Localization and Its Absence: A New Metallic State for Conducting Polymers

Abstract

A widely held view in solid-state physics is that disorder precludes the presence of long-range transport in one dimension. A series of models has been recently proposed that do not conform to this view. The primary model is the random dimer model, in which the site energies for pairs of lattice sites along a linear chain are assigned one of two values at random. This model has a set of conducting states that ultimately allow an initially localized particle to move through the lattice almost ballistically. This model is applicable to the insulator-metal transition in a wide class of conducting polymers, such as polyaniline and heavily doped polyacetylene. Calculations performed on polyaniline demonstrate explicitly that the conducting states of the random dimer model for polyaniline are coincident with recent calculations of the location of the Fermi level in the metallic regime. A random dimer analysis on polyparaphenylene also indicates the presence of a set of conducting states in the vicinity of the band edge. The implications of this model for the metallic state in other polymers, including heavily doped polyacetylene, are discussed.

Enhancement of Localization in One-Dimensional Random Potentials with Long-Range Correlations

“…This opinion is based on the fact that correlations, reducing the degree of disorder, typically give rise to extended states. In the dimer model, short-range correlations result in two resonant extended states [8,9], that was observed in a semiconductor superlattice [10]. A continuum of extended states for potentials with long-range correlations was predicted in Refs.…”

A suitable model for emeraldine salt

“…The first one refers to the dimensionality ͑1D versus 3D͒ of the metallic states. Conduction and charge transport mechanisms in doped PANI have historically been explained by means of 1D ͑disordered-amorphous͒ 11,31 and 3D ͑ordered-crystalline͒ 32 models. MacDiarmid and co-workers have stressed the experimental evidence that while metallic Pauli susceptibility is detected in the ordered regions, predominantly spinless bipolarons are present in the disordered doped PANI.…”

Bias driven coherent carrier dynamics in a two-dimensional aperiodic potential

“…[10,11,12,13,14,17,15,16,18] Thus, a short-range correlated disorder was found to stabilize the extended states at special resonance energies. In the thermodynamic limit, such extended states form a set of null measure in the density of states, [10,11,12,13,14] implying the absence of mobility edges in these systems. In contrast, systems with long-range correlations of disorder support a set of delocalized states within a finite bandwidth, [15,16] giving rise to mobility edges.…”