Y. Tomkiewicz

On a new conducting polymer-pyrolyzed Kapton

Abstract Transport and magnetic properties of pyrolyzed Kapton are studied for different pyrolysis temperatures. We show that well below the graphitization level, pyrolyzed Kapton exhibits isotropic conductivities comparable to those measured for doped polyacetylene. Variable range hopping is suggested as a possible mechanism for this high conductivity. The effect of pyrolysis on carrier number and mobility has been investigated. It has been shown that for low pryolysis temperatures (T p p > 650°C the most significant changes occur in the carrier mobility.

Spin dynamics in trans-polyacetylene

Abstract Detailed spin-echo and cw EPR measurements on trans -(CD) x and trans -(CH) x are reported. The magnitude of the on-chain diffusion rate, and its temperature dependence are evaluated. The results do not agree with predictions of the soliton model nor with results evaluated from recent NMR measurements.

Chemistry and Electrocrystallization of Organic Metals and Superconductors

Abstract Considerable variation in the conditions of electrochemical crystal growth of TMTSF2X (i.e., constant current versus constant potential, ambient versus inert atmosphere, etc.) and in the purity of the constituents (donor, electrolyte, solvent) does not significantly affect the unusual low-temperature properties of this class of materials. Our results suggest that the electrocrystallization procedure may be self-purifying by selecting for conducting crystal phases with constituents having specific oxidation potentials and solubility properties. However, doping solutions with structurally and chemically similar constituents (i.e., TMTTF, and IO− 4 in CIO− 4) leads to their incorporation in the crystal structure where they have a profound effect. Several mole percent of these dopants suppress superconductivity in the PF− 6 and CIO− 4 salts, and increase and broaden the metal-insulator phase transition.

Electron-electron interactions in the fulvalene family of organic metals

Abstract Magnetic susceptibility as well as optical reflectivity data in the metallic regime of the conductivity of HMTTF-TCNQ are presented. It is shown that: 1. (1) The spin susceptibility of the donor stack is temperature independent in the temperature range of the metallic conductivity 50 ⩽T⩽300 K. 2. (2) By comparing the magnitude of this susceptibility with the value estimated from plasma frequency measurements based on the model of non-interacting electrons, we conclude that the donor stack susceptibility is not enhanced. 3. (3) A comparison of data on the three compounds (HMTTF, TTF, TSeF)-TCNQ leads to the conclusion that the donor stack susceptibility is enhanced only in TTF-TCNQ.

Charge-density waves in TSeF-TCNQ — an x-ray study

Abstract In an X-ray study of TSeF-TCNQ, two- and three-dimensional short-range ordering of the charge-density waves were found below ∼100 K and ∼50 K, respectively. The periodicity of the short-range order along the a-axis developing below ∼50 K is a, although that of the superlattice below 29 K is 2a From this behavior it is concluded that charge-density wave amplitudes on both kinds of stacks are comparable and that fourth-order interaction terms are important near as well as below the phase transition. Evidence is also given that it is a 2kF-CDW, not a 4kF-CDW, that is seen in TSeF-TCNQ. The differences between the roles of the fulvalene CDWs in TSeF-TCNQ and in TTF-TCNQ are discussed.

Two phase transitions in TSeF-TCNQ?—The effects of doping with small concentrations of TTF-TCNQ

Abstract The single phase transition which had been observed in TSeF-TCNQ is seen to split into two phase transitions when TSeF-TCNQ is doped with small amounts of TTF-TCNQ (

Magnetic resonance properties of polyacetylene

Abstract Electron-nuclear double resonance (ENDOR) spectra have been recorded for samples of pristine cis- and trans-polyacetylene at temperatures ranging from 15°K to 290°K. The ENDOR spectra of cis-polyacetylene consist of resolved contributions arising from distant and local ENDOR mechanisms. The temperature dependence of the peak positions of the local ENDOR spectra is analyzed in terms of a simple two-site electron jump model resulting in modulation of a 1.8 MHz isotropic hyperfine coupling. This analysis leads to an activation energy of 34°K. The ENDOR spectra of trans-polyacetylene at temperature above 70°K consist of a single line centered at the free proton Larmor frequency and exhibiting a temperature-dependent linewidth. These spectra are assumed to be composed of distant and local ENDOR contributions with motional narrowing of the local ENDOR structure accounting for the temperature dependence of the ENDOR linewidths. This assumption is supported by observation of two-component ENDOR spectra b...

Spin and charge excitations in polyacetylene

Abstract Spin and charge excitations in pristine and AsF5-doped polyacetylene have been investigated to resolve the issue of whether or not topological solitons have been observed. We present experimental results for both materials suggesting that the simple model from which the soliton-based interpretations have been derived is inapplicable. In pristine trans-polyacetylene, both protonated and deuterated, conclusions are deduced from spin-echo and continuous-wave EPR about the spin diffusion rate, its temperature dependence and anisotropy that contradict predictions of the soliton theory. In AsF5-doped cis-polyacetyelene, we conclude from magnetic and transport measurements that the doping is very inhomogeneous, the doped and undoped parts being largely distinct with the doped regions metallic. The conductivity changes previously interpreted as arising from a transition wthin the doped region are attributed to a change in the topology of these regions, i. e. the crossing of a percolation threshold to a r...

Transport and EPR studies at ambient pressure in (TMTSF)2-PF6 —Pristine and doped with TMTTF

Abstract We report on transport and EPR studies of (TMTSF)2-PF6, both in its pristine form and when doped with the sulfur analog TMTTF. All samples undergo a single phase transition which can be seen in both the transport and magnetic data and which, in the undoped material, occurs at ∼11.5 K. The effects of a 3% dopant concentration are dramatic: a 40% rise in the transition temperature, a 130% rise in the room- temperature spin susceptibility and a 30% fall in the low-temperature g shift. Possible causes are discussed.

2kF and 4kF charge-density waves in TTF0.4TSeF0.6-TCNQ — An X-ray study

Abstract In the quasi one-dimensional conductor TTF0.4TSeF0.6-TCNQ, it is found that the 2kF CDW has three-dimensional correlations of relatively long range below the metal-insulator transition temperature ∼40K, although the 4kF CDWs are only one-dimensional in character at all temperatures. This result supports the speculation that the 2kF CDWs and the 4kF CDWs are present predominantly on the TCNQ and the fulvalene stacks, respectively. The 2kF-value of the present material is similar to that of TTF-TCNQ but far from that of TSeF-TCNQ. The change of the value with the concentration of TSeF (or TTF) is nonlinear. The transverse period of the ordered 2kF CDW along the a-axis is a just below the transition temperature and becomes 4 a below 34.5K. A discussion is given based on the two-chain model.