposed to high electric fields, regularly named “dark current,” is commonly described in modern literature using an analytical approximate solution provided by Murphy and Good [Phys. Rev. 102, 1464 (1956)]. This expression, which corrects earlier work by Fowler and Nordheim, is a Fowler-Nordheim-type equation: $I\sim E^2\exp (-a/E)$, where $I$ is the dark current, $E$ is the local electric field, and $a$ is a system-specific constant. In this paper, a numerical approximation, rather than the analytical one given by Murphy and Good, is presented. This approximation is accurate over a wide range of fields, and is used to derive the effective field enhancement factor $\beta$. On the basis of this approximation, and considering local field and current fluctuations, two alternative methods for $\beta$ estimation are presented. These methods allow instantaneous field-specific estimation of $\beta$, rather than the average estimate derived with current methods. The applicability of fluctuation-based methods is demonstrated by numerical simulation in a variety of conditions. The methods are applied to estimate $\beta$ using fluctuation analysis in experimental measurements that were not dedicated for this purpose. An open-source code for the implementation of fluctuation-derived $\beta$ estimation is provided, with an analysis of possible future experimental opportunities using dedicated experiments.

Ultrasound focusing, with strong penetrability and superior biocompatibility, has attracted substantial interest in biomedical science. Conventional focusing techniques generally rely on an active transducer array or curved lens, and acoustic metamaterials have recently emerged as an alternative approach. However, the low operating frequency of an ultrasound focusing metasurface below 1 MHz and the narrow bandwidth hinder its broad applications in practice. Here, we experimentally realize broadband ultrasound focusing at megahertz frequencies based on a compact and passive metasurface. Three-dimensional spot focusing, ranging from 0.5 to 1.4 MHz, with high spatial resolution (Full width at half maximum of 0.63λ and full length at half maximum of 2.75λ at 1 MHz) and tunable focal length (18.5–71 mm), is demonstrated. The focusing pattern is further enriched by realizing a broadband ultrasound scalpel, which effectively converges sound energy within a sharp “needle” area. Additionally, accompanied by an enhancement of ultrasound intensity, a prominent temperature rise appears in the focal region, the dependence of which on the ultrasound frequency is investigated and the optimized operating frequency is revealed. Broadband and subwavelength ultrasound spot focusing and a scalpel at megahertz frequencies would promote applications in particle manipulation, nondestructive detection, diagnostic sonography, and ultrasound therapy.

Modeling of large-scale research facilities is extremely challenging due to complex physical processes and engineering problems. Here, we adopt a data-driven approach to model the longitudinal phase-space-diagnostic beamline at the photoinector of the European XFEL with an encoder-decoder neural-network model. A deep convolutional neural network (decoder) is used to build images, measured on the screen, from a small feature map generated by another neural network (encoder). We demonstrate that the model, trained only with experimental data, can make high-fidelity predictions of megapixel images for the longitudinal phase-space measurement without any prior knowledge of photoinjectors or electron beams. The prediction significantly outperforms existing methods. We also show the scalability and interpretability of the model by sharing the same decoder with more than one encoder, used for different setups of the photoinjector, and propose a pragmatic way to model a facility with various diagnostics and working points. This opens the door to a way of accurately modeling a photoinjector using neural networks and experimental data. The approach can possibly be extended to the whole accelerator and even other types of scientific facility.

The investigation of the electronic properties of semiconductors from transport measurements (i.e., resistivity, Hall, and Seebeck coefficient measurements) is challenging due to the averaging of charge-carrier dynamics inherent in such measurements. Here, we investigate the incorporation of a fourth measurement of electronic transport, the Nernst coefficient, into the analysis, termed the method of four coefficients. This approach yields the Fermi level, effective mass, scattering exponent, and relaxation time prefactor. We begin with a review of the underlying mathematics and investigate the mapping between the four-dimensional material property and transport coefficient spaces. We then investigate how the traditional single-parabolic band method yields a single, potentially incorrect point on the solution subspace. This uncertainty can be resolved through Nernst coefficient measurements and we map the span of the ensuing subspace. We conclude with an investigation of how sensitive the analysis of transport coefficients is to experimental error for different sample types.

Noncollinear antiferromagnets have promising potential for replacing ferromagnets in the field of spintronics as high-density devices with ultrafast operation. To take full advantage of noncollinear antiferromagnets in spintronics applications, it is important to achieve efficient manipulation of noncollinear antiferromagnetic spin. Here, using the anomalous Hall effect as an electrical signal of the triangular magnetic configuration, spin-orbit-torque switching with no external magnetic field is demonstrated in noncollinear antiferromagnetic antiperovskite manganese nitride ${\mathrm{Mn}}_3\mathrm{Ga}\mathrm{N}$ at room temperature. The pulse-width dependence and subsequent relaxation of Hall signal behavior indicate that the spin-orbit torque plays a more important role than the thermal contribution due to pulse injection. In addition, multistate memristive switching with respect to pulse current density is observed. The findings advance the effective control of noncollinear antiferromagnetic spin, facilitating the use of such materials in antiferromagnetic spintronics and neuromorphic computing applications.

Recent reports on spin structures produced in nanomaterials due to confinement of spins imposed by geometrical restrictions are at the center of rising scientific interest. Topological curling magnetic structures (vortices) exhibit unique properties, regarding the energy profile, good colloidal stability in suspensions, manipulation under a low-frequency magnetic field, and torque exertion. The last property provides the potential to mechanically eradicate cancer cells via magnetomechanical actuation using remote ac magnetic fields. Here, we study, theoretically and by micromagnetic simulations, the magnetic energy evolutions for vortex nanosystems, i.e., ${\mathrm{Fe}}_3{\mathrm{O}}_4$ nanodisks (NDs) and nanorings (NRs). The obtained results for magnetic energy, magnetic susceptibility, and magnetization reversal confirm that the vortex-domain structure in NRs exhibits better stability and avoids agglomeration in solution, owing to the presence of a central hole, whereas the presence of a vortex core in NDs induces magnetic remanence. Although NDs are found to exert slightly higher torques than NRs, this weakness can be compensated for by a small increase (i.e., approximately equals 20%) in the amplitude of the applied field. Our results provide evidence of the magnetic stability of the curling ground states in NRs and open the possibility of applying these systems to magnetomechanical actuation on single cells for therapeutics in biomedicine, such as cancer-cell destruction by low-frequency torque transduction.

Recent studies have revealed that Janus structures and heterobilayers made from them might have properties superior to those of two-dimensional (2D) materials. We construct 2D monolayer boron phosphide (MBP)/$\mathrm{Mo}\mathrm{S}\mathrm{Se}$ and $\mathrm{Mo}\mathrm{S}\mathrm{Se}$/MBP heterobilayers and describe comprehensively their optoelectronic properties in the presence of biaxial in-plane and uniaxial out-of-plane strain and the effects of electric fields using first-principles methods. The electronic bands of both MBP/$\mathrm{Mo}\mathrm{S}\mathrm{Se}$ and $\mathrm{Mo}\mathrm{S}\mathrm{Se}$/MBP heterobilayers display a peculiarity in the direct gap, with different band-gap values in the pristine forms. Remarkably, it is shown that different varieties of band alignments are induced for different orders of the van der Waals (vdW) heterobilayers. Electric fields and in-plane and out-of-plane strain give rise to important changes in the electronic and optical properties. The band alignments can transform from type I to type II under the influence of an electric field and strain. The main absorption peaks of pristine forms of both heterobilayers have in the visible region (approximately 2.67 eV), while in the presence of biaxial strain are redshifted. Since the absorption peaks of the pristine forms of the heterobilayers are in the visible region, the heterobilayers recommended here can be used in photovoltaic applications, and the presence of effects of external electric fields and strain promises functional optoelectronic devices.

We propose a mechanism to substantially rectify radiative heat flow by having one side of the thermal diode offering a large contrast in the local density of electromagnetic states, while the opposite side serving as a narrow bandpass filter that limits the transport to a desired spectral range. To demonstrate, we realize such a scenario by matching thin films of metal-to-insulator transition materials and polar dielectrics in the near field. By leveraging the distinct scaling behaviors of the local density of states with film thickness for metals and insulators, we theoretically achieve rectification ratios over 140, a 10-fold improvement over the state of the art, with nanofilms of vanadium dioxide and cubic boron nitride in the parallel-plane geometry at experimentally feasible gap sizes (∼100 nm). Our rational design offers relative ease of fabrication, flexible choice of materials, and robustness against deviations from optimal film thicknesses. We expect this work to facilitate the application of thermal diodes in solid-state thermal circuits and energy-conversion devices.

We evaluate the field-dependent energy barriers associated with annihilation of a Néel skyrmion in ultrathin magnetic nanodots with interfacial Dzyaloshinskii-Moriya interaction. Three scenarios of the skyrmion annihilation are considered: expansion or contraction of the radius of the skyrmion and skyrmion escape through the dot boundary. For typical parameters of $\mathrm{Co}$/$\mathrm{Pt}$ circular nanodots, we find that at zero magnetic field, the energy barrier associated with a displacement of the skyrmion core is always lower than that corresponding to its expansion or contraction. Importantly, the energy barrier value is small, so that the skyrmion in ultrathin dots is unstable versus thermal fluctuations at room temperature for a long time scale. However, we find that the skyrmion can be stabilized by an out-of-plane magnetic field applied parallel to its core. The increase of the energy barriers corresponds to a magnetic field interval, where skyrmions of small and large radius coexist and the large-core-radius skyrmion is more stable than the small one. In this field region, the skyrmion can be stable for hours before it is annihilated by thermal fluctuations escaping through the dot edge.

Terahertz (THz) absorbers have recently attracted extensive attention for their promising potential in various applications; however, many existing THz absorbers are restrained by their narrow bandwidth and complicated and costly fabrication process that renders them unfavorable for practical devices. Herein, we propose a stereoscopic multilayered ultrabroadband THz metamaterial absorber by stacking multilayer concentric resonators on different-level top surfaces of a monolithic three-dimensional (3D) pagodalike substrate. By taking full advantage of the 3D printing technique, the proposed ultrabroadband absorber can be produced efficiently in an easy three-step process that overcomes the fabrication complexities of traditional multistep photolithography processes. Additionally, the feasibility and robustness of the proposed fabrication method for common out-of-plane THz narrowband absorbers are also validated, and the absorption capacities of the 3D printed absorbers are numerically and experimentally elucidated. These results might provide an efficient concept and fabrication technique to stimulate many potential applications in emerging THz technologies, such as sensing, imaging, and wireless communications.

When a circularly polarized (CP) Gaussian beam normally irradiates a solid plasma target, the spin angular momentum of the CP beam transforms into the orbital angular momentum (OAM) of the high-order harmonics through the spin-orbital interaction; this provides a promising way to obtain intense attosecond pulses carrying OAM. However, normal irradiation faces realistic challenges in experiments, as one cannot extract the harmonic without interfering with the driving laser. Here, we propose a feasible scheme to generate vortex high-order harmonics by using a CP Gaussian beam obliquely incident to the target. Theoretical analyses and simulation results show that the $n\mathrm{th}$-order harmonic is composed of vortex modes with topological charges from $l=0$ to $|l|=n-1$. The composition ratio depends on the laser focal size and the incident angle. The obtained number of vortex photons is comparable to the normal incidence case at up to an incident angle of ${10}^{\circ }$, which greatly facilitates the experimental arrangement.

A Bloch surface wave (BSW) confined at the surface of a truncated dielectric multilayer is a low-loss platform for chip-scale photonic integration. The guided BSW in a dielectric ridge is confined by the photonic band gap vertically, yet couples into the continuum modes of the multilayered substrate horizontally. We demonstrate that a low-loss BSW waveguide can be realized with the quasibound BSW modes in the continuum by engineering the geometric parameters, due to the destructive interference of multiple leakage channels in the multilayered substrate. In addition, a high-Q BSW resonator is obtained by optimizing the geometric parameter of the multilayered substrate, which provides an alternative way to realize ultrasmall resonators with a high-Q factor. The photonic integrated circuits on a low-loss BSW platform can be utilized for optical communications, enhancing light-matter interactions, and nonlinear optical applications.

Control of the dynamics of mechanical resonators is central to quantum science and metrology applications. Optomechanical control of diamond resonators is attractive owing to the excellent physical properties of diamond and its ability to host electronic spins that can be coherently coupled to mechanical motion. Using a confocal microscope, we demonstrate tunable amplification and damping of the motion of a diamond nanomechanical resonator. Observation of both normal-mode cooling from room temperature to 80 K and amplification into self-oscillations with $60\mu \mathrm{W}$ of optical power is observed via waveguide optomechanical readout. This system is promising for quantum spin optomechanics, as it is predicted to enable optical control of stress-spin coupling with rates of approximately $1$ MHz (100 THz) to ground (excited) states of diamond nitrogen-vacancy centers.

The complex electrical permittivity and magnetic permeability of composite materials made of a polymer binder filled with micron-scale carbonyl iron powder (CIP) are measured between 0.1 and 39 GHz. Permeability is measured in overlapping frequency subbands using two different techniques: a free-space method from 3 to 39 GHz and a coaxial impedance cell from 0.1 to 5 GHz. The dependence on filler concentration is studied for composites based on phosphated CIP R-100F-2. It is found that the static permittivity and permeability of the composites increase logarithmically with increasing percentage of CIP volume loading; this corresponds to Lichtenecker’s law for a mixture of two components. It is demonstrated that by using the R-100F-2 type CIP it is possible to produce single-layer radar-absorbing materials with a relatively small thickness (less than 1.5 mm) and a deep and broad normal-incidence reflectivity minimum (less than −20 dB) from 10 to 30 GHz.

We present the autofocusing and self-healing properties of aberration laser beams (ALBs) in a turbulent media. The ALB is generated by a diffractive optical element (DOE) whose phase profile consists of radial and periodic angular dependence, and then propagated through a turbulent media under weak, moderate, and strong turbulence conditions. The theoretical calculations, numerical simulations, and experimental results are presented. The results show that the presence of turbulence leads to distortion in the spatial structure of ALBs, as well as causes beam wandering. However, the autofocusing properties of ALBs remain invariant. Further, ALBs also possess self-healing abilities in free space as well as in a turbulent media. The self-healing sustains reasonably well even when a large portion of ALB (approximately $60\mathrm{\%}$) is truncated. The distance requires for the self-healing of truncated ALBs remains the same for various percentage of truncation. Particularly, for any amount of truncation up to approximately $60\mathrm{\%}$, ALB self-heals by propagating up to autofocusing distance corresponding to a nontruncated beam.

There has been a growing interest in ultrahigh-purity low-background $\mathrm{Na}\mathrm{I}$($\mathrm{Tl}$) crystals for dark-matter direct searches. Past research indicates that zone refining is an efficient and scalable way to purify $\mathrm{Na}\mathrm{I}$. In particular, $\mathrm{K}$ and $\mathrm{Rb}$—two elements with radioisotopes that can cause scintillation backgrounds—can be efficiently removed by zone refining. However, zone refining has never been demonstrated for ultrahigh-purity $\mathrm{Na}\mathrm{I}$, which has become commercially available recently. In this paper, we show that many common metallic impurities can be efficiently removed via zone refining. A numerical model for predicting the final impurity distribution is developed and used to fit the inductively coupled plasma mass spectrometry (ICPMS) measurement data to determine the segregation coefficient and the initial concentration. Under this scheme, the segregation coefficient for $\mathrm{K}$ is estimated to be $0.57$, indicating that zone refining is still effective in removing $\mathrm{K}$ from ultrahigh-purity $\mathrm{Na}\mathrm{I}$. As zone refining tends to move the impurities to one end, elements with concentrations too low to be measured directly in the unprocessed powder can potentially be detected in the end due to the enrichment. We also present an analysis technique to estimate the initial concentrations of impurities with partial data, which effectively enhances the sensitivity of the spectrometer. Using this technique, the initial concentration of ${}^{85}\mathrm{Rb}$ is estimated to be between 5 parts per trillion (ppt) and 14 ppt at the 90% confidence level (CL), at least 14 times lower than the detection limit of ICPMS and 7 times lower than the current most stringent limit set by the DAMA collaboration by direct counting of radioactive ${}^{87}\mathrm{Rb}$. These results imply that zone refining is a key technique in developing next-generation $\mathrm{Na}\mathrm{I}$-based crystal scintillators for dark-matter direct detection.

In a subcritical reactor, each neutron produces only $k_{\mathrm{eff}}<1$ neutrons per generation (asymptotically and on average), and thus, the neutron population decreases exponentially in the absence of an external source. The chain reaction is thus easy to stop, making such reactors inherently stable. Interestingly, and contrary to common wisdom, there is no relation between $k_{\mathrm{eff}}$, the external source intensity, and the output power of the reactor. Here, various possible strategies to exploit this fact are presented, and they are applied to the design of a rudimentary multilayer system that allows an arbitrarily large number of fissions per source neutron to be reached, while keeping $k_{\mathrm{eff}}<1$ fixed (here, $k_{\mathrm{eff}}=0.97$). The behavior of the amplifier is verified with simple Monte-Carlo transport simulations. The proposal admittedly brings complications and subtleties that need to be studied further, but, if developed, could allow subcritical reactors to be driven with faint neutron sources, e.g., radioisotope based.