The generation of electromechanical frequency combs in both air and liquid environments using a capacitive microresonator array is presented in this paper. In contrast to frequency-comb generation in purely mechanical resonators, we show that the damping-dependent threshold for comb generation can be reduced by parametrically coupling a resonant electrical circuit to the mechanical resonator. A one-dimensional lumped parameter model of the proposed system is presented and semianalytical solutions are developed to investigate the parameters influencing frequency-comb formation under various operating conditions. The results obtained with numerical simulations are experimentally validated using a commercially available microelectromechanical resonator, and frequency combs with a repetition rate sensitive to the force on the mechanical resonator are generated with a single electrical drive in air and in a liquid-filled microfluidic channel. In contrast to prior work on electromechanical frequency combs, this work represents a simple yet robust approach to generating stable combs, thereby enabling its practical use in applications, such as gas sensing and microfluidics.

Laser-driven neutrons arouse outstanding interest because of their promising uses in several fields, from basic science to materials inspection. Many experiments achieved neutron yields [${10}^8-{10}^{10}\mathrm{n}/(\text{sr s})$] suitable for applications. These results were obtained by exploiting high-energy (approximately $10$–$100\mathrm{J}$) lasers working at low repetition rates. Instead, adopting advanced target configurations like near-critical double-layer targets (DLTs) and compact, commercial lasers was slightly considered. Here, a theoretical study is performed to address neutron generation with commercial ($40$–$400\mathrm{TW}$, $1$–$15\mathrm{J}$) systems and DLTs. We investigate proton acceleration and interaction with various materials to induce ($p,n$) reactions. DLTs allow achieving $1$–$2$ orders of magnitude larger neutron yields and maximum energies 3 times higher than with single-layer targets. Then, the feasibility of two materials characterization techniques, namely fast neutron activation analysis and pulsed fast neutron resonance radiography, is assessed. The results indicate that they can be performed with commercial lasers and DLTs.

Dispersion forces (van der Waals force and Casimir force) originating from quantum fluctuations are crucial in the cohesion of microscale and nanoscale particles. The nonadditivity of these forces and the complex geometry of realistic particles make conventional additive algorithms produce unacceptable errors, and it is experimentally challenging to identify the contribution of dispersion forces from the many forces that constitute the cohesion. In this study, we use the fluctuating surface current technique, an exact scattering theory-based nonadditive algorithm, to accurately quantify the influence of geometry on dispersion forces. To characterize the complex geometries, we employ a data-adaptive spatial filtering method and three-level descriptors. Our results determine the influence of multiscale surface fluctuations on dispersion forces. Additionally, we establish a convenient formula for predicting the dispersion forces between realistic particles with complex shapes from the exact Lifshitz solution via multistage corrections.

We report depth-resolved measurements of the Meissner screening profile in several surface-treated $\mathrm{Nb}$ samples using low-energy muon spin rotation. In these experiments, implanted positive muons, whose stopping depths below $\mathrm{Nb}$’s surface are adjusted between approximately $10\mathrm{nm}$ and $150\mathrm{nm}$, reveal the field distribution inside the superconducting element via their spin precession (communicated through their radioactive decay products). We compare how the field screening is modified by different surface treatments commonly used to prepare superconducting radio-frequency cavities used in accelerator beamlines. In contrast to an earlier report [A. Romanenko et al., Appl. Phys. Lett. 104, 072601 (2014)], we find no evidence for any “anomalous” modifications to the Meissner profiles, with all data being well described by a London model. Differences in screening properties between surface treatments can be explained by changes to the carrier mean free paths resulting from dopant profiles near the material’s surface.

We investigate the thermoelectric properties of a hybrid nanodevice composed of a two-dimensional carbon-based material and a superconductor. This system presents nonlinear bipolar thermoelectricity, as induced by the spontaneous breaking of the particle-hole (PH) symmetry in a tunnel junction between bilayer graphene (BLG) and a Bardeen-Cooper-Schrieffer superconductor. In this scheme, the nonlinear thermoelectric effect, predicted and observed in superconductor-insulator-${\mathrm{superconductor}}^{\prime }$ junctions, is not affected by the competitive effect of the Josephson coupling. From a fundamental perspective, the most intriguing feature of this effect is its bipolarity. The capability to open and control the BLG gap guarantees improved thermoelectric performances that reach up to 1 mV/K, regarding the Seebeck coefficient, and a power density of $1\mathrm{nW}/\text{μ}{\mathrm{m}}^2$ for temperature gradients of tens of kelvin. Furthermore, the externally controlled gating can also dope the BLG, which is otherwise intrinsically PH symmetric, giving us the opportunity to investigate the bipolar thermoelectricity, even in the presence of the controlled suppression of the PH symmetry. The predicted robustness of this system could foster further experimental investigations and applications in the near future, thanks to the available nanofabrication techniques.

Open-path dual-comb spectroscopy has emerged as a promising technique for regional multigas monitoring with its conspicuous advantages of broadband spectral coverage, high spectral resolution, and rapid update rate. However, it is challenging to realize its full potential due to the undesirable mutual coherence of the dual-comb source and turbulence in the air path, which hinder it from field-deployed open-path applications. Here, phase-sensitive open-path dual-comb spectroscopy based on free-running combs is reported, in which dual-purpose compensation is proposed to provide immunity against both the time jitter of comb sources and turbulent noise. Broadband and high-fidelity atmospheric amplitude and phase spectra containing gas absorption and dispersion information over a 900-m turbulent air path are acquired. For the rovibrational resonances of ${\mathrm{CO}}_2$ and ${\mathrm{H}}_2\mathrm{O}$ in 6250–6660 ${\mathrm{cm}}^{-1}$, the achieved residual of the amplitude spectrum is no more than 0.01, and the average residual of the phase spectrum is 0.2 mrad, corresponding to about 0.2 as of relative timing noise or a refraction-index change of about 6 × ${10}^{-14}$ over the target path. The precision of concentration retrieval is about 3 ppm for ${\mathrm{CO}}_2$ in 30 s. A simulated gas-leakage measurement validates the dynamic monitoring capability of this system. This highly effective noise-compensation method provides the possibility of deploying the portable configuration and holds the potential to propel environmental protection and atmospheric science.

This paper presents an analytical framework for the analysis of time-varying metal-based metamaterials. Specifically, we particularize the study to time-modulated metal-air interfaces embedded between two different semi-infinite media that are illuminated by monochromatic plane waves of frequency ${\omega }_0$. The formulation is based on a Floquet-Bloch modal expansion, which takes into account the time periodicity of the structure ($T_s=2\pi /{\omega }_s)$ and integral-equation techniques. It allows us to extract the reflection and transmission coefficients as well as to derive nontrivial features about the dynamic response and dispersion curves of time-modulated metal-based screens. In addition, the proposed formulation has an associated analytical equivalent circuit that gives a physical insight into the diffraction phenomenon. Similarities and differences between space- and time-modulated metamaterials are discussed via the proposed circuit model. Finally, some analytical results are presented to validate the present framework. Good agreement is observed with numerical computations provided by a self-implemented finite-difference time-domain (FDTD) method. Interestingly, the present results suggest that time-modulated metal-based screens can be used as pulsed sources (when ${\omega }_s\ll {\omega }_0$), beam formers (${\omega }_s\sim {\omega }_0$) to redirect energy in specific regions of space, and analog samplers (${\omega }_s\gg {\omega }_0$).

The interfacial Dzyaloshinskii-Moriya interaction (iDMI) originating from a heavy-metal/ferromagnet (FM) interface plays a crucial role in stabilizing chiral spin textures in magnetic multilayers. Recently, graphene based on the low-spin-orbit-coupling (SOC) element carbon ($\mathrm{C}$) has shown a significant iDMI owing to the Rashba SOC when deposited on top of $\mathrm{Co}$. A similar enhanced SOC has been reported earlier for other $\mathrm{C}$ allotropes, namely fullerene (${\text{C}}_{60}$) and carbon nanotubes, and a sizeable DMI can be expected to emerge from them. In this context, we elucidate the magnetization-reversal mechanism, the domain-wall (DW) dynamics, and the strength of the iDMI in a $\mathrm{Pd}(4.0\mathrm{nm})/\mathrm{Co}(0.5\mathrm{nm})/{\text{C}}_{60}(t_{{\mathrm{C}}_{60}})/\text{Pd}(2.0\mathrm{nm})$ system with varying $t_{{\mathrm{C}}_{60}}$. The field-induced DW velocity measured in the creep region shows a DW velocity 2 orders of magnitude higher for a sample with 1.6 nm of ${\text{C}}_{60}$, due to a lower depinning field and spin-dependent hybridization at the $\text{Co}/{\text{C}}_{60}$ interface. Further, DMI measurements performed by use of the asymmetric DW expansion method exhibit a systematic increase in the iDMI from $-0.07$ to $-0.46\mathrm{mJ}/{\mathrm{m}}^2$ with an increase in $t_{{\mathrm{C}}_{60}}$. Such an enhanced iDMI turns an achiral Bloch wall into a chiral Néel wall. The emergence of a finite DMI ($D_{\mathrm{Co}/{\mathrm{C}}_{60}}\sim -0.11\mathrm{mJ}/{\mathrm{m}}^2$) from the $\text{Co}/{\text{C}}_{60}$ interface is particularly encouraging for the use of $\mathrm{C}$-based materials in chiral-DW-based device applications.

Although the field of two-dimensional (2D) materials has democratized materials science by making high-quality samples accessible cheaply, due to the atomically thin nature of these systems, an integration with nanostructures is almost always required to obtain a significant optical response. Traditionally, these nanostructures are fabricated via electron beam lithography or focused ion beam milling, which are expensive, and large-area fabrication can be further time-consuming. In order to overcome this problem, we report the integration of 2D semiconductors on a cost-effective and large-area fabricated nanocone platform. We show that the plasmon modes of our nanocone structures lead to photoluminescence enhancement of monolayer ${\text{WSe}}_2$ by about eight to ten times compared with the nonplasmonic case, consistent with finite-difference time-domain simulations. Excitation power-dependent measurements reveal that our nanocone platform enables a versatile route to engineering the relative exciton trion contributions to the emission.

Inhomogeneous broadening is a major limitation for the application of quantum emitters in hexagonal boron nitride to integrated quantum photonics. Here we demonstrate that so-called blue emitters with an emission wavelength of 436 nm are less sensitive to electric fields than other quantum emitters in hexagonal boron nitride. Our measurements reveal a weak, predominantly quadratic Stark effect that indicates a negligible transition dipole moment of the defect. Using these results, we discuss implications for the symmetry of the defect and use density-functional-theory simulations to identify a likely atomic structure of blue emitters in hexagonal boron nitride.

Charge noise is one of the main sources of environmental decoherence for spin qubits in silicon, presenting a major obstacle in the path towards highly scalable and reproducible qubit fabrication. Here we demonstrate in-depth characterization of the charge noise environment experienced by a quantum dot in a CMOS-fabricated silicon nanowire. We probe the charge noise for different quantum dot configurations, finding that it is possible to tune the charge noise over two orders of magnitude, ranging from $1\text{μ}{\mathrm{eV}}^2/\mathrm{Hz}$ to $100\text{μ}{\mathrm{eV}}^2/\mathrm{Hz}$. In particular, we show that the top interface and the reservoirs are the main sources of charge noise, and their effect can be mitigated by controlling the quantum dot extension. Additionally, we demonstrate a method for the measurement of the charge noise experienced by a quantum dot in the few-electron regime. We measure a comparatively high charge noise value of $40\text{μ}{\mathrm{eV}}^2/\mathrm{Hz}$ at the first electron, and demonstrate that the charge noise is highly dependent on the electron occupancy of the quantum dot.

Manipulating charge-to-spin conversion in a single magnetic layer holds great promise for optimizing efficiency of spintronic devices. Here we present experimental evidence uncovering a giant conversion anisotropy of up to 400% in fourfold-symmetric anisotropy $\mathrm{Fe}$ films on $\mathrm{Mg}\mathrm{O}(100)$ substrate by changing the direction of applied radio frequency microwave currents from the magnetic hard axis to the easy one. Together with the anisotropy of Gilbert damping and anisotropic magnetoresistance, the fourfold-symmetric anisotropy is found to stem from large emergent spin-orbit coupling in $\mathrm{Fe}$ films with local lattice distortions, which is further confirmed by selected area electron diffraction patterns in different regions. Given that $\mathrm{Fe}$ films are deposited by magnetron sputtering at room temperature, our results suggest a simple route for designing future energy-efficient magnetic devices for mass production.

We present a quantitative microscopic analysis of the formation of exciton-polaritons, the composite particles possessing light and material components, polariton-polariton interactions, and resonant pumping dynamics in cylindrical semiconductor micropillars. We obtain that, due to the exciton-exciton interaction, the polaritonic states’ populations become redistributed among pumped and pristine states. We discuss how the redistribution effect can be used in devices generating photons with nonzero orbital angular momenta.

We propose a nontrivial two-qubit gate scheme in which Rydberg atoms are subject to designed pulses resulting from geometric evolution processes. By utilizing hybrid robust nonadiabatic and adiabatic geometric operations on the control atom and target atom, respectively, we improve the robustness of the two-qubit Rydberg gate against Rabi control errors as well as blockade errors in comparison with the conventional two-qubit blockade gate. Numerical results with the current state-of-the-art experimental parameters corroborates the above mentioned robustness. We also evaluate the influence induced by the motion-induced dephasing and the dipole-dipole interaction and imperfection-excitation-induced leakage errors, which both could decrease the gate fidelity. Our scheme provides a promising route towards systematic control error (Rabi error) as well as blockade-error-resilient geometric quantum computation on neutral atom system.

Further research on organic molecule crystals with spontaneous polarization as a possible candidate for the bulk photovoltaic effect is urgently needed to meet the requirements for optoelectric devices. We demonstrate the shift current in a type of molecular crystal that acts at high temperatures and contains organic molecules with electron donor-acceptor groups, such as paranitroaniline derivatives. We find an extremely strong multicomponent shift-current response along the parallel and perpendicular to polar molecules of the crystals using density-functional simulations, which is more than 10 times larger than the previously reported value for shift current over a broad spectral range from visible to ultraviolet spectrum. Even more intriguing, the remarkable dependency of the conduction- and valence-band dispersions on the lattice constant along polarization allows the shift current in these organic molecule structures to be tuned under stress and doping. Our findings not only suggest a class of organic compounds for realizing the highly controllable shift-current response, but also open up a channel for the search for solar materials.

The squeeze-film-damping effect is one of the main limits in many precision experiments, especially in the study of short-range interactions, and its physical mechanism is still not very clear. We develop a torsion pendulum to measure the damping effects caused by the thermal motion of residual gas between a cubic test mass and a pair of symmetric movable plates. The measured results are consistent with the model that synthesized the elastic and inelastic collisions by introducing a thermal accommodation coefficient (TAC). In our research, the pressure dependence of the TAC, which can be expressed as an exponential decay function, is also observed. This dependence may imply that the elastic collisions correspond to the simple bouncing of residual gas molecules, while the inelastic collisions correspond to the adsorption and desorption cycle of gas molecules on the surface. The method in this work and the discovered TAC pressure dependence are instructive for the analysis of residual-gas effects in weak-force measurements, as well as the deviation correction of rarefied gas pressure measurements based on the squeeze-film-damping effect.

Photophoretic levitation is a propulsion mechanism by which lightweight objects can be lifted and controlled through their interactions with light. Since photophoretic forces on macroscopic objects are usually maximized at low pressures, they may be tested in a vacuum chamber in close proximity to the chamber floor and walls. We report experimental evidence that the terrain under levitating microflyers, including the chamber floor or the launchpad from which the microflyer lifts off, can greatly increase the photophoretic lift forces relative to their free-space (midair) values. To characterize this so-called “ground effect” during vacuum-chamber tests, we introduce a miniature launchpad composed of three J-shaped (candy-cane-like) wires that minimize the microflyer’s extraneous interactions with the underlying surfaces. We compare our J-shaped-wire launchpad with previously used wire-mesh launchpads for simple levitating Mylar-based disks with diameters of 2, 4, and 8 cm. Importantly, we discover that wire-mesh launchpads increase the photophoretic lift force by up to sixfold. A significant ground effect is also associated with the bottom of the vacuum chamber, particularly when the distance to the bottom surface is less than the diameter of the levitating disk. We provide guidelines to minimize the ground effect in vacuum-chamber experiments, which are necessary to test photophoretic microflyers intended for high-altitude exploration and surveillance on Earth or on Mars.

The ability to perform fast, high-fidelity entangling gates is a requirement for a viable quantum processor. In practice, achieving fast gates often comes with the penalty of strong-drive effects that are not captured by the rotating-wave approximation. These effects can be analyzed in simulations of the gate protocol, but those are computationally costly and often hide the physics at play. Here, we show how to efficiently extract gate parameters by directly solving a Floquet eigenproblem using exact numerics and a perturbative analytical approach. As an example application of this toolkit, we study the space of parametric gates generated between two fixed-frequency transmon qubits connected by a parametrically driven coupler. Our analytical treatment, based on time-dependent Schrieffer-Wolff perturbation theory, yields closed-form expressions for gate frequencies and spurious interactions, and is valid for strong drives. From these calculations, we identify optimal regimes of operation for different types of gates including $i$swap, controlled-Z, and cnot. These analytical results are supplemented by numerical Floquet computations from which we directly extract drive-dependent gate parameters. This approach has a considerable computational advantage over full simulations of time evolutions. More generally, our combined analytical and numerical strategy allows us to characterize two-qubit gates involving parametrically driven interactions, and can be applied to gate optimization and cross-talk mitigation such as the cancelation of unwanted $ZZ$ interactions in multiqubit architectures.

Carrier thermalization in a superlattice solar cell made of polar semiconductors is studied theoretically by considering a minimal model where electron-phonon scattering is the principal channel of carrier energy loss. Of note, the effect of an intrinsic quantum mechanical property, the phonon coherence, on carrier thermalization is investigated, within a semiclassical picture in terms of phonon wave packet. It turns out that coherent longitudinal optical (LO) phonons weaken the effective electron-phonon coupling, thus supposedly lowering the carrier-energy-loss rate in the solar cell. The resulting thermalization power is indeed significantly reduced by the coherent phonons, resulting in an enhanced hot-carrier effect, particularly for thin enough well layer where carrier confinement is also strong. A recent experiment on a superlattice solar-cell prototype is shown to manifest the coherent phonon-driven phenomenon. Our results demonstrate the practical implications of the fundamental quantum coherence property of phonons in semiconductors for improving superlattice solar-cell performance, via hot-carrier effect.