Based on a quantum modeling of the electronic transport, this work shows that ultra-thin solar cells can exhibit an improved open-circuit voltage $V_{\mathrm{OC}}$, without current reduction. This improvement is obtained when an energy-selective contact is considered between the absorber and the reservoir, and is attributed to a hot-carrier effect. While extraction with a nonselective contact does not generate hot carriers, the use of energy-selective contact induces an increase of carrier temperature up to 130 K and a corresponding $V_{\mathrm{OC}}$ enhancement of 41 meV, considering an ($\mathrm{In},\mathrm{Ga})\mathrm{As}$ absorber. This enhancement agrees with a simple and general expression formulated in the quantum thermal machine field. Concerning the current, we show that current through an energy-selective contact is of the same order of magnitude as the one obtained without selectivity. This remarkable behavior, which is explained by the hybridization of states in the absorber with the state of the contact, requires a quantum confinement and thus an ultrathin absorber. Finally, we propose a simple rate model enabling an intuitive interpretation of the numerical results.

We report on a graphical domain engineering technique with the capability to fabricate macroscale domain structures with nanoscale spatial resolution in nonpolar-cut lithium-niobate thin film on insulators through the biased probe tip of scanning atomic force microscopy. It is found that the domain writing process is asymmetric with respect to the spontaneous polarization ${\mathrm{P}}_s$ even though the tip-induced poling field is mirror symmetric. Various domain structures, with a dimension larger than millimeters while consisting of nanoscale domain elements and with arbitrary domain-wall inclination angle with respect to ${\mathrm{P}}_s$ , are designed graphically and then written directly into nonpolar-cut lithium-niobate crystals. As a proof-of-principle demonstration, periodically poled $x$-cut lithium-niobate thin film on insulators with a period of 600 nm, a depth of 460 nm, and a length of approximately $1$ mm is fabricated. This technique could be useful for device applications in integrated optics and optoelectronics and domain-wall nanoelectronics based on lithium niobate on insulators.

We report on experimental and simulation-based results using $(\mathrm{In},\mathrm{Ga})\mathrm{N}$ alloy quantum barriers in c-plane green light-emitting diode (LED) structures as a means to improve vertical carrier transport and reduce forward voltage $(V_F)$. Three-dimensional device simulations that include random alloy fluctuations are used to understand carrier behavior in a disordered potential. The simulated current density–voltage (J-V) characteristics and modified electron-hole overlap $|F_{\mathrm{mod}}{|}^2$ indicate that increasing the indium fraction in the $(\mathrm{In},\mathrm{Ga})\mathrm{N}$ quantum barriers leads to a reduced polarization discontinuity at the interface between the quantum barrier and quantum well, thereby reducing $V_F$ and improving $|F_{\mathrm{mod}}{|}^2$. Maps of electron and hole current through the device show a relatively homogenous distribution in the $XY$ plane for structures using $\mathrm{Ga}\mathrm{N}$ quantum barriers; in contrast, preferential pathways for vertical transport are identified in structures with $(\mathrm{In},\mathrm{Ga})\mathrm{N}$ barriers as regions of high and low current. A positive correlation between hole (electron) current in the p-side (n-side) barrier and indium fraction reveals that preferential pathways exist in regions of high indium content. Furthermore, a negative correlation between the strain ${\epsilon }_{zz}$ and indium fraction shows that high indium content regions have reduced strain-induced piezoelectric polarization in the Z direction due to the mechanical constraint of the surrounding lower indium content regions. Experimentally, multiple quantum well green LEDs with $(\mathrm{In},\mathrm{Ga})\mathrm{N}$ quantum barriers exhibit lower $V_F$ and blue-shifted wavelengths relative to LEDs with $\mathrm{Ga}\mathrm{N}$ quantum barriers, consistent with simulation data. These results can be used to inform heterostructure design of low $V_F$, long-wavelength LEDs and provide important insight into the nature of carrier transport in III-nitride alloy materials.

The electric field tunable band gap and optical properties in low-dimensional materials (quantum-confined Stark effect) are very useful in applications of optoelectronics. In this paper, based on the many-body perturbation method, we investigate the evolution of the quasiparticle electronic structure, exciton, and optical properties of two-dimensional (2D) Hittorf’s phosphorene under an out-of-plane electric field. Compared to other 2D monolayers, the relatively large thickness of Hittorf’s phosphorene leads to a significant reduction in the quasiparticle band gap when an electric field is applied along the quantum confinement direction. The unique bilayer structure, on the other hand, guarantees a well spatial separation of photon-excited electron-hole pairs and, consequently, reduced exciton binding energy under an out-of-plane electric field. These combined effects lead to an almost fixed exciton energy and optical absorption edge at low applied electric fields. However, when the field is larger than 0.1 V/Å, substantial reductions in the exciton energy and optical absorption edge are identified. For the higher-order exciton states, the involvement of more complex band-to-band electron-hole pair formation results in a nonmonotonic electric field dependence. The effective optical modulation accompanied with these giant Stark effects shows potential applications of Hittorf’s phosphorene in 2D optoelectronic devices.

Field-of-view and resolution trade-offs in x-ray micro-computed-tomography (micro-CT) imaging limit the characterization, analysis, and model development of multiscale porous systems. To this end, we develop an applied methodology utilizing deep learning to enhance low-resolution (LR) images over large sample sizes and create multiscale models capable of accurately simulating experimental fluid dynamics from the pore (microns) to continuum (centimeters) scale. We develop a three-dimensional (3D) enhanced deep-superresolution (EDSR) convolutional neural network to create superresolution (SR) images from LR images, which alleviates common micro-CT hardware and/or reconstruction defects in high-resolution (HR) images. When paired with pore-network simulations and parallel computation, we can create large 3D continuum-scale models with spatially varying flow and material properties. We quantitatively validate the workflow at various scales using direct HR and SR image similarity, pore-scale material and/or flow simulations, and continuum-scale multiphase-flow experiments (drainage-immiscible flow pressures and 3D fluid-volume fractions). The SR images and models are comparable to the HR ground truth and generally accurate to within experimental uncertainty at the continuum scale across a range of flow rates. They are found to be significantly more accurate than their LR counterparts, especially in cases where a wide distribution of pore sizes are encountered. The applied methodology opens up the possibility to image, model, and analyze truly multiscale heterogeneous systems that are otherwise intractable.

In quasi-two-dimensional (quasi-2D) perovskites, inorganic quantum wells are sandwiched between organic spacers (barriers) from the energy-level perspective. In addition to quantum confinement, dielectric confinement occurs in the layered perovskites due to a large difference in the dielectric constants of the inorganic framework and the organic spacer layer. While quantum confinement increases the exciton binding energy, deconfinement in the dielectric constant results in a decrease in exciton binding energy and also an increase in carrier mobility. In this work, we investigate the effects of quantum confinement and dielectric deconfinement on the dynamic response of light-emitting diodes (LEDs) based on several series of quasi-2D perovskites and operated under an alternating voltage. With pulsed electroluminescence emission appearing in both bias sections of sinusoidal ac voltages, we show how the quantum confinement and dielectric deconfinement affect and increase the frequency range of ac LED operation, that is, the −3 dB cutoff frequency. This −3-dB frequency could be seen to increase due to a relaxation of quantum and dielectric confinements, leading to an enhancement of the effective carrier mobility in the active emitting material of the devices.

We examine coupling interactions used to synchronize macroscopic Josephson oscillations induced in intrinsic Josephson-junction (IJJ) mesa stacks made of a $\mathrm{Bi}2212$ single crystal. Synchronized radiations of terahertz electromagnetic (EM) waves are detected under common voltage and current bias operations of two connected mesas with close individual radiation frequencies, while uncoupled and bimodal radiations are frequently observed in two mesas with different individual radiation frequencies. Detailed observations of the polarizations of the EM waves emitted when two mesas are biased in parallel or series allow us to reveal the coupling matrix components, which include ratios of synchronized IJJs in the mesas and phase delay between the macroscopic Josephson oscillations. A frequency evolution of the phase delay implies that the coupling between the Josephson oscillations is mediated by the small amplitude Josephson plasmon inside the superconducting substrate. This finding stimulates systematic survey on polarization of EM wave emitted from synchronized multiple mesa devices in order to realize powerful terahertz emissions from superconductors.

Accurate control of light polarization represents a core building block in polarization metrology, imaging, and optical and quantum communications. Voltage-controlled liquid crystals offer an efficient way of polarization transformation. However, common twisted nematic liquid crystals are notorious for lacking an accurate theoretical model linking control voltages and output polarization. An inverse model, which would predict control voltages required to prepare a target polarization, is even more challenging. Here we report both the direct and inverse models based on deep neural networks, radial basis functions, and linear interpolation. We present an inverse–direct compound model solving the problem of control voltages ambiguity. We demonstrate an order of magnitude improvement in accuracy using deep learning compared to the radial basis function method and 2 orders of magnitude improvement compared to the linear interpolation. Errors of the deep neural network model also decrease faster than the other methods with an increasing number of training data. The best direct and inverse models reach average infidelities of $4\times {10}^{-4}$ and $2\times {10}^{-4}$, respectively, which are accuracy levels not reported yet. Furthermore, we demonstrate local and remote preparation of an arbitrary single-photon polarization state using deep learning models. The results will impact the application of twisted-nematic liquid crystals, increasing their control accuracy across the board. The presented bidirectional learning can be used for optimal classical control of complex photonic devices and quantum circuits beyond interpolation.

The perpendicular magnetic anisotropy at magnetic transition-metal–oxide interfaces is a key element in building out-of-plane magnetized magnetic tunnel junctions for spin-transfer-torque magnetic random access memory (STT MRAM). Size downscaling renders magnetic properties more sensitive to thermal effects. Thus, understanding the temperature dependence of the magnetic anisotropy becomes crucial. In this work, we theoretically address the correlation between the temperature dependence of magnetic anisotropy and magnetization in typical $\mathrm{Fe}$/$\mathrm{Mg}\mathrm{O}$-based structures. In particular, the possible mechanisms behind the experimentally reported deviations from the Callen and Callen scaling power law are analyzed. At ideal interfaces, first-principles calculations reveal (i) small high-order anisotropy constants compared to first order and (ii) enhanced exchange constants. Considering these two intrinsic effects in the atomistic simulations, the temperature-dependence of the total and layer-resolved anisotropy are found to follow the Callen and Callen scaling power law, thus ruling out an intrinsic microscopic mechanism underlying deviations from this law. Besides, two possible extrinsic macroscopic mechanisms are unveiled, namely the influence of the dead layer, often present in the storage layer of STT-MRAM cells and the spatial inhomogeneities of the interfacial magnetic anisotropy. About the first mechanism, we show that the presence of a dead layer tends to reduce the scaling exponents. In the second mechanism, increasing the percentage of inhomogeneity in the interfacial perpendicular magnetic anisotropy is revealed to decrease the scaling exponent. These results allow us to coherently explain the difference in scaling exponents relating anisotropy and magnetization thermal variations reported in earlier experiments. This is crucial for the understanding of the thermal stability of the storage-layer magnetization in STT-MRAM applications.

Nonvolatile memory devices have received a lot of interest in both industry and academia in the last decade. Transition metal oxide-based memories offer potential applications as universal memory and artificial synapses. Here we focus on the one-time conditioning of metal/oxide/metal structures leading to the formation of a conducting filament in $\mathrm{Ti}\mathrm{N}/{\mathrm{Ta}}_x{\mathrm{O}}_{1-x}/\mathrm{Ti}\mathrm{N}$ structures and develop a finite-element model of this process. The process considered here consists of two steps. First the thermal runaway increases the temperature of the device and sets up large temperature gradients. In the second step, the lateral temperature gradient drives the ion motion forming a $\mathrm{Ta}$-rich and $\mathrm{O}$-poor filament. The process comes to steady state when the ion flux due to concentration and stress gradients balances the thermophoretic fluxes. The model replicated the structure of the filament including the size of the $\mathrm{Ta}$-rich filament core (20 nm diameter), the surrounding $\mathrm{Ta}$-depleted ring (50 nm), and the compositions of both regions. In addition, the model reproduced characteristic dynamics of the electroformation with slow changes of conductance during the incubation period, rapid increase of conductance during compositional runaway, and saturation. The range of critical material parameters, namely transport heats for $\mathrm{Ta}$ and $\mathrm{O}$, is discussed.

We study the coherence in time and space of electromagnetic fields propagated through complex media. Whether for localization, imaging, or telecommunication, the development of dedicated numerical techniques is generally based on the exploitation of simplified models considering either coherent or diffuse fields. The optimization of such applications in conditions of partial coherence can therefore be particularly challenging, requiring the development of hybrid algorithms adaptable to prior knowledge on the processed fields. The objective of this work is to provide numerical techniques for decomposing an electromagnetic field into subspaces that can then be filtered according to their level of spatial and temporal coherence. In contrast to the studies carried out on space-space transfer matrices notably used for the calculation of Wigner-Smith operators, these decompositions are carried out on space-time matrices in order to facilitate the study of temporal dispersion. The theory is developed for illustrative purposes using experimental results from a leaky resonant system, but seem to be applicable to any scattering and reverberating media capable of transforming localized and coherent excitations into complex and diffuse distributions. To conclude this work, the proposed technique is exploited to improve image reconstruction in a millimeter-wave computational imaging demonstration. In the studied context and from a more general perspective, we propose a technique to select the most suitable subspaces for each application operating under conditions of partial coherence, whether these correspond in the most extreme cases to ballistic paths or to diffuse fields.

We propose a realistic setup, inspired by already existing experiments, within which we develop a general formalism for the implementation of distributed quantum gates. Mediated by a quantum link that establishes a bidirectional quantum channel between distant nodes, our proposal works both for inter- and intranode communication and handles scenarios ranging from the few to the many modes limit of the quantum link. We are able to design fast and reliable state transfer protocols in every regime of operation, which, together with a detailed description of the scattering process, allows us to engineer two sets of deterministic universal distributed quantum gates: gates whose implementation in quantum networks does not need entanglement distribution nor measurements. By employing a realistic description of the physical setup we identify the most relevant imperfections in the quantum links as well as optimal points of operation with resulting infidelities of $1-F\approx {10}^{-2}$–${10}^{-3}$.

We demonstrate that time-delayed nonlinear effects in exciton polaritons can be used to construct neural networks where information is coded in optical pulses arriving consecutively on the sample. The highly nonlinear effects are induced by time-dependent interactions with the excitonic reservoir. These nonlinearities allow the creation of a nonlinear XOR logic gate that can perform operations on the picosecond timescale. An optoelectronic neural network based on the constructed logic gate performs the classification of spoken digits with a high accuracy rate.

The recently discovered antisymmetric interlayer exchange coupling (AIEC) plays pivotal roles in the magnetization switching of a synthetic antiferromagnet by inducing magnetization canting. Large AIEC is reported for perpendicularly magnetized $\mathrm{Pt}/\mathrm{Co}/\mathrm{Ir}/\mathrm{Co}/\mathrm{Pt}$ with wedge-shaped layers. The effective field of AIEC is related with symmetric interlayer exchange coupling, providing a guide for enhancing AIEC. We develop an extended Stoner–Wohlfarth model for a synthetic antiferromagnet, revealing key factors in its magnetization switching. Combining the theoretical knowledge and the experimental results, perpendicular magnetization switching is achieved solely by an in-plane magnetic field.

Memristors are among the most promising elements for modern microelectronics, having unique properties such as quasicontinuous change of conductance and long-term storage of resistive states. However, identifying the physical mechanisms of resistive switching and of the evolution of conductive filaments in such structures still remains a major challenge. In this work, aiming at a better understanding of these phenomena, we experimentally investigate an unusual effect of enhanced conductive filament stability in memristors with copper filaments under the applied voltage and present a simplified theoretical model of the effect of a quantum current through a filament on its shape. Our semiquantitative, continuous model indeed predicts that, for a thin filament, the “quantum pressure” exerted on its walls by the recoil of charge carriers can well compete with the surface tension and crucially affect the evolution of the filament profile at voltages of around 1 V. At lower voltages, the quantum pressure is expected to provide extra stability to the filaments supporting quantized conductance, which we also reveal experimentally using a methodology focusing on retention statistics. Our results indicate that the recoil effects could be potentially important for resistive switching in memristive devices with metallic filaments and that taking them into account in the rational design of memristors could help achieve their better retention and plasticity characteristics.

A dead layer without superconductivity, i.e., zero resistance and Meissner effect, commonly exists at the interface between substrate and superconducting film. It is demonstrated that the dead layer can significantly degrade the performance of superconducting devices at microwave frequencies. Knowing the thickness of the dead layer, $d_{\mathrm{dead}}$, is necessary when using a two-coil mutual-inductance (TCMI) technique to measure the absolute value of the magnetic penetration depth, $\lambda$, in superconducting films. However, there is still no established method to determine $d_{\mathrm{dead}}$ in superconducting films. Here, we propose employing the TCMI technique to determine $d_{\mathrm{dead}}$, based on a multilayered model in which the dead layer is sandwiched between superconducting layer and substrate. Using this method, we find that $d_{\mathrm{dead}}=43\pm 10\mathrm{nm}$ for our superconducting $\mathrm{FeSe}$ films, in agreement with the absence of diamagnetism for the 42-nm-thick film. With the $d_{\mathrm{dead}}$ determined, we accurately determine the absolute value of $\lambda$ in superconducting $\mathrm{FeSe}$ films.

Reverse annealing is a variant of quantum annealing, in which the system is prepared in a classical state, reverse annealed to an inversion point, and then forward annealed. We report on reverse annealing experiments using the D-Wave 2000Q device, with a focus on the $p=2p$-spin problem, which undergoes a second-order quantum phase transition with a gap that closes polynomially in the number of spins. We concentrate on the total and partial success probabilities, with the latter being the probabilities of finding each of two degenerate ground states of all spins up or all spins down, and the former being their sum. The empirical partial success probabilities exhibit a strong asymmetry between the two degenerate ground states, depending on the initial state of the reverse anneal. To explain these results, we perform open-system simulations using master equations in the limits of weak and strong coupling to the bath. The former, known as the adiabatic master equation, with decoherence in the instantaneous energy eigenbasis, predicts perfect symmetry between the two degenerate ground states, thus failing to agree with the experiment. In contrast, the latter, known as the polaron transformed Redfield equation, is in close agreement with experiment. Thus, our results present a challenge to the sufficiency of the weak system-bath coupling limit in describing the dynamics of current experimental quantum annealers, at least for reverse annealing on timescales of a microsecond or longer.

The all-d-metal $\mathrm{Ni}\text{−}(\mathrm{Co})\text{−}\mathrm{Mn}\text{−}\mathrm{Ti}$-based Heusler alloys are found to show a giant magnetocaloric effect near room temperature and are thereby potential materials for solid-state refrigeration. However, the relative large thermal hysteresis and the moderate ferromagnetic magnetization provides limitations for real applications. In the present study, we demonstrate that introducing interstitial B atoms within ${\mathrm{Ni}}_{36.5}{\mathrm{Co}}_{13.5}{\mathrm{Mn}}_{35}{\mathrm{Ti}}_{15}$ alloys can effectively decrease the thermal hysteresis Δ$T_{\mathrm{hys}}$ (down to 4.4 K), and simultaneously improve the saturation magnetization (maximum 40% enhancement) for low concentrations of B doping (up to 0.4 at. %). In comparison to the undoped reference material, the maximum magnetic entropy change (Δ$S_m$) for the ${\mathrm{Ni}}_{36.5}{\mathrm{Co}}_{13.5}{\mathrm{Mn}}_{35}{\mathrm{Ti}}_{15}{\mathrm{B}}_{0.4}$ alloy shows a remarkable improvement from 9.7 to 24.3 J ${\mathrm{kg}}^{-1}{\mathrm{K}}^{-1}$ for an applied magnetic field change (Δ${\mu }_{0}H$) of 5 T (30.2 J ${\mathrm{kg}}^{-1}{\mathrm{K}}^{-1}$ for Δ${\mu }_{0}H$ = 7 T). Additionally, due to the obtained low thermal hysteresis Δ$T_{\mathrm{hys}}$, the maximum reversible Δ$S_m^{\mathrm{rev}}$ amounts to 18.9 J ${\mathrm{kg}}^{-1}{\mathrm{K}}^{-1}$ at 283 K for Δ${\mu }_{0}H$ = 5 T (22.0 J ${\mathrm{kg}}^{-1}{\mathrm{K}}^{-1}$ at 281 K for Δ${\mu }_{0}H$ = 7 T), which is competitive to the traditional $\mathrm{Ni}\text{−}\mathrm{Mn}$-$X$-based Heusler alloys (X = $\mathrm{Ga}$, $\mathrm{In}$, $\mathrm{Sn}$, $\mathrm{Sb}$). The enhancement of the magnetic moments by B doping is also observed in first-principles calculations. These calculations clarify the atomic occupancy of B and the changes in the electronic configuration. Our current study indicates that interstitial doping with a light element (boron) is an effective method to improve the magnetocaloric effect in these all-d-metal $\mathrm{Ni}\text{−}\mathrm{Co}\text{−}\mathrm{Mn}\text{−}\mathrm{Ti}$-based magnetic Heusler compounds.