We study ion partitioning and self-charging of nanoporous electrodes with room-temperature ionic liquids using a classical density-functional theory that accounts for molecular-excluded volume effects and electrostatic correlations. Nanopores of zero electrical potential are predicted to favor adsorption of small ions even without specific surface attraction, and the imbalanced distributions of cations and anions inside the pore induces a net surface charge that promotes further enrichment of small ions. The self-amplified ion partitioning is most significant when the nanopore and the ionic species are of comparable dimension.

Tunnel field-effect transistors based on van der Waals heterostructures are emerging device concepts for low-power applications, auguring $\mathrm{sub}\text{−}60\mathrm{mV}/\mathrm{dec}$ subthreshold swing values. In these devices, the channel is built from a stack of several different two-dimensional materials whose nature allows tailoring the band alignments and enables a good electrostatic control of the device. In this work, we propose a theoretical study of the variability of the performances of a ${\mathrm{MoS}}_2\text{−}{\mathrm{ZrS}}_2$ tunnel field-effect transistor induced by fluctuations of the relative position or the orientation of the layers. Our results indicate that although a steep subthreshold slope ($20\mathrm{mV}/\mathrm{dec}$) is achievable, fluctuations in the relative orientation of the ${\mathrm{ZrS}}_2$ layer with respect to the ${\mathrm{MoS}}_2$ one lead to a significant variability in the tunneling current by about one decade. This arises from changes in the orbital overlap between the layers and from the modulation of the transport direction.

We propose to achieve manipulation of mechanical vibrations at the micron scale by exploiting the interaction of individual, isolated mechanical resonators with surface acoustic waves. We experimentally investigate a sample consisting of cylindrical pillars individually grown by focused-ion-beam-induced deposition on a piezoelectric substrate, exhibiting different geometrical parameters and excited by a long-wavelength surface elastic wave. The mechanical displacement is strongly confined in the resonators, as shown by field maps obtained by laser scanning interferometry. A tenfold displacement field enhancement compared to the vibration at the surface is obtained, revealing that the energy is efficiently coupled. The spatial distribution of the elastic energy at the surface is governed by the geometrical characteristics of the resonators and can therefore be controlled by frequency tuning the elastic wave source. The results show the potential of the proposed approach to achieve dynamic control of surface phonons at the microscale or nanoscale.

The distinct preferential alignment, i.e., horizontal orientation with respect to the substrate plane, of the optical transition dipole moment vectors (TDMVs) of organic dye molecules is of paramount importance for extracting the internally generated power of organic light-emitting diodes (OLEDs) to the outside world. This feature is one of the most promising approaches for the enhancement of the electrical efficacy in state-of-the-art OLEDs, as their internal quantum efficiencies are already close to the ultimate limit. If one can achieve complete horizontal orientation of the TDMVs, it is possible to increase the efficiency by at least 50% because alignment strongly influences the power dissipation into the different optical modes present in such a thin-film device. Thus, this feature of organic light-emitting molecules can lead to advanced performance for future applications. Therefore, we present here a review of recent achievements, ongoing research, and future tasks in this particular area of organic electronics.

Systems that exhibit topologically protected edge states are interesting both from a fundamental point of view as well as for potential applications, the latter because of the absence of backscattering and robustness to perturbations. It is desirable to be able to control and manipulate such edge states. Here, we show that artificial square ices can incorporate both features: an interfacial Dzyaloshinskii-Moriya interaction gives rise to topologically nontrivial magnon bands, and the equilibrium state of the spin ice is reconfigurable with different configurations having different magnon dispersions and topology. The topology is found to develop as odd-symmetry bulk and edge magnon bands approach each other so that constructive band inversion occurs in reciprocal space. Our results show that topologically protected bands are supported in square spin ices.

Nanocomposites of ferroelectric ceramic filler and polymer matrix show considerable promise as high energy-storage dielectric capacitors. However, the influence of the microstructure of the ferroelectric filler on the electric energy-storage performance in the nanocomposite has not been quantitatively studied, yet it is a key element in understanding the methods employed to improve the performance of capacitors. We demonstrate a strategy to enhance the energy-storage density with topological vortex structures in nanocomposites. Using three-dimensional phase field calculations, we show that multivortex structures can exist in ferroelectric nanowires without charge defects or free charges at the interface between the filler and matrix. The switching behavior of the topological structure (vortex and antivortex pair) under external electric field is calculated in nanocylinder wires. The small remnant polarization and very narrow hysteresis loop due to the vortex structure in the nanocomposites can lead to a large enhancement of energy density, as high as $5\mathrm{J}/{\mathrm{cm}}^3$ compared to $1–2\mathrm{J}/{\mathrm{cm}}^3$ for commercial capacitors, and high energy-storage efficiency (over 95%) at a relatively low electric field of $140\mathrm{MV}/\mathrm{m}$.

The influence of the carrier concentration on the elasticity is measured for a microscale silicon resonator. UV radiation is used to generate a surface charge that gates the underlying carrier concentration, as indicated by the device resistance. Correlated with the carrier concentration change is a drop in the resonant frequency that persists for 60 h following exposure. Model calculations show that the change in resonant frequency is due to the modification of the elastic modulus in the near-surface region. This effect becomes increasingly important as device dimensions are reduced to the nanometer scale, and contributes an important source of instability for microscale and nanoscale electromechanical devices operating in radiation environments.

The structural and magnetic properties are the most important parameters for practical applications of Co-based Heusler alloys. The correlations between the crystallization degree, chemical order, magnetic coercivity, saturation magnetization ($M_S$), and in-plane magnetic anisotropies are systematically investigated for ${\mathrm{Co}}_2\mathrm{FeSi}$ (CFS) films fabricated at different temperatures ($T_S$). XRD shows that the CFS layer changes progressively from a disordered crystal structure into a chemically disordered $A2$ structure and further into a chemically ordered $B2$ and even $L{2}_1$ structures when increasing $T_S$ up to $480°\mathrm{C}$. Meanwhile, the static angular remanence magnetization curves show a clear transition of magnetic anisotropy from twofold to fourfold symmetry, due to the competition effect between the uniaxial anisotropy field $H_U$ and biaxial anisotropy field $H_B$. The $H_U$ value is found to be weakly dependent on $T_S$, while $H_B$ shows a continuous enhancement at $T_S>300°\mathrm{C}$, implying that the enhancement of the $L{2}_1$ ordering degree would not weaken the biaxial anisotropy. The varying trend of $H_B$ is similar to $M_S$, which can be respectively attributed to the improved crystal structure and chemical order. The anisotropic fields and their variation behaviors determined by a vibrating sample magnetometer are highly consistent with the results by a time-resolved magneto-optical Kerr effect study. Our findings provide a better understanding of the structural ordering and magnetic anisotropy, which will be helpful for designing advanced spintronic devices.

${\mathrm{Eu}}^{2+}$-doped alkali or alkali earth iodide scintillators with energy resolutions $\le 3\%$ at 662 keV promise the excellent discrimination ability for radioactive isotopes required for homeland-security and nuclear-nonproliferation applications. To extend their applications to x-ray imaging, such as computed tomography scans, the intense afterglow which delays the response time of such materials is an obstacle that needs to be overcome. However, a clear understanding of the origin of the afterglow and feasible solutions is still lacking. In this work, we present a combined experimental and theoretical investigation of the physical insights of codoping-based defect engineering which can reduce the afterglow effectively in ${\mathrm{KCaI}}_3:{\mathrm{Eu}}^{2+}$ single-crystal scintillators. We illustrate that ${\mathrm{Sc}}^{3+}$ codoping greatly suppresses the afterglow, whereas ${\mathrm{Y}}^{3+}$, ${\mathrm{Gd}}^{3+}$, or ${\mathrm{La}}^{3+}$ codoping enhances the afterglow. Meanwhile, a light yield of $57000\mathrm{photons}/\mathrm{MeV}$ and an energy resolution of 3.4% at 662 keV can be maintained with the appropriate concentration of ${\mathrm{Sc}}^{3+}$ codoping, which makes the material promising for medical-imaging applications. Through our thermoluminescence techniques and density-functional-theory calculations, we are able to identify the defect structures and understand the mechanism by which codoping affects the scintillation performance of ${\mathrm{KCaI}}_3:{\mathrm{Eu}}^{2+}$ crystals. The proposed defect-engineering strategy is further validated by achieving afterglow suppression in ${\mathrm{Mg}}^{2+}$ codoped ${\mathrm{KCaI}}_3:{\mathrm{Eu}}^{2+}$ single crystals.

Despite numerous experimental and theoretical studies on the highly miscible $\mathrm{Sn}/\mathrm{Cu}(001)$ system, a conclusive detailed atomic picture of this rich alloy surface phase diagram is still warranted. Depending of the surface coverage of Sn, a rich variety of $\mathrm{Sn}/\mathrm{Cu}(001)$ surface structures may be expected, ranging from the so-called phase I to phase IV. An extreme lack of atomic details about them hinders the overall comprehension of this alloy system. We focus on examining the surface energetics and thermodynamics of the $\mathrm{Sn}/\mathrm{Cu}(001)$ structures to identify the low-energy structures for the experimentally observed phases I–IV. We also discuss the surface electronic structure of these low-energy $\mathrm{Sn}/\mathrm{Cu}(001)$ structures in terms of their surface work functions and surface dipole moments. Finally, we compare the simulated scanning-tunneling-microscopy (STM) images of these $\mathrm{Sn}/\mathrm{Cu}(001)$ phases with available experimental STM measurements. We believe this work sets a good theoretical platform for an accurate further investigation of the $\mathrm{Sn}/\mathrm{Cu}$ bimetallic surface-alloy system for surface-sensitive applications in, e.g., heterogeneous nanocatalysis.

Nonreciprocal microwave devices, such as circulators, are useful in routing quantum signals in quantum networks and protecting quantum systems against noise coming from the detection chain. However, commercial, cryogenic circulators, now in use, are unsuitable for scalable superconducting quantum architectures due to their appreciable size, loss, and inherent magnetic field. We report on the measurement of a key nonreciprocal element, i.e., the gyrator, which can be used to realize a circulator. Unlike state-of-the-art gyrators, which use a magneto-optic effect to induce a phase shift of $\pi$ between transmitted signals in opposite directions, our device uses the phase nonreciprocity of a Josephson-based three-wave-mixing device. By coupling two of these mixers and operating them in noiseless frequency-conversion mode, we show that the device acts as a nonreciprocal phase shifter whose phase shift is controlled by the phase difference of the microwave tones driving the mixers. Such a device could be used to realize a lossless, on-chip, superconducting circulator suitable for quantum-information-processing applications.

While producing comparable efficiencies and showing similar properties when probed by conventional techniques, such as Raman spectroscopy, photoluminescence, and x-ray diffraction, two thin-film solar-cell materials with complex structures, such as quaternary compound ${\mathrm{Cu}}_2{\mathrm{ZnSnSe}}_4$ (CZTSe), may, in fact, differ significantly in their microscopic structures. In this work, laser-induced-modification Raman spectroscopy coupled with high spatial resolution and high-temperature capability is demonstrated as an effective tool to reveal the existence of microscopic scale variations between nominally similar alloys and, thus, to obtain additional structure information beyond what the conventional characterization techniques can offer. Specifically, CZTSe films prepared by sputtering and coevaporation methods that exhibit similar Raman and XRD features are found to behave very differently under high laser power and high-temperature Raman probe because the difference in their microscopic structures leads to different structure modifications in response to the external stimuli, such as light illumination and temperature. They are also shown to undergo different degrees of plastic changes and have different thermal conductivities as revealed by spatially resolved Raman spectroscopy. This technique provides a convenient way to assess and compare materials and provides complementary information for other structural characterization techniques, such as TEM and XRD.

The present communication shows the possibility of observing microdischarges under cathodic polarization during plasma electrolytic oxidation at high frequency. Cathodic microdischarges can ignite beyond a threshold frequency found close to 2 kHz. The presence (respectively, absence) of an electrical double layer is put forward to explain how the applied voltage can be screened, which therefore prevents (respectively, promotes) the ignition of a discharge. Interestingly, in the conditions of the present study, the electrical double layer requires between 175 and $260\mu \mathrm{s}$ to form. This situates the expected threshold frequency between 1.92 and 2.86 kHz, which is in good agreement with the value obtained experimentally.

The discovery of the unprecedented figure of merit $ZT$ of SnSe has sparked a large number of studies on the fundamental physics of this material and further improvement through guided materials design and optimization. Motivated by its rich chemical-bonding characters, unusual multivalley electronic structure, and the sensitivity of the band-edge states to lattice strains, we carry out accurate quasiparticle calculations for the low-temperature phase SnSe under strains. We illustrate how the band-edge states can be engineered by lattice strains, including the size and the nature of the band gap, the positions of the band extrema in the Brillouin zone, and the control of the number of electron and/or hole valleys. The distinct atomic origin and orientation of the wave functions of the different band-edge states dictates the relative shift in their band energy, enabling active control of the near-edge electronic structure of this material. Our work demonstrates that strain engineering is a promising way to manipulate the low-energy electronic structure of SnSe, which can have profound influences on the optical and transport properties of this material.

The manipulation of spin current is a promising solution for low-power devices beyond CMOS. However, conventional methods, such as spin-transfer torque or spin-orbit torque for magnetic tunnel junctions, suffer from large power consumption due to frequent spin-charge conversions. An important challenge is, thus, to realize long-distance transport of pure spin current, together with efficient manipulation. Here, the mechanism of gate-driven pure spin current in graphene is presented. Such a mechanism relies on the electrical gating of carrier-density-dependent conductivity and spin-diffusion length in graphene. The gate-driven feature is adopted to realize the pure spin-current demultiplexing operation, which enables gate-controllable distribution of the pure spin current into graphene branches. Compared with the Elliott-Yafet spin-relaxation mechanism, the D’yakonov-Perel spin-relaxation mechanism results in more appreciable demultiplexing performance. The feature of the pure spin-current demultiplexing operation will allow a number of logic functions to be cascaded without spin-charge conversions and open a route for future ultra-low-power devices.

We investigate the ability of an antineutrino detector to determine the fuel type of a reactor. A hypothetical 5-ton antineutrino detector is placed 25 m from the core and measures the spectral shape and rate of antineutrinos emitted by fission fragments in the core for a number of 90-d periods. Our results indicate that four major fuel types can be differentiated from the variation of fission fractions over the irradiation time with a true positive probability of detection at approximately 95%. In addition, we demonstrate that antineutrinos can identify the burnup at which weapons-grade mixed-oxide (MOX) fuel would be reduced to reactor-grade MOX, on average, providing assurance that plutonium-disposition goals are met. We also investigate removal scenarios where plutonium is purposefully diverted from a mixture of MOX and low-enriched uranium fuel. Finally, we discuss how our analysis is impacted by a spectral distortion around 6 MeV observed in the antineutrino spectrum measured from commercial power reactors.

We optimize the design of NbN nanowire superconducting single-photon detectors using the recently discovered position-dependent detection efficiency in these devices. This optimized design of meandering wire NbN detectors maximizes absorption at positions where photon detection is most efficient by altering the field distribution across the wire. In order to calculate the response of the detectors with different geometries, we use a monotonic local detection efficiency from a nanowire and optical absorption distribution via finite-difference-time-domain simulations. The calculations predict a trade-off between average absorption and absorption at the edge, leading to a predicted optimal wire width close to 100 nm for a 1550-nm wavelength, which drops to a 50-nm wire width for a 600-nm wavelength. The absorption at the edges can be enhanced by depositing a silicon nanowire on top of the superconducting nanowire, which improves both the total absorption efficiency and the internal detection efficiency of meandering wire structures. The proposed structure can be integrated in a relatively simple cavity structure to reach absorption efficiencies of 97% for perpendicular and 85% for parallel polarization.

Voltage-control effects provide an energy-efficient means of tailoring material properties, especially in highly integrated nanoscale devices. However, only insulating and semiconducting systems can be controlled so far. In metallic systems, there is no electric field due to electron screening effects and thus no such control effect exists. Here, we demonstrate that metallic systems can also be controlled electrically through ionic rather than electronic effects. In a $\mathrm{Pt}/\mathrm{Co}$ structure, the control of the metallic $\mathrm{Pt}/\mathrm{Co}$ interface can lead to unprecedented control effects on the magnetic properties of the entire structure. Consequently, the magnetization and perpendicular magnetic anisotropy of the Co layer can be independently manipulated to any desired state, the efficient spin toques can be enhanced about 3.5 times, and the switching current can be reduced about one order of magnitude. This ability to control a metallic system may be extended to control other physical phenomena.