Realistic multiport interferometers (beamsplitter meshes) are sensitive to component imperfections, and this sensitivity increases with size. Self-configuration techniques can be employed to correct these imperfections, but not all techniques are equal. This paper highlights the importance of algorithmic stability in self-configuration. Naïve approaches based on sequentially setting matrix elements are unstable and perform poorly for large meshes, while techniques based on power ratios perform well in all cases, even in the presence of large errors. Based on this insight, we propose a self-configuration scheme for triangular meshes that requires only external detectors and works without prior knowledge of the component imperfections. This scheme extends to the rectangular mesh by adding a single array of detectors along the diagonal.

We investigate a W-Ta alloying route to reduce the auto-oscillation threshold current densities and the power consumption of nanoconstriction based spin Hall nano-oscillators. Using spin-torque ferromagnetic resonance measurements on microbars of ${\mathrm{W}}_{100-x}{\mathrm{Ta}}_x$(5 nm)/$\mathrm{Co}$-$\mathrm{Fe}$-$\mathrm{B}$($t$)/$\mathrm{Mg}\mathrm{O}$ stacks with $t=1.4$, 1.8, and 2.0 nm, we measure a substantial improvement in both the spin-orbit torque efficiency and the spin Hall conductivity. We demonstrate a 34% reduction in auto-oscillation threshold current density, which translates into a 64% reduction in power consumption as compared with pure W-based spin Hall nano-oscillators. Our work demonstrates the promising aspects of W-Ta alloying for the energy-efficient operation of emerging spintronic devices.

Magnetic force microscopy (MFM) is a characterization method used to obtain images of the stray field emanating from the surface of magnetic samples with high spatial resolution. The MFM signal arises from a convolution of the magnetic moment distribution of the MFM tip with the stray field of the sample. Therefore, deconvolution of the stray field of a magnetic sample requires a well-defined micromagnetic structure of a tip. While the magnetic moment distribution of the tip remains challenging to assess, it is convenient to calibrate the tip-equivalent magnetic charge in a plane parallel to the sample surface running through the tip apex using a suitable tip-calibration method. Once the tip-equivalent magnetic charge is calibrated, MFM data can be deconvolved to obtain the stray field of the sample or the equivalent magnetic surface charge, for example, at the sample surface. Here we use low-noise MFM data obtained with high-quality-factor cantilevers and a modified L-curve method for the optimization of a Tikhonov deconvolution procedure used for the calibration of the MFM tip, and—once the tip is calibrated—for obtaining the stray field or magnetic surface charge of the sample.

We investigate the ferromagnetic resonance (FMR) response of microfabricated microwave resonators loaded with small ${\mathrm{Co}}_{16}{\mathrm{Pd}}_{84}$ alloy rectangles. A major increase in the FMR signal-to-noise ratio is achieved by employing the microwave-resonator structure. A FMR peak shift similar to that of ${\mathrm{Co}}_{16}{\mathrm{Pd}}_{84}$ continuous films is measured in the presence of hydrogen gas in the sample environment. We show that the very high sensitivity of the FMR signal of the ${\mathrm{Co}}_{16}{\mathrm{Pd}}_{84}$ alloy rectangle to hydrogen exposure can be used to measure relatively small hydrogen-concentration steps near 100% ${\mathrm{H}}_2$. Additionally, we also demonstrate that this structure can measure hydrogen over a concentration range from 3% to 100% ${\mathrm{H}}_2$ in ${\mathrm{N}}_2$. In time-dependent FMR measurements, we discover a temperature dependence of the FMR signal, which we relate to intrinsic temperature-dependent changes in saturation magnetization and the magnetic anisotropy of the $\mathrm{Co}$-$\mathrm{Pd}$ alloy.

Quantum computing promises to provide exponential speed-ups to certain classes of problems. In many such algorithms, a classical vector $\mathbf{b}$ is encoded in the amplitudes of a quantum state $|b⟩$. However, efficiently preparing $|b⟩$ is known to be a difficult problem because an arbitrary state of $Q$ qubits generally requires approximately $2^Q$ entangling gates, which results in significant decoherence on today’s noisy intermediate-scale quantum (NISQ) computers. We present a deterministic (nonvariational) algorithm that allows us to flexibly reduce the quantum resources required for state preparation in an entanglement-efficient manner. Although this comes at the expense of reduced theoretical fidelity, actual fidelities on current NISQ computers might actually be higher due to reduced decoherence. We show this to be true for various cases of interest, such as the normal and log-normal distributions. For low-entanglement states, our algorithm can prepare states with more than an order of magnitude fewer entangling gates as compared with isometric decomposition.

Nonmagnetic nitride-based van der Waals magnetic heterostructures, introducing magnetism by constructing heterostructures with a two-dimensional room-temperature intrinsic ferromagnet and manipulating both the charge and the spin degrees of freedom, are important materials for developing high-performance low-dimensional spintronic devices. To obtain the unique physical properties of novel devices, a fundamental physical understanding of this material system is highly desired. The electronic and magnetic properties of the $\mathrm{Al}\mathrm{N}/{\mathrm{VSe}}_2$ van der Waals heterostructure are studied systematically by combining first-principles calculations and Schrödinger-Poisson simulations. The $\mathrm{Al}\mathrm{N}/{\mathrm{VSe}}_2$ van der Waals heterostructure presents superior physical properties such as a large conduction band offset, ferromagnetic ground state, magnetic anisotropy, and high Curie temperature. Meanwhile, the band alignment and Curie temperature of the $\mathrm{Al}\mathrm{N}/{\mathrm{VSe}}_2$ van der Waals heterostructure can be modulated by biaxial strain or an electric field, while the easy magnetization axis remains the in-plane direction. Varying the sheet carrier density changes the energy levels, shifts the average charge position, and tilts the conduction band profile. Our method can also be applied to study the interface properties of other van der Waals heterostructure systems.

Terahertz scanning tunneling microscopy (THz-STM) has enabled spatiotemporal imaging with femtosecond temporal resolution and angstrom spatial resolution. In THz-STMs, the junction bias is modulated by coupling THz pulses, which results in transient voltage bias and extremely high transient tunneling currents. In order to have efficient imaging of the sample surface, it is important to understand the nonlinear tunneling current response and its parametric dependence. In this work, we theoretically investigate the basic scaling of rectified electrons in a THz-STM junction. We use a self-consistent quantum model that includes both space-charge potential and exchange-correlation potential, which were ignored in previous studies. Since THz-STMs are operated at high transient voltage in field emission regime, the effects of exchange-correlation potential become crucial. We validate our calculation with recently reported experimental data and investigate the rectification property of the tip-sample junction for different parameters. We find that the time-dependent tunneling current and the electron transport can be manipulated by varying the dc bias voltage (polarity, amplitude), incident THz field (polarity, shape, peak amplitude), work functions of STM tip and sample—especially their difference $\mathrm{\Delta }W$, and the tip-sample separation. Our study provides an important framework that can be used in the future to characterize, control, and improve THz-induced currents and probing techniques at the nanometer scale over subpicosecond time periods.

In many applications, the ability to measure the vector information of a magnetic field with high spatial resolution and low cost is essential, but remains a challenge for existing magnetometers composed of multiple sensors. Here, we report a single-device based vector magnetometer, which is enabled by spin-orbit torque, capable of measuring a vector magnetic field using the harmonic Hall resistances of a ferromagnet (FM)/heavy metal (HM) bilayer with superparamagnetic behavior. Under an ac driving current, the first- and second-harmonic Hall resistances of the FM/HM bilayer show a linear relationship with the vertical and longitudinal component (along the current direction) of the magnetic field, respectively. By employing an L-shaped Hall device with two orthogonal arms, we can measure all the three field components simultaneously, thereby detecting both the amplitude and direction of magnetic field in a three-dimensional space. As proofs of concepts, we demonstrate both angular position sensing on the three coordinate planes and vector mapping of magnetic field generated by a permanent magnet, both of which are in good agreement with the simulation results. Crosstalk between vertical and longitudinal field components at large field is discussed using theoretical models.

We present a systematic method to implement a perturbative Hamiltonian diagonalization based on the time-dependent Schrieffer-Wolff transformation. Applying our method to strong parametric interactions we show how, even in the dispersive regime, full Rabi model physics is essential to describe the dressed spectrum. Our results unveil several qualitatively different results, including realization of large energy-level shifts, tunable in magnitude and sign with the frequency and amplitude of the pump mediating the parametric interaction. Crucially, Bloch-Siegert shifts, typically thought to be important only in the ultrastrong or deep-strong coupling regimes, can be rendered large even for weak dispersive interactions to realize points of exact cancelation of dressed shifts (“blind spots”) at specific pump frequencies. The framework developed here highlights the rich physics accessible with time-dependent interactions and serves to significantly expand the functionalities for control and readout of strongly interacting quantum systems.

Acoustic metasurfaces have great potential in the field of beam-forming acoustic antennas due to their thin, two-dimensional, and miniature artificial materials that can freely control sound waves. Here, we propose an acoustic metasurface, with multibeam acoustic radiation serving as leaky-wave antennas used for acoustic beam steering. Based on the holographic principle, we pattern a sinusoidal modulated admittance surface, which is designed as a periodic arrangement of cylindrical holes with varying depth profiles. An omnidirectional sound generated from a tiny hole located at the center of the patterned plate creates acoustic surface waves on the modulated surface while simultaneously generating leaky-wave radiation. For multibeam steering, the holographic admittance surfaces are designed with multiple subregions of the metasurface, with each subregion emitting single-beam radiation in the desired direction. Furthermore, the holographic metasurfaces are programmed separately to have forward and backward leaky-wave radiation. In this study, we use three-dimensional additive fabrication to print acoustic holograms to craft the surface-admittance variation at subwavelength resolution. Experimental results on these printed holograms show multidirectional acoustic beam steering in close agreement with the theoretical analysis and numerical results. Forward and backward multibeam frequency scanning is also demonstrated by frequency variation. Thus, we expect that this planar surface can be used in applications such as acoustic communications, levitation, and imaging.

The anomalous Nernst effect (ANE), the generation of a transverse electric voltage by a longitudinal temperature gradient, has attracted increasing interest from researchers recently, due to its potential in thermoelectric power conversion and close relevance to the Berry curvature of the band structure. Avoiding the stray field of ferromagnets, the ANE in antiferromagnets (AFMs) has the advantage of realizing highly efficient and densely integrated thermopiles. Here, we report the observation of the ANE in an antiperovskite noncollinear AFM, ${\mathrm{Mn}}_3\mathrm{Sn}\mathrm{N},$ experimentally, which is triggered by the enhanced Berry curvature from Weyl points located close to the Fermi level. Considering that antiperovskite ${\mathrm{Mn}}_3\mathrm{Sn}\mathrm{N}$ has a rich magnetic phase transition, we modulate the noncollinear AFM configurations by the biaxial strain, which enables us to control its ANE. Our findings provide a potential class of materials to explore the Weyl physics of noncollinear AFMs as well as realizing antiferromagnetic spin caloritronics that exhibits promising prospects for energy conversion and information processing.

The complex nonlinear dynamics of colliding counterpropagating ultrashort laser pulses circulating in a vertical external-cavity surface-emitting ring laser cavity are captured by a first-principles quantum-mechanical microscopic model coupled to an intracavity beam propagator. Our numerical simulations establish that overall mode locking stability is sensitive to relative phases and delays of the pulses impinging on the semiconductor saturable absorber mirror. Delay offsets of the counterpropagating pulses lead to asymmetric carrier recovery and instability, often leading in turn to single unidirectional circulating pulses. Although computationally challenging, we also include transverse effects and show that the influence of interference patterns from spatial gratings written by the counterpropagating pulses is significant in stabilizing the modelocked pulses.

We report a strain effect on spin transport in semiconductors that exhibit $\mathrm{Ge}$-like conduction bands at room temperature. Using four-terminal nonlocal spin-transport measurements in lateral spin-valve devices, we experimentally estimate the spin diffusion length ($\lambda$) of $\mathrm{Ge}$ and strained ${\mathrm{Si}}_{0.1}{\mathrm{Ge}}_{0.9}$ with two different carrier concentrations. Despite the $\mathrm{Ge}$-like electronic band structure, the obtained $\lambda$ of a strained ${\mathrm{Si}}_{0.1}{\mathrm{Ge}}_{0.9}$ is comparable to that of a $\mathrm{Si}$ channel. We discuss a possible mechanism of the strain-induced enhancement of $\lambda$ at room temperature. As a consequence, we demonstrate the electrical detection of 5-$\mu \mathrm{m}$ lateral spin transport in the strained ${\mathrm{Si}}_{0.1}{\mathrm{Ge}}_{0.9}$ by applying an electric field at room temperature.

Recent progress in the application of color centers to nanoscale spin sensing makes the combined use of noise spectroscopy and scanning probe imaging an attractive route for the characterization of arbitrary material systems. Unfortunately, the traditional approach to characterizing environmental magnetic field fluctuations from the measured probe signal typically requires the experimenter’s input, thus complicating the implementation of automated imaging protocols based on spectrally resolved noise. Here, we probe the response of color centers in diamond in the presence of externally engineered random magnetic signals and implement a deep neural network to methodically extract information on their associated spectral densities. Building on a long sequence of successive measurements under different types of stimuli, we show that our network manages to efficiently reconstruct the spectral density of the underlying fluctuating magnetic field with good fidelity under a broad set of conditions and with only a minimal measured data set, even in the presence of substantial experimental noise. These proof-of-principle results create opportunities for the application of machine-learning methods to color-center-based nanoscale sensing and imaging.

Strain engineering is a promising method for improving the optical and optoelectrical properties of two-dimensional (2D) materials. However, the local strain applied by the patterned substrate is highly localized, resulting in a small average local strain that can be detected using traditional optical methods. Here, we report a simple strategy to efficiently amplify the local-strain signal of 2D materials, by mechanically integrating monolayer ${\mathrm{Mo}\mathrm{S}}_2$ on $\mathrm{Au}$ nanoparticles of predetermined size. Using Raman and photoluminescence measurements, it is proved that ${\mathrm{Mo}\mathrm{S}}_2$ wrapped around $\mathrm{Au}$ nanoparticles with an average diameter of 39 nm can achieve the strongest strain-plasmonic coupling. Within this structure, the significant amplification effect of plasmonic coupling on the local strain signal is studied in detail by Lorentzian fitting of the Raman and PL spectra. The detected maximum local strain signal can reach 2.78%, and the corresponding local strain signal is amplified by more than 650%. Furthermore, the physical mechanism of local-strain-signal amplification is analyzed due to the robust strain-plasmonic coupling. This result provides a simple method for amplifying the local strain signal, paving the way for high-performance 2D optical and optoelectronic devices. This method provides a structure to amplify the local strain signal combined with plasmonic coupling, which can promote the development of high-performance broadband photodetectors.

We study the role of time-reversal symmetry on the dynamical response of nonlinear optical systems that behave as unidirectional (one-way) devices. It is shown that lossless nonlinear materials, despite being nonreciprocal, are typically time-reversal invariant. This property raises an apparent paradox because time-reversal-invariant systems are forcibly bidirectional. Here, we present a solution for this conundrum, and theoretically explain why the one-way behavior can indeed be compatible with the time-reversal invariance. It is found that in the time-reversed problem the incident waves have a variation in time that is generally incompatible with the adiabatic approximation. For this reason, the adiabatic approximation fails to predict the bidirectional nature of nonlinear systems. We discuss the implications of this finding on the performance of practical nonlinear one-way devices.

We present and characterize a Rb atomic vapor cell with silicon-ring electrodes embedded between borosilicate glass tubes. The ring electrodes can be externally connectorized for application of electric fields to the inside of the cell. An atom-based, all-optical, laser-spectroscopic field-sensing method is employed to measure electric fields in the cell. Here, the Stark effect of electric-field-sensitive rubidium Rydberg atoms is exploited to measure dc electric fields in the cell of approximately $5$ V/cm, with a relative uncertainty of 10%. Measurement results are compared with dc field calculations, allowing us to quantify electric field attenuation due to free surface charges inside the cell. We further measure the propagation of microwave fields into the cell, using Autler-Townes splitting of Rydberg levels as a field probe. Results are obtained for a range of microwave powers and polarization angles relative to the cell’s ring electrodes. We compare the results with microwave-field calculations.