The emeraldine base (EB) form of polyaniline can be varied from insulating (o < 10-1°ohm-tcm l) to conducting (o-~ 10 + lohm-lcm -1) through protonation. That is, the number of electrons on the polymer backbone is constant while the number of protons is increased. We present here extensive magnetic, optical and transport data that demonstrate that the resulting emeraldine salt (ES) is metallic with axfinite density of states at the Fermi energy. The results are consistent with segregation into fully protonated emeraldine salt and unprotonated emeraldine base polymer regions. It is proposed that the observed transition is an isolated bipolaron-to-polaron lattice transition. The correspondence of this concept to the disproportionation between protonated imine plus amine to form two semiquinones is shown.

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We report detailed measurements of the interlayer magnetoresistance of the layered organic superconductor -(BEDT-TTF) 2 Cu(SCN) 2 for temperatures down to 0.5 K and fields up to 30 T. The upper critical field is determined from the resistive transition for a wide range of temperatures and field directions. For magnetic fields parallel to the layers, the upper critical field increases approximately linearly with decreasing temperature. The upper critical field at low temperatures is compared to the Pauli paramagnetic limit, at which singlet superconductivity should be destroyed by the Zeeman splitting of the electron spins. The measured value is comparable to a value for the paramagnetic limit calculated from thermodynamic quantities but exceeds the limit calculated from BCS theory. The angular dependence of the upper critical field shows a cusplike feature for fields close to the layers, consistent with decoupled layers.

In this paper, we report direct evidence of a structural transition in the organic superconductor -͑BEDT-TTF͒ 2 Cu͓N͑CN͒ 2 ͔Br near 80 K and the effect of disorder on the superconducting transition temperature. By cooling the sample from above 80 K, the interlayer magnetoresistance displays a bumplike feature, which increases sharply with increasing cooling rate. The rapidly cooled sample has a much larger resistivity and a lower transition temperature, which decreases linearly with increasing resistivity near the transition temperature. We propose that rapid cooling quenches the sample into a disordered state. Localized moments in the disordered state reduce the superconducting transition temperature. ͓S0163-1829͑98͒51522-6͔

Magnetization as a function of field and temperature for a ferromagnetic Heusler alloy Ni 2 MnGa is reported. Magnetization above the Martensitic transition displays a field-dependent peak effect, a direct magnetic evidence of premartensitic phase. At low fields, the peak effect occurs at a temperature consistent with the observations of the micromodulated structure transition seen from neutron scattering, electron microscopy, and ultrasonic studies in this compound. At high fields, the peak effect is suppressed. The strong field dependence of the peak temperature suggests a large magnetoelastic interaction in the intermediate phase.

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