Spectroscopic studies of polyaniline in solution and in spin-cast films

It is demonstrated experimentally and theoretically that through the use of an active (feedback) controller one can dramatically modify the nature of the flow in a toroidal thermal convection loop heated from below and cooled from above. In particular, we show how a simple control strategy can be used to suppress (laminarize) the naturally occurring chaotic motion or induce chaos in otherwise time-independent flow. The control strategy consists of sensing the deviation of fluid temperatures from desired values at a number of locations inside the loop and then altering the wall heating to either counteract or enhance such deviations.

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Controlling Chaos in a Thermal Convection Loop

An unusual interlayer coupling, recently discovered in layered magnetic systems, is analysed from the experimental and theoretical points of view. This coupling favours the 90 orientation of the magnetization of the adjacent magnetic films. It can be phenomenologically described by a term in the energy expression, which is biquadratic with respect to the magnetizations of the two films. The main experimental findings, as well as the theoretical models, explaining the phenomenon are discussed.

https://iopscience.iop.org/article/10.1088/0022-3727/31/8/003/pdf

Biquadratic interlayer coupling in layered magnetic systems

Novel inorganic/organic self-powered UV–vis photodetectors based on single Se microtube and conducting polymers—polyaniline (PANI), polypyrrole (PPy), and poly(3,4-ethylenedioxythiophene) (PEDOT)—are fabricated. The conducting polymers are directly coated on the surface of a single Se microtube via a facile and low-cost in situ polymerization method. The integrated Se/PANI photodetector with 45-nm-thick PANI layer shows excellent self-powered behavior under UV–vis light illumination. In particular, it exhibits high on/off ratio of 1.1 × 103, responsivity (120 mA W−1), large detectivity (3.78 × 1011 Jones), and ultrafast response speed (rise time of 4.5 µs and fall time of 2.84 ms) at zero bias at 610 nm (0.434 mW cm−2)-light illumination. Moreover, the individual Se/PPy and Se/PEDOT self-powered photodetectors also exhibit fast and stable responses, including responsivity of 70 and 5.5 mA W−1, rise time of 0.35 and 1.00 ms, fall time of 16.97 and 9.78 ms, respectively. Given the simple device architecture and low cost fabrication process, this work provides a promising way to fabricate inorganic/organic, high-performance, self-powered photodetectors.

Novel p–p Heterojunctions Self-Powered Broadband Photodetectors with Ultrafast Speed and High Responsivity

Sulfonic acid ring-substituted polyaniline has been synthesized and characterized by elemental analysis, FTIR, UV-vis and electron spin resonance spectroscopy. Approximately 50% of the total number of phenyl rings in the polymer are mono-substituted by -SO− 3 groups. Without external doping, the ring-sulfonated polyaniline has a conductivity of ∼0.03 S/cm; it is therefore a “self-doped” conducting polymer. The water soluble sodium salt of the ring-sulfonated polyaniline is an insulator and can be reversibly converted back to the self-doped conducting form by treatment with aqueous acid.

Sulfonic acid ring-substituted polyaniline, a self-doped conducting polymer

We report the first observation of the photoinduced absorption spectrum of the emeraldine base form of polyaniline. The spectrum contains both subgap photoinduced bleaching at 1.8 eV and photoinduced absorptions at 0.9, 1.4, and 3.0 eV, as well as band-edge bleaching above 3.5 eV. The existence of the subgap bleaching peak is consistent with a model of optically excited localized molecular excitons. The energies of the three photoinduced absorption peaks and the dependence of the peak amplitudes on pump power give evidence for the photoexcitation of polaron pairs that recombine bimolecularly.

Photoexcitation of polarons and molecular excitons in emeraldine base.

We have developed longitudinal pumping and angular dependent ferromagnetic resonance techniques to investigate the temperature dependence of the interlayer exchange field ${H}_{\mathrm{ex}}$ on a series of Co/Ru/Co trilayer films. Our results indicate, for the first time, that the variation of ${H}_{\mathrm{ex}}$ with temperature follows roughly the relationship ${H}_{\mathrm{ex}}\frac{\ensuremath{\propto}(\frac{T}{{T}_{0}})}{sinh(\frac{T}{{T}_{0}})}$ as predicted by theoretical models. In addition, no evidence of the biquadratic exchange term was observed in these samples.

Using ferromagnetic resonance as a sensitive method to study temperature dependence of interlayer exchange coupling.

The nonlinear dynamics of the main magnetoexchange branch of a circular thin (0.45 \ensuremath{\mu}m) film of yttrium iron garnet are investigated at perpendicular resonance. Distinct ``fingers of auto-oscillation'' arise from each magnetoexchange mode in the low-power spectrum. A set of equations of motion derived from a microscopic Hamiltonian are integrated to compare with experiment. In the first finger, the model predicts the power for onset of auto-oscillation to within 1 dB of the experiment and the frequency to within 15%.

High-power ferromagnetic resonance without a degenerate spin-wave manifold.

It is shown that in ferromagnets, an antisymmetric Dzialoshinsky-Moriya interaction will introduce an additional phase shift (dynamical canting) between spins in their spinwave precession motion. This phase shift will have a most pronounced influence on the resonance intensities of the standing spin-wave mode in a thin film of a material having such an interaction.

P. E. Wigen

Papers

Photoexcitation of polarons and molecular excitons in emeraldine base.

Using ferromagnetic resonance as a sensitive method to study temperature dependence of interlayer exchange coupling.

High-power ferromagnetic resonance without a degenerate spin-wave manifold.

Effect of Dzialoshinsky-Moriya Interactions on Propagation of Spin Waves in Ferromagnets: Dynamical Canting

Photoinduced Absorption in the La2−xSrxCuO4–5 System: 2 eV Bandgap and Long-Lived Defect States