We investigate the effects of atomistic interfaces on the device performance of ${\mathrm{Hf}\mathrm{O}}_2$-based ferroelectric tunnel junctions (FTJs) using density-functional calculations. The atomistic structures of ${\mathrm{Hf}\mathrm{O}}_2$ FTJs with different interfacial conditions are constructed and their device performances, such as on:off current ratio are evaluated. We find that without the external effects such as dissimilar metal electrodes or composite layers inserted to make an asymmetric potential barrier, the intrinsic effects stemming from the interface dipoles can be tailored toward achieving high device performance. Especially, the atomistic asymmetry effect, which gives the same effect as the built-in electric field, can be exploited for a high on:off current ratio at low external bias voltages. We demonstrate that the asymmetrically terminated $\mathrm{Zr}$-doped ${\mathrm{Hf}\mathrm{O}}_2$ FTJ exhibits a current ratio of 12, which is higher than any previously reported values, theoretically or experimentally, for ${\mathrm{Hf}\mathrm{O}}_2$-based FTJs with symmetric metal electrodes.

We show that a space-time-varying medium can function as a front-end transceiver, i.e., an antenna-mixer-amplifier. Such a functionality is endowed by space-time surface waves associated with complex space-time wave vectors in a subluminal space-time medium. The proposed structure introduces pure frequency up and down conversions, with very weak undesired time harmonics. In contrast to other recently proposed space-time mixers, a large frequency up- to down-conversion ratio, associated with gain, is achievable. Furthermore, as the structure does not operate based on progressive energy transition between the space-time modulation and the incident wave, it possesses a subwavelength thickness (metasurface). Such a multifunctional, highly efficient, and compact medium is expected to find various applications in modern wireless telecommunication systems.

As a problem in acoustics, sound insulation finds wide applications in diverse situations. We design and experimentally implement an omnidirectional broadband acoustic ventilation barrier using a topology-optimization method. We propose a broadband acoustic barrier constructed of interlaced contraction structure with a central hollow channel, utilizing Bragg’s scattering mechanism happening at impedance discontinuities. Based on this barrier, a density-based topology-optimization method is employed for the inverse design to broaden the sound-insulation band nearly 90% while keeping a low transmission. The topology-optimization method systematically optimizes the scatterers distributed at intervals in our initial barrier in dimensions of size, shape, and orientation to possess many complicated features, which produces the maximum reflection at a desired frequency range. Experiments with acoustic waves of different incident angles are conducted to validate the optimized design, whose results are consistent with the simulations. The measured ventilation rate of the optimized barrier reaches 60.8% compared to initial barrier (49.2%) due to the reduction of fill rate of solid material and aerodynamic loss, demonstrating a high ventilation effect. Our design opens routes to design sound insulators that enable applications to be air permeable yet sound proofing simultaneously.

The presence of nondegenerate valley states in silicon can drastically affect electron dynamics in silicon-based heterostructures, leading to electron spin relaxation and spin-valley coupling. In the context of solid-state spin qubits, it is important to understand the interplay between spin and valley degrees of freedom to avoid or alleviate these decoherence mechanisms. Here we report the observation of relaxation from the excited valley state to the ground state in a $\mathrm{Si}/\mathrm{Si}$-$\mathrm{Ge}$ quantum dot at zero magnetic field. Valley-state readout is aided by a valley-dependent tunneling effect, which we attribute to valley-orbit coupling. We find a long intervalley relaxation time of 12.0 ± 0.3 ms, a value that is unmodified when a magnetic field is applied. Furthermore, we compare our findings with the spin relaxation time and find that the spin-valley “hot spot” relaxation is roughly 4 times slower than intervalley relaxation, consistent with established theoretical predictions. The precision of this technique, adapted from electron spin readout via energy-dependent tunneling, is an improvement over indirect valley relaxation measurements and could be a useful probe of valley physics in spin-qubit and valley-qubit implementations.

Defect-induced interfacial states are created in an atomically thin two-dimensional molybdenum-disulfide channel by underlying a narrow pattern of a graphene layer in a field effect transistor. A nondestructive method for the generation of charge states allowed a highly sensitive molecular interaction with sensitivity of nearly 3 orders of magnitude at room temperature. The presence of interfacial states in the channel leads to a conductance fluctuation and its magnitude is modulated using nitrogen-dioxide gas molecules in the subthreshold region. The study provides a systematic approach to establish a correlation between modulated conductance fluctuation and the molecular concentration up to parts per billion. First-principles density-functional theory further explains the role of unique interfacial configuration on conductance fluctuation. Therefore, our study demonstrates an experimental approach to induce charge state for the modulation of carrier concentration and exploits the role of defect-induced interfacial states in atomically thin interfaces for molecular interaction.

We propose a method to track signals from quadrant photodiodes (QPDs) in heterodyne laser interferometers that employ digital phase-locked loops for phase readout. Instead of separately tracking the four segments from the QPD and then combining the results into length and differential-wavefront-sensing signals, this method employs a set of coupled tracking loops that operate directly on the combined length and angular signals. The benefits are an increased signal-to-noise ratio in the loops and the possibility of adapting the loop bandwidths to the differing dynamical behavior of the signals being tracked, which now correspond to physically meaningful observables. We demonstrate an improvement of up to 6 dB over single-segment tracking, which makes this scheme an attractive solution for applications in precision intersatellite laser interferometry in ultralow light conditions.

The interplay of high operating temperatures and good heat dissipation is crucial for high-performance terahertz quantum cascade lasers. We therefore study the influence on the cross-plane thermal conductivity of different aluminum concentrations in the barrier of ${\mathrm{Ga}\mathrm{As}/\mathrm{Al}}_x{\mathrm{Ga}}_{1-x}\mathrm{As}$ active regions. The thermal conductivity is decreasing from $30\mathrm{W}{\mathrm{K}}^{-1}{\mathrm{m}}^{-1}$ to $12\mathrm{W}{\mathrm{K}}^{-1}{\mathrm{m}}^{-1}$ if the aluminum concentration is increased from 15% to 24%. The temperature during pulsed-laser operation is obtained by measuring the variation of the emission frequency for different laser pulse lengths. This shows, that besides the thermal conductivity, the amount of electric input power has a strong influence on the temperature reached internally during laser operation and is critical for creating high-power devices operating at high temperatures. We show that active regions with thin but high ${\mathrm{Al}}_x{\mathrm{Ga}}_{1-x}\mathrm{As}$ barriers fulfill this need and are well suited for high-temperature operation. A thermal model of the devices allows prediction of the active-region temperature increase for very short pulse durations. For the structure with 24% Al barriers and a starting temperature of 10 K, the model shows an increase by 24 K for a pulse length of only 300 ns.

We demonstrate a multilayer nanoscale cryogenic heater-based switch (M-hTron) that uses a normal-metal heater overlapping a thin-film superconductor separated by a thin insulating layer. The M-hTron eliminates leakage current found in three-terminal superconducting switches and applies heat locally to the superconductor, reducing the energy required to switch the device. Modeling using the energy-balance equations and the acoustic mismatch model demonstrates reasonable agreement with experiment. The M-hTron is a promising device for digital superconducting electronics that require high fan-out and offers the possibility of enhancing readout for superconducting-nanowire single-photon detectors.

Rare-earth doped nanocrystals possess optical transitions with significant either electric or magnetic dipole characters. They are of considerable interest for understanding and engineering light-matter interactions at the nanoscale with numerous applications in nanophotonics. Here, we study the ${}^5{D}_0{\to }^7{F}_1$ transition dipole vector in individual ${\mathrm{NaYF}}_4:{\mathrm{Eu}}^{3+}$ nanorod crystals by Fourier and confocal microscopies. A single-crystal host matrix leads to narrow emission lines at room temperature that permit separation of the Stark sublevels resulting from the crystal-field splitting. We observe a fully magnetic transition and low variability of the transition dipole orientation over several single nanorods. We estimate the proportion of the dipole transitions for the Stark sublevels. We also determine an effective altitude of the rod with respect to the substrate. The narrow emission lines characteristic of ${\mathrm{NaYF}}_4:{\mathrm{Eu}}^{3+}$ ensure well-defined electric or magnetic transitions, and are thus instrumental for probing locally their electromagnetic environment by standard confocal microscopy.

Imbibition in a porous medium plays a central role in diverse energy, environmental, and industrial processes. In many cases, the medium has multiple parallel strata of different permeabilities; however, how this stratification impacts imbibition is poorly understood. We address this gap in knowledge by directly visualizing forced imbibition in three-dimensional porous media with two parallel strata. We find that imbibition is spatially heterogeneous: for a small capillary number Ca, the wetting fluid preferentially invades the fine stratum, while for Ca above a threshold value, the fluid instead preferentially invades the coarse stratum. This threshold value depends on the medium geometry, the fluid properties, and the presence of residual wetting films in the pore space. These findings are well described by a linear stability analysis that incorporates crossflow between the strata. Thus, our work provides quantitative guidelines for predicting and controlling flow in stratified porous media.

Large-scale spiking neural networks have been designed based on superconducting-nanowire single-photon detectors (SNSPDs) as receiver elements for photonic communication between artificial neurons and synapses. Such large-scale artificial neural networks will require thousands of waveguide-integrated SNSPD devices on a single chip. Efficient waveguide-coupled SNSPDs and high-throughput methods for their characterization at this scale are very challenging. Here, we design, fabricate, and measure SNSPDs that are compatible with these large-scale networks. We demonstrate integrated-photonic circuits to simultaneously characterize many waveguide-coupled SNSPDs with a single-fiber input. We achieve up to 15 waveguide-coupled SNSPDs in a single integrated-photonic circuit and a total of 49 waveguide SNSPDs with a detection plateau out of 49 tested. This is perhaps the largest number of SNSPDs integrated in a photonic circuit to date. We employ several types of photonic circuits to enable rapid and reliable characterization of detector performance. We further demonstrate several important synaptic functions of these detectors. These include a binary response in the detectors with average incident photon numbers ranging from less than ${10}^{-3}$ to greater than 10, indicating that synaptic responses based on these detectors are independent of the number of incident photons in a communication pulse. Such a binary response is ideal for communication in neural systems. We further demonstrate that the response has a linear dependence of the output-current pulse height on the bias current with up to a factor of 1.7 tunability in pulse height, a function that cannot be obtained without a detection plateau.

The realization of robust strong coupling and entanglement between distant quantum emitters (QEs) is necessary for scalable quantum-information processes. However, it is hard to achieve it based on conventional systems. Here, we propose theoretically and demonstrate numerically a scheme to realize such strong coupling and entanglement. Our scheme is based on the photonic crystal platform with topologically protected edge state and zero-dimensional topological corner cavities. When the QEs are put into topological cavities, the strong coupling between them can be fulfilled with the assistance of the topologically protected interface state. Such a strong coupling can maintain a very long distance and be robust against various defects. In particular, we numerically prove that the topologically protected entanglement between two QEs can also be realized. Moreover, the duration of quantum beats for such entanglement can reach several orders longer than that for the entanglement in a conventional photonic cavity, making it beneficial for a scalable quantum-information process.

Harvesting of solar energy by hot carriers from optically induced intraband transitions offers new perspectives for photovoltaic energy conversion. Clearly, mechanisms slowing down hot-carrier thermalization constitute a fundamental core of such pathways of third-generation photovoltaics. The intriguing concept of hot polarons stabilized by long-range phonon correlations in charge-ordered strongly correlated three-dimensional metal-oxide perovskite films has emerged and been demonstrated for ${\mathrm{Pr}}_{0.7}{\mathrm{Ca}}_{0.3}{\mathrm{Mn}\mathrm{O}}_3$ at low temperature. In this work, a tailored approach to extending such processes to room temperature is presented. It consists of a specially designed epitaxial growth of two-dimensional Ruddlesden-Popper ${\mathrm{Pr}}_{0.5}{\mathrm{Ca}}_{1.5}{\mathrm{Mn}\mathrm{O}}_4$ films on $\mathrm{Nb}$:${\mathrm{Sr}\mathrm{Ti}\mathrm{O}}_3$ with a charge-ordering transition at $T_{\mathrm{CO}}$ ∼ 320 K. This opens the route to a different phonon-bottleneck strategy of slowing down carrier relaxation by strong coupling of electrons to cooperative lattice modes.

Practical implementations of quantum technologies require preparation of states with a high degree of purity—or, in thermodynamic terms, very low temperatures. Given finite resources, the third law of thermodynamics prohibits perfect cooling; nonetheless, attainable upper bounds for the asymptotic ground-state population of a system repeatedly interacting with quantum thermal machines have been derived. These bounds apply within a memoryless (Markovian) setting, in which each refrigeration step proceeds independently of those previous. Here, we expand this framework to study the effects of memory on quantum cooling. By introducing a memory mechanism through a generalized collision model that permits a Markovian embedding, we derive achievable bounds that provide an exponential advantage over the memoryless case. For qubits, our bound coincides with that of heat-bath algorithmic cooling, which our framework generalizes to arbitrary dimensions. We lastly describe the adaptive stepwise optimal protocol that outperforms all standard procedures.

Magnetic shields that use both active and passive components to enable the generation of a tailored low-field environment are required for many applications in science, engineering, and medical imaging. Until now, accurate field nulling, or field generation, has only been possible over a small fraction of the overall volume of the shield. This is due to the interaction between the active field-generating components and the surrounding high-permeability passive shielding material. In this paper, we formulate the interaction between an arbitrary static current flow on a cylinder and an exterior closed high-permeability cylinder. We modify the Green’s function for the magnetic vector potential and match boundary conditions on the shield’s interior surface to calculate the total magnetic field generated by the system. We cast this formulation into an inverse optimization problem to design active–passive magnetic field shaping systems that accurately generate any physical static magnetic field in the interior of a closed cylindrical passive shield. We illustrate this method by designing hybrid systems that generate a range of magnetic field profiles to high accuracy over large interior volumes, and simulate them in real-world shields whose passive components have finite permeability, thickness, and axial entry holes. Our optimization procedure can be adapted to design active–passive magnetic field shaping systems that accurately generate any physical user-specified static magnetic field in the interior of a closed cylindrical shield of any length, enabling the development and miniaturization of systems that require accurate magnetic shielding and control.

Metasurfaces with in-plane structure asymmetry have been widely explored for realizing high-Q resonances. However, the structure asymmetry parameter, which governs the magnitude of the Q factor, is difficult to modulate directly. This hinders the Q factor from being tuned actively and efficiently. In this paper, a mechanism to excite the high-Q resonance in the in-plane symmetric hole lattice is proposed. The resonance is achieved by use of two perforated metasurfaces, where two localized modes in one layer couple differently with a third mode in the other layer, exhibiting an interplane coupling asymmetry. We further show that, by lateral displacement of one metasurface (approximately λ/20), the electromagnetic coupling asymmetry can be manipulated. The high-Q resonance can thus be switched on or off and tuned dynamically, leading to efficient modulation of the transmission intensity, the Q factor, etc. The results open a way for excitation and dynamic tuning of the high-Q resonance.

For the application of a ferroelectric thin film to optical and electric devices, it is important to align its ferroelectric polarization parallel or perpendicular to the film surface. In this context, an effective method to characterize the polarization directions in the whole area of devices is needed. Here, we report an alternative method to determine a polarization vector in three dimensions using an emission of terahertz waves. The target material is an organic hydrogen-bonded molecular ferroelectric 2-methylbenzimidazole (MBI). By the irradiation of a femtosecond laser pulse on a bulk single-crystal thin film of MBI, we observe the emission of the terahertz waves originating from the polarization modulation of infrared-active phonons via the impulsive stimulated Raman-scattering mechanism. By measuring the terahertz electric field parallel to the ferroelectric polarization with rotating the single-crystal film around two independent axes, we succeed in determining the polarization vector in three dimensions, the direction of which is tilted by 45° normal to the film surface and substrate. This method can be a powerful tool to characterize directions of ferroelectric polarizations in any kinds of thin-film samples.

Reliable in situ control of spin wave (SW) excitation between localized and propagating SW modes is of great interest for both fundamental and applied spintronics and magnonics. While spin-transfer-torque-generated SWs can typically be tuned directly via the driving current, the frequency of the highest intensity SWs, achieved in the strongly self-localized magnetic droplet soliton, is virtually current independent, as the droplet frequency is given by the intrinsic material properties. Here, we demonstrate, using micromagnetic simulations, how the droplet frequency can be efficiently tuned by an applied voltage through the effect of electric field (E-field)-dependent perpendicular magnetic anisotropy (PMA). It is found that as the PMA decreases, the droplet begins to distort and eventually collapses to give way to propagating SWs. However, due to the geometrically confined structures, the radially propagating SWs are reflected by the periphery boundary of the sample, and then the forward and backward SWs superpose to produce a series of standing SWs. The node number of the standing SWs strongly depends on the sample size as well as the applied E field. These findings provide a deeper understanding of magnetic excitation properties, which will be helpful for designing advanced spintronic devices.

This work is focused on understanding the reasons behind the large electrostrictive strain of poly(vinylidene fluoride-trifluoroethylene-chlorotrifluoroethylene) terpolymer. Although a few explanations have been proposed in the literature, it remains largely unclear. Here, the role of an electrically induced phase transition is investigated. The strain in the crystalline part of the polymer is monitored using XRD while an electric field is applied onto the sample. Three regions of interest are clearly evidenced and, of particular interest, we observe a change in crystal symmetry located on the 30–70 V µ${\mathrm{m}}^{-1}$ range. In that region, the lattice progressively loses its hexagonal symmetry and moves toward the phase usually observed at lower temperature, with a higher polar order. In parallel, we conduct macroscopic strain measurements to compare to the XRD data. Three different regimes are also observed with a sudden increase in electrostrictive coefficient on the 30–70 V µ${\mathrm{m}}^{-1}$ interval, going from 19 to 33 ${\mathrm{m}}^4{\mathrm{C}}^{-1}$. This corresponds to a 1% strain, i.e., 25% of the total deformation measured at 100 V µ${\mathrm{m}}^{-1}$. By thoroughly comparing macroscopic strain and x-ray measurements, we are able to single out and quantify the impact of this field-induced phase transition in the polymer overall strain.

Real-time tracking of high-speed objects in cognitive tasks is challenging in the present artificial intelligence techniques because the data processing and computation are time consuming, resulting in impeditive time delays. A brain-inspired continuous attractor neural network (CANN) can be used to track fast moving targets, where the time delays are intrinsically compensated if the dynamical synapses in the network have short-term plasticity. Here, we show that synapses with short-term depression can be realized by a magnetic tunnel junction, which perfectly reproduces the dynamics of the synaptic weight in a widely applied mathematical model. Then, these dynamical synapses are incorporated into one-dimensional and two-dimensional CANNs, which are demonstrated to have the ability to predict a moving object via micromagnetic simulations. This portable spintronics-based hardware for neuromorphic computing needs no training and is therefore very promising for the tracking technology of moving targets.