We address the tunneling current in a graphene/$h$-BN/graphene heterostructure as a function of the twisting between the crystals. The twisting induces a modulation of the hopping amplitude between the graphene layers that provides the extra momentum necessary to satisfy momentum and energy conservation and to activate coherent tunneling between the graphene electrodes. Conservation rules limit the tunneling to states with wave vectors lying at the conic curves defined by the intersection of two Dirac cones shifted in momentum and energy. There is a critical voltage where the intersection is a straight line, and the joint density of states presents a maximum. This peak in the number of states where both the wave vectors and the energies are matched reflects in a peak in the tunneling current and in a negative differential conductivity.

We introduce an electrically driven scheme to tune the polariton condensate energy in a high-finesse GaAs microcavity. In contrast to the conventional redshift observed in semiconductor quantum wells (QWs) under applied electrical bias arising from the quantum-confined Stark effect (QCSE), we report here the blueshift of a polariton condensate caused by controlled reduction of the Rabi splitting due to tunneling-induced charge buildup and fractional bleaching of QWs. At larger electrical bias, the QCSE becomes dominant, leading to a redshift in the linear regime, while in the nonlinear regime to the eventual quenching of the condensate emission. This ability to tune the polariton condensate energy brings within reach the realization of voltage-controlled polariton condensate devices and variable-wavelength sources of coherent light.

Titanium dioxide (${\mathrm{TiO}}_2$) plays a central role in the study of artificial photosynthesis, owing to its ability to perform photocatalytic water splitting. Despite over four decades of intense research efforts in this area, there is still some debate over the nature of the first water monolayer on the technologically relevant anatase ${\mathrm{TiO}}_2(101)$ surface. In this work, we use first-principles calculations to reverse engineer the experimental high-resolution x-ray photoelectron spectra measured for this surface by Walle et al. [J. Phys. Chem. C 115, 9545 (2011)] and find evidence supporting the existence of a mix of dissociated and molecular water in the first monolayer. Using both semilocal and hybrid functional calculations, we revise the current understanding of the adsorption energetics by showing that the energetic cost of water dissociation is reduced via the formation of a hydrogen-bonded hydroxyl-water complex. We also show that such a complex can provide an explanation of an unusual superstructure observed in high-resolution scanning tunneling microscopy experiments.

We have investigated the strain-dependent magnetic properties of epitaxial narrow-bandwidth manganite, ${\mathrm{Eu}}_{0.7}{\mathrm{Sr}}_{0.3}{\mathrm{MnO}}_3$ (ESMO), thin films on various substrates traversing from compressive ($-2.4\%$) to tensile ($+1.5\%$) strain. The stoichiometry and crystalline quality of the films were confirmed with diffraction and spectroscopic-based characterization. Using field- and temperature-dependent magnetometry, we find that the epitaxial strain acts to suppress the ferromagnetic state, although films under compressive strain retain a greater propensity for ferromagnetic behavior. However, temperature-dependent resistivity reveals that ESMO thin films exhibit a ferromagnetic insulating state in contrast to wide-bandwidth ${\mathrm{La}}_{0.7}{\mathrm{Sr}}_{0.3}{\mathrm{MnO}}_3$ films under a comparable strain. The combination of a highly strain-dependent ferromagnetic state and robust insulating behavior may offer novel applications in spin filtering, sensing, or electronics.

A semiclassical simulation approach is presented for studying quantum noise in large-scale photonic circuits incorporating an ideal Kerr nonlinearity. A circuit solver is used to generate matrices defining a set of stochastic differential equations, in which the resonator field variables represent random samplings of the Wigner quasiprobability distributions. Although the semiclassical approach involves making a large-photon-number approximation, tests on one- and two-resonator circuits indicate satisfactory agreement between the semiclassical and full-quantum simulation results in the parameter regime of interest. The semiclassical model is used to simulate random errors in a large-scale circuit that contains 88 resonators and hundreds of components in total and functions as a four-bit ripple counter. The error rate as a function of on-state photon number is examined, and it is observed that the quantum fluctuation amplitudes do not increase as signals propagate through the circuit, an important property for scalability.

Multiphoton transitions in $\mathrm{GaSb}/\mathrm{GaAs}$ quantum-dot intermediate-band solar cells are investigated at variable temperature and excitation intensity. A transition temperature is observed that corresponds to the crossover between quantum-dot intraband transitions dominated by thermal escape due to infrared photogeneration. The transition temperature follows an Arrhenius relation with an activation energy of 220 meV that corresponds to the energy barrier observed by holes in the quantum dots. The transition temperature is in the range of 160–225 K for the temperature range studied, significantly higher than observed in previous type-I quantum-dot systems. These results illustrate the potential of type-II structures with deep confinement potentials and strong intraband absorption for future intermediate-band solar cells and quantum devices.

An analytic potential energy function is developed for simulating clusters and nanoparticles of aluminum and its hydride. An embedded-atom method is used which modulates the background electron density as a function of the number of nearest-neighbor atoms. The method is parametrized and tested using an extensive training set computed from first-principle density-functional theory. The potential energy function is found to be reliable for clusters of arbitrary size, shape, and composition ratio. The force field obtained from the analytic potential energy function is computationally efficient and well suited for simulating large systems of aluminum and aluminum hydride particles. A proposed molecular dynamics simulation related to hydrogen-storage technologies for onboard automotive applications is briefly discussed.

We report the existence of confined electronic states at the (110) and (111) surfaces of ${\mathrm{SrTiO}}_3$. Using angle-resolved photoemission spectroscopy, we find that the corresponding Fermi surfaces, subband masses, and orbital ordering are different from the ones at the (001) surface of ${\mathrm{SrTiO}}_3$. This occurs because the crystallographic symmetries of the surface and subsurface planes and the effective electron masses along the confinement direction influence the symmetry of the electronic structure and the orbital ordering of the $t_{2g}$ manifold. Remarkably, our analysis of the data also reveals that the carrier concentration and thickness are similar for all three surface orientations, despite their different polarities. The orientational tuning of the microscopic properties of two-dimensional electron states at the surface of ${\mathrm{SrTiO}}_3$ echoes the tailoring of macroscopic (e.g., transport) properties reported recently in ${\mathrm{LaAlO}}_3/{\mathrm{SrTiO}}_3$ (110) and (111) interfaces, and is promising for searching new types of two-dimensional electronic states in correlated-electron oxides.

We explore the Fe(001)-$\mathit{p}(1\times 1)\mathrm{O}$ surface for use as a spin-polarizing mirror to implement spin sensitivity in display-type electron analyzers. We perform spin-dependent low-energy electron reflection measurements over a wide range of incident electron energies and angles. We derive maps of reflectivity, the Sherman function, and figure of merit. Several working points are identified. A maximum figure of merit of $2.8\times {10}^{-2}$ is found at an incidence angle of 58° and a kinetic energy of 2.8 eV.

Silicene zigzag nanoribbons are studied using ab initio simulation methods. We find novel structure-property relations influenced by several factors, such as the magnitude of the width, spin polarization, spin-orbit coupling, and extended topological defects. It is obtained that while defect-free silicene nanoribbons experience antiferromagnetic-ferromagnetic transition as a function of the width, all defective nanoribbons are ferromagnets. At the same time, the spin-orbit coupling role is significant as it leads to spin-dependent energy gaps in the electronic structure. The origin of edged spin polarization is also studied in terms of the balance between the exchange correlation and kinetic energy contributions. The uncovered unique spin-dependent properties may be useful for the application of silicene nanoribbons in spintronic applications.

Switching the direction of the magnetic moment in a nanomagnet is studied within a modified Slonczewski model that permits torsional oscillations of the magnet. We show that mechanical freedom may inhibit or assist magnetization switching, depending on parameters. Three regimes are studied: switching by torsional oscillations alone, switching by the spin-polarized current with torsional oscillations permitted, and magnetization switching by the current combined with mechanical twist. We show that switching of the magnetic moment is possible in all three cases and that allowing torsional oscillations of the magnet may have certain advantages for applications. Phase diagrams are computed that show the range of parameters required for the switching.

The ability to manipulate microscale and nanoscale particles is highly desirable for various applications ranging from targeting drug delivery to additive manufacturing. Here we report an acoustic sieve that is capable of aligning, trapping, sorting, and transferring a large number of particles according to their size or mass density, all in a tunable manner. The concept is based on the highly localized periodic radiation force induced by the resonance transmission of an acoustic wave across a phononic crystal plate, a phenomenon analogous to the surface-phonon-enhanced optical force, yet the physical concept has not been explored in acoustics. The acoustic sieve demonstrates the effective manipulation of massive particles using an artificially engineered acoustic field by a phononic crystal, and it has potential application for a wide range of applications.

An all-x-ray pump and probe capability is highly desired for the free-electron laser community. A possible implementation involves the use of an x-ray mirror downstream of the sample to backreflect the pump beam onto itself. We expose silicon single crystals, a candidate for this hard-x-ray mirror, to the hard-x-ray beam of the Linac Coherent Light Source (SLAC National Acceleration Laboratory) to assess its suitability. We find that silicon is an appropriate mirror material, but its reflectivity at high x-ray fluences is somewhat unpredictable. We attribute this behavior to x-ray-induced local damage in the mirror, which we have characterized post mortem via microdiffraction, scanning electron microscopy, and Raman spectroscopy. We demonstrate a strategy to reduce local damage by using a structured silicon-based mirror. Preliminary results suggest that the latter yields reproducible Bragg reflectivity at high x-ray fluences, promising a path forward for silicon single crystals as x-ray backreflectors.

A theory of parametric excitation of spin waves (SWs) in ultrathin ferromagnetic strips by a microwave electric field is developed. The excitation uses the effect of voltage-controlled magnetic anisotropy in ferromagnet-dielectric heterostuctures. The characteristic values of the electric field necessary for parametric excitation of propagating SWs of 5–10 GHz frequency in $\mathrm{Fe}/\mathrm{MgO}$ structure are found to be $0.1–1.5\mathrm{V}/\mathrm{nm}$. The minimum excitation threshold is achieved in narrow strip (strip width $w_x\sim 10–20\mathrm{nm}$) for relatively long dipole-dominated SWs. In wider strips ($w_x\gtrsim 100\mathrm{nm}$) electric parametric pumping excites mostly short exchange-dominated SWs having higher excitation thresholds, but substantially wider range of possible SW frequencies.

We report micron-sized Ge crystal arrays grown on deeply patterned Si substrates that yield a surge of the interband photoluminescence intensity by more than 2 orders of magnitude with respect to that typical for epitaxial layers directly grown on planar substrates. This finding is ascribed to the strongly modified internal quantum efficiency induced by controlling the nonradiative recombination at dislocations and to the improved light extraction offered by the array architecture. By spectrally resolving the interband and the dislocation-related luminescence, we address the parasitic activity of extended defects and its impact on the optical properties of the heterosystem. Such results are then exploited along with band gap engineering to design SiGe reflectors and Ge quantum wells that are effective in further amplifying the emission yield.

Spin transport efficiency in heterostructures depends on the spin conductances of each constituent and their interfaces. We report a comparative study of spin pumping in ${\mathrm{Y}}_3{\mathrm{Fe}}_5{\mathrm{O}}_{12}/\mathrm{Cu}/\mathrm{Pt}$ and ${\mathrm{Y}}_3{\mathrm{Fe}}_5{\mathrm{O}}_{12}/\mathrm{Cu}/\mathrm{W}$ trilayers. Surprisingly, the insertion of a Cu interlayer between ${\mathrm{Y}}_3{\mathrm{Fe}}_5{\mathrm{O}}_{12}$ and W substantially improves (over a factor of 4) the spin current injection into W while similar insertion between ${\mathrm{Y}}_3{\mathrm{Fe}}_5{\mathrm{O}}_{12}$ and Pt degrades the spin current. This is a consequence of a much improved ${\mathrm{Y}}_3{\mathrm{Fe}}_5{\mathrm{O}}_{12}/\mathrm{Cu}$ spin mixing conductance relative to that for ${\mathrm{Y}}_3{\mathrm{Fe}}_5{\mathrm{O}}_{12}/\mathrm{W}$. This result implies the possibility of engineered heterostructures with matching spin conductances to enable optimal spin transport efficiency.

We present design, fabrication, and characterization of THz stereometamaterial perfect absorber devices. Stereometamaterials consist of a three-dimensional lattice of meta-atoms, which exhibits distinct optical properties depending on the spatial arrangement of the meta-atoms in the unit cell. Integrated into a perfect absorber structure, the absorption and reflection properties of such devices at resonant frequencies reveal unique characteristics that are controlled by the polarization of the incident wave. Because of the break in reflection symmetry, such devices act as polarization-dependent passive switches, with the frequency response of a single band or a dual band absorber. The observed behavior is interpreted in terms of dipole-dipole interaction between the dipoles of the meta-atoms and their mirror images with respect to the metal backplane.

We propose an advanced formulation of standard antenna theory for the basic investigation and design of resonant nanoparticles. This methodology is based on transforming the original scattering problem into a radiation configuration by invoking the induction theorem. Then applying basic antenna theory principles, such as the suppression of any reactive power, the properties of the resonances are engineered. This nanoantenna approach has been validated by revisiting a number of well-known multilayered core-shell structures. It provides additional important physical insights into how the core-shell structures operate and it enables combinations of different resonant phenomena associated with them, e.g., plasmonic and high-$ϵ$ resonances, in an intuitive manner. Its efficacy is demonstrated by designing a multilayered nanoparticle that achieves lasing with a maximum directivity in the forward direction and a null in the backward direction, i.e., a Huygens source nanoparticle laser.

Metal whiskers often grow across leads of electric equipment and electronic package causing current leakage or short circuits and raising significant reliability issues. The nature of metal whiskers remains a mystery after several decades of research. Here, the existence of metal whiskers is attributed to the energy gain due to electrostatic polarization of metal filaments in the electric field. The field is induced by surface imperfections: contaminations, oxide states, grain boundaries, etc. A proposed theory provides closed form expressions and quantitative estimates for the whisker nucleation and growth rates, explains the range of whisker parameters and effects of external biasing, and predicts statistical distribution of their lengths.

The first-principles simulations are applied to study a photoinduced metastability in amorphous selenium and contribution of the valence-alteration pair (VAP) defects in this process. The VAP defect is confirmed to be the equilibrium defect; it minimizes the destabilizing interaction induced by disorientation of the lone-pair (LP) electrons along the Se chains, and, thus, relieves tension in a system. The photoexcitation involves the LP electrons, and it is proposed to be described by two coexisting processes, namely, single- and double-electron excitations. Both processes are found to induce the defect states in the band gap and cause experimentally observed photodarkening; however, only double-electron excitation is capable of triggering bond rearrangement and structural transformation. Lattice relaxation, which follows bond rearrangement, occurs with characteristic energy of $-0.9\pm 0.3\mathrm{eV}$. It is found to promote formation of energetically favorable VAP defects and to trigger the ringlike to helixlike transformations, thus, ultimately stimulating the photoinduced crystallization. The photoinduced crystallization is directly simulated in a system characterized by increased crystalline order.