The mechanism of surface restructuring by ultrashort laser pulses involves a number of fast, nonequilibrium, and interrelated processes while the solid is in a transient state. As a result, the analysis of the experimental data cannot address all of the mechanisms of nanostructuring. In this paper, we present a direct comparison of a simulation and the experimental results of surface nanomodifications induced by a single laser pulse. The experimental results are obtained by using a mask-projection setup with a laser wavelength of 248 nm and a pulse length of 1.6 ps. Two-beam interference of this short wavelength allows for producing a large-area intensity grating of $40\mu \mathrm{m}$ in diameter on a gold surface with a sinusoidal shape and a period of 500 nm. The formed structures are analyzed at the surface and in a cross section by a scanning electron microscope (SEM) and transmission electron microscope (TEM), respectively. Then a hybrid atomistic-continuum model capable of capturing the essential mechanisms responsible for the nanostructuring process is used to model the interaction of the laser pulse with a thick gold target. The good agreement between the modeling results and the experimental data justifies the proposed approach as a powerful tool revealing the physics behind the nanostructuring process at a gold surface and providing a microscopic insight into the dynamics of the structuring processes of metals in general. The presented model, therefore, is an important step towards a computational tool for predicting a materials response to an ultrashort laser pulse on the atomic scale. This detailed understanding of the dynamics of the process will pave the way towards predesigned topologies for functionalized surfaces on nanoscales and microscales.

The inconclusive results of the previous first-principles studies on the $\mathrm{\Sigma }3$ grain boundaries (GBs) in ${\mathrm{CuInSe}}_2$ reveal the importance of employing a method that can correctly describe the electronic structure of this solar-cell material. We employ hybrid functional calculations to study the $\mathrm{\Sigma }3(112)$ and $\mathrm{\Sigma }3(114)$ GBs in ${\mathrm{CuInSe}}_2$ and ${\mathrm{CuGaSe}}_2$. The electronic structure changes introduced by the formation of GBs are threefold: the creation of gap states, a shift in band edges, and the alteration of band-gap sizes. Gap states commonly behave as recombination centers, but the band alignment and the change in the band-gap size induced by GBs mitigate the destructive effect of these states in ${\mathrm{CuInSe}}_2$. That means that $\mathrm{\Sigma }3$ GBs are not detrimental for the carrier transport in devices based on ${\mathrm{CuInSe}}_2$. Conversely, these GBs are destructive for the carrier transport in ${\mathrm{CuGaSe}}_2$. The different behaviors of the $\mathrm{\Sigma }3$ GBs in CISe and CGSe might be considered by experimentalists to optimize the device fabrication to achieve high-performance solar cells.

Spintronic device performance depends critically on magnetization reversal mechanisms, but these are rarely imaged in order to verify correct operation. Here we use magnetometry and magnetic imaging to study thin films and patterned elements of highly spin-polarized ${\mathrm{La}}_{0.67}{\mathrm{Sr}}_{0.33}\mathrm{Mn}{\mathrm{O}}_3$ grown epitaxially on $\mathrm{Nd}\mathrm{Ga}{\mathrm{O}}_3$ substrates whose crystallographic orientation determines magnetic anisotropy strength. Small anisotropy yields gradual magnetization reversal via nucleation and propagation of small-needle domains, whereas large anisotropy yields a single nucleation event resulting in sharp and complete magnetization reversal. We explain these observed differences using micromagnetic simulations, and exploit them in order to quantify the effect of ${\mathrm{La}}_{0.67}{\mathrm{Sr}}_{0.33}\mathrm{Mn}{\mathrm{O}}_3$ electrode behavior on spin signals from hypothetical devices. Our work, therefore, highlights the dramatic discrepancies that can arise between the design and performance of spintronic devices.

The possible existence of short-channel effects in oxide field-effect transistors is investigated by exploring field-effect transistors with various gate lengths fabricated from ${\mathrm{LaAlO}}_3\text{−}{\mathrm{SrTiO}}_3$ heterostructures. The studies reveal the existence of channel-length modulation and drain-induced barrier lowering for gate lengths below $1\mu \mathrm{m}$, with a characteristic behavior comparable to semiconducting devices. With the fabrication of field-effect transistors with gate lengths as small as 60 nm, the results demonstrate the possibility to fabricate by electron-beam lithography functional devices based on complex oxides with characteristic lengths of several tens of nanometers.

Current state-of-the-art Na-ion battery cathodes are selected from the broad chemical space of layered first-row transition-metal (TM) oxides. Unlike their lithium-ion counterparts, seven first-row layered TM oxides can intercalate Na ions reversibly. Their voltage curves indicate significant and numerous reversible phase transformations during electrochemical cycling. These transformations are not yet fully understood but arise from Na-ion vacancy ordering and metal oxide slab glide. In this study, we investigate the nature of vacancy ordering within the $O3$ host lattice framework. We generate predicted electrochemical voltage curves for each of the Na-ion intercalating layered TM oxides by using a high-throughput framework of density-functional-theory calculations. We determine a set of vacancy-ordered phases appearing as ground states in all ${\text{Na}}_{x}M{\mathrm{O}}_2$ systems and investigate the energy effect of the stacking of adjacent layers.

Optical trapping of light-absorbing particles in a gaseous environment is governed by a laser-induced photophoretic force, which can be orders of magnitude stronger than the force of radiation pressure induced by the same light intensity. In spite of many experimental studies, the exact theoretical background underlying the photophoretic force and the prediction of its influence on the particle motion is still in its infancy. Here, we report the results of a quantitative analysis of the photophoretic force and the stiffness of trapping achieved by levitating graphite and graphite-coated glass shells of calibrated sizes in an upright diverging hollow-core vortex beam, which we refer to as an “optical funnel”. The measurements of forces are conducted in air at various gas pressures in the range from 5 mbar to 2 bar. The results of these measurements lay the foundation for mapping the optically induced force to the intensity distribution in the trap. The mapping, in turn, provides the necessary information to model flight trajectories of particles of various sizes entering the beam at given initial speed and position relative to the beam axis. Finally, we determine the limits of the parameter space for the particle speed, size, and radial offset to the beam axis, all linked to the laser power and the particular laser-beam structure. These results establish the grounds for developing a touch-free optical system for precisely positioning submicrometer bioparticles at the focal spot of an x-ray free-electron laser, which will significantly enhance the efficiency of studying nanoscale morphology of proteins and biomolecules in femtosecond coherent diffractive imaging experiments.

The intrinsic Josephson junctions (IJJs) in the high-$T_c$ superconductor ${\mathrm{Bi}}_2{\mathrm{Sr}}_2{\text{CaCu}}_2{\mathrm{O}}_{8+\delta }$ (Bi2212) are shown to have great potential for the construction of an oscillator emitting in the terahertz-frequency $f$ regime. However, earlier devices with Bi2212 substrates exhibit strong self-heating effects detrimental to their operation and limiting the maximum $f$ to approximately 1 THz. Here we describe an efficient fabrication procedure for a stand-alone-mesa IJJ terahertz oscillator with considerably reduced self-heating effects, greatly expanding the tunability and maximum value of $f$, potentially even to 15 THz. Their typical current-voltage characteristics, radiation tunability and power, and some practical uses are also presented.

The performance of organic light-emitting diodes (OLEDs) has traditionally been understood on the basis of one-dimensional (1D) models that exploit their planar symmetry. Recently, however, full 3D models have predicted that the current density in these devices is in fact laterally inhomogeneous and highly filamentary on the nanoscale. Here, we implement a 3D kinetic Monte Carlo model to understand the factors that underlie electrical inhomogeneity in OLEDs and explore how it affects their quantum efficiency roll-off and operational lifetime. We find that current filaments initiate at both injecting contacts and internal organic-organic layer interfaces, driven by local injection barrier minima and propagated by percolation paths that naturally occur within the disordered molecular-site distribution. In a classic bilayer OLED, electron and hole filaments are observed to coexist in the same layer and can bypass one another, resulting in substantial efficiency loss due to charge imbalance. In the case of a double-heterostructure phosphorescent OLED, inhomogeneity leads to locally enhanced exciton-polaron annihilation rates that account for an approximately threefold reduction in the operating lifetime and an order-of-magnitude decrease in the critical current density for quantum efficiency roll-off. These results underscore the importance of considering the 3D nature of current transport in OLEDs and point to an unexpected role of organic heterojunctions in exacerbating the degree of inhomogeneity in multilayer devices.

At separations below 100 nm, Casimir-Lifshitz forces strongly influence the actuation dynamics of microelectromechanical systems (MEMS) in dry vacuum conditions. For a micron-size plate oscillating near a surface, which mimics a frequently used setup in experiments with MEMS, we show that the roughness of the surfaces significantly influences the qualitative dynamics of the oscillator. Via a combination of analytical and numerical methods, it is shown that surface roughness leads to a clear increase of initial conditions associated with chaotic motion, that eventually lead to stiction between the surfaces. Since stiction leads to a malfunction of MEMS oscillators, our results are of central interest for the design of microdevices. Moreover, stiction is of significance for fundamentally motivated experiments performed with MEMS.

By coupling magnetic elements to metamaterials, hybrid metamolecules can be created with useful properties such as photon-magnon mode mixing. Here, we present results for a split-ring resonator (SRR) placed in close proximity to a thin crystalline film of magnetically hard FeCo. Eddy-current shielding is suppressed by patterning the FeCo into $100\text{−}\mu \mathrm{m}$ disks. At the ferromagnetic resonance (FMR) condition of FeCo, photon-magnon coupling strengths of 5% are observed. Altogether, three distinct features are presented and discussed: (i) remanent magnets allow FMR to be performed in a near-zero field, partially eliminating the need for applied fields; (ii) the anisotropic FMR permits angular control over hybrid SRR and FMR resonances; and (iii) the in-plane and out-of-plane magnetization of FeCo opens the door to “magnetically configurable metamaterials” in real time. Finally, a special study is presented of how best to excite the numerous transverse magnetic and electric modes of the SRR by using near-field excitation from a coplanar waveguide.

Optical phase-array antennas can be used to control not only the angular distribution but also the polarization of fluorescence from quantum emitters. The emission pattern of the resulting system is determined by the properties of the antenna, the properties of the emitters, and the strength of the antenna-emitter coupling. Here we show that Fourier polarimetry can be used to characterize these three contributions. To this end, we measure the angle- and Stokes-parameter-resolved emission of bullseye plasmon antennas as well as spiral antennas excited by an ensemble of emitters. We estimate the average antenna-emitter coupling on the basis of the degree of polarization and determine the effect of anisotropy in the intrinsic emitter orientation on polarization of the resulting emission pattern. Our results provide not only new insights into the behavior of bullseye and spiral antennas but also demonstrate the potential of Fourier polarimetry when characterizing antenna-mediated fluorescence.

The efficient internal mixing of colliding droplets upon coalescence is critical to various technological processes such as color manipulation in ink-jet printing and the initiation of the liquid-phase reaction of gelled hypergolic propellants in rocket engines. Recognizing that such processes can be optimized by varying the impact inertia as well as employing fluids of non-Newtonian rheology, the head-on collision, coalescence, and internal mixing pattern between two impacting equal-sized droplets of non-Newtonian fluids is computationally investigated by using the lattice Boltzmann method. Results show that, with increasing non-Newtonian effects, droplet deformation and separation following coalescence is promoted for shear-thinning fluids, while permanent coalescence allowing an extended duration for mixing is promoted for shear-thickening fluids. Furthermore, large-scale internal mixing is promoted for the colliding droplets with larger shear-thinning disparity, while coalescence and mixing is synergistically facilitated for the collision between a shear-thinning droplet and a shear-thickening droplet. The individual and coupled influences of viscosity on the droplet deformation and impact inertia, internal motion, viscous loss, and merging of the colliding interfaces leading to the observed outcomes are mechanistically identified and described.

Complex doping schemes in $R_3{\mathrm{Al}}_5{\mathrm{O}}_{12}$ (where $R$ is the rare-earth element) garnet compounds have recently led to pronounced improvements in scintillator performance. Specifically, by admixing lutetium and yttrium aluminate garnets with gallium and gadolinium, the band gap is altered in a manner that facilitates the removal of deleterious electron trapping associated with cation antisite defects. Here, we expand upon this initial work to systematically investigate the effect of substitutional admixing on the energy levels of band edges. Density-functional theory and hybrid density-functional theory (HDFT) are used to survey potential admixing candidates that modify either the conduction-band minimum (CBM) or valence-band maximum (VBM). We consider two sets of compositions based on ${\mathrm{Lu}}_3{B}_5{\mathrm{O}}_{12}$ where $B$ is Al, Ga, In, As, and Sb, and $R_3{\mathrm{Al}}_5{\mathrm{O}}_{12}$, where $R$ is Lu, Gd, Dy, and Er. We find that admixing with various $R$ cations does not appreciably affect the band gap or band edges. In contrast, substituting Al with cations of dissimilar ionic radii has a profound impact on the band structure. We further show that certain dopants can be used to selectively modify only the CBM or the VBM. Specifically, Ga and In decrease the band gap by lowering the CBM, while As and Sb decrease the band gap by raising the VBM, the relative change in band gap is quantitatively validated by HDFT. These results demonstrate a powerful approach to quickly screen the impact of dopants on the electronic structure of scintillator compounds, identifying those dopants which alter the band edges in very specific ways to eliminate both electron and hole traps responsible for performance limitations. This approach should be broadly applicable for the optimization of electronic and optical performance for a wide range of compounds by tuning the VBM and CBM.

We report an experimental technique for Kelvin probe force microscopy using the dissipation signal of frequency-modulation atomic force microscopy for bias-voltage feedback. It features a simple implementation and faster scanning as it requires no low-frequency modulation. The dissipation is caused by the oscillating electrostatic force that is coherent with the tip oscillation, which is induced by a sinusoidally oscillating voltage applied between the tip and sample. We analyze the effect of the phase of the oscillating force on the frequency shift and dissipation and found that the relative phase of 90° that causes only the dissipation is the most appropriate for Kelvin-probe-force-microscopy measurements. The present technique requires a significantly smaller ac-voltage amplitude by virtue of enhanced force detection due to the resonance enhancement and the use of fundamental flexural-mode oscillation for electrostatic force detection. This feature will be of great importance in the electrical characterizations of technically relevant materials whose electrical properties are influenced by the externally applied electric field as is the case in semiconductor electronic devices.

We report on the realization of an open plane-concave Fabry-Perot resonator with a mode volume below ${\lambda }^3$ at optical frequencies. We discuss some of the less-common features of this microcavity regime and show that the ultrasmall mode volume allows us to detect cavity resonance shifts induced by single nanoparticles even at quality factors as low as 100. Being based on low-reflectivity micromirrors fabricated on a silicon cantilever, our experimental arrangement provides broadband operation, tunability of the cavity resonance, and lateral scanning. These features are interesting for a range of applications including biochemical sensing, modification of photophysics, and optomechanical studies.

We demonstrate the application of reliable methods to determine both the average micromotion energies and the number of sympathetically cooled ions (SCIs) embedded in mixed-ion Coulomb crystals in a linear Paul trap. The number of the SCIs and the micromotion energies for the observed mixed-ion crystals are determined by comparing experimentally obtained images with molecular-dynamics simulations, where the kinetic energies of SCIs trapped in rf fields are averaged in cold elastic collisions between the laser-cooled ions and virtual very light atoms. This combined method quickly achieves the quasiequilibrium state of large mixed Coulomb crystals with over ${10}^3$ ions, regardless of the initial conditions, and shows that the previously used pseudopotential-based adiabatic approximations should be replaced by such molecular-dynamics simulations. In addition, a pattern-matching recognition procedure is introduced which objectively ascertains the number of ions. We also apply the presented characterization method to determine the reaction-rate constant between slow acetonitrile molecules and sympathetically cooled ${\mathrm{Ne}}^{+}$ ions at a translational temperature lower than 10 K.

${\mathrm{Li}}_7{\mathrm{P}}_3{\mathrm{S}}_{11}$ metastable crystal attracts much attention as a solid electrolyte for all-solid-state Li-ion batteries (LIBs) because of its extremely high ionic conductivity at room temperature. Knowing the relationship between the structural framework and dynamics of Li ions will deepen the understanding of lithium-superionic conductors as solid electrolytes in LIBs. However, clarifying the dynamics of Li ions in ${\mathrm{Li}}_7{\mathrm{P}}_3{\mathrm{S}}_{11}$ metastable crystal is hindered by the lack of adequate techniques. In this study, we directly observe the fast Li-ion diffusion in ${\mathrm{Li}}_7{\mathrm{P}}_3{\mathrm{S}}_{11}$ metastable crystal by using state-of-the-art quasielastic neutron scattering. A signal for the self-diffusion of Li ions is clearly observed at 473 K. The jump diffusion model is used to determine the self-diffusion constant $D$, mean residence time ${\tau }_0$, and jump length $⟨l⟩$ of the Li ions. We use crystal-structure analysis to successfully identify $⟨l⟩$ as the average bond length ${l^{\mathrm{av}}}_{\mathrm{Li}─\mathrm{Li}}$ for the nearest-neighbor $\mathrm{Li}─\mathrm{Li}$ bonds. The Li ions are located within stable regions in the conduction pathways, and the potential barrier is low between them. Consequently, we suggest that the Li ions migrate between stable regions within $⟨l⟩=4.3\AA$ in ${\mathrm{Li}}_7{\mathrm{P}}_3{\mathrm{S}}_{11}$ metastable crystal.

We propose a quantum-sensing scheme for measuring weak forces based on a symmetry-breaking adiabatic transition in the quantum Rabi model. We show that the system described by the Rabi Hamiltonian can serve as a sensor for extremely weak forces with sensitivity beyond the yoctonewton (yN) per $\sqrt{\mathrm{Hz}}$ range. We propose an implementation of this sensing protocol using a single trapped ion. A major advantage of our scheme is that the force detection is performed by projective measurement of the population of the spin states at the end of the transition, instead of the far slower phonon number measurement used hitherto.

${\mathrm{Cu}}_2{\mathrm{ZnGeSe}}_4$ is of interest for the development of next-generation thin-film photovoltaic technologies. To understand its electronic structure and related fundamental optical properties, we perform first-principles calculations for three structural variations: kesterite, stannite, and primitive-mixed CuAu phases. The calculated data are compared with the room-temperature dielectric function $ϵ={ϵ}_1+i{ϵ}_2$ spectrum of polycrystalline ${\mathrm{Cu}}_2{\mathrm{ZnGeSe}}_4$ determined by vacuum-ultraviolet spectroscopic ellipsometry in the photon-energy range of 0.7 to 9.0 eV. Ellipsometric data are modeled with the sum of eight Tauc-Lorentz oscillators, and the best-fit model yields the band-gap and Tauc-gap energies of 1.25 and 1.19 eV, respectively. A comparison of overall peak shapes and relative intensities between experimental spectra and the calculated $ϵ$ data for three structural variations suggests that the sample may not have a pure (ordered) kesterite phase. The complex refractive index $N=n+ik$, normal-incidence reflectivity $R$, and absorption coefficients $\alpha$ are calculated from the modeled $ϵ$ spectrum, which are also compared with those of ${\mathrm{Cu}}_2{\mathrm{ZnSnSe}}_4$. The spectral features for ${\mathrm{Cu}}_2{\mathrm{ZnGeSe}}_4$ appear to be weaker and broader than those for ${\mathrm{Cu}}_2{\mathrm{ZnSnSe}}_4$, which is possibly due to more structural imperfections presented in ${\mathrm{Cu}}_2{\mathrm{ZnGeSe}}_4$ than ${\mathrm{Cu}}_2{\mathrm{ZnSnSe}}_4$.

The magnetic switching behavior in continuous NiFe films patterned with IrMn gratings is investigated experimentally and with micromagnetic simulations. The samples made by a two-step deposition process consist of a 10-nm-thick NiFe layer on which is placed 10-nm-thick IrMn stripes with width from 100 to 500 nm and period from 240 nm to $1\mu \mathrm{m}$. Exchange bias is introduced by field cooling in directions parallel or perpendicular to the IrMn stripes. The samples display a two-step hysteresis loop for higher stripe width and period, as the pinned and unpinned regions of the NiFe reverse independently but a one-step loop for lower stripe periods. The transition between these regimes is reproduced by micromagnetic modeling.