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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]

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]