A. B. Kaiser

Metallic behaviour in highly conducting polymers

Abstract We give a brief review of transport data in highly conducting polymers and suggest specific microscopic and macroscopic pictures that can account for the data. The conductivity temperature dependence of new polyacetylene is generally consistent with a combination of Shengs expression for fluctuation-induced tunnelling and highly anisotropic metallic conduction, with a maximum intrinsic conductivity exceeding 10 6 S/cm. Despite the high conductivity, localization effects sometimes appear to occur at low temperatures: these could be due to charging energy effects in interchain transfer at low temperature, or to quantum corrections as in disordered metals. The surprisingly linear thermopower observed for new polyacetylene argues against conduction models involving a change in the number of charge carriers with temperature. It also indicates a smaller interaction between electrons and phonons than in normal metals, which is consistent with the remarkably high intrinsic conductivities inferred.

‘Metallic’ polymers: Interpretation of the electronic transport properties

Abstract We summarize experimental data on the conductivity and thermoelectric power of highly-conducting polymers. The necessary requirements for models to give a quantitative account of the temperature dependence of these transport properties are then deduced. It appears that there are thin ‘barrier’ regions in which temperature-assisted conduction (e.g. by hopping or tunnelling) occurs in parallel with ‘metallic’ conduction. We point out that the observation of a linear thermopower in many of the highly-conducting polymers is a striking departure from usual metallic behaviour, which shows a change in slope due to electron-phonon enhancement. We suggest that this absence of a detectable electron-phonon interaction for thermopower in these polymers is another indicator of a huge intrinsic conductivity.

Experimental evidence for multi-band conduction in highly-doped La―Sr―Cu―O superconductors

Abstract We have measured the Hall coefficient in highly-doped La 2- x Sr x CuO 4 superconductors ( x =0.2 and 0.25), finding a large decrease with temperature. A good fit to the data is obtained using the two-band model involving both holes and electrons (as suggested by our earlier thermopower measurements), with the carrier-phonon interaction being relatively stronger for the holes.

Anion-rotation slow down on approach to the Peierls transition of a 12:7 pyrene hexafluoroantimonate radical cation salt

Abstract Nuclear magnetic resonance of 19F nuclei in the radical cation salt (pyrene)12(SbF6)7 is analysed in dependence of temperature and frequency. Linewidth and spin-lattice relaxation reflect the rotational motion of the SbF6 octahedra that is frozen-in on approaching the Peierls transition. The relevance of this mode of molecular motions for the conduction electrons is demonstrated via Overhauser-shift detected pyrene-1H spin-lattice relaxation.

Determination of all relevant conduction electron scattering times in an exceptionally anisotropic arene conductor, the 12:7 pyrene hexafluoroantimonate radical cation salt

For (pyrene)12(SbF6)7 a 104:1 anisotropy of the microwave conductivity is observed. We show that the timescale for the end of the one-dimensional motion of the conduction electron spins derived by analysis of the frequency and temperature dependence of the proton spin–lattice relaxation and of the electron spin resonance linewidth and relaxation times is governed to a comparable degree by perpendicular-to-stack hopping and by interaction with intrastack paramagnetic localized defects in these crystals. The electron spin self-diffusion coefficient derived for the magnetic field gradient parallel to the pyrene stacking direction is therefore influenced by perpendicular-to-stack motion as well.

Proton longitudinal relaxation rate in the 12:7 radical cation salt pyrene hexafluoroantimonate

The organic conductor (Py) 12 (SbF 6 ) 7 shows a 10 4 :1 anisotropy of the conductivity. It is thus an extremely one-dimensional conductor and undergoes a Peierls transition at about 116 K. Temperature dependent measurements at different Larmor frequencies as well as frequency dependent measurements at 210 K of the longitudinal relaxation rate of the protons are presented including the relaxation rate in the rotating frame (T 1ρ ), together with their quantitative theoretical description.

Thermoelectric power of Ba 1 − x K x BiO 3

We report measurements of thermoelectric power of two superconducting samples of Ba{sub 1{minus}{ital x}}K{sub {ital x}}BiO{sub 3} with differing resistivity behaviors. The thermopower is negative between 50 and 300 K. We find in both samples an unusual temperature dependence similar to that in high-oxygen-content samples of Y-Ba-Cu-O with negative thermopower, suggesting a common mechanism producing this behavior in Ba{sub 1{minus}{ital x}}K{sub {ital x}}BiO{sub 3} and the Cu-O-plane superconductors. Our sample with an activated temperature dependence of resistivity shows a change to positive thermopower before becoming superconducting, possibly due to the effect of nonmetallic grain boundaries.