C. R. Fincher

Doped polyacetylene forms a new class of conducting polymers in which the electrical conductivity can be systematically and continuously varied over a range of eleven orders of magnitude. Transport studies and far-infrared transmission measurements imply a metal-to-insulator transition at dopant concentrations near 1%.

We prepared by vapor deposition at room temperature thin (500 Å) Co/Pt multilayers or layered structures directly onto glass or Si substrates. They show a preferential magnetization perpendicular to the film plane for Co thicknesses below 12 Å and a 100% perpendicular remanence for Co thicknesses below 4.5 Å. The magnetic anisotropy can be explained by an interface contribution to the anisotropy. We also investigated the magneto-optical (MO) polar Kerr effect of these multilayers. Because of their excellent magnetic properties and their potentially high oxidation and corrosion resistance, these Co/Pt-layered structures are very promising candidates for MO recording. The Kerr rotation θk at λ=820 nm for a 35×(4.0 Å Co+12.7 Å Pt)-layered structure, which has 100% magnetic remanence, is modest (−0.12°), but the reflectivity R is high (70%), which results in a respectable figure of merit Rθ2k. Furthermore, the Kerr effect increases towards shorter wavelengths and thus favors future higher-density recording.

The magnitude of the symmetry-breaking dimerization distortion has been determined by analysis of x-ray scattering data from trans-${(\mathrm{CH})}_{x}$ ${u}_{0}\ensuremath{\simeq}0.03$ \AA{}.This distortion can account for the magnitude of the energy gap, implying that electron-electron interactions do not dominate the physics of long-chain polyenes.

The results of an experimental study of the infrared absorption of the ${(\mathrm{CH})}_{x}$ system, lightly doped ( 0.1%) with acceptors and donors, are presented. Additional absorptions are observed near 1370 ${\mathrm{cm}}^{\ensuremath{-}1}$ (width \ensuremath{\sim} 50 ${\mathrm{cm}}^{\ensuremath{-}1}$) and 900 ${\mathrm{cm}}^{\ensuremath{-}1}$ (width \ensuremath{\sim} 400 ${\mathrm{cm}}^{\ensuremath{-}1}$) upon doping with iodine, As${\mathrm{F}}_{5}$, and sodium. The two additional absorption maxima appear to be general features of lightly doped ${(\mathrm{CH})}_{x}$ independent of specific dopant or of cis (trans) content. Measurements on stretch-oriented films demonstrate that these absorption maxima are polarized primarily along the polymer chains. The narrow mode at 1370 ${\mathrm{cm}}^{\ensuremath{-}1}$ is attributed to a molecular vibration made ir active by the doping. The broader absorption centered at 900 ${\mathrm{cm}}^{\ensuremath{-}1}$ is discussed in terms of quasi-one-dimensional donor and acceptor bound states along the ${(\mathrm{CH})}_{x}$ chain. We include an experimental determination of the room-temperature dielectric constant in undoped ${(\mathrm{CH})}_{x}$; ${\ensuremath{\epsilon}}_{\ensuremath{\parallel}}\ensuremath{\simeq}10\ensuremath{-}12$, with uncertainty arising from the incomplete orientation of the ${(\mathrm{CH})}_{x}$ films. The bound-state energy is thus consistent with a one-dimensional hydrogenic model in which the Coulomb potential along the chain is reduced by ${\ensuremath{\epsilon}}_{\ensuremath{\parallel}}$. Alternatively viewing semiconducting ${(\mathrm{CH})}_{x}$ as a Peierls distorted one-dimensional metal, we discuss localized domain-wall-like charged donor (acceptor) states induced by charge-transfer doping.

The band structure and electronic properties of pure and heavily doped polyacetylene (both as grown and stretch oriented) have been investigated by a combination of optical-absorption and -reflection measurements in the frequency range from the middle ir (0.1 eV) through the visible (4.0 eV). The absorption data are consistent with a direct gap of approximately 1.4 eV in the trans-${(\mathrm{CH})}_{x}$. A Kramers-Kronig analysis of the reflection data has been carried out to obtain $\ensuremath{\sigma}(\ensuremath{\omega})$ and $\ensuremath{\epsilon}(\ensuremath{\omega})$. We find that for the undoped semiconducting polymer, the strong transition observed in the visible exhausts the oscillator strength sum rule for $\ensuremath{\pi}$ electrons consistent with an interband transition. The frequency-dependent conductivity obtained from Kramers-Kronig analysis of the metallic polymer reflection data suggests "interrupted-strand" behavior. Application of effective-medium theory implies an intrinsic dc conductivity for metallic ${[\mathrm{CH}{(\mathrm{As}{\mathrm{F}}_{5})}_{0.15}]}_{x}$ of $\ensuremath{\sigma}>2\ifmmode\times\else\texttimes\fi{}{10}^{4}$ ${\mathrm{\ensuremath{\Omega}}}^{\ensuremath{-}1}$ ${\mathrm{cm}}^{\ensuremath{-}1}$. The measured dc values have thus far been limited by the low-density fibril morphology.