We study the voltage control of perpendicular magnetic anisotropy in an ultrathin Fe layer sandwiched between the Cr buffer and MgO tunneling barrier layers. A high-interface magnetic anisotropy energy of $2.1\mathrm{mJ}/{\mathrm{m}}^2$ is achieved in the Cr/ultrathin $\mathrm{Fe}/\mathrm{MgO}$ structure. A large voltage-induced perpendicular magnetic anisotropy change is observed under the negative-bias voltage applications for the case of the Fe layer thinner than 0.6 nm. The amplitude of the voltage-induced anisotropy energy change exhibits a strong Fe-thickness dependence and it reaches as high as $290\mathrm{fJ}/\mathrm{Vm}$. The observed high values of the surface anisotropy and voltage-induced anisotropy energy change demonstrate the feasibility of voltage-driven spintronic devices.

To avoid surface recombination at the contacts and ensure efficient charge collection and high open-circuit voltages ($V_{\mathrm{OC}}$) in organic bulk heterojunction and perovskite solar cells, selective contacts with optimized energy levels are needed. However, a detailed theoretical understanding of how the device performance is affected by surface recombination at the contacts is still lacking. In this work, the influence of surface recombination on the open-circuit voltage in sandwich-type solar cells, with optically thin active layers, is clarified using numerical simulations. Furthermore, analytical expressions are derived, directly relating $V_{\mathrm{OC}}$ to relevant device parameters, such as surface recombination velocity ($S_p$), mobility, and active layer thickness. At large $S_p$, the surface recombination is determined by diffusion-limited transport in the bulk. By reducing $S_p$, thus increasing the charge selectivity of the electrode, the surface recombination is eventually reduced as the transport becomes limited by interface kinetics at the contact. Depending on the interplay between surface recombination and bulk recombination, and the properties of the contacts, different operating regimes are identified featuring different light ideality factors and thickness dependences.

We present superconducting microwave-frequency resonators based on NbTiN nanowires. The small cross section of the nanowires minimizes vortex generation, making the resonators resilient to magnetic fields. Measured intrinsic quality factors exceed $2\times {10}^5$ in a 6-T in-plane magnetic field and $3\times {10}^4$ in a 350-mT perpendicular magnetic field. Because of their high characteristic impedance, these resonators are expected to develop zero-point voltage fluctuations one order of magnitude larger than in standard coplanar waveguide resonators. These properties make the nanowire resonators well suited for circuit QED experiments needing strong coupling to quantum systems with small electric dipole moments and requiring a magnetic field, such as electrons in single and double quantum dots.

We report the experimental demonstration of a magnetologic gate built on graphene at room temperature. This magnetologic gate consists of three ferromagnetic electrodes contacting a single-layer graphene spin channel and relies on spin injection and spin transport in the graphene. We utilize electrical bias tuning of spin injection to balance the inputs and achieve “exclusive or” (xor) logic operation. Furthermore, a simulation of the device performance shows that substantial improvement towards spintronic applications can be achieved by optimizing the device parameters such as the device dimensions. This advance holds promise as a basic building block for spin-based information processing.

The electric-field-modulated nonvolatile resistivity and magnetization switching in elastically coupled ${\mathrm{La}}_{0.67}{\mathrm{Ca}}_{0.33}{\mathrm{MnO}}_3$ films grown on (111)-oriented $0.7\mathrm{Pb}({\mathrm{Mg}}_{1/3}{\mathrm{Nb}}_{2/3}){\mathrm{O}}_3\text{−}0.3{\mathrm{PbTiO}}_3$ substrates is achieved through the ferroelastic effect. By taking advantage of the 180° ferroelectric and non-180° ferroelastic domain switching, we identify that such changes in order parameters stem from domain-switching-induced strain rather than accumulation or depletion of charge carriers at the interface. Specifically, the strong correlation between the ferroelastic strain and the magnetic field is manifested not only by the strain-tunable magnetoresistance effect but also by the magnetically manipulated strain effect, which is essentially driven by the electronic phase separation. These findings present a potential strategy for elucidating the essential physics of the ferroelastic-strain effect and delivering prototype devices for energy-efficient and nonvolatile information storage.

We study high-field (up to $B\sim 8.5\mathrm{T}$) magnetophotocurrent (MPC) related to photogenerated polaron pairs (PPs) in the temperature range $T=10–320\mathrm{K}$ in organic bulk heterojunction photovoltaic cells. We find that in the high-field regime ($B>1\mathrm{T}$), $\mathrm{MPC}(B)$ response increases with $B$ for temperature $T>200\mathrm{K}$ but decreases with $B$ at $T<200\mathrm{K}$. $\mathrm{MPC}(B)$ response does not saturate even at the highest field studied, at all $T$. We attribute the observed high-field $\mathrm{MPC}(B)$ response to two competing mechanisms within the PP spin states: (a) a spin-mixing mechanism caused by the difference in the donor-acceptor (or positive-negative polarons) $g$ factors (the so-called “$\mathrm{\Delta }g$ mechanism”), and (b) the spin polarization induced by thermal population of the PP Zeeman split levels. The nonsaturating $\mathrm{MPC}(B)$ response at high fields and high temperatures indicates that there exist charge-transfer excitons (CTEs) with decay time in the subnanosecond time domain. With decreasing temperature, the CTE decay time sharply increases, thereby promoting an increase of the thermal spin-polarization contribution to the $\mathrm{MPC}(B)$ response.

We show that the magnetic moment of a nanoparticle embedded in the surface of a solid can be switched by surface acoustic waves in the GHz frequency range via a universal mechanism that does not depend on the structure of the particle and the structure of the substrate. It is based upon the generation of the effective ac magnetic field in the coordinate frame of the nanoparticle by the shear deformation of the surface due to surface acoustic waves. The magnetization reversal occurs via a consecutive absorption of surface phonons of the controlled variable frequency. We derive analytical equations governing this process and solve them numerically for the practical range of parameters.

Local oscillatory rheology from echography consists of a traditional rheology experiment synchronized with high-frequency ultrasonic imaging which gives access to the local material response to oscillatory shear. Besides classical global rheological quantities, this method provides quantitative time-resolved information on the local displacement across the entire gap of the rheometer. From the local displacement response, we compute and decompose the local strain in its Fourier components and measure the spatially resolved viscoelastic moduli. After benchmarking our method on homogeneous Newtonian fluids and soft solids, we demonstrate that this technique is well suited to characterize spatially heterogeneous samples, wall slip, and the emergence of nonlinearity under large-amplitude oscillatory stress in soft materials.

We present a density-matrix-based transport model applicable to quantum cascade lasers which computes both linear and nonlinear optical properties coherently and nonperturbatively. The model is applied to a dual-active-region midinfrared quantum cascade laser which generates terahertz radiation at the difference frequency between two midinfrared pumps. A new mechanism for terahertz generation is identified as self-detection, ascribed to the beating of current flow following the intensity, associated with stimulated emission. This mechanism peaks at optical rectification but exhibits a bandwidth reaching significantly into the terahertz range, which is primarily limited by the subpicosecond intersubband lifetimes. A metric is derived to assess the strength of self-detection in candidate active regions through experiment alone, and suggestions are made for improvement of the performance at frequencies below 2 THz.

Soft acoustic metamaterials utilizing mesoporous structures have been proposed recently as a means for tuning the overall effective properties of the metamaterial and providing better coupling to the surrounding air. In this paper, the use of silica aerogel is examined theoretically and experimentally as part of a compact soft acoustic metamaterial structure, which enables a wide range of exotic effective macroscopic properties to be demonstrated, including negative density, density near zero, and nonresonant broadband slow-sound propagation. Experimental data are obtained on the effective density and sound speed using an air-filled acoustic impedance tube for flexural metamaterial elements, which have been investigated previously only indirectly due to the large contrast in acoustic impedance compared to that of air. Experimental results are presented for silica aerogel arranged in parallel with either one or two acoustic ports and are in very good agreement with the theoretical model.

The frequency dependence of the electric-field-induced magneto-optical Faraday effect is investigated in the magnetoelectric antiferromagnet chromia. Two electrically induced Faraday signals superimpose in proportion to the linear magnetoelectric susceptibility $\alpha$ and the antiferromagnetic order parameter $\eta$. The relative strength of these contributions is determined by the frequency of the probing light and can be tuned between extreme characteristics following the temperature dependence of $\alpha$ or $\eta$. The frequency dependence is analyzed in terms of electric dipole transitions of perturbed ${\mathrm{Cr}}^{3+}$ crystal-field states. The results allow us to measure voltage-controlled selection, isothermal switching, and temperature dependence of $\eta$ in a tabletop setup. The voltage-specific Faraday rotation is independent of the sample thickness, making the method scalable and versatile down to the limit of dielectric breakdown.

Electrical readout of spin qubits requires fast and sensitive measurements, which are hindered by poor impedance matching to the device. We demonstrate perfect impedance matching in a radio-frequency readout circuit, using voltage-tunable varactors to cancel out parasitic capacitances. An optimized capacitance sensitivity of $1.6\mathrm{aF}/\sqrt{\mathrm{Hz}}$ is achieved at a maximum source-drain bias of $170\text{−}\mu \mathrm{V}$ root-mean-square and with a bandwidth of 18 MHz. Coulomb blockade in a quantum-dot is measured in both conductance and capacitance, and the two contributions are found to be proportional as expected from a quasistatic tunneling model. We benchmark our results against the requirements for single-shot qubit readout using quantum capacitance, a goal that has so far been elusive.

Nitrogen-vacancy (NV) centers can couple to confined phonons in diamond mechanical resonators via the effect of lattice strain on their energy levels. Access to the strong spin-phonon coupling regime with this system requires resonators with nanoscale dimensions in order to overcome the weak strain response of the NV ground-state spin sublevels. In this work, we incorporate photostable NVs in diamond cantilevers with lateral dimensions of a few hundred nanometers. Coupling of the NV ground-state spin to the mechanical mode is detected in electron spin resonance, and its temporal dynamics are measured via spin echo. Our small mechanical-mode volume leads to a $10\times -100\times$ enhancement in the spin-phonon coupling strength over previous NV-strain coupling demonstrations. This is an important step towards strong spin-phonon coupling, which can enable phonon-mediated quantum-information processing and quantum metrology.

The study of interface and surface states in photonic crystals provides new approaches to control the flow of light. In this work, a type of interface state in two-dimensional photonic crystals is experimentally realized and characterized. The existence of this type of interface state is solely dependent on the reflection properties of the bulk photonic crystals without any surface or interface decorations. Our experimental results are in agreement with numerical simulations, and the propagation properties of these measured interface states indicate that this type of interface state will be a good candidate for future optical waveguiding applications.

We describe a high resolving power technique for Hall-effect measurements, efficient in determining Hall mobility and carrier density in organic field-effect transistors and other low-mobility systems. We utilize a small low-frequency ac magnetic field ($B_{\mathrm{rms}}<0.25\mathrm{T}$) and a phase-sensitive (lock-in) detection of Hall voltage, with the necessary corrections for Faraday induction. This method significantly enhances the signal-to-noise ratio and eliminates the necessity of using high magnetic fields in Hall-effect studies. With the help of this method, we are able to obtain the Hall mobility and carrier density in organic transistors with a mobility as low as $\mu \sim 0.3{\mathrm{cm}}^2{\mathrm{V}}^{-1}{\mathrm{s}}^{-1}$ by using a compact desktop apparatus and low magnetic fields. We find a good agreement between Hall-effect and electric-field-effect measurements, indicating that, contrary to the common belief, certain organic semiconductors with mobilities below $1{\mathrm{cm}}^2{\mathrm{V}}^{-1}{\mathrm{s}}^{-1}$ can still exhibit a fully developed, band-semiconductor–like Hall effect, with the Hall mobility and carrier density matching those obtained in longitudinal transistor measurements. This suggests that, even when $\mu <1{\mathrm{cm}}^2{\mathrm{V}}^{-1}{\mathrm{s}}^{-1}$, charges in organic semiconductors can still behave as delocalized coherent carriers. This technique paves the way to ubiquitous Hall-effect studies in a wide range of low-mobility materials and devices, where it is typically very difficult to resolve the Hall effect even in very high dc magnetic fields.

Recent advances in quantum technology have led to the development and manufacturing of experimental programmable quantum annealers that promise to solve certain combinatorial optimization problems of practical relevance faster than their classical analogues. The applicability of such devices for many theoretical and real-world optimization problems, which are often constrained, is severely limited by the sparse, rigid layout of the devices’ quantum bits. Traditionally, constraints are addressed by the addition of penalty terms to the Hamiltonian of the problem, which, in turn, requires prohibitively increasing physical resources while also restricting the dynamical range of the interactions. Here, we propose a method for encoding constrained optimization problems on quantum annealers that eliminates the need for penalty terms and thereby reduces the number of required couplers and removes the need for minor embedding, greatly reducing the number of required physical qubits. We argue the advantages of the proposed technique and illustrate its effectiveness. We conclude by discussing the experimental feasibility of the suggested method as well as its potential to appreciably reduce the resource requirements for implementing optimization problems on quantum annealers and its significance in the field of quantum computing.

Asymmetric transmission devices that allow acoustic waves to propagate only along one direction receive significant attention because of the rich physics and many potential applications. However, most proposed designs, such as acoustic superlattices and nonlinear acoustic diodes, are much larger than the wavelength, which are impractical for aqueous applications that require unidirectionality at low frequencies. In this work, subwavelength asymmetric acoustic transmission (SAAT) is realized based on low-frequency forbidden transmission of solid-fluid acoustic superlattices. Our design is comprised of a 1D superlattice serving as a low-frequency forbidden transmitter and a periodic rectangular grating serving as the wave-front converter. The SAAT structure and superlattice constant are only $0.6\lambda$ and $0.128\lambda$, respectively, where $\lambda$ is the wavelength, while a very high rectifying efficiency of over ${10}^8$ is achieved. This SAAT design breaks the size-wavelength limitation to allow unidirectional low-frequency acoustic wave transmissions, which is desirable for the miniaturization of integrated acoustic devices in unidirectional signal-transmission applications.

Bulk hexagonal boron nitride (hBN) is a highly nonlinear natural hyperbolic material that attracts major attention in modern nanophotonics applications. However, studies of its optical properties in the visible part of the spectrum and quantum emitters hosted by bulk hBN have not been reported to date. In this work, we study the emission properties of hBN crystals in the red spectral range using sub-band-gap optical excitation. Quantum emission from defects is observed at room temperature and characterized in detail. Our results advance the use of hBN in quantum nanophotonics technologies and enhance our fundamental understanding of its optical properties.

Quantum levitation enabled by repulsive Casimir force has been desirable due to the potential exciting applications in passive-suspension devices and frictionless bearings. In this paper, dynamically tunable stable levitation is theoretically demonstrated based on the configuration of dissimilar gratings separated by an intervening fluid using exact scattering theory. The levitation position is insensitive to temperature variations and can be actively tuned by adjusting the lateral displacement between the two gratings. This work investigates the possibility of applying quantum Casimir interactions into macroscopic mechanical devices working in a noncontact and low-friction environment for controlling the position or transducing lateral movement into vertical displacement at the nanoscale.

We present a spectral analysis of Rydberg atoms in strong microwave fields using electromagnetically induced transparency (EIT) as an all-optical readout. The measured spectroscopic response enables optical, atom-based electric-field measurements of high-power microwaves. In our experiments, microwaves are irradiated into a room-temperature rubidium vapor cell. The microwaves are tuned near the two-photon $65D–66D$ Rydberg transition and reach an electric-field strength of $230\mathrm{V}/\mathrm{m}$, about 20% of the microwave-ionization threshold of these atoms. A Floquet treatment is used to model the Rydberg-level energies and their excitation rates. We arrive at an empirical model for the field-strength distribution inside the spectroscopic cell that yields excellent overall agreement between the measured and calculated Rydberg EIT-Floquet spectra. Using spectral features in the Floquet maps, we achieve an absolute strong-field measurement precision of 6%.