Metamaterial-based acoustic wave manipulation shows great potential in effective acoustic energy confinement and low-frequency acoustic isolation. We numerically and theoretically propose here a concept based on multilateral metasurfaces for reflected acoustic focusing and energy confinement. The theoretical phase-shift profile required for reflected wave focusing and governed by the generalized Snell’s law can be discretely realized by appropriately arraying the labyrinthine units in the right sequences. Based on this design, multilateral metasurfaces for acoustic wave focusing and energy confinement under point-source incidence are considered and sufficiently investigated. The coupling effects and multiple reflections between or among metasurfaces, which play a significant role in the energy confinement, are initially analyzed and discussed. We show that the acoustic focusing and confinement increase with the sides of the multilateral metasurfaces as anticipated. In addition to the contribution of the first reflection, multiple reflections also contribute to the acoustic focusing and energy confinement, especially when the metasurfaces are configured in parallel. The proposed multilateral metasurfaces should have excellent performance in acoustic energy confinement in various situations due to the variable designs and strong acoustic focusing capabilities.

We report a giant perpendicular exchange bias $H_{\mathrm{ex}}^{\mathrm{per}}$ exhibited in the $R\text{−}H$ curves of an inverted $(\mathrm{Co}/\mathrm{Pt}{)}_n/\mathrm{Co}/\mathrm{IrMn}$ multilayer and an inverted $(\mathrm{Co}/\mathrm{Pt}{)}_n/\mathrm{Co}/\mathrm{Cu}/(\mathrm{Co}/\mathrm{Pt}{)}_4/\mathrm{Co}/\mathrm{IrMn}$ spin-valve structure, in which the Pt layers are only $1/4\mathrm{nm}$ in thickness and thinner than the Co layers. $H_{\mathrm{ex}}^{\mathrm{per}}$ shows an unusual linear scaling with the repetition $n+1$ of the $(\mathrm{Co}/\mathrm{Pt}{)}_n/\mathrm{Co}$ free layer, reaching 792 Oe for $n+1=9$. In contrast, the in-plane exchange bias $H_{\mathrm{ex}}^{\mathrm{in}}$ is almost constant. This effect originates from $\mathrm{Co}/\mathrm{IrMn}$ interfacial magnetic coupling in the inverted $(\mathrm{Co}/\mathrm{Pt}{)}_4/\mathrm{Co}/\mathrm{IrMn}$ reference layer and is magnified by a near-perpendicular exchange coupling between the interface Co layer and the multilayer $(\mathrm{Co}/\mathrm{Pt}{)}_n$ ($n=4$ for the spin-valve structures) in the reference layer. The latter also leads to a linear scaling of the exchange bias with the number of repetitions of $\mathrm{Co}/\mathrm{Pt}$ in the reference layer.

The stepwise growth of epitaxial Fe on $\mathrm{Cu}(001)/\mathrm{Si}(001)$, investigated by in situ polarized neutron reflectometry is presented. A sputter deposition system was integrated into the neutron reflectometer AMOR at the Swiss neutron spallation source SINQ, which enables the analysis of the microstructure and magnetic moments during all deposition steps of the Fe layer. We report on the progressive evolution of the accessible parameters describing the microstructure and the magnetic properties of the Fe film, which reproduce known features and extend our knowledge on the behavior of ultrathin iron films.

Chiroptical effects characterized by different optical responses for left- (LCP) and right-handed circularly polarized light (RCP) are powerful and valuable tools in optics with wide applications in polarization-resolved imaging and sensing. Previously observed strong chiroptical effects are limited to metamaterials with complex three-dimensional chiral structures at the subwavelength scale. Although asymmetrical transmission of LCP and RCP have been investigated in planar chiral metasurfaces, the observed weak chiroptical effects result from anisotropic Ohmic dissipation of the metal constituents. Here, we demonstrate by theory and proof-of-concept experiments that a large difference in transmittances of LCP and RCP can be attained in a single-layer planar chiral metamaterial with a subwavelength thickness. Without violating the reciprocity and mirror symmetry, the strong chiroptical effect, independent of dielectric loss, arises from a mechanism of multimode interference. The described effect may lead to a gateway towards chiral manipulations of light and chiral optical devices.

We investigate synchronization in a population of mobile pulse-coupled agents with a view towards implementations in swarm-robotics systems and mobile sensor networks. Previous theoretical approaches dealt with range and nearest-neighbor interactions. In the latter case, a synchronization-hindering regime for intermediate agent mobility is found. We investigate the robustness of this intermediate regime under practical scenarios. We show that synchronization in the intermediate regime can be predicted by means of a suitable metric of the phase response curve. Furthermore, we study more-realistic $K$-nearest-neighbor and cone-of-vision interactions, showing that it is possible to control the extent of the synchronization-hindering region by appropriately tuning the size of the neighborhood. To assess the effect of noise, we analyze the propagation of perturbations over the network and draw an analogy between the response in the hindering regime and stable chaos. Our findings reveal the conditions for the control of clock or activity synchronization of agents with intermediate mobility. In addition, the emergence of the intermediate regime is validated experimentally using a swarm of physical robots interacting with cone-of-vision interactions.

The efficiency of terahertz emission by the photo-Dember effect from bare InAs, GaAs, and InSb samples as a function of the excitation-light wavelength is investigated to demonstrate that an electron-hole kinetic-energy equilibrium is rapidly reached, before the radiation peak. Numerical and experimental results show clear features that match the onset of electron scattering from the $\mathrm{\Gamma }$ valley to the $L$ and $X$ valleys. An ambipolar-based simulation model is developed to describe the above characteristics and to explain the experimental pump-fluence dependence of radiation emission from InAs. Results illustrate the potential of terahertz excitation spectroscopy to probe the electron density of states deep in the bands of semiconductor materials and obtain a depth profile of the dynamic forces acting on carriers at a subpicosecond time scale. Moreover, the potential of the further development of numerical models intrinsically including electron-hole attraction to describe photoinduced hot-carrier dynamics is explained.

Pancharatnam-Berry (PB) metasurfaces exhibit strong abilities to control spin-polarized light, but transmission-mode PB functional devices exhibiting simultaneously nearly 100% efficiencies and ultrathin thicknesses are rarely seen. Here, we show that 100%-efficiency photonic spin Hall effect (PSHE) can be realized in ultrathin PB metasurfaces exhibiting both electric and magnetic responses satisfying certain criteria, and then we design and fabricate a microwave transmissive PB metasurface with thickness of approximately $\lambda /8$ yet exhibiting a maximum PSHE efficiency of approximately 91% in experiments (approximately 94% in full-wave simulations). Our results can stimulate the realizations of high-performance PB metadevices with diversified functionalities at different frequencies. As an example, we fabricate several vortex-beam generators and demonstrate that they not only exhibit ultrahigh working efficiencies but also are immune from normal-mode background interferences.

We demonstrate hybrid photoelectric pulse protocols for reading the spin states of nitrogen-vacancy ($\mathrm{N}\text{−}V$) centers in diamond, compatible with coherent spin control and performed on shallow nitrogen-implanted electronic grade diamond. The measurements are carried out on spin ensembles from 1000 to just five $\mathrm{N}\text{−}V$ centers as a first step towards the fabrication of scalable photoelectric quantum chips. Specific microwave protocols are developed that suppress background photocurrent related to ionization of ${{\mathrm{N}}_S}^0$ defects and provide a high contrast and SNR. The technique is demonstrated on Rabi and Ramsey sequences.

Superconducting spintronics in hybrid superconductor-ferromagnet ($S\text{−}F$) heterostructures provides an exciting class of device. The prototypical superspintronic device is the superconducting spin valve, where the critical temperature $T_c$ of the $S$ layer can be controlled by the relative orientation of two (or more) $F$ layers. Here, we show that such control is also possible in a simple $S\text{−}F$ bilayer. Using field history to set the remanent magnetic state of a thin Er layer, we demonstrate for a $\mathrm{Nb}/\mathrm{Er}$ bilayer a high level of control of both $T_c$ and the shape of the resistive transition $R(T)$ to zero resistance. We are able to model the origin of the remanent magnetization, treating it as an increase in the effective exchange field of the ferromagnet and link the remanent magnetization using conventional $S\text{−}F$ theory to the suppression of $T_c$. We observe stepped features in the $R(T)$, which we argue is due to a fundamental interaction of superconductivity with inhomogeneous ferromagnetism, a phenomena currently lacking theoretical description.

We propose a model that describes the operation characteristics of a two-dimensional electron gas (2DEG) in a monolayer transition-metal dichalcogenide thin-film transistor (TFT) having trapped charges near the channel interface. We calculate the drift mobility of the carriers scattered by charged defects located in the channel or near the channel interfaces. The calculated drift mobility is a function of the 2DEG areal density of interface traps. Finally, we calculate the model transfer ($I_D-V_{GS}$) and output ($I_D-V_{SD}$) characteristics and verify them by comparing with the experimental results performed with monolayer ${\mathrm{MoS}}_2$ TFTs. We find the modeled results to be excellently consistent with the experiments. This proposed model can be utilized for measuring the interface-trapped charge and trap site densities from the measured transfer curves directly, avoiding more complicated and expensive measurement methods.

We present a method that enables performing x-ray phase-contrast imaging (XPCI) computed tomography with a laboratory setup using a single image per projection angle, eliminating the need to move optical elements during acquisition. Theoretical derivation of the method is presented, and its validity conditions are provided. The object is assumed to be quasihomogeneous, i.e., to feature a ratio between the refractive index and the linear attenuation coefficient that is approximately constant across the field of view. The method is experimentally demonstrated on a plastics phantom and on biological samples using a continuous rotation acquisition scheme achieving scan times of a few minutes. Moreover, we show that such acquisition times can be further reduced with the use of a high-efficiency photon-counting detector. Thanks to its ability to substantially simplify the image-acquisition procedure and greatly reduce collection times, we believe this method represents a very important step towards the application of XPCI to real-world problems.

We provide further theoretical insights and experimental verification of the modal-dispersion-induced effective surface-plasmon polaritons (ESPPs) by engineering the transverse-electric (TE) modes in conventional rectangular waveguides. The complete field distributions, dispersion relations, and asymptotic frequency of the ESPPs are derived analytically. Wave-port excitations and smooth bridges are designed for the mode conversion between propagating modes in rectangular waveguides and the ESPPs. Analytical calculations and numerical simulations are performed for ${\mathrm{TE}}_{10}$- and ${\mathrm{TE}}_{20}$-mode-induced ESPPs, showing excellent agreement. Moreover, we design a double-layered substrate-integrated waveguide showing that ESPPs are supported at the interface between the two layers with different dielectric constants. This work opens up an avenue for low-frequency designer surface plasmons and may find potential applications in the design of compact filters, resonators, and sensors of ESPPs in the microwave and terahertz frequencies.

Complex-network theory has attracted considerable attention for nearly a decade, and it enables us to encompass our understanding of nonlinear dynamics in complex systems in a wide range of fields, including applied physics and mechanical, chemical, and electrical engineering. We conduct an experimental study using a pragmatic online detection methodology based on complex-network theory to prevent a limiting unstable state such as blowout in a confined turbulent combustion system. This study introduces a modified version of the natural visibility algorithm based on the idea of a visibility limit to serve as a pragmatic online detector. The average degree of the modified version of the natural visibility graph allows us to detect the onset of blowout, resulting in online prevention.

We present the generation of continuous-wave optical squeezing from a titanium–in-diffused lithium niobate waveguide resonator. We directly measure $2.9\pm 0.1\mathrm{dB}$ of single-mode squeezing, which equates to a produced level of $4.9\pm 0.1\mathrm{dB}$ after accounting for detection losses. This device showcases the current capabilities of this waveguide architecture and precipitates more complicated integrated continuous-wave quantum devices in the continuous-variable regime.

We use first-principles methods to investigate the effects of collective lattice excitations on the formation of oxygen vacancies in perovskite thin films. We find that phonons play a crucial role in the strain-mediated control of defect chemistry at finite temperatures. Specifically, zero-temperature oxygen-vacancy formation trends deduced as a function of epitaxial strain can be fully reversed near room temperature. Our first-principles calculations evidence a direct link between the lattice contribution to the oxygen-vacancy free energy and the volume expansion that the system undergoes when it is chemically reduced: The larger the resulting volume expansion, the more favorable thermal excitations are to point-defect formation. However, the interplay between the vibrational vacancy entropy—or, equivalently, chemical expansion—and epitaxial strain is difficult to generalize, as it can be strongly influenced by underlying structural and magnetic transitions. In addition, we find that vacancy ordering can be largely hindered by the thermal lattice excitations.

We study high-frequency generation in a system of electromagnetically coupled semiconductor superlattices fabricated on the same doped substrate. Applying a bias voltage to a single superlattice generates high-frequency current oscillations. We demonstrate that within a certain range of the applied voltage, the current oscillations within the superlattices can be self-synchronized, which leads to a dramatic rise in the generated microwave power. These results, which are in good agreement with our numerical model, open a promising practical route towards the design of high-power miniature microwave generators.

Injection-type ballistic rectifiers are realized as asymmetric cross junctions. Their output-voltage ($V_{\text{out}}$) input-current ($I_{\text{in}}$) characteristics are known as parabola $V_{\text{out}}={\alpha }^2{I}_{\text{in}}^2$, where ${\alpha }^2$ is the curvature coefficient. Previous experiments on widely separated dual-stage ballistic rectifiers yielded the total output signal to be the sum of that of the single stages $V_{\text{out}}={\alpha }_1^2{I}_{\text{in}1}^2+{\alpha }_2^2{I}_{\text{in}2}^2$. Starting from an ostensive model containing the ballistic length as critical size, we show that sufficiently closely positioned stages lead to input-current addition $V_{\text{out}}={({\alpha }_1{I}_{\text{in}1}+{\alpha }_2{I}_{\text{in}2})}^2$. We demonstrate two consequences of current addition. First, the excess voltage with respect to voltage addition $V_{\text{exc}}=2{\alpha }_1{\alpha }_2{I}_{\text{in}1}{I}_{\text{in}2}$ is proportional to the product $I_{\text{in}1}{I}_{\text{in}2}$. Second, sign-dependent current cancellation results in a shift of the $V_{\text{out}}(I_{\text{in}1})$ parabola vertex at fixed $I_{\text{in}2}$ or, at $I_{\text{in}1}=-I_{\text{in}2}$, to vanishing output voltage. The regime of intermediate-stage separation is discussed in terms of partial overlap of the injected charge clouds, leading to a smaller excess voltage. The results should provide both a deeper insight into the microscopic mechanism of ballistic rectification and novel device applications.

We describe a macroscopic beam splitter for polar neutral molecules. A complex electrode structure is required for the beam splitter which would be very difficult to produce with traditional manufacturing methods. Instead, we make use of a nascent manufacturing technique: 3D printing of a plastic piece, followed by electroplating. This fabrication method opens a plethora of avenues for research, since 3D printing imposes practically no limitations on possible shapes, and the plating produces chemically robust, conductive construction elements with an almost free choice of surface material. It has the added advantage of dramatically reduced production cost and time. Our beam splitter is an electrostatic hexapole guide that smoothly transforms into two bent quadrupoles. We demonstrate the correct functioning of this device by separating a supersonic molecular beam of ${\mathrm{ND}}_3$ into two correlated fractions. It is shown that this device can be used to implement experiments with differential detection wherein one of the fractions serves as a probe and the other as a reference. Reverse operation would allow the merging of two beams of polar neutral molecules.

We discuss heat transport in a thermally biased superconducting quantum-interference device (SQUID) in the presence of an external magnetic flux, when a non-negligible inductance of the SQUID ring is taken into account. A properly sweeping driving flux causes the thermal current to modulate and behave hysteretically. The response of this device is analyzed as a function of both the hysteresis parameter and the degree of asymmetry of the SQUID, highlighting the parameter range over which hysteretic behavior is observable. Markedly, the temperature of the SQUID also shows hysteretic evolution, with sharp transitions characterized by temperature jumps up to, e.g., approximately 0.02 K for a realistic Al-based setup. In view of these results, the proposed device can effectively find an application as a temperature-based superconducting memory element, working even at gigahertz frequencies by suitably choosing the superconductor on which the device is based.