The optical and electronic properties of semiconductors are strongly affected by structural and stoichiometric defects. The precise incorporation of dopants and the control of impurities are essentially what makes semiconductors useful materials for a broad range of devices. The standard defect and impurity characterization methods are sensitive only on a macroscopic scale, like the most widely used method of deep-level transient spectroscopy (DLTS). We perform time-resolved measurements of the resonance fluorescence of a single self-assembled $(\mathrm{In},\mathrm{Ga})$$\mathrm{As}$ quantum dot (QD) at low temperatures ($4.2\mathrm{K}$). By pulsing the applied gate voltage, we are able to selectively occupy and unoccupy individual defects in the vicinity of the dot. We address the exciton transition of the QD with a tunable diode laser. Our time-resolved measurements exhibit a shift of the resonance energy of the optical transition. We attribute this to a change of the electric field in the dot’s vicinity, caused by electrons tunneling from a reservoir to the defect sites. Furthermore, we are able to characterize the defects concerning their position and activation energy by modeling our experimental data. Our results thus demonstrate how a quantum dot can be used as a quantum sensor to characterize the position and activation energy of individual shallow defects on the nanoscale.

We demonstrate second-harmonic generation in a fiber-coupled periodically poled titanium in-diffused lithium niobate waveguide over the temperatures range from 292 to 4.4 K. We observe a reproducible shift in the phase-matched pump wavelength within the telecom band, which is at odds with the phase matching arising from existing models of the refractive indices in this temperature regime. We further investigate transient discontinuities while temperature cycling, which we attribute to the build-up and discharge of pyroelectric fields. Our results show that modifications of the Sellmeier coefficients for lithium niobate are required in the cryogenic temperature regime for our waveguides, but also establish titanium in-diffused waveguides in lithium niobate as a versatile nonlinear photonic integration platform compatible with cryogenic quantum technologies.

Acoustic metasurfaces display unprecedented potential for their unique and flexible capabilities of wave front manipulations. Although rapid progress and significant developments have been achieved in this field, it still remains a significant challenge to obtain an acoustic metasurface that can perform different functions in three-dimensional (3D) space, as desired. Here, a tunable acoustic metasurface composed of Helmholtz-resonator-like digital-coding meta-atoms is presented to overcome the limitation and realize 3D dynamic wave manipulations. The digital meta-atom is constructed from two cylindrical cavities controlled by a motor. By changing the depth of the bottom cavity with the motor automatically, the reflection phase of the meta-atom can be adjusted continuously over the range of nearly 360°, realizing the required digital states. To build the relationship between the digital-coding profiles and scattering patterns, convolution and addition operations are implemented. Based on such operations, various fascinating functionalities, such as arbitrary scattering-pattern shift and superposition of different scattering beams, are achieved. This study paves the way for studying acoustic metasurfaces from the digital perspective and provides efficient methods for realizing spatial acoustic wave control.

Quantum non-Gaussian gate is a missing piece to the realization of continuous-variable universal quantum operations in an optical system. In a measurement-based implementation of the cubic phase gate, a lowest-order non-Gaussian gate, non-Gaussian ancillary states that have a property we call nonlinear squeezing are required. This property, however, has never been experimentally verified. In this paper, we generate a superposition between a vacuum state and a single-photon state whose nonlinear squeezing is maximized by the optimization of the superposition coefficients. The nonlinear squeezing is observed via real-time quadrature measurements, meaning that the generated states are compatible with real-time feedforward and are suitable as ancillary states for the cubic phase gate in the time domain. Moreover, by increasing the number of photons, it is expected that nonlinear squeezing can be further improved. The idea presented here can be readily extended to higher-order phase gates [P. Marek et al., Phys. Rev. A 97, 022329 (2018)]. As such, this work presents an important step to extend continuous-variable quantum information processing from Gaussian regime to non-Gaussian regime.

Magnetic forces in oscillating electronic circuits provide a versatile way to remotely actuate objects. However, practical control of these forces is challenging due to variations in the circuit configuration and noise. We show that these challenges can be overcome using a bistable parity-time-symmetric circuit. Due to nonlinear parity-time symmetry, the circuit automatically tracks one of two eigenmodes corresponding to resonant attraction and repulsion during variations in the operating configuration. We show that noise facilitates controllable switching between the modes, and experimentally demonstrate practical actuation by remotely driving a pendulum and a soft pump membrane.

A hallmark of near-zero-index materials is the uniform phase profile that the waves experience as they propagate with staticlike properties in these media. This feature is exploited in electromagnetics for several applications, including for the realization of compact highly directive antennas illuminated by a single source. Here, we discuss the promise of near-zero metamaterials for acoustic radiation, based on the cutoff of a higher-order mode in a waveguide with small holes. We show that the near-zero nature of this phenomenon ensures unusual robustness to disorder, which is analyzed using a statistical description of the radiation patterns for randomly generated location of the holes. The radiation pattern can be made more directive by simply increasing the overall radiating aperture, without having to worry about the specific location of individual holes and their relative distance. The proposed zero-compressibility material offers a promising alternative to traditional acoustic directive sources due to its design simplicity, inherent robustness, and compact profile.

Correct parameterization of the Gaussian disorder model (GDM) on spatially random sites is necessary for a complete description of charge transport in disordered materials and concomitant device characteristics. Because the GDM on spatially random sites considers both energetic and spatial disorder, it is superior to the GDM on a cubic lattice. However, analytical arguments and experimental evidence are still lacking for correct parameterization of the model over a wide range of model parameters, energetic and spatial disorder, and electric fields. We show that the model requires a set of parameters to correctly account for high mobility and its charge density dependence, and we develop such a model. The model is implemented in a numerical simulation tool for comparison with the measured device characteristics. Accurate agreement with experimental data, particularly with the high mobility values in organic field-effect transistors, is achieved throughout a wide range of temperature by adjusting both the localization length and the attempt-to-escape frequency.

In contrast to the valley-selective Berry curvatures of the conduction band of monolayer ${\mathrm{Mo}\mathrm{S}}_2$, bilayer (2L-) ${\mathrm{Mo}\mathrm{S}}_2$ (i.e., in 2H-phase) has vanishing Berry curvatures at both K and K′ valleys due to the inversion symmetry. When a vertical electric field is applied to 2L-${\mathrm{Mo}\mathrm{S}}_2$, however, conduction bands are split due to a potential difference and the valley-selective Berry curvatures are restored. Especially, for an electron-doped 2L-${\mathrm{Mo}\mathrm{S}}_2$, application of a vertical electric field together with an in-plane bias electric field enables one to induce valley magnetization from the valley magnetoelectric effect and further bring about its real-space distribution, i.e., the valley magnetic domain (VMD), through a competition with the valley Hall effect. In particular, the uniaxial strain induces a transfer of electrons from the Q valleys and increases the radii of the electron pockets at the K and K′ valleys. In this context, therefore, at fixed values of the bias field and strain strengths, VMD moving and the consequent modulation of the anomalous transverse current perpendicular to the bias field are found to be achieved via an electrical method, that is, by controlling the vertical electric field.

Valley interface states, resulting in acoustic valley Hall topological insulators, have recently become a hot topic in the study of acoustic systems. On the basis of structural diversity and potential applications, we construct a two-dimensional triangular-lattice phononic crystal with $C_{3v}$-symmetric scatterers, and obtain two distinct valley Hall phases with nonvanishing valley Chern indices by rotating the scatterers. We numerically calculate the dispersion relations of these valley Hall phases including two kinds of interfaces, and find that valley interface states exist at not only the zigzag interface but also the armchair interface. We demonstrate the acoustic splitting and merging of valley interface states in the cross-waveguides, and numerically achieve the xor and or logic functions. We also design three complicated waveguides by assembling phononic crystals with distinct valley Hall phases. By experimental measurements in these waveguides, we successfully implement one-, two-, and three-channel topological transport. This research possibly provides a design route exploiting valley interface states to fabricate multichannel acoustic communication devices.

The localized release of energy at the surface of plasma-exposed materials plays a crucial role in many plasma-driven processes. Unfortunately, the interaction between low-temperature plasmas and surfaces is both poorly understood and difficult to characterize. Here, we use Raman thermometry to measure the temperature of graphene during plasma exposure. We observe a significant increase in the graphene temperature even for modest input plasma powers. These results suggest that two-dimensional materials are effective surface probes for the quantitative characterization of plasma-induced heating effects and for further investigation of the plasma-surface interaction.

Magnetic tunnel junctions with perpendicular anisotropy form the basis of the spin-transfer torque magnetic random-access memory (STT MRAM), which is nonvolatile, fast, dense, and has quasi-infinite write endurance and low power consumption. Based on density-functional-theory (DFT) calculations, we propose an alternative design of magnetic tunnel junctions comprising $\mathrm{Fe}(n)\mathrm{Co}(m)\mathrm{Fe}(n)|\mathrm{Mg}\mathrm{O}$ storage layers [n and m denote the number of monolayers (ML)] with greatly enhanced perpendicular magnetic anisotropy (PMA) up to several mJ/m${}^2$, leveraging the interfacial perpendicular anisotropy of $\mathrm{Fe}|\mathrm{Mg}\mathrm{O}$ along with a strain-induced bulk PMA discovered within bcc $\mathrm{Co}$. This giant enhancement dominates the demagnetizing energy when increasing the film thickness. The tunneling magnetoresistance (TMR) estimated from the Julliere model is comparable with that of the pure $\mathrm{Fe}|\mathrm{Mg}\mathrm{O}$ case. We discuss the advantages and pitfalls of a real-life fabrication of the structure and propose the $\mathrm{Fe}(3\mathrm{ML})\mathrm{Co}(4\mathrm{ML})\mathrm{Fe}(3\mathrm{ML})$ as a storage layer for $\mathrm{MgO}$-based STT MRAM cells. The large PMA in strained bcc $\mathrm{Co}$ is explained in the framework of second-order perturbation theory by the $\mathrm{MgO}$-imposed strain and consequent changes in the energies of $d_{yz}$ and $d_{z^2}$ minority-spin bands.

We demonstrate a simple, robust, and contactless method for nondestructive testing of magnetic materials such as steel. This uses a fiber-coupled magnetic sensor based on nitrogen-vacancy centers (NVCs) in diamond without magnetic shielding. Previous NVC magnetometry has sought a homogeneous bias magnetic field on the diamond to improve the sensitivity. In contrast, here we show that the spatial resolution for imaging is improved by applying an inhomogeneous magnetic field to the steel even though this leads to an inhomogeneous magnetic field on the diamond. Structural damage in the steel distorts the inhomogeneous magnetic field and by detecting this distortion we reconstruct the damage profile through quantifying the shifts in the NVC Zeeman splitting. With a 1-mm magnet as the source of our inhomogeneous magnetic field, we achieve a high spatial resolution of 1 mm in the plane parallel and 0.1 mm in the direction perpendicular to the surface of the steel. This works even when the steel is covered by a nonmagnetic material. The lift-off distance of our sensor head from the surface of 316 stainless steel is up to 3 mm.

The possibility of measuring in situ operando photoluminescence spectroscopy within a photonic atom probe allows for the real-time study of the mechanical stress state within a field emitter either statically, as a function of the field-induced tensile stress, or dynamically, as a result of the evolution of the shape of the emitter upon its evaporation. Dynamic evolution results from the relaxation of strain induced by lattice mismatch and by the propagation of stress from the apex, while the morphology of the field emitter changes. Optomechanical information can be interpreted through the three-dimensional atomic scale images of the chemical composition of the emitter obtained through standard atom probe analysis. Here, the photoluminescence signal of a $\mathrm{Zn}\mathrm{O}/(\mathrm{Mg},\mathrm{Zn})\mathrm{O}$ quantum well allows for the local measurement of strain within the well and of the electrostatic field applied to the apex of the nanoscale field emitter.

Increasing the signal-to-noise ratio in dynamic atomic force microscopy plays a key role in nanomechanical mapping of materials with atomic resolution. In this work, we develop an experimental procedure for increasing the sensitivity of higher harmonics of an atomic-force-microscope cantilever without modifying the cantilever geometry but instead by utilizing dynamical mode coupling between its flexural modes of vibration. We perform experiments on different cantilevers and samples and observe that via nonlinear resonance frequency tuning we can obtain a frequency range where strong modal interactions lead to 7-fold and 16-fold increases in the sensitivity of the 6th and 17th harmonics while reducing sample indentation. We derive a numerical model that captures the observed physics and confirms that nonlinear mode coupling is the reason for the increase of the amplitude of higher harmonics during tip-sample interactions.

Permanent magnets generally require a favorable, but difficult-to-achieve combination of high magnetization, Curie point, and magnetic anisotropy. Thus there have been few, if any, viable permanent magnets developed since the 1982 discovery of ${\mathrm{Nd}}_2{\mathrm{Fe}}_{14}\mathrm{B}$ [M. Sagawa, S. Fujimura, H. Yamamoto, Y. Matsuura, and S. Hirosawa, J. Appl. Phys. 57, 4094 (1985)]. Here we point out, both by direct first-principles calculations on the iron carbides and silicides ${\mathrm{Fe}}_5{\mathrm{C}}_2$, ${\mathrm{Fe}}_5\mathrm{Si}\mathrm{C},$ and ${\mathrm{Fe}}_7{\mathrm{C}}_3$ as well as a discussion of recent experimental findings, that there are numerous rare-earth-free iron-rich potential permanent-magnet materials with sufficient intrinsic magnetic properties to reasonably achieve room-temperature energy products of 20–25 MG Oe. This is substantially better than the performance of the best available rare-earth-free magnets based on ferrite, as well as shape-anisotropy-employing alnico. These magnets could plausibly fill, at low cost, the present performance “gap” [J. M. D. Coey, Scr. Mater. 67, 524 (2012)] between the best rare-earth-free magnets and rare-earth magnets such as ${\mathrm{Nd}}_2{\mathrm{Fe}}_{14}\mathrm{B}$ and $\mathrm{Sm}\text{−}\mathrm{Co}$.