A. Denenstein

Semiconductor-metal transition in doped (CH)x: Thermoelectric power☆

Thermoelectric power studies of polyacetylene have been carried out as a function of dopant concentration and temperature. The thermopower of pure trans-(CH)x is large (S = +850 μ V°K) and positive consistent with p-type material. With iodine doping, (CHIy)x, the thermopower remains positive over the full range of concentration 0 < y < 0.22. The semiconductor-metal transition is clearly observed at nc ≅ 3 mole %; S falls dramatically from S = +850 μ V°K at y = 0.003 to S = +30 μ V°K at y = 0.03. At higher concentrations, S remains nearly constant saturating at +18 μ V°K in the heavily doped metallic polymer. Temperature dependences are consistent with metallic behavior at the highest dopant concentrations and hopping transport in the undoped and lightly doped polymer.

Specific heats of pure and doped polyacetylene

The temperature dependences (0.8 K ⩽ T < 7 K) of the specific heats of pure polyacetylene (both cis and trans isomers) and heavily doped metallic [CH(AsF5)0.12]X are reported. Results for undoped cis-(CH)X indicate behavior typical of crystalline polymers, whereas isomerization to the trans-form leads to a small term linear in temperature signifying increased disorder. Comparison of the data from cis and trans starting material indicates that isomerization is induced during doping. The increased coefficient of the linear term in the metallic polymer is discussed in terms of two contributions; an electronic term expected for a metal, and the increased effect of disorder in the doped polymer.

Inter-soliton electron hopping transport in trans-(CH)x

Abstract The electrical conductivity (pressure and temperature dependence) and the thermopower ( n -type and p -type) are compared with hopping theories of transport. We find excellent quantitative agreement with Kivelsons theory of intersoliton electron hopping.

Anomalous electrical properties of linear chain mercury compounds

Abstract The electrical resistivity of Hg 2.86 AsF 6 has been studied as a function of temperature. At room temperature, the resistivity along the chain direction is 10 −4 Ω-cm with an anisotropy of about 10 2 . This incommensurate linear chain system remains metallic at low temperatures with resistance ratio ϱ ab (300 K)/ ϱ ab (1.4 K) ≅ 3000 and still increasing with no apparent sign of residual resistivity. A large anisotropic magnetic field dependence of the resistivity is observed below 30 K. Near 4 K, the c-axis resistance drops abruptly more than three orders of magnitude, apparently to zero, while ϱ ab is continuous. The c-axis transition is suppressed in a small magnetic field.

Superconductivity in Hg3-δAsF6: Absence of surface mercury

Abstract Low temperature resistance and X-ray fluorescence measurements indicate that the anisotropic superconductivity of Hg3-δAsF6 does not arise from a surface film of extruded mercury. Meissner effect data demomstrate that the superconductivity goes to complete flux exclusion below 0.43 K.

Magnetic field induced residual resistivity in quasi-one-dimensional Hg3−δAsF6☆

Abstract The â-b plane resistivity of the Hg-chain compound, Hg3−δAsF6, has been measured as a function of temperature and magnetic field using a contactless ac technique. The data imply a magnetic field dependent residual resistivity which goes to zero as the field is reduced to zero. The results are discussed in terms of the quasi-one-dimensional electronic and structural features of this novel linear chain compound.

Pressure dependence of the photoabsorption of polyacety-lene

Abstract The optical absorption spectra of pure cis and trans-polyacetylene at various hydrostatic pressures, in the region 0–9 Kbar, are reported. We find a “red shift” of the absorption edge indicating a reduction of the energy gap as a function of pressure. At 9 Kbar, E is reduced by about 0.1 eV. In addition a decrease ing the slope of α (ω) is observed at high pressures. These changes result from an increase in the interchain transfer integrals, t1. Comparison of the data with the results of tight binding calculations lead to estimates of t1≃ 0.025 − 0.05 eV (ambient) and t1/t1≃ 10−2.

Thermoelectric Power of Linear Mercury Chain Compound

We have studied the thermoelectric power of the incommensurate linear chain compound Hg3−δ AsF6. At high temperatures, the electron diffusion thermopower in this anisotropic metal is linear in temperature. At low temperatures, phonon drag thermopower contributions can be identified.