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.

Formation of an intermediate band in ${\mathrm{GaN}}_x{\mathrm{P}}_{0.4}{\mathrm{As}}_{0.6-x}$ alloys due to the isovalent doping by nitrogen is studied by photoreflectance and absorption spectroscopy. The fundamental energy gap transition ($E_0$) observed for an N-free alloy is replaced by two optical transitions ($E_{-}$ and $E_{+}$) in GaNPAs layers. The $E_{-}$ and $E_{+}$ transitions are explained within the band anticrossing model, where the localized level of nitrogen interacts with the conduction band of the GaPAs host, splitting it into two subbands. The valence band (VB) is mostly unaffected by nitrogen incorporation as confirmed by the same spin-orbit splitting for N-free and N-containing alloys. The energy position of the $E_{-}$ subband and a strong optical absorption between the VB and the $E_{-}$ subband indicates the GaNPAs alloys have an electronic structure suitable for intermediate-band solar cells. Such an electronic structure is not observed for other III–V alloys like GaInAs, GaInAsP, etc., for which the virtual crystal approximation can be applied to describe the evolution of the electronic structure with the alloy content. Results obtained in this work clearly show that GaNPAs with a few percent of nitrogen is an unusual material system, for which the electronic structure properties differ very significantly from properties of well-known III–V alloys, and the application of virtual crystal approximation in this case is inappropriate or very limited.

We present measurements of rubrene single-crystal field-effect transistors with textbooklike transfer characteristics, as one would expect for intrinsically trap-free semiconductor devices. Particularly, the high purity of the crystals and the defect-free interface to the gate dielectric are reflected in an unprecedentedly low subthreshold swing of $65\mathrm{mV}/\mathrm{decade}$, remarkably close to the fundamental limit of $58.5\mathrm{mV}/\mathrm{decade}$. From these measurements, we quantify the residual density of traps by a detailed analysis of the subthreshold regime, including a full numerical simulation. An exceedingly low trap density of $D_{\text{bulk}}=1\times 10^{13}{\mathrm{cm}}^{-3}{\mathrm{eV}}^{-1}$ at an energy of approximately 0.62 eV is found. This result corresponds to one trap per eV in $10^8$ rubrene molecules. The equivalent density of traps located at the interface ($D_{\text{it}}=3\times 10^9{\mathrm{cm}}^{-2}{\mathrm{eV}}^{-1}$) is as low as in the best crystalline Si/Si field-effect transistors. These results highlight the benefit of having van der Waals bonded semiconducting crystals without electronically active states due to broken bonds at the surface.

The most general response of a small dipolar particle to electromagnetic fields is determined not by its electric and magnetic polarizabilities only but also by magnetoelectric coupling coefficients. We are interested in creating particles with engineered magnetoelectric coupling of nonreciprocal nature, including artificial “moving particles.” Recently, it has been shown theoretically that using small nonreciprocal bianisotropic particles, one can realize perfect electromagnetic isolators, nonreciprocal twist polarizers, one-way transparent sheets, and other devices. This paper is a necessary step towards these applications. For two electrically small nonreciprocal scatterers, an analytical electromagnetic model of polarizabilities is developed. Both particles are bianisotropic: the so-called Tellegen omega particle and moving chiral particle. Analytical results are compared to the full-wave numerical simulations. Both models satisfy to main physical restrictions and leave no doubts in the possibility to realize these particles experimentally.

Surface plasmon polaritons (SPPs) are electromagnetic excitations coupled to electron-charge density waves at metal-dielectric interfaces. They have recently been found to enable a range of nanophotonic devices for controlling systems at the quantum level, including single-photon sources, transistors, and ultracompact quantum circuitry. An important quantum feature of SPPs yet to be fully explored is their bosonic nature. In this work, we report direct evidence of the bosonic nature of SPPs in a scattering-based beam splitter. A parametric down-conversion source is used to produce two indistinguishable photons, each of which is converted into a SPP on a metal-stripe waveguide and then made to interact through a semitransparent Bragg mirror. In this plasmonic analog of the Hong-Ou-Mandel experiment, we measure a coincidence dip with a visibility of 72%, a key signature that SPPs are bosons and that quantum interference is clearly involved. Our work opens up possibilities for the study of fundamental quantum effects in plasmonic systems and their related applications.

Liquid-crystal–based biosensors utilize the high sensitivity of liquid-crystal alignment to the presence of amphiphiles adsorbed to one of the liquid-crystal surfaces from water. They offer inexpensive, easy optical detection of biologically relevant molecules such as lipids, proteins, and cells. Present techniques use linear polarizers to analyze the alignment of the liquid crystal. The resulting images contain information not only about the liquid-crystal tilt with respect to the surface normal, the quantity which is controlled by surface adsorption, but also on the uncontrolled in-plane liquid-crystal alignment, thus making the detection largely qualitative. Here we show that detecting the liquid-crystal alignment between circular polarizers, which are only sensitive to the liquid-crystal tilt with respect to the interface normal, makes possible quantitative detection by measuring the transmitted light intensity with a spectrophotometer. Following a new procedure, not only the concentration dependence of the optical path difference but also the film thickness and the effective birefringence can be determined accurately. We also introduce a new “dynamic” mode of sensing, where (instead of the conventional “steady” mode, which detects the concentration dependence of the steady-state texture) we increase the concentration at a constant rate.

We investigate the dynamics of molecular transport across the gas boundary layer formed above a cold substrate used in the organic vapor phase deposition of small organic molecules. The boundary layer properties ultimately determine film thickness uniformity and morphology, and material utilization efficiency. We use laser-induced fluorescence to spatially resolve the temperature and the concentration of organic molecules within the boundary layer. Under conditions typically used in organic vapor phase deposition of chamber pressures less than 5 torr, we find that the boundary layer extends to a remarkable distance of over 10 cm from the cooled substrate surface. Analytical and numerical models of molecular transport processes are developed to understand the transport of organic molecules to the substrate. Our models provide insights into conditions required to optimize film uniformity and material utilization efficiency in the growth of organic electronic devices.

Nanopore DNA sequencing holds great promise for producing long read lengths from small amounts of starting material, however, high error rates are a problem. We perform nonequilibrium electron transport calculations within an effective tight-binding model of the DNA molecule to study the intrinsic structural noise in DNA sequencing via transverse current in nanopores. The structural noise arises from the effect of neighboring bases on the tunneling current. We find that it could be comparable to the environmental noise, which is caused by changes of the position of the molecule with respect to the electrodes in the nanopore. Moreover, while the environmental noise can be reduced by continuous measurement and by improving the measurement setup, the structural noise is intrinsic. With the help of our methodology we optimize the dependence of the structural noise on the measurement parameters, such as the type of the electrodes and the applied bias. We also propose a statistical technique, utilizing not only the currents through the nucleotides but also the correlations in the currents, to improve the fidelity of the sequencing.

Despite the lack of reproducible experimental confirmation, group-V elements have been considered as possible sources of $p$-type doping in ZnO in the form of simple and complex defects. Using ab initio calculations, based on state-of-the-art hybrid exchange-correlation functional, we study a wide range of defects and defect complexes related with N, P, As, and Sb impurities. We show that none of the candidates for $p$-type doping can be considered a good source of holes in the valence band due to deep acceptor levels and low formation energies of compensating donor defects. In addition, we discuss the stability of complexes in different regimes.

We demonstrate and characterize a scalable optical subsystem for detecting ion qubit states in a surface electrode ion trap. An array of lithographically fabricated diffractive lenses located below the plane of the trap images ions at multiple locations, relaying the collected light out of the vacuum chamber through multimode fibers. The lenses are designed with solid angle collection efficiencies of 3.58%; with all losses included, a detection efficiency of 0.388% is measured. We measure a minimal effect of the dielectric optical substrate on the temporal variation of stray electric fields and the motional heating rate of the ion.