We present an approach to infer the surface density of magnetic moments $I_s$ in ultrathin ferromagnetic films with perpendicular anisotropy. It relies on quantitative stray-field measurements with an atomic-size magnetometer based on the nitrogen-vacancy center in diamond. The method is applied to microstructures patterned in a 1-nm-thick film of CoFeB. We report measurements of $I_s$ with a few percent uncertainty and a spatial resolution in the range of $(100\mathrm{nm}{)}^2$, an improvement by several orders of magnitude over existing methods. As an example of application, we measure the modifications of $I_s$ induced by local irradiation with ${\mathrm{He}}^{+}$ ions in an ultrathin ferromagnetic wire. This method offers a route to study variations of magnetic properties at the nanoscale.

We find that the motion of the valley electrons—electronic states close to the $K$ and $K^{\prime }$ points of the Brillouin zone—is confined into two dimensions when the layers of ${\mathrm{MoS}}_2$ form the $3R$ stacking, while in the $2H$ polytype, the bands have dispersion in all three dimensions. According to our first-principles band-structure calculations, the valley states have no interlayer hopping in $3R\text{−}{\mathrm{MoS}}_2$, which is proven to be the consequence of the rotational symmetry of the Bloch functions. By measuring the reflectivity spectra and analyzing an anisotropic hydrogen-atom model, we confirm that the valley excitons in $3R\text{−}{\mathrm{MoS}}_2$ have two-dimensional hydrogenlike spectral series, and the spreads of the wave function are smaller than the interlayer distance. In contrast, the valley excitons in $2H\text{−}{\mathrm{MoS}}_2$ are well described by the three-dimensional model and, thus, not confined in a single layer. Our results indicate that the dimensionality of the valley degree of freedom can be controlled simply by the stacking geometry, which can be utilized in future valleytronics.

Core-shell nanoparticle systems of Zn-ZnO and $\mathrm{ZnO}\text{−}{\mathrm{TiO}}_2$ are studied computationally using finite-element methods. The inclusion of a surface free energy and the elastic mismatch of the core and shell create an imprinting effect within the shell structure that produces a wide variation of strains. Due to this diversity of strains, the sharp, direct, band-gap edges of the bulk semiconductor are observed to be broadened. We show that a variety of factors, such as particle size, core-to-shell volume ratio, applied hydrostatic pressure, shell microstructure, as well as the effect of elastic anisotropy, can influence the distribution of optical band-gap values throughout the particle.

Near-field heat engines are devices that convert the evanescent thermal field supported by a primary source into usable mechanical energy. By analyzing the thermodynamic performance of three-body near-field heat engines, we demonstrate that the power they supply can be substantially larger than that of two-body systems, showing their strong potential for energy harvesting. Theoretical limits for energy and entropy fluxes in three-body systems are discussed and compared with their corresponding two-body counterparts. Such considerations confirm that the thermodynamic availability in energy-conversion processes driven by three-body photon tunneling can exceed the thermodynamic availability in two-body systems.

We present a detailed theoretical analysis of the electrostatic built-in fields and the electronic structures of polar and semipolar dot-in-a-well systems. Our theory is based on a symmetry-adapted multiband $\mathbf{k}·\mathbf{p}$ model, parametrized by the incline angle to the wurtzite $c$ axis, that accounts fully for the three-dimensional quantum-dot structure. As an example, we apply the model to the experimentally relevant semipolar plane ($11\overline{2}2$). We show here that the built-in fields in isolated ($11\overline{2}2$) semipolar quantum dots are strongly reduced compared to an equivalent $c$-plane structure. Our analysis further reveals that in terms of ground-state transition oscillator strength, the semipolar ($11\overline{2}2$) dot-in-a-well systems show a superior behavior compared with their polar counterpart. We also find that increasing the InN content in the quantum dot up to a critical value leads to the unusual behavior that the ground-state electron and hole wave-function overlap increases and therefore the corresponding oscillator strength. This effect can be attributed to changes in the built-in potential profile inside the semipolar ($11\overline{2}2$) quantum dot.

This paper reports the design, fabrication, and experimental validation of a broadband acoustic cloak for the concealing of three-dimensional (3D) objects placed inside an open cavity with arbitrary surfaces. This 3D cavity cloak represents the acoustic analogue of a magician hat, giving the illusion that a cavity with an object is empty. Transformation acoustics is employed to design this cavity cloak, whose parameters represent an anisotropic acoustic metamaterial. A practical realization is made of 14 perforated layers fabricated by drilling subwavelength holes on 1-mm-thick Plexiglas plates. In both simulation and experimental results, concealing of the reference object by the device is shown for airborne sound with wavelengths between 10 cm and 17 cm.

The evolution of ferroelectric domains in the lead-free $\mathrm{Ba}({\mathrm{Zr}}_{0.2}{\mathrm{Ti}}_{0.8}){\mathrm{O}}_3\text{−}x({\mathrm{Ba}}_{0.7}{\mathrm{Ca}}_{0.3}){\mathrm{TiO}}_3$ (abbreviated as $\mathrm{BZT}\text{−}x\mathrm{BCT}$) piezoelectric ceramic is investigated in situ under an applied electric field using transmission electron microscopy (TEM). Poling-induced, reversible, transformation from a multidomain to a single-domain state is monitored for a large variety of compositions. For all studied materials, this transformation occurs with the appearance of an intermediate nanodomain state at moderate poling fields. According to our results, under high poling fields, a single-domain state vanishes and multiple domains reappear within the grains. Upon further cycling, switching between two different multidomain states occurs. For all $\mathrm{BZT}\text{−}x\mathrm{BCT}$ compositions that we investigate, no sign of the electric-field-induced structural changes is detected using the selected area electron-diffraction (SAED) patterns, which are devoid of the reflection splitting or any detectable changes during electrical poling. The extrinsic contribution to the piezoelectric properties is found to dominate in the $\mathrm{BZT}\text{−}x\mathrm{BCT}$ piezoceramic.

It was recently suggested that molecular junctions would be excellent elements for efficient and high-power thermoelectric energy-conversion devices. However, experimental measurements of thermoelectric conversion in molecular junctions indicate rather poor efficiency, raising the question of whether it is indeed possible to design a setup for molecular junctions that will exhibit enhanced thermoelectric performance. Here we suggest that hybrid single-molecule–nanoparticle junctions can serve as efficient thermoelectric converters. The introduction of a semiconducting nanoparticle introduces new tuning capabilities, which are absent in conventional metal-molecule-metal junctions. Using a generic model for the molecule and nanoparticle with realistic parameters, we demonstrate that the thermopower can be of the order of hundreds of microvolts per degree kelvin and that the thermoelectric figure of merit can reach values close to 1, an improvement of 4 orders of magnitude over existing measurements. This favorable performance persists over a wide range of experimentally relevant parameters and is robust against disorder (in the form of surface-attached molecules) and against electron decoherence at the nanoparticle-molecule interface.

By adding photonic wire structures to an organic microcavity, we create an additional confinement and a Bloch-like band structure in the dispersion of periodically structured cavities. We experimentally observe spontaneous and stimulated emission from the ground and different excited discrete modes at room temperature. By changing the spatial gain distribution via a two-beam interference, we are able to directly control the laser emission from both extended and confined modes of such organic photonic wires. Both spatial distribution and dispersion exhibit coherent emission from tunable modes, which we describe with an analytical model and numerical simulations, in agreement with our measurements.

Searching for solar-absorbing materials containing earth-abundant elements with chemical stability is of critical importance for advancing photovoltaic technologies. Mott insulators have been theoretically proposed as potential photovoltaic materials. In this paper, we evaluate their performance in solar cells by exploring the photovoltaic properties of Mott insulator ${\mathrm{LaVO}}_3$ (LVO). LVO films show an indirect band gap of 1.08 eV as well as strong light absorption over a wide wavelength range in the solar spectrum. First-principles calculations on the band structure of LVO further reveal that the $d\text{−}d$ transitions within the upper and lower Mott-Hubbard bands and $p\text{−}d$ transitions between the O $2p$ and V $3d$ band contribute to the absorption in visible and ultraviolet ranges, respectively. Transport measurements indicate strong carrier trapping and the formation of polarons in LVO. To utilize the strong light absorption of LVO and to overcome its poor carrier transport, we incorporate it as a light absorber in solar cells in conjunction with carrier transporters and evaluate its device performance. Our complementary experimental and theoretical results on such prototypical solar cells made of Mott-Hubbard transition-metal oxides pave the road for developing light-absorbing materials and photovoltaic devices based on strongly correlated electrons.

The acoustic diode (AD) can provide brighter and clearer ultrasound images by eliminating acoustic disturbances caused by sound waves traveling in two directions at the same time and interfering with each other. Such an AD could give designers new flexibility in making ultrasonic sources like those used in medical imaging or nondestructive testing. However, current AD designs, based on nonlinear effects, only partially fill this role by converting sound to a new frequency and blocking any backward flow of the original frequency. In this work, an AD model that preserves the frequencies of acoustic waves and has a relatively high forward-power-transmission rate is proposed. Theoretical analysis indicates that the proposed AD has forward, reverse, and breakdown characteristics very similar to electrical diodes. The significant rectifying effect of the proposed AD is verified numerically through a one-dimensional example. Possible schemes for experimental realization of this model as well as more complex and efficient AD designs are also discussed.

Simulations and experiments are used to study extreme-ultraviolet (EUV) laser drilling of submicrometer holes. The ablation process is studied with a 2D Eulerian hydrodynamic code that includes bound-free absorption processes relevant to the interaction of EUV lasers with a solid material. Good agreement is observed between the simulated and measured ablated depths for on-target irradiances of up to ${1\times 10}^{10}\mathrm{W}{\mathrm{cm}}^{-2}$. An increase in the irradiance to ${1\times 10}^{12}\mathrm{W}{\mathrm{cm}}^{-2}$ is predicted to ablate material to a depth of $3.8\mu \mathrm{m}$ from a single pulse with a hole diameter 3 to 4 times larger than the focal spot size. The model allows for the simulation of the interaction of a laser pulse with the crater created by a previous shot. Multiple-pulse lower-fluence irradiation configurations under optimized focusing conditions, i.e., approaching the diffraction limit, are shown to be advantageous for applications requiring mesoscale [$(100\mathrm{nm})–(1\mu \mathrm{m})$] features and a high level of control over the ablation profile.

We theoretically study the influence of a predominant fieldlike spin-orbit torque on the magnetization switching of small devices with a uniform magnetization. We show that for a certain range of ratios (0.23–0.55) of the Slonczewski torque to the fieldlike torque, it is possible to deterministically switch the magnetization without requiring any external assist field. A precise control of the pulse length is not necessary, but the pulse edge sharpness is critical. The proposed switching scheme is numerically verified by micromagnetic simulations to be effective in devices. Switching without any external assist field is of great interest for the application of spin-orbit torques to magnetic memories.

Titanium dioxide is an important material that has found applications in various fields of technology. Many of its functionalities originate from intrinsic defects. In this paper, by using state-of-the-art hybrid density-functional calculations, a defect structure associated with the Ti vacancy in anatase $\mathrm{Ti}{\mathrm{O}}_2$ is identified. This structure is significantly more stable than the existing model and accounts for experimental observations. We find that the four electron holes resulting from a Ti vacancy are self-compensated by the formation of a dioxygen molecule ${\mathrm{O}}_2$ from two lattice oxygen anions ${\mathrm{O}}^{2-}$. The oxygen molecule has a triplet-spin $S=1$ electronic state and, hence, can be the source of local magnetic moments observed experimentally in undoped samples. In addition to the typical idea of cation vacancies being acceptor-type defects that can accept electrons, we find that the Ti vacancy can actually accommodate extra holes (lose electrons) by forming another oxygen molecule and, therefore, also behave as a donorlike defect (or a hyperdeep donor). Furthermore, the presence of the Ti vacancy causes an up-shift of the electronic states of the neighboring oxygen anions, which can account for a unique feature in the optical absorption spectrum of “yellow” anatase $\mathrm{Ti}{\mathrm{O}}_2$.

Despite the ever-increasing interest in multiscale porous materials, the chemophysical origin of their thermal properties at the nanoscale and its connection to the macroscale properties still remain rather obscure. In this paper, we link the atomic- and macroscopic-level thermal properties by combining tools of statistical physics and mean-field homogenization theory. We begin with analyzing the vibrational density of states of several calcium-silicate materials in the cement paste. Unlike crystalline phases, we indicate that calcium silicate hydrates (CSH) exhibit extra vibrational states at low frequencies ($<2\mathrm{THz}$) compared to the vibrational states predicted by the Debye model. This anomaly is commonly referred to as the boson peak in glass physics. In addition, the specific-heat capacity of CSH in both dry and saturated states scales linearly with the calcium-to-silicon ratio. We show that the nanoscale-confining environment of CSH decreases the apparent heat capacity of water by a factor of 4. Furthermore, full thermal conductivity tensors for all phases are calculated via the Green-Kubo formalism. We estimate the mean free path of phonons in calcium silicates to be on the order of interatomic bonds. This satisfies the scale separability condition and justifies the use of mean-field homogenization theories for upscaling purposes. Upscaling schemes yield a good estimate of the macroscopic specific-heat capacity and thermal conductivity of cement paste during the hydration process, independent of fitting parameters.

Cement paste has a complex distribution of pores and molecular-scale spaces. This distribution controls the hysteresis of water sorption isotherms and associated bulk dimensional changes (shrinkage). We focus on two locations of evaporable water within the fine structure of pastes, each having unique properties, and we present applied physics models that capture the hysteresis by dividing drying and rewetting into two related regimes based on relative humidity (RH). We show that a continuum model, incorporating a pore-blocking mechanism for desorption and equilibrium thermodynamics for adsorption, explains well the sorption hysteresis for a paste that remains above approximately 20% RH. In addition, we show with molecular models and experiments that water in spaces of $\lesssim 1\mathrm{nm}$ width evaporates below approximately 20% RH but reenters throughout the entire RH range. This water is responsible for a drying shrinkage hysteresis similar to that of clays but opposite in direction to typical mesoporous glass. Combining the models of these two regimes allows the entire drying and rewetting hysteresis to be reproduced accurately and provides parameters to predict the corresponding dimensional changes. The resulting model can improve the engineering predictions of long-term drying shrinkage accounting also for the history dependence of strain induced by hysteresis. Alternative strategies for quantitative analyses of the microstructure of cement paste based on this mesoscale physical model of water content within porous spaces are discussed.

Effects of disorder scattering critically influence quantum-transport properties of nanostructures both fundamentally and practically. In this work, we report a theoretical analysis of the important issue of device-to-device quantum-transport variability (DDV) induced by random configurations of discrete dopants. Instead of calculating many impurity configurations by brute force, which is practically impossible to accomplish from first principles, here we use a state-of-the-art atomistic technique where the configurational average is carried out analytically, thereby, DDV can be predicted for any impurity concentration. The DDV we quantitatively analyze is the off-state tunnel conductance variability in Si nanosized field-effect transistor channels with channel lengths ranging from 6.5 to 15.2 nm doped with different concentrations of boron impurity atoms. The variability is predicted by varying the doping concentration, channel length, and the doping positions. We find that doping away from the source or drain contacts of the channel very significantly reduces variability, and doping close to the source or drain produces a nonintuitive outcome of increasing variability. The physics is understood by analyzing the microscopic details of the potential profile in the tunnel barrier. Finally, we organize the ab initio data by a Wentzel-Kramers-Brillouin model.

Liquid-crystal devices are a promising cheap alternative for terahertz light modulation, albeit they suffer from problems associated with thick cells. Here we describe a few-micron-thick polarization-independent nematic liquid-crystal metamaterial device displaying terahertz reflectance modulation depths above 23 dB, millisecond response times, low operating voltages, and a spectral tuning of more than 15%. The dramatic performance improvement is based on invoking critical coupling with external fields, which rests on a suitable choice of resonator geometry. We analyze the coupling mechanism to conclude that perfect absorption can be reached with a wide range of parameters and liquid-crystal materials. The proposed device performance, microscopic details, and the nematic molecule switching dynamics are evaluated with the use of a rigorous tensorial formulation of the Landau–de Gennes theory and shown to be robust to small parameter deviations.

We numerically demonstrate a practical means of systematically controlling topological transport on the surface of a three-dimensional topological insulator, by introducing strong disorder in a layer of depth $d$ extending inward from the surface of the topological insulator. The dependence on $d$ of the density of states, conductance, scattering time, scattering length, diffusion constant, and mean Fermi velocity are investigated. The proposed control via disorder depth $d$ requires that the disorder strength be near the large value which is necessary to drive the topological insulator into the nontopological phase. If $d$ is patterned using masks, gates, ion implantation, etc., then integrated circuits may be fabricated. This technique will be useful for experiments and for device engineering.

Modern electronic devices are unthinkable without the well-controlled formation of interfaces at heterostructures. These structures often involve at least one amorphous material. Modeling such interfaces poses a significant challenge, since a meaningful result can be expected only by using huge models or by drawing from many statistically independent samples. Here we report on the results of high-throughput calculations for interfaces between crystalline silicon ($c\text{−}\mathrm{Si}$) and amorphous silicon nitride ($a\text{−}{\mathrm{Si}}_3{\mathrm{N}}_{3.5}:\mathrm{H}$), which are omnipresent in commercially available solar cells. The findings reconcile only partly understood key features. At the interface, threefold-coordinated Si atoms are present. These are caused by the structural mismatch between the amorphous and crystalline parts. The local Fermi level of undoped $c\text{−}\mathrm{Si}$ lies well below that of $a\text{−}\mathrm{Si}\mathrm{N}:\mathrm{H}$. To align the Fermi levels in the device, charge is transferred from the $a\text{−}\mathrm{Si}\mathrm{N}:\mathrm{H}$ part to the $c\text{−}\mathrm{Si}$ part resulting in an abundance of positively charged, threefold-coordinated Si atoms at the interface. This explains the existence of a positive, fixed charge at the interface that repels holes.