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The electric Freedericksz transition is a second-order quadratic effect, which, in a planarly aligned nematic liquid crystal layer, manifests above a threshold field as a homogeneous symmetric distortion with maximum director-tilt in the midplane. We find that, upon excitation by a low frequency (<0.2Hz) square-wave field, the instability becomes spatially and temporally varying. This is demonstrated using calamitic liquid crystals, initially in the 90°-twisted planar configuration. The distortion occurs close to the negative electrode following each polarity switch and, for low-voltage amplitudes, decays completely in time. We use the elastically favorable geometry of Brochard-Leger walls to establish the location of maximum distortion. Thus, at successive polarity changes, the direction of extension of both annular and open walls switches between the alignment directions at the two substrates. For high voltages, this direction is largely along the midplane director, while remaining marginally oscillatory. These results are broadly understood by taking into account the time-varying and inhomogeneous field conditions that prevail soon after the polarity reverses. Polarity dependence of the instability is traced to the formation of intrinsic double layers that lead to an asymmetry in field distribution in the presence of an external bias. Momentary field elevation near the negative electrode following a voltage sign reversal leads to locally enhanced dielectric and gradient flexoelectric torques, which accounts for the surface-like phenomenon observed at low voltages. These spatiotemporal effects, also found earlier for other instabilities, are generic in nature. [ABSTRACT FROM AUTHOR]

Electric-field-induced patterns of diverse morphology have been observed over a wide frequency range in a recently synthesized bent-core nematic (BCN) liquid crystal. At low frequencies (up to ~25 Hz), the BCN exhibited unusual polarity-dependent patterns. When the amplitude of the ac field was enhanced, these two time-asymmetrical patterns turned into time-symmetrical prewavylike stripes. At ac frequencies in the middle-frequency range (~50-3000 Hz), zigzag patterns were detected whose obliqueness varied with the frequency. Finally, if the frequency was increased above 3 kHz, the zigzag pattern was replaced by another, prewavylike pattern, whose threshold voltage depended on the frequency; however, the wave vector did not. For a more complete characterization, material parameters such as elastic constants, dielectric permittivities, and the anisotropy of the diamagnetic susceptibility were also determined. [ABSTRACT FROM AUTHOR]

It was recently observed that the two antiferromagnetically coupled sublattices of a rare earth-transition metal ferrimagnet can temporarily align ferromagnetically during femtosecond laser heating, but always with the transition metal aligning in the rare earth direction. This behavior has been attributed to the slower magnetization dynamics of the rare earth sublattice. The aim of this work was to assess how the difference in the speed of the transition metal and rare earth dynamics affects the formation of the transient ferromagneticlike state and consequently controls its formation. Our investigation was performed using extensive atomistic spin simulations and analytic micromagnetic theory of ferrimagnets, with analysis of a large area of parameter space such as initial temperature, Gd concentration, and laser fluence. Surprisingly, we found that at high temperatures, close to the Curie point, the rare earth dynamics become faster than those of the transition metal. Subsequently we show that the transient state can be formed with the opposite polarity, where the rare earth aligns in the transition metal direction. Our findings shed light on the complex behavior of this class of ferrimagnetic materials and highlight an important feature which must be considered, or even exploited, if these materials are to be used in ultrafast magnetic devices. [ABSTRACT FROM AUTHOR]

The electroluminescence of a polythiophene wire suspended between a metallic surface and the tip of a scanning tunneling microscope is reported. Under positive sample voltage, the spectral and voltage dependencies of the emitted light are consistent with the fluorescence of the wire junction mediated by localized plasmons. This emission is strongly attenuated for the opposite polarity. Both emission mechanism and polarity dependence are similar to what occurs in organic light emitting diodes (OLED) but at the level of a single molecular wire. [ABSTRACT FROM AUTHOR]

Surface sensitive synchrotron x-ray photoelectron spectroscopy (XPS) and real-time in situ XPS were used to study the thermal stability of the hydroxyl termination and downward band bending on the polar surfaces of ZnO single crystals. On the O-polar face, the position of the Fermi level could be reversibly cycled between the conduction band and the band gap over an energetic distance of approximately 0.8 eV (∼ 1 /4 of the band gap) by controlling the surface H coverage using simple ultrahigh vacuum (UHV) heat treatments up to 750 °C, dosing with H2O/H2 and atmospheric exposure. A metallic to semiconductorlike transition in the electronic nature of the O-polar face was observed at an H coverage of approximately 0.9 monolayers. For H coverage less than this, semiconducting (depleted) O-polar surfaces were created that were reasonably stable in UHV conditions. In contrast, the downward band bending on the Zn-polar face was significantly more resilient, and depleted surfaces could not be prepared by heat treatment alone. [ABSTRACT FROM AUTHOR]

The observation of two polarity-sensitive electrical responses found in the low-frequency (<1 Hz) regime of a square wave field is reported for an achiral rodlike smectic-C liquid crystal with negative dielectric and conductivity anisotropies and in the 90°-twisted configuration. The first involves a transient director modulation appearing at each polarity reversal and vanishing under steady field conditions. The instability is polarity sensitive, with the maximum distortion localized near the negative electrode instead of the sample midplane. This is inferred from the wave-vector orientation alternating in the two halves of the driving cycle between the alignment directions at the two substrates. Various electro-optic characteristics of this temporal phenomenon are also described. Following a similar observation in nematic liquid crystals, we associate the transient periodic order with the Carr-Helfrich mechanism assisted by quadrupolar flexoelectric polarization obtaining under electric field gradients. The second polarity-sensitive effect manifests in the relative shift of the periodic Fréedericksz pattern upon field reversal. The shift, which is linear in field for low fields, tends to saturate for large fields. It is interpreted as due to flexoelectric polarization associated primarily with the c director twist about the layer normal. A model involving a periodic wedgelike band, which has the twist localized within it and is flanked by two uniformly and transversely aligned regions, accounts for the flexoelectric shift of the optical pattern. [ABSTRACT FROM AUTHOR]

By a combined angle-resolved photoemission spectroscopy and density functional theory study, we discover that the surface metallicity is polarity driven in SmB6. Two surface states, not accounted for by the bulk band structure, are reproduced by slab calculations for coexisting B6 and Sm surface terminations. Our analysis reveals that a metallic surface state stems from an unusual property, generic to the (001) termination of all hexaborides: the presence of boron 2p dangling bonds, on a polar surface. The discovery of polarity-driven surface metallicity sheds new light on the 40-year old conundrum of the low-temperature residual conductivity of SmB6, and raises a fundamental question in the field of topological Kondo insulators regarding the interplay between polarity and nontrivial topological properties. [ABSTRACT FROM AUTHOR]

The modified reduced Ostrovsky equation is a reduction of the modified Korteweg-de Vries equation, in which the usual linear dispersive term with a third-order derivative is replaced by a linear nonlocal integral term, representing the effect of background rotation. Here we study the case when the cubic nonlinear term has the same polarity as the rotation term. This equation is integrable provided certain slope constraints are satisfied. We demonstrate, through theoretical analysis and numerical simulations, that when this constraint is not satisfied at the initial time, wave breaking inevitably occurs. [ABSTRACT FROM AUTHOR]

Melamine on Cu(001) is mechanically unstable under the current of a scanning tunneling microscope tip and can switch among configurations. However, these are not equally accessible, and the switching critical current depends on the bias polarity. In order to explain such rich phenomenology, we have developed a scheme to evaluate the evolution of the reaction paths and activation barriers as a function of bias, which is rooted in the nonequilibrium Green's function method implemented within density functional theory. This, combined with the calculation of the inelastic electron tunneling spectroscopy signal, allows us to identify the vibrational modes promoting the observed molecular conformational changes. Finally, once our ab initio results are used within a resonance model, we are able to explain the details of the switching behavior, such as its dependence on the bias polarity, and the noninteger power relation between the reaction rate constants and both the bias voltage and the electric current. [ABSTRACT FROM AUTHOR]

Noise measurements are a probe beyond simple electronic transport that can reveal additional information about electronic correlations and inelastic processes. Here we report noise measurements in individual electromigrated nanojunctions, examining the evolution from the many-channel regime to the tunneling regime, using a radio frequency technique. While we generally observe the dependence of noise on bias expected for shot noise, in approximately 12% of junction configurations we find discrete changes in the bias dependence at threshold values of the bias, consistent with electronic excitation of local vibrational modes. Moreover, with some regularity we find significant mesoscopic variation in the magnitude of the noise in particular junctions even with small changes in the accompanying conductance. In another ~17% of junctions we observe pronounced asymmetries in the inferred noise magnitude as a function of bias polarity, suggesting that investigators should be concerned about current-driven ionic motion in the electrodes even at biases well below those used for deliberate electromigration. [ABSTRACT FROM AUTHOR]

With ellipsometry, x-ray diffraction, and resistance measurements we investigated the electric-field effect on the confined electrons at the LaAlO3/SrTiO3 interface. We obtained evidence that the localization of the electrons at negative gate voltage is induced, or at least enhanced, by a polar phase transition in SrTiO3 which strongly reduces the lattice polarizability and the subsequent screening. In particular, we show that the charge localization and the polar order of SrTiO3 both develop below ~50 K and exhibit similar, unipolar hysteresis loops as a function of the gate voltage. [ABSTRACT FROM AUTHOR]

When wurtzite ZnO is sliced perpendicular to the (0001) axis, two different polar surfaces, the (0001)-Zn and (0001)-O terminated surfaces, are formed. In a simple ionic picture, both surfaces are electrostatically unstable due to a diverging electrostatic energy. Although the ionic picture is an oversimplification, the surfaces adopt a modified surface structure to compensate for the polarity. In close collaboration with experiment, a hexagonal honeycomblike reconstruction has been suggested [J. V. Lauritsen et al., ACS Nano 5, 5987 (2011)]. The remarkable observation is that the (0001)-O surface behaves very differently than the (0001)-Zn surface. Here, we present a detailed density functional theory investigation of the (000i)-O surface, including a systematic investigation of H and Zn coverage as well as an investigation of various surface reconstructions. The difference between the two polar surfaces is explained by the different bonding preferences of Zn and O atoms: as a d element, Zn atoms are more flexible in their bond formation than O atoms. [ABSTRACT FROM AUTHOR]

Bulk magnetoresistance quantum oscillations are observed in high quality single crystal samples of BiTel. This compound shows an extremely large internal spin-orbit coupling, associated with the polarity of the alternating Bi, Te, and I layers perpendicular to the c axis. The corresponding areas of the inner and outer Fermi surfaces around the A point show good agreement with theoretical calculations, demonstrating that the intrinsic bulk Rashba-type splitting is nearly 360 meV, comparable to the largest spin-orbit coupling generated in heterostructures and at surfaces. [ABSTRACT FROM AUTHOR]

We theoretically describe the dynamics of swimmer populations in rigidly confined thin liquid films. We first demonstrate that hydrodynamic interactions between confined swimmers depend solely on their shape and are independent of their specific swimming mechanism. We also show that, due to friction with the nearby rigid walls, confined swimmers do not just reorient in flow gradients but also in uniform flows. We then quantify the consequences of these microscopic interaction rules on the large-scale hydrodynamics of isotropic populations. We investigate in detail their stability and the resulting phase behavior, highlighting the differences with conventional active, three-dimensional suspensions. Two classes of polar swimmers are distinguished depending on their geometrical polarity. The first class gives rise to coherent directed motion at all scales, whereas for the second class we predict the spontaneous formation of coherent clusters (swarms). [ABSTRACT FROM AUTHOR]

We present a computational study on MgO(100) and MgO(l11) nanoribbons with poiar edges as a generic example of poiar two-dimensional irreducible oxide nanostructures. Four inequivalent polarity compensation mechanisms-edge metallization, nonstoichiometry, hydroxylation, and screening by a metal substrate-are analyzed, giving basis for a discussion on their relative efficiency, and enabling a comparison with known polarity characteristics of three-dimensional objects such as bulk surfaces or thin films. Aside from the similarities, we point out that polarity in 2D objects may require smaller compensating charges and thus may be stabilized more easily than in 3D objects. Moreover, compensating charges can be provided more efficiently by the metal substrate and/or humid environment. This suggests that such polar objects may be fabricated and tuned more easily and in a more controlled manner than their 3D counterparts. [ABSTRACT FROM AUTHOR]

We study a Vicsek-style model of self-propelled particles where ferromagnetic and nematic alignment compete in both the usual "metric" version and in the "metric-free" case where a particle interacts with its Voronoi neighbors. We show that the phase diagram of this out-of-equilibrium XY model is similar to that of its equilibrium counterpart: The properties of the fully nematic model, studied before by Ginelli et al. [Phys. Rev. Lett. 104, 184502 (2010)], are thus robust to the introduction of a modest bias of interactions toward ferromagnetic alignment. The direct transitions between polar and nematic ordered phases are shown to be discontinuous in the metric case, and continuous, belonging to the Ising universality class, in the metric-free version. [ABSTRACT FROM AUTHOR]

The interplay of topological defects with curvature is studied for out-of-surface magnetic vortices in thin spherical nanoshells. In the case of an easy-surface Heisenberg magnet it is shown that the curvature of the underlying surface leads to a coupling between the localized out-of-surface component of the vortex with its delocalized in-surface structure, i.e., polarity-chirality coupling. [ABSTRACT FROM AUTHOR]

We studied H-atom migration in SiO2/Si systems triggering O vacancy generation based on first-principles calculations. The migrating H atoms in SiO2 have been found to be mostly negatively charged irrespective of the charge-state polarity in the systems because strong charge transfer from the Si to the O atoms substantially increases the O 2p energy levels. Although the SiO2 systems with no defect are not degraded by the migrating H atoms, 2Si=0-H complex formation in the SiO2/Si systems has been found to generate an O vacancy via H2O molecule desorption after alternate electron transfer between the 2Si=0-H complex and the Si substrate, leading to the final electrical breakdown of SiO2 films. The migration of negatively charged H atoms and their degradation agree well with the recently observed gate bias polarity dependent Si02 film degradation. [ABSTRACT FROM AUTHOR]

In the B2 phase formed by bent-shaped molecules a dense line texture is frequently observed. For the texture description a model is proposed consisting of a periodic system of anticlinic antiferroelectric bulk domains with opposite chiralities separated by π walls in which polarization rotates. The bulk domains are situated between layers of synclinic ferroelectric phase near the upper and lower surfaces. In the surface layers induced by polar anchoring domains of opposite chirality are separated by defect lines. Under the electric field the ferroelectric layer near one sample surface is growing against the antiferroelectric structure in the sample bulk and the stripe texture is fading out, but still in the saturated field surface lines or walls persist separating ferroelectric domains of opposite chirality. The proposed model is adapted also for a case when the anticlinic antiferroelectric structure in the sample bulk is sandwiched between anticlinic ferroelectric layers near the surfaces. In that case the applied electric field eliminates all the lines because the resulting anticlinic ferroelectric structure is racemic, with no chiral domains. [ABSTRACT FROM AUTHOR]

Using density-functional-based total-energy calculations and the LDA-1/2 method to compute approximately quasiparticle band structures, we have studied clean relaxed InN surfaces with varying polarity. More in detail, the c-plane, a-plane, and m-plane surfaces have been investigated. In contrast to the polar faces, which allow Fermi level pinning, the projected fundamental bulk gap of about 0.71 eV is free of surface states in the nonpolar cases. Consequently, freshly cleaved InN surfaces cannot lead to a surface accumulation layer. The different electronic structures modify the surface dipole and hence the ionization energy and electron affinity significantly when varying the surface normal from [0001] via [11́OO] or [112́0] to [0001́]. [ABSTRACT FROM AUTHOR]

We observe a magnetic-field-induced enhancement of the nonlocal spin signal in Ni80Fe20/Ag lateral spin valves. The enhancement depends on the bias current polarity but not on the field direction. We present a theoretical model that explains our experimental results, taking into account the electron-spin relaxation of magnetic impurities. We find that the relaxation is about an order of magnitude weaker than Elliott-Yafet relaxation. [ABSTRACT FROM AUTHOR]

We present an ellipsometry study of the interface between thermotropic liquid crystals and liquid phases consisting of various binary mixtures of water and glycerol. The liquid-crystal samples contain a small constant amount of a surfactant which induces a homeotropic anchoring at the interface. We determine the smectic or nematic order at the interface in the temperature range above the liquid-crystal-isotropic transition while the water to glycerol ratio is varied, corresponding to a systematic modification of the polarity of the liquid phase. The surface-induced order becomes less pronounced with increasing glycerol concentration in the liquid phase. The observed behavior is compared with previous studies in which the surfactant concentration in the liquid-crystal bulk phase was varied. The results indicate that in both cases the magnitude of the surfactant coverage at the interface is the key quantity which determines the liquid-crystal surface order at the interface. [ABSTRACT FROM AUTHOR]

Spin-polarized radio frequency (RF) currents and RF-Oersted fields resonantly excite a magnetic vortex core confined in a micron-scale soft magnetic disk. In this study, we measured the rectifying voltage spectra caused by the anisotropic magnetoresistance oscillation due to the gyration of the vortex with different polarity and chirality. The measured spectra are presented such that we can determine the vortex properties and strength of the spin torques and Oersted field accurately and directly through analytical calculation. [ABSTRACT FROM AUTHOR]

We explore size-dependent magnetic states of sub-100-nm Permalloy nanomagnets. The geometry is suitable for independent setting and readout of vortex polarity and chirality by applying in-plane magnetic fields only. Micromagnetic calculations show that in "Pac-Man"-like magnetic nanodots the relaxation channels to specific chirality and polarity states from uniform magnetization state are deterministic and are not influenced by the presence of moderate out-of-plane fields. The particular geometry opens a straight channel for magnetization relaxation toward a stable closure-domain vortex state with specific chirality and polarity. We show that in these nanomagnets the write process is simple and the signal is easily readable. [ABSTRACT FROM AUTHOR]

Relying on classical electrostatics, we extend the understanding of the electrostatic forces responsible for polarity to nano-objects. We show that while no actual divergence of the electrostatic potential occurs in polar nano-objects, beyond a critical object size, the divergent behavior of the potential differences as a function of size induces polar instability analogous to that at semi-infinite surfaces. We highlight several new and generic properties of polar nano-objects: (1) the dependence of the instability behavior on object dimensionality; (2) the driving role of the smallest structural parameter, not necessarily the thickness; (3) the inhomogeneity of the electrostatic potential within a polar termination and its divergent behavior as a function of the termination size; and (4) the polar character of symmetric stoichiometric objects with a vanishing total dipole moment. We give simple analytical arguments in order to build a generic framework, valid independent of the precise object dimensionality, structure, and ionocovalent characteristics. [ABSTRACT FROM AUTHOR]

This paper studies the propagation of solitons in the nonlinear LC transmission line (NLCTL) with capacitor impurity. Based on Kirchhoff's laws, the numerical simulation shows that the amplitude of the soliton will be increased or decreased when it is close to the positive or negative impurity. Then, it will be split into reflected and transmitted waves by both the positive and negative impurities. Furthermore, their final amplitude and propagating speeds are almost independent of the impurity polarity. The observations near the impurity can be understood in the physical picture of the linear uncoupled energy absorption. By these results, we find that the impurity-soliton interactions (ISIs) in NLCTLs for both inductance and capacitance impurities, which have been seen to be different before, actually can be unified. They also indicate that the ISIs in NLCTLs essentially can be integrated with those in many other soliton systems, such as the Frenkel-Kontorova model and hydrodynamics. Moreover, the impurity-induced influence on the NLCTL solitons can also be well interpreted in the framework of the nonlinear Schrödinger model with an impurity term derived from the discrete voltage propagation equations by means of the perturbation method. [ABSTRACT FROM AUTHOR]

Oxide growth with semiconductorlike accuracy has led to atomically precise thin films and interfaces that exhibit a plethora of phases and functionalities not found in the oxide bulk material. This has yielded spectacular discoveries such as the conducting, magnetic, and even superconducting LaAlO3/SrTiO3 interfaces separating two prototypical insulating perovskite materials. All these investigations, however, consider the static state at the interface, although studies on fast oxide interface dynamics would introduce a powerful degree of freedom to understanding the nature of the LaAlO3/SrTiO3 interface state. Here, we show that the polarization state at the LaAlO3/SrTiO3 interface can be optically enhanced or attenuated within picoseconds. Our observations are explained by a model based on charge propagation effects in the interfacial vicinity and transient polarization buildup at the interface. [ABSTRACT FROM AUTHOR]

We have measured the K-shell and total internal conversion coefficients (ICCs), aK and aT, for the 109.3-keV M4 transition in 125Te to be 185.0(40) and 350.0(38), respectively. Previously this transition's ICCs were considered to be anomalous, with measured values lying below calculated ones. When compared with Dirac-Fock calculations, our new results show good agreement. The aK result agrees well with the version of the theory that takes account of the K-shell atomic vacancy and disagrees with the one that does not. This is consistent with our conclusion drawn from a series of measurements on high multipolarity transitions. [ABSTRACT FROM AUTHOR]

Triboelectric charging, the phenomenon by which electrical charge is exchanged during contact between two surfaces, has been known to cause significant charge separation in granular mixtures, even between chemically identical grains. This charging is a stochastic size-dependent process resulting from random collisions between grains. The prevailing models and experimental results suggest that, in most cases, larger grains in a mixture of dielectric grains acquire a positive charge, while smaller grains charge negatively. These models are typically restricted to mixtures of two discrete grain sizes, which are not representative of most naturally occurring granular mixtures, and neglect the effect of grain size on individual charging events. We have developed a model that predicts the average charge distribution in a granular mixture, for any continuous size distribution of dielectric grains of a single material. Expanding to continuous size distributions enables the prediction of charge separation in many natural granular phenomena, including terrestrial dust storms and industrial powder handling operations. The expanded model makes predictions about the charge distribution, including specific conditions under which the usual size-dependent polarity is reversed such that larger grains charge negatively. [ABSTRACT FROM AUTHOR]

Vortex core polarity switching in NiFe disks has been evidenced using an all-electrical magnetoresistive rectification scheme. Simulation and experiments yield a consistent rectified signal loss when driving core gyration at high powers. With increasing power, the frequency range over which the loss occurs grows and the resonance downshifts in frequency, consistent with nonlinear core dynamics and periodic core polarity switching induced by the core reaching its critical velocity. Core-polarity-dependent rectification signals enable an independent verification of the switched core polarity. We also demonstrate the ability to impede core polarity switching by displacing the core towards the disk's edge where an increased core stiffness reduces the core velocity. [ABSTRACT FROM AUTHOR]

We study, by means of an atomistic toy model, the interplay of ferroelastic twin patterns and electrical polarization. Our molecular dynamics simulations reproduce polarity in straight twin walls as observed experimentally. We show, by making contact with continuum theory, that the effect is governed by linear flexoelectricity. Complex twin patterns, with very high densities of kinks and/or junctions, produce winding structures in the dipolar field, which are reminiscent of polarization vortices. By means of a "cold shearing" technique, we produce patches with high vortex densities; these unexpectedly show a net macroscopic polarization even if neither the original sample nor the applied mechanical perturbation breaks inversion symmetry by itself. These results may explain some puzzling experimental observations of "parasitic" polarity in the paraelectric phase of BaTiO3 and LaAlO3. [ABSTRACT FROM AUTHOR]

We demonstrate tunable transverse rectification in a density-modulated two-dimensional electron gas (2DEG). The density modulation is induced by two surface gates, running in parallel along a narrow stripe of 2DEG. A transverse voltage in the direction of the density modulation is observed, i.e., perpendicular to the applied source-drain voltage. The polarity of the transverse voltage is independent of the polarity of the source-drain voltage, demonstrating rectification in the device. We find that the transverse voltage Uy depends quadratically on the applied source-drain voltage and nonmonotonically on the density modulation. The experimental results are discussed in the framework of a diffusion thermopower model. [ABSTRACT FROM AUTHOR]