Transport and Magnetic Resonance Studies of Polyaniline

Conductivity, thermoelectric power, spin concentration and spin dynamics has been measured in polyaniline samples equilibrated at different pH. Data provide evidence for Metal-Insulator transition upon pH variation.     

Non-linear applications of microstructured optical fibres

The enhancement of different non-linear processes in microstructured optical fibres can be achieved through manipulation of the dispersion characteristics of the fibre. This is demonstrated by extending the region of short wavelength operation of high power supercontinuum generation through four wave mixing in a cascaded fibre geometry where the dispersion of each fibre decreased on propagation. The technique is further refined in a demonstration utilizing long lengths of dispersion decreasing tapered microstructured fibres, where the supercontinuum extends to around 300 nm with spectral power densities in excess of 2 mW/nm in the uv. These long length tapers can also be utilized for adiabatic soliton pulse compression in new spectral regions, allowing the compression of 655 fs pulses to 45 fs.     

Phosphor und Vanadin

Ohne Zusammenfassung     

Exact Synchronization for Finite-State Sources

We analyze how an observer synchronizes to the internal state of a finite-state information source, using the ϵ-machine causal representation. Here, we treat the case of exact synchronization, when it is possible for the observer to synchronize completely after a finite number of observations. The more difficult case of strictly asymptotic synchronization is treated in a sequel. In both cases, we find that an observer, on average, will synchronize to the source state exponentially fast and that, as a result, the average accuracy in an observer’s predictions of the source output approaches its optimal level exponentially fast as well. Additionally, we show here how to analytically calculate the synchronization rate for exact ϵ-machines and provide an efficient polynomial-time algorithm to test ϵ-machines for exactness.     

Asymptotic Synchronization for Finite-State Sources

We extend a recent synchronization analysis of exact finite-state sources to nonexact sources for which synchronization occurs only asymptotically. Although the proof methods are quite different, the primary results remain the same. We find that an observer’s average uncertainty in the source state vanishes exponentially fast and, as a consequence, an observer’s average uncertainty in predicting future output converges exponentially fast to the source entropy rate.     

Role of nitrogen in the mobility drop of electrons in modulation-doped GaAsN/AlGaAs heterostructures

We report transport properties of a 2 dimension electron gas (2DEG) in molecular beam epitaxy-grown GaAs_1−xN_x/AlGaAs modulation-doped heterostructures. Quantum oscillations in far infrared cyclotron resonance prove the efficient electron transfer and formation of the 2DEG. The 2DEG mobility strongly depends on the N concentration in the channel layer. It shows a drastic decrease as compared to N-free samples, even for the smallest amount of N (0.02%). For this N composition, the electron effective mass was found to be 0.073m₀. Reduced growth temperature (450 °C) was found to improve the mobility of N-containing channels. Examination of transport properties from 4 to 300 K and cyclotron resonance experiments give evidence of the presence of ionised impurity-like scattering centres in GaAsN.