Weak localization, electron-electron interaction, and metal-insulator transition in ion-implanted polymers

Kr⁺ ion implanted rigid rod PBO, PBT and ladder BBL polymers show similar transport and optical behavior, with room temperature conductivities increasing from 10^-12 S/cm to ∼ 10² S/cm after implantation. A metal-insulator transition is observed at T_c ∼ 30 K. Above T_c, the results of a negative magnetoresistance (ΔR/R(H,T)), a weakly temperature dependent conductivity (σ(T) α T^p, 0.5 < p < 1), a linearly T-dependent thermopower (S(T)), a Pauli spin susceptibility (χ^Pauli), and a large microwave dielectric constant (ε_mw) resemble those of disordered metals in which weak localization and e-e interactions dominate the charge transport. Below T_c, the changed behavior of positive ΔR/R, stronger T-dependent σ, and 1/T-dependent S with a continued large and positive ε_mw all suggest an increased e-e interaction effect which opens up a 'Coulomb' gap (∼2meV), likely due to the enhanced localization. The density of states (DOS) is zero only at the center of the gap in accord with the theoretical prediction of the 'Coulomb' gap.