The temperature dependence of the conductivity in the critical regime of the metal - insulator transition in conducting polymers

Abstract

The metal-insulator (M-I) transition in conducting polymers is particularly interesting; critical behaviour has been observed over a relatively wide temperature range in a number of systems, including polyacetylene (CH) x , polypyrrole (PPy), poly(pphenylenevinylene) (PPV), and polyaniline (PANI). In each case, the metallic, critical, and insulating regimes near the M-I transition have been identified from plots of W = ( ln σ/ ln T ) versus T : in the metallic and insulating regimes W (T ) exhibits positive and negative temperature coefficients, respectively, while in the critical regime W (T ) is temperature independent. Thus, in the critical regime, the conductivity follows a power law, σ (T ) = AT β , where β is the critical exponent which is predicted by scaling theory to be between one-third and unity. In conducting polymers, the critical regime is easily tunable by varying the extent of disorder, or by applying external pressure and/or magnetic fields. The transitions from metallic to critical behaviour and from critical to insulating behaviour have been induced with a magnetic field, and that from insulating to critical and then to metallic behaviour with increasing external pressure. The detailed evolution of σ (T ) in the critical regime at low temperatures can be observed in W (T ) plots as the system is changed from metal to insulator. Although W (T ) is temperature independent for a wide range of temperatures below 50 K, β systematically increases from values less than one-third on the metallic side towards unity as the system is moved toward the insulating side.

Spin charge carrier dynamics in poly(bis-alkylthioacetylene)

“…Besides, this process leads to the extremal-temperature dependence with T~ ~ 160 K at which the semiconductor-metal transition takes place. The analogous phenomena were registered in the study of polyaniline [31,41], poly(phenylenevinylene) [42], poly(3,4-ethylenedioxythiophene) [43], and other OPS [44].…”

Doping-dependent studies of the Anderson-Mott localization in polyaniline at the metal-insulator boundary

“…In conjunction with the activation energy, a less robust empirical parameter, the resistivity ratio r ϭ(295 K)/(10 K͒ is used for a quick estimate of the sample quality. According to previous studies, 17,18 for PANi samples in the metallic regime r Ͻ2, while the corresponding limits for critical and insulating behavior are 2Ͻ r Ͻ5 and 10Ͻ r , respectively.…”

Magnetoconductance anisotropy of a polymer thin film at the onset of metallicity

“…This strongly suggests that these two samples are at the onset of metallic state, with metallicity fully established when b < 0.33. 1,16 The normalized plot in Figure 2(c) emphasizes the transition from convex conductivity versus temperature curves for insulating-type samples, to concave curves found for highly conductive samples. This relates to the change of the second derivative r 00 (T) sign when moving from the insulator towards the transition to the metallic side, as deduced from Eqs.…”