Magnetoresistance of iodine-doped polyacetylene at low temperatures

Abstract

We have measured the magnetoresistance of metallic polyacetylene at temperatures T between 4.2 K and 0.3 K and in fields B up to 3.4 Tesla both parallel and perpendicular to the average current flow. We found an exclusively positive magnetoresistance scaling in low fields as B2/T 2 for T< I K and as B2/T 3 for T> 2 K. At high doping levels the magnetoresistance is small and saturates in high fields at low temperatures. These results are independent of the particular geometry used and will be discussed in terms of the magnetic field effect on the hopping conductivity.One of the intriguing properties of polyacetylene is its virtually metallic electrical conductivity when doped with suitable acceptors like iodine. In spite of a wealth of information that has been obtained about this fascinating organic conductor I a detailed understanding of the electrical transport mechanism is still lacking, mostly because of the complicated morphology and the inhomogeneities associated with it.In our present work we have measured the conductivity of iodine-doped polyacetylene (CHIy) x in the temperature range between 4.2 K and 0.33 K. We mostly studied samples having large iodine concentrations (0.15~y~0.32), i.e. the metallic regime. Magnetic fields up to 3.4 Tesla were employed in both longitudinal and transverse geometries, including the Corbino arrangement. In all of our samples we found a positive magnetoreslstance (MR) becoming very large at low temperatures. The more conducting samples had a smaller MR which saturated at low temperatures, irrespective of the particular geometry. Our results are thus different from those obtained earlier by Kwak et al. 2 and Gould et al. 3 on AsF 5 doped (CH) x and by RSss et el. 4 who find a negative MR at 4.2 K in very metallic samples. Obviously, the detailed form and even the sign of the MR depends on how metallic the samples are prepared.We used samples which were polymerized either according to Luttinger 5 or Shirakawa 6. Treating a 0.6 mm thick layer of Luttinger catalyst solution containing l mg cobalt nitrate and 3 mg sodium boronhydride per ml ethanol at -78°C with acetylene gas results in a sponge llke red polymer layer that was washed twice with ethanol and dried under vacuum. The resuiting shiny film was about 40 ~m thick and weighed 3 mg/cm 2. Alternatively, the wall of a 20 cm long round bottomed, evacuated flask of 5 cm diameter was wettet at -78°C with a degassed Shirakawa catalyst solution containing 3.0 ml aluminiumtriethyl and 1.9 ml titanium tetrabutoxide in 20 ml toluene, aged for one hour. Allowing acetylene gas to flow immediately into the flask, a film was formed, which, after removal of the unreacted catalyst solution, was washed 15 times with dry pentane and then dried. The resulting film was 80 ~m thick and weighed 2 mg/cm 2. Both types of samples were stored under vacuum on dry ice. Doping was performed by repeatedly exposing the samples to iodine vapor and consecutive vacuum pumping at room temperature. Iodine content of the samples was determined by weighing ...

Variable-range hopping conduction in doped germanium at very low temperatures and high magnetic fields

“…1 up to 1.5 K even if one expands the scales considerably. The MR is more sensitive to changes in ~c(0) than the temperature dependence of R in zero field because B~ oc ~c(0)-3, see (8). The width 6 of the Coulomb gap can now be estimated by comparing it with the available energy range AEr at 0.5 K:…”

“…Initially the temperature was recorded by a second germanium thermometer which had been calibrated against a 3He melting curve thermometer [1][2][3][4][5][6][7][8][9][10][11][12][13][14][15]. For thermometry in a magnetic field a carbon resistor was used [16] which also followed (2) but which had only a very small MR [17,18].…”

“…(18) Close to the metal-insulator transition the variation of f(B) in (17) is predicted to be slightly altered [12] while (18) will remain valid. The weak field result (8) should not be affected by the subbarrier scattering.…”

“…By inserting the zero-field result r e = a 04/2 one finds a small correction A~c(B) for the resistivity exponent, viz. : (8) where B02 = (~h2/eZa~) (T/To) 3/2 and a,,~6.6.102 is a numerical factor [19].…”

Non-ohmic conduction in doped polyacetylene at low temperatures

“…More detailed information may be drawn from the magnetic field dependence of ~. For low dopant concentration a large positive magnetoresistance (MR) is observed below 4K which varies as B 2 at low fields and tends to saturate in high fields below 1 K [5], This result is attributed to orbital shrinking of the localized wavefunction in the magnetic field. From a fit to the data the localization length and the DOS can be determined.…”